umu.sePublikationer
Ändra sökning
Avgränsa sökresultatet
123456 101 - 150 av 291
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 101. Holkundkar, Amol R.
    et al.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Transition from wakefield generation to soliton formation2018Ingår i: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, nr 4, artikel-id 043204Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    It is well known that when a short laser pulse propagates in an underdense plasma, it induces longitudinal plasma oscillations at the plasma frequency after the pulse, typically referred to as the wakefield. However, for plasma densities approaching the critical density, wakefield generation is suppressed, and instead the EM-pulse (electromagnetic pulse) undergoes nonlinear self-modulation. In this article we have studied the transition from the wakefield generation to formation of quasi-solitons as the plasma density is increased. For this purpose we have applied a one-dimensional relativistic cold fluid model, which has also been compared with particle-in-cell simulations. A key result is that the energy loss of the EM-pulse due to wakefield generation has its maximum for a plasma density of the order 10% of the critical density, but that wakefield generation is sharply suppressed when the density is increased further.

  • 102. Hubert, Benoit
    et al.
    Aikio, Anita T.
    Amm, Olaf
    Pitkänen, Timo
    Kauristie, Kirsti
    Milan, Steve M.
    Cowley, Stan W. H.
    Gérard, Jean-Claude
    Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations2010Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, s. 883-892Artikel i tidskrift (Refereegranskat)
  • 103.
    Hubert, Benoit
    et al.
    Liège, Belgium.
    Aikio, Anita T.
    Oulu, Finland.
    Amm, Olaf
    Helsinki, Finland.
    Pitkänen, Timo
    Oulu, Finland.
    Kauristie, Kirsti
    Helsinki, Finland.
    Milan, Steve M.
    Leicester, UK.
    Cowley, Stan W. H.
    Leicester, UK.
    Gérard, Jean-Claude
    Liège, Belgium.
    Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations2010Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, s. 883-892Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from observations of the EISCAT radar facility are used to estimate the latitude of the PCB along the meridian of the EISCAT VHF beam. The second method utilizes global images of proton aurora obtained by the IMAGE satellite FUV SI12 instrument. These methods are applied to three different intervals. In two events, the agreement between the methods is good and the mean of the difference is within the resolution of the observations. In a third event, the PCB estimated from EISCAT data is located several degrees poleward of that obtained from the IMAGE FUV SI12 instrument. Comparison of the reconnection electric field estimated from the two methods shows that highresolution measurements both in time and space are needed to capture the variations in reconnection electric field during substorm expansion. In addition to the two techniques introduced above to determine the PCB location, we also use a search for the location of the reversal of the east-west component of the equivalent current known as the magnetic convection reversal boundary (MCRB). The MCRB from the MIRACLE magnetometer chain mainly follows the motion of the polar cap boundary during different substorm phases, but differences arise near the Harang discontinuity.

  • 104. Hølland, Vidar
    et al.
    Pitkänen, Timo
    Baumann, Gisela
    Mellemstrand, Gard
    Søreide, Asbjørn
    Njåstad, S.
    Slettebakken, K.
    Enell, Carl-Fredrik
    Virtanen, Ilkka I.
    Vierinen, Juha
    Tanskanen, Eija I.
    Nordic ionospheric sounding rocket seeding experiment (NISSE)2009Ingår i: Proc. 19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7-11 June 2009, 2009, s. 467-472Konferensbidrag (Övrigt vetenskapligt)
  • 105.
    Hølland, Vidar
    et al.
    Bergen, Norway.
    Pitkänen, Timo
    Oulu, Finland.
    Baumann, Gisela
    Copenhagen, Denmark; Helsinki, Finland.
    Mellemstrand, Gard
    Bergen, Norway.
    Søreide, Asbjørn
    Bergen, Norway.
    Njåstad, S.
    Bergen, Norway.
    Slettebakken, K.
    Bergen, Norway.
    Enell, Carl-Fredrik
    Sodankylä, Finland.
    Virtanen, Ilkka I.
    Oulu, Finland.
    Vierinen, Juha
    Sodankylä, Finland.
    Tanskanen, Eija I.
    Bergen, Norway; Helsinki, Finland.
    Nordic ionospheric sounding rocket seeding experiment (NISSE)2009Ingår i: Proc. 19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7-11 June 2009, 2009, s. 467-472Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    The Nordic Ionospheric Sounding rocket Seeding Experiment (NISSE) is a student research project in the REXUS student rocket experiment program. The NISSE experiment flew onboard a sounding rocket, the REXUS 6, which was launched at the Esrange rocket range on March 12, 2009. In the NISSE experiment about 8.3 kg of water was to be released into the ionosphere at the REXUS 6 apogee altitude of about 95 km. The EISCAT UHF incoherent scatter radar system located in Northern Fennoscandia, was in action for detection and observation of the effects of the released water on the upper atmosphere. Although NISSE was only partially successful, we are motivated to present here the conceptual description of the experiment and discuss the experience gained from an educational point of view.

  • 106.
    Janson, Oskar
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gravitational Wave Interaction in a Vlasov Plasma2013Självständigt arbete på avancerad nivå (yrkesexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Gravitationsvågor förutses i Einsteins allmänna relativitetsteori och har så här

    långt endast blivit indirekt detekterade. Den första direkta detektionen är dock

    bara en tidsfråga och observatorier runtom i världen arbetar hårt för att lyckas

    med den. Då gravitationsvågor väl är detekterade sägs de ha stor potential inom

    astronomi.

    För att man ska kunna tolka mätdata från gravitationsvågor behöver man veta

    hur vågen beter sig i propagationsmediet. Av den anledningen är syftet med detta

    examensarbete att undersöka hur gravitationsvågor beter sig i ett magnetiserat

    plasma.

    Med hjälp av en modell för kinetisk plasma, Einsteins fältekvationer och tetradformalism

    härleds en allmän lösning för en gravitationsvåg som propagerar i en

    magnetiserad plasma fram. Lösningen används därefter för att hitta dispersionsrelationen

    för gravitationsvågen givet två specialfall: fallet för Alfvénresonans och

    fallet för cyklotronresonens. Alfvénfallet är redan studerat i tidigare litteratur och

    resultatet man hittar visar sig stämma överens med det tidigare funna resultatet

    som säger att det inte har en märkbar påverkan på vågen. Cyklotronresonansfallet

    är nytt och valdes eftersom att det kan förstärka de effekter som partiklarna har på

    gravitationsvågen.

    De båda specialfallen studeras närmare med avseende på detektion av en dispersion

    inducerad av mediet. Påverkan på gravitationsvågens propagation sluts

    dock till att vara för liten för att den ska bli uppmätt i de undersökta fallen.

  • 107.
    Jenab, S. M. Hosseini
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Head-on collision of nonlinear solitary solutions to Vlasov-Poisson equations2019Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, nr 2, artikel-id 022303Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nonlinear solitary solutions to the Vlasov-Poisson set of equations are studied in order to investigate their stability by employing a fully kinetic simulation approach. This study is carried out in the ion-acoustic regime for a collisionless, electrostatic, and Maxwellian electron-ion plasma. The trapped population of electrons is modeled based on the well-known Schamel distribution function. Head-on mutual collisions of nonlinear solutions are performed in order to examine their collisional stability. The findings include three major aspects: (I) These nonlinear solutions are found to be divided into three categories based on their Mach numbers, i.e., stable, semi-stable, and unstable. Semi-stable solutions indicate a smooth transition from stable to unstable solutions for the increasing Mach number. (II) The stability of solutions is traced back to a condition imposed on averaged velocities, i.e., net neutrality. It is shown that a bipolar structure is produced in the flux of electrons, early in the temporal evolution. This bipolar structure acts as the seed of the net-neutrality instability, which tips off the energy balance of nonlinear solution during collisions. As the Mach number increases, the amplitude of the bipolar structure grows and results in a stronger instability. (III) It is established that during mutual collisions, a merging process of electron holes can occur to a variety of degrees, based on their velocity characteristics. Specifically, the number of rotations of electron holes around each other (in the merging phase) varies. Furthermore, it is observed that in the case of a non-integer number of rotations, two electron holes exchange their phase space cores.

  • 108.
    Jenab, S. M. Hosseini
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Centre for Space Research, North-West University, Potchefstroom Campus, Potchefstroom, South Africa.
    Spanier, F.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Centre for Space Research, North-West University, Potchefstroom Campus, Potchefstroom, South Africa.
    Scattering of electron holes in the context of ion-acoustic regime2019Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, nr 3, artikel-id 034502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mutual collisions between ion-acoustic solitary waves are studied based on a fully kinetic simulation approach. Two cases, small and large relative velocities, are studied, and the effect of trapped electron population on the collision process is focused upon. It is shown that, for the case of small relative velocity, the repelling force between the trapped populations of electrons results in scattering of electron holes. However, this phenomenon cannot be witnessed if the relative velocity is considerably high since the impact of trapped population remains very weak.

  • 109.
    Jenab, Seyyed Mehdi Hosseini
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Spanier, F.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    A study of the stability properties of Sagdeev solutions in the ion-acoustic regime using kinetic simulations2018Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 25, nr 7, artikel-id 072304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Sagdeev pseudo-potential approach has been employed extensively in theoretical studies to determine large-amplitude (fully) nonlinear solutions in a variety of multi-species plasmas. Although these solutions are repeatedly considered as solitary waves (and even solitons), their temporal stability has never been proven. In this paper, a numerical study of the Vlasov-Poisson system is made to follow their temporal evolution in the presence of numerical noise and thereby test their long-time propagation stability. Considering the ion-acoustic regime, both constituents of the plasma, i.e., electrons and ions are treated following their distribution functions in these sets of fully-kinetic simulations. The findings reveal that the stability of the Sagdeev solution depends on a combination of two parameters, i.e., velocity and trapping parameter. It is shown that there exists a critical value of trapping parameter for both fast and slow solutions which separates stable from unstable solutions. In the case of stable solutions, it is shown that these nonlinear structures can propagate for long periods, which confirms their status as solitary waves. Stable solutions are reported for both Maxwellian and Kappa distribution functions. For unstable solutions, it is demonstrated that the instability causes the Sagdeev solution to decay by emitting ion-acoustic wave-packets on its propagation trail. The instability is shown to take place in a large range of velocities and even for Sagdeev solutions with a velocity much higher than the ion-sound speed. Besides, in order to validate our simulation code, two precautionary measures are taken. First, the well-known effect of the ion dynamics on a stationary electron hole solution is presented as a benchmarking test of the approach. Second, In order to verify the numerical accuracy of the simulations, the conservation of energy and entropy is presented.

  • 110.
    Johansson, Petter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Contribution from Spin-Orbit Coupling to the Langmuir Wave Dispersion Relation in Magnetized Plasmas2010Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [en]

    This thesis analyses the effect spin-orbit coupling has on the dispersion of Langmuir waves in magnetized plasmas, using recently developed kinetic theories of plasmas including quantummechanical and relativistic effects. Two new wave modes appearclose to the resonance  = ( g/2 − 1) , where  is the cyclotron frequency and g is the electron gyromagnetic ratio. Forconsidered long wave lengths the deviation from this resonanceis very small. The wave modes are also very weakly damped.

  • 111. Juusola, Liisa
    et al.
    Kubyshkina, Marina
    Nakamura, Rumi
    Pitkänen, Timo
    Amm, Olaf
    Kauristie, Kirsti
    Partamies, Noora
    Rème, Henry
    Snekvik, Kristian
    Whiter, Daniel
    Ionospheric signatures of a plasma sheet rebound flow during a substorm onset2013Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 350-363Artikel i tidskrift (Refereegranskat)
  • 112. Karimov, A. R.
    et al.
    Shatokhin, V. L.
    Yu, M. Y.
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Linköping University, Linköping ,SE-58183, Sweden.
    The processes of nonequilibrium exchange in rotating plasma flows2016Ingår i: II CONFERENCE ON PLASMA & LASER RESEARCH AND TECHNOLOGIES, 2016, Vol. 747, artikel-id 012077Konferensbidrag (Refereegranskat)
    Abstract [en]

    The mechanisms of energy/momentum exchange in rotating and compressing plasma flows have been discussed. It has been shown that such flows are capable of transforming the energy of different degrees of freedom into the energy of one degree owing to the interaction of the coupled nonlinear radial, axial and azimuthal electron-ion oscillations. These processes may lead to the additional acceleration of the flow in azimuthal or axial direction so they might be instrumental for the creation of space thrusters employing pulse transformations for propulsion.

  • 113. Karimov, A. R.
    et al.
    Yu, M. Y.
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics, Linköping University, SE-58183 Linköping, Sweden.
    Expansion of a cold non-neutral plasma slab2014Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, nr 12, artikel-id 122304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Expansion of the ion and electron fronts of a cold non-neutral plasma slab with a quasi-neutral core bounded by layers containing only ions is investigated analytically and exact solutions are obtained. It is found that on average, the plasma expansion time scales linearly with the initial inverse ion plasma frequency as well as the degree of charge imbalance, and no expansion occurs if the cold plasma slab is stationary and overall neutral. However, in both cases, there can exist prominent oscillations on the electron front. 

  • 114. Kawamoto, Tatsuro
    et al.
    Rosvall, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Estimating the resolution limit of the map equation in community detection2015Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, nr 1, s. 012809-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A community detection algorithm is considered to have a resolution limit if the scale of the smallest modules that can be resolved depends on the size of the analyzed subnetwork. The resolution limit is known to prevent some community detection algorithms from accurately identifying the modular structure of a network. In fact, any global objective function for measuring the quality of a two-level assignment of nodes into modules must have some sort of resolution limit or an external resolution parameter. However, it is yet unknown how the resolution limit affects the so-called map equation, which is known to be an efficient objective function for community detection. We derive an analytical estimate and conclude that the resolution limit of the map equation is set by the total number of links between modules instead of the total number of links in the full network as for modularity. This mechanism makes the resolution limit much less restrictive for the map equation than for modularity; in practice, it is orders of magnitudes smaller. Furthermore, we argue that the effect of the resolution limit often results from shoehorning multilevel modular structures into two-level descriptions. As we show, the hierarchical map equation effectively eliminates the resolution limit for networks with nested multilevel modular structures.

  • 115. Keiling, A.
    et al.
    Marghitu, O.
    Vogt, J.
    Amm, O.
    Bunescu, C.
    Constantinescu, V.
    Frey, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, T.
    Nakamura, R.
    Nilsson, H.
    Semeter, J.
    Sorbalo, E.
    Magnetosphere-ionosphere coupling of global Pi2 pulsations2014Ingår i: Journal of Geophysical Research. Space Physics, ISSN 2169-9380, Vol. 119, nr 4, s. 2717-2739Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This super global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfven and fast modes, field line resonance, and possibly a forced cavity-type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.

  • 116.
    Kero, Johan
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Szasz, Csilla
    National Institute of Polar Research.
    Nakamura, Takuji
    National Institute of Polar Research.
    Terasawa, T.
    Institute for Cosmic Ray Research, University of Tokyo.
    Miyamoto, H.
    Department of Earth Science and Astronomy, University of Tokyo.
    Nishimura, K.
    National Institute of Polar Research.
    A meteor head echo analysis algorithm for the lower VHF band2012Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 30, nr 4, s. 639-659Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have developed an automated analysis scheme for meteor head echo observations by the 46.5 MHz Middle and Upper atmosphere (MU) radar near Shigaraki, Japan (34.85A degrees N, 136.10A degrees E). The analysis procedure computes meteoroid range, velocity and deceleration as functions of time with unprecedented accuracy and precision. This is crucial for estimations of meteoroid mass and orbital parameters as well as investigations of the meteoroid-atmosphere interaction processes. In this paper we present this analysis procedure in detail. The algorithms use a combination of single-pulse-Doppler, time-of-flight and pulse-to-pulse phase correlation measurements to determine the radial velocity to within a few tens of metres per second with 3.12 ms time resolution. Equivalently, the precision improvement is at least a factor of 20 compared to previous single-pulse measurements. Such a precision reveals that the deceleration increases significantly during the intense part of a meteoroid's ablation process in the atmosphere. From each received pulse, the target range is determined to within a few tens of meters, or the order of a few hundredths of the 900 m long range gates. This is achieved by transmitting a 13-bit Barker code oversampled by a factor of two at reception and using a novel range interpolation technique. The meteoroid velocity vector is determined from the estimated radial velocity by carefully taking the location of the meteor target and the angle from its trajectory to the radar beam into account. The latter is determined from target range and bore axis offset. We have identified and solved the signal processing issue giving rise to the peculiar signature in signal to noise ratio plots reported by Galindo et al. (2011), and show how to use the range interpolation technique to differentiate the effect of signal processing from physical processes.

  • 117.
    Kero, Johan
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Determination of meteoroid physical properties from tristatic radar observations2008Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, s. 2217-2228Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work we give a review of the meteor head echo observations carried out with the tristatic 930MHz EISCAT UHF radar system during four 24 h runs between 2002 and 2005 and compare these with earlier observations. A total number of 410 tristatic meteors were observed. We describe a method to determine the position of a compact radar target in the common volume monitored by the three receivers and demonstrate its applicability for meteor studies. The inferred positions of the meteor targets have been utilized to estimate their velocities, decelerations and directions of arrival as well as their radar cross sections with unprecedented accuracy. The velocity distribution of the meteoroids is bimodal with peaks at 35–40 km/s and 55–60 km/s, and ranges from 19–70 km/s. The estimated masses are between 10−9–10−5.5 kg. There are very few detections below 30 km/s. The observations are clearly biased to high-velocity meteoroids, but not so biased against slow meteoroids as has been presumed from previous tristatic measurements. Finally, we discuss how the radial deceleration observed with a monostatic radar depends on the meteoroid velocity and the angle between the trajectory and the beam. The finite beamwidth leads to underestimated meteoroid masses if radial velocity and deceleration of meteoroids approaching the radar are used as estimates of the true quantities in a momentum equation of motion.

  • 118.
    Kero, Johan
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Meisel, David
    SUNY-Geneseo, NY, USA.
    Three-dimensional radar observation of a submillimeter meteoroid fragmentation2008Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, nr L04101, s. 5-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present and discuss two examples of pulsating meteor events observed with the tristatic 930 MHz EISCAT UHF radar system. One of them provides the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. The received power fluctuates regularly in the time profiles of all three receivers, but the fluctuation frequencies are different. The pulsations are interpreted as being due to interference from two distinct scattering centers and the three detected pulsation rates are utilized to calculate the differential velocity of the fragments. The result is consistent with interference from two fragments of unequal cross-sectional area over mass ratio, separating from each other due to different deceleration along the trajectory of their parent meteoroid. The other meteor event is an example of a meteoroid undergoing quasi-continuous disintegration. This manifests itself as simultaneous pulsations at all three receivers. Both observations indicate a head echo target upper size limit of the order of half the 32 cm radar wavelength.

  • 119.
    Kero, Johan
    et al.
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    On the meteoric head echo radar cross section angular dependence2008Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, nr L07101, s. 5-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present radar cross section (RCS) measurements of meteor head echoes observed with the tristatic 930 MHz EISCAT UHF radar system. The three receivers offer a unique possibility to accurately compare the monostatic RCS of a meteor target with two simultaneously probed bistatic RCSs at different aspect angles. Meteoroids from all possible directions entering the common volume monitored by the three receivers are detected, out to an aspect angle of 130° from the meteoroid trajectories. The RCS of individual meteors as observed by the three receivers are equal within the accuracy of the measurements. This is consistent with an essentially isotropic scattering process as has previously been inferred from polarization measurements by S. Close et al. (2002). There is a very weak trend present in our data suggesting that the RCS may decrease at a rate of 0.2 dB per 10° with increasing aspect angle.

  • 120. Keshmiri, V.
    et al.
    Larsen, Christian
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Edman, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Forchheimer, R.
    Tu, D.
    A Current Supply with Single Organic Thin-Film Transistor for Charging Supercapacitors2016Ingår i: THIN FILM TRANSISTORS 13 (TFT 13) / [ed] Kuo, Y, ELECTROCHEMICAL SOC INC , 2016, Vol. 75, nr 10, s. 217-222Konferensbidrag (Refereegranskat)
    Abstract [en]

    We present a current supply, comprising a single organic thin-film transistor (OTFT), for the charging of supercapacitors. The current supply takes power from the electric grid (115 V AC, US standard), converts the AC voltage to a quasi-constant DC current (similar to 0.1 mA) regardless of the impedance of the load, and charges the supercapacitor. Solution-processed OTFTs based on the popular polymeric semiconductor poly(3-hexylthiophene- 2,5-diyl) have been developed to rectify the 115 V AC voltage. A diodeconfigured OTFT was used as a half-wave rectifier. The single OTFT current supply was demonstrated to charge a 220 mF supercapacitor to 1 V directly using 115 V AC voltage as the input. This work paves the road towards all-printable supercapacitor energy-storage systems with integrated chargers, which enable direct charging from a power outlet.

  • 121. Khrennikov, K.
    et al.
    Wenz, J.
    Buck, A.
    Xu, Jiancai
    Heigoldt, M.
    Veisz, Laszlo
    MPI für Quantenoptik, Garching, Germany.
    Karsch, S.
    Tunable All-Optical Quasimonochromatic Thomson X-Ray Sourcein the Nonlinear Regime2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, nr 19, artikel-id 195003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an all-laser-driven, energy-tunable, and quasimonochromatic x-ray source based on Thomson scattering from laser-wakefield-accelerated electrons. One part of the laser beam was used to drive a few-fs bunch of quasimonoenergetic electrons, while the remainder was backscattered off the bunch at weakly relativistic intensity. When the electron energy was tuned from 17–50 MeV, narrow x-ray spectra peaking at 5–42 keV were recorded with high resolution, revealing nonlinear features. We present a large set of measurements showing the stability and practicality of our source.

  • 122.
    Khurana, Krishan K.
    et al.
    Institute of Geophysics and Planetary Physics and Dept. of Earth, Planetary and Space Sciences, University of California at Los Angeles, CA, 90095, USA.
    Fatemi, Shahab
    Space Sciences Laboratory, University of California, Berkeley, California, USA.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Roussos, Elias
    Max Planck Institute, Göttingen, Germany.
    Krupp, Norbert
    Max Planck Institute, Göttingen, Germany.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Russell, Christopher T.
    Institute of Geophysics and Planetary Physics and Dept. of Earth, Planetary and Space Sciences, University of California at Los Angeles, CA, 90095, USA.
    Dougherty, Michele K.
    Imperial College, London, U.K.
    The role of plasma slowdown in the generation of Rhea's Alfvén wings2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 2, s. 1778-1788Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Alfvén wings are known to form when a conducting or mass-loading object slows down a flowing plasma in its vicinity. Alfvén wings are not expected to be generated when an inert moon such as Rhea interacts with Saturn's magnetosphere, where the plasma impacting the moon is absorbed and the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea, Cassini clearly observed magnetic field signatures consistent with Alfvén wings. In addition, observations from a high-inclination flyby (Distance > 100 R Rh ) of Rhea on 3 June 2010 showed that the Alfvén wings continue to propagate away from Rhea even at this large distance. We have performed three-dimensional hybrid simulations of Rhea's interaction with Saturn's magnetosphere which show that the wake refilling process generates a plasma density gradient directed in the direction of corotating plasma. The resulting plasma pressure gradient exerts a force directed toward Rhea and slows down the plasma streaming into the wake along field lines. As on the same field lines, outside of the wake, the plasma continues to move close to its full speed, this differential motion of plasma bends the magnetic flux tubes, generating Alfvén wings in the wake. The current system excited by the Alfvén wings transfers momentum to the wake plasma extracting it from plasma outside the wake. Our work demonstrates that Alfvén wings can be excited even when a moon does not possess a conducting exosphere.

  • 123.
    Kobyakov, Dmitry N.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Applied Physics of the Russian Academy of Sciences, 603950 Nizhny Novgorod, Russia; Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany.
    Pethick, C. J.
    Two-component Superfluid Hydrodynamics of Neutron Star Cores2017Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 836, nr 2, artikel-id 203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the hydrodynamics of the outer core of a neutron star under conditions when both neutrons and protons are superfluid. Starting from the equation of motion for the phases of the wave functions of the condensates of neutron pairs and proton pairs, we derive the generalization of the Euler equation for a one-component fluid. These equations are supplemented by the conditions for conservation of neutron number and proton number. Of particular interest is the effect of entrainment, the fact that the current of one nucleon species depends on the momenta per nucleon of both condensates. We find that the nonlinear terms in the Euler-like equation contain contributions that have not always been taken into account in previous applications of superfluid hydrodynamics. We apply the formalism to determine the frequency of oscillations about a state with stationary condensates and states with a spatially uniform counterflow of neutrons and protons. The velocities of the coupled sound-like modes of neutrons and protons are calculated from properties of uniform neutron star matter evaluated on the basis of chiral effective field theory. We also derive the condition for the two-stream instability to occur.

  • 124. Krasheninnikov, SI
    et al.
    Shevchenko, VI
    Shukla, Padma Kant
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institut für Theoretische Physik IV and Centre for Plasma Science and Astrophysics, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom; Centre for Fundamental Physics (CfFP), Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom; Centro de Fisica, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal.
    Spinning of a charged dust particle in a magnetized plasma2007Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 361, nr 1-2, s. 133-135Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a novel mechanism for spinning of a charged dust particle in a magnetized plasma. Specifically, we show that the dust particle spinning is caused by a torque that is produced due to interactions between the electric field E and an asymmetric dust particle electric dipole that is induced by the cross-field (viz. the E x B, where B is the external magnetic field) plasma flow. The torque acting on the dust particle is proportional to vertical bar E vertical bar(2), so that both laminar and turbulent electric fields can cause dust particle spinning. For plasma parameters typical of tokamak edges. the characteristic dust spinning frequency is of the order of several hundred kHz.

  • 125.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Plasma Interactions with Icy Bodies in the Solar System2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Här studerar jag “plasmaväxelverkan med isiga kroppar i solsystemet”, det vill säga, min strävan är att förstå de grundläggande processerna som styr sådana interaktioner. Genom att använda numerisk modellering i kombination med observationer på plats vid himlakropparna kan man förstå sig på deras interna strukturer och rymdmiljöer.

    Efter en bred översikt över de fysiska lagar som styr ett rymdplasma följer en mer detaljerad del. Denna innehåller metoder för hur man kan modellera växelverkan mellan rymdplasma och isiga kroppar. Numerisk modellering av rymdplasma appliceras på de isiga himlakropparna Callisto (en måne kring Jupiter), dvärgplaneten Ceres (lokaliserad i asteroidbältet mellan Mars och Jupiter) och kometen 67P/Churyumov-Gerasimenko.

    Det tidsvarierande magnetiska fältet kring Jupiter inducerar strömmar inuti den elektriskt ledande månen Callisto. Dessa strömmar skapar magnetfältsstörningar som tros vara relaterade till ett elektriskt ledande hav under Callistos yta. Plasmaflödet i närheten av Callisto påverkas i hög grad av dessa magnetfältsstörningar. Genom att använda en hybrid-plasma-lösare har växelverkan modellerats, där effekten av magnetisk induktion har inkluderats. Resultaten stämmer väl överens med magnetfältsdata från förbiflygningarna av Callisto (C3 och C9) som gjordes av den obemannade rymdfarkosten Galileo i dess bana kring Jupiter. Den magnetiska konfigurationen som uppstår möjliggör ett inflöde av laddade joner på Callistos baksida. Plasma som träffar ytan slår bort materia och skapar Callistos tunna atmosfär.

    En långtidsstudie av solvindsprotoner sett från rymdfarkosten Rosetta utfördes då kometen 67P/Churyumov-Gerasimenko närmade sig solen. Ultraviolett strålning från solen joniserar det neutrala vattnet i kometens koma (kometens atmosfär). Nyligt joniserade vattenmolekyler plockas upp av solvindsflödet och tvingar solvindsprotonernas banor att böjas av, så att rörelsemängden bevaras. Denna effekt ökar stadigt då kometen närmar sig solen. Solvinden böjs av kraftigt, men förlorar inte mycket energi. Hybridmodellering av solvindens växelverkan bekräftar att kraften som verkar på solvinden till störst del får den att böjas av, medan kraften som verkar till att sänka dess fart är mycket lägre.

    Ceres har enligt observationer av rymdteleskopet Herschel under 2012 och 2013 haft högt utflöde av vattenånga från dess yta. Där har två regioner identifierats som källor för vattenångan. Eftersom Ceres roterar kommer källornas regioner göra det också. Plasmaväxelverkan i närheten av Ceres beror i hög grad på vattenångskällans placeringen, medan det inte gör det långt ifrån Ceres. På global nivå har Ceres en kometliknande växelverkan med solvinden, där störningar i solvinden propagerar långt nedströms från Ceres.

  • 126.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Nilsson, Hans
    Swedish Institute of Space Physics.
    Simon Wedlund, Cyril
    University of Oslo, Department of Physics, Oslo, Norway.
    Kallio, Esa
    Aalto University, Department of Electronics and Nanoengineering, Espoo, Finland.
    Mann, Ingrid
    University of Tromsø, Department of Physics and Technology, Tromsø, Norway.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, Tomas
    KTH Royal Institute of Technology, School of Electrical Engineering, Stockholm, Sweden.
    Energy conversion in cometary atmospheres: Hybrid modeling of 67P/Churyumov-Gerasimenko2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets.

    Methods. We used a hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov-Gerasimenko at a heliocentric distance of 1.5 AU.

    Results. We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads.

    Conclusions. The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.

  • 127.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Fatemi, Shahab
    Space Sciences Laboratory, UC Berkeley.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Ceres interaction with the solar wind2017Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 5, s. 2070-2077Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The solar wind interaction with Ceres is studied for a high water vapor release from its surface using a hybrid model including photoionization. We use a water vapor production rate of 6 kg/s, thought to be due to subsurface sublimation, corresponding to a detection on 6 March 2013 by the Herschel Space Observatory. We present the general morphology of the plasma interactions, both close to Ceres and on a larger scale. Mass loading of water ions causes a magnetic pileup region in front of Ceres, where the solar wind deflects up to 15 ∘ and slows down by 15%. The global plasma interaction with Ceres is not greatly affected by the source location of water vapor nor on gravity, only on the production rate of water vapor. On a global scale, Ceres has a comet-like interaction with the solar wind with observable perturbations farther than 250 Ceres radii downstream of the body.

  • 128.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Khurana, Krishan K.
    Department of Earth and Space Sciences, University of California, Los Angeles.
    Fatemi, Shahab
    Space Sciences Laboratory, University of California, Berkeley.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Callisto plasma interactions: Hybrid modeling including induction by a subsurface ocean2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 6, s. 4877-4889Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    By using a hybrid plasma solver (ions as particles and electrons as a fluid), we have modeled the interaction between Callisto and Jupiter's magnetosphere for variable ambient plasma parameters. We compared the results with the magnetometer data from flybys (C3, C9, and C10) by the Galileo spacecraft. Modeling the interaction between Callisto and Jupiter's magnetosphere is important to establish the origin of the magnetic field perturbations observed by Galileo and thought to be related to a subsurface ocean. Using typical upstream magnetospheric plasma parameters and a magnetic dipole corresponding to the inductive response inside the moon, we show that the model results agree well with observations for the C3 and C9 flybys, but agrees poorly with the C10 flyby close to Callisto. The study does support the existence of a subsurface ocean at Callisto.

  • 129.
    Lue, Charles
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Solar Wind Proton Interactions with Lunar Magnetic Anomalies and Regolith2015Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Rymdmiljön runt månen formas av den växelverkan som sker mellan månen och solvinden. I den föreliggande avhandlingen undersöker vi två aspekter av denna växerverkan, nämligen växelverkan mellan solvindsprotoner och magnetiserade områden i månskorpan, och växelverkan mellan solvindsprotoner och månens ytdamm. Vi använder oss av partikelsensorer på månsatelliten Chandrayaan-1 för att analysera solvindsprotoner som reflekteras från månen, även de protoner som fångar upp en elektron från ytan och reflekteras som neutrala väteatomer. Vi använder oss också av datorsimuleringar för att bygga vidare på de uppmätta resultaten.

    Observationerna från Chandrayaan-1 visar att reflektion av solvindsprotoner från magnetiserade områden är ett vanligt förekommande fenomen på månen, som inträffar även vid magnetiseringar som är utbredda över mindre än 100 km. Vid det största magnetiserade området på månen (1000 km i diameter), reflekteras i genomsnitt ~10% av de infallande solvindsprotonerna. Våra datorsimuleringar visar att dessa protonflöden har globala effekter på månens plasmamiljö. De reflekterade protonerna kan nå månens nattsida. De kan också nå långt uppströms om månen och störa solvindsflödet. I den lokala plasmamiljön vid ett magnetiserat område av storleken 200 km visar våra simuleringar ett förändrat solvindsflöde, där det skapas områden som delvis skyddas från solvinden, likväl som områden som utsätts för mer solvind.

    Vi observerar även solvindsprotoner som reflekterats från ytdammet på månen. Dessa protonflöden är lägre än de från de magnetiska fälten. Reflektionen från ytan varierar mellan ~0.01% och 1% av solvindsflödet, i samband med förändringar i solvindshastigheten. Vi förklarar detta med att partiklarnas laddning bestäms av den hastighet de har när de lämnar måndammet. Vidare undersöker vi hur egenskaperna hos de reflekterade neutrala väteatomerna beror på solvindstemperaturen. Vi skapar en modell för att beskriva sambandet och använder sedan denna modell för att studera hur solvinden faller in mot månens yta när den befinner sig i jordens magnetoskikt, där jordens magnetfält orsakar en upphettning av solvindsflödet. Resultaten från dessa och andra studier använder vi sedan för att modellera solvindsreflektion från planeten Merkurius yta, för jämförelse med framtida observationer.

  • 130.
    Lue, Charles
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaan, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bhardwaj, Anil
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Dhanya, M. B.
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern, Switzerland.
    Strong influence of lunar crustal fields on the solar wind flow2011Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 38, artikel-id L03202Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We discuss the influence of lunar magnetic anomalies on the solar wind and on the lunar surface, based on maps of solar wind proton fluxes deflected by the magnetic anomalies. The maps are produced using data from the Solar WInd Monitor (SWIM) onboard the Chandrayaan-1 spacecraft. We find a high deflection efficiency (average ∼10%, locally ∼50%) over the large-scale (>1000 km) regions of magnetic anomalies. Deflections are also detected over weak (<3 nT at 30 km altitude) and small-scale (<100 km) magnetic anomalies, which might be explained by charge separation and the resulting electric potential. Strong deflection from a wide area implies that the magnetic anomalies act as a magnetosphere-like obstacle, affecting the upstream solar wind. It also reduces the implantation rate of the solar wind protons to the lunar surface, which may affect space weathering near the magnetic anomalies.

  • 131.
    Lue, Charles
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Saito, Yoshifumi
    Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan.
    Nishino, Masaki
    Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan.
    Asamura, Kazushi
    Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bhardwaj, Anil
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern, Switzerland.
    Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheathManuskript (preprint) (Övrigt vetenskapligt)
  • 132.
    Lue, Charles
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Barabash, Stas
    Saito, Yoshifumi
    Nishino, Masaki
    Wieser, Martin
    Asamura, Kazushi
    Bhardwaj, Anil
    Wurz, Peter
    Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath2016Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 1, s. 432-445Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study hydrogen energetic neutral atom (ENA) emissions from the lunar surface, when the Moon is inside the terrestrial magnetosheath. The ENAs are generated by neutralization and backscattering of incident protons of solar wind origin. First, we model the effect of the increased ion temperature in the magnetosheath (>10 times larger than that in the undisturbed solar wind) on the ENA scattering characteristics. Then, we apply these models to ENA measurements by Chandrayaan-1 and simultaneous ion measurements by Kaguya at the Moon, in the magnetosheath. We produce maps of the ENA scattering fraction, covering a region at the lunar near-side that includes mare and highland surfaces and several lunar magnetic anomalies. We see clear signatures of plasma shielding by the magnetic anomalies. The maps are made at different lunar local times, and the results indicate an extended influence and altered morphology of the magnetic anomalies at shallower incidence angles of the magnetosheath protons. The scattering fraction from the unmagnetized regions remains consistent with that in the undisturbed solar wind (10%-20%). Moreover, the observed ENA energy spectra are well reproduced by our temperature-dependent model. We conclude that the ENA scattering process is unchanged in the magnetosheath. Similarly to the undisturbed solar wind case, it is only magnetic anomalies that provide contrast in the ENA maps, not any selenomorphological features such as mare and highland regions.

  • 133.
    Lue, Charles
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna.
    Bhardwaj, Anil
    Swedish Institute of Space Physics, Kiruna.
    Wurz, Peter
    Swedish Institute of Space Physics, Kiruna.
    Chandrayaan-1 observations of backscattered solar wind protons from the lunar regolith: Dependence on the solar wind speed2014Ingår i: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 119, nr 5, s. 968-975Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the backscattering of solar wind protons from the lunar regolith using the Solar Wind Monitor of the Sub-keV Atom Reflecting Analyzer on Chandrayaan-1. Our study focuses on the component of the backscattered particles that leaves the regolith with a positive charge. We find that the fraction of the incident solar wind protons that backscatter as protons, i.e., the proton-backscattering efficiency, has an exponential dependence on the solar wind speed that varies from similar to 0.01% to similar to 1% for solar wind speeds of 250km/s to 550km/s. We also study the speed distribution of the backscattered protons in the fast (similar to 550km/s) solar wind case and find both a peak speed at similar to 80% of the solar wind speed and a spread of similar to 85km/s. The observed flux variations and speed distribution of the backscattered protons can be explained by a speed-dependent charge state of the backscattered particles.

  • 134.
    Lue, Charles
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Box 812, SE-98128 Kiruna, Sweden; Department of Physics and Astronomy, University of Iowa, 30 N Dubuque St, IA 52242 Iowa City, United States.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bhardwaj, Anil
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern, Switzerland.
    Asamura, Kazushi
    Institute of Space and Astronautical Science, Japan Exploration Agency, Sagamihara, Japan.
    Solar wind scattering from the surface of Mercury: Lessons from the Moon2017Ingår i: Icarus, ISSN 0019-1035, Vol. 296, s. 39-48Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We discuss the surface-scattering of solar wind protons at Mercury based on observed scattering characteristics from lunar regolith. The properties of the impinging plasma are expected to be different between different regions on Mercury, and between Mercury and the Moon. Here, we review the expected Hermean plasma conditions and lunar empirical scattering models. We present observed and modeled energy spectra for scattered protons and hydrogen energetic neutral atoms (ENAs) for three cases of very different plasma conditions at the Moon. Then, we simulate scattering from the Hermean surface by applying the empirical models to four different scenarios of plasma precipitation on Mercury. The results suggest that surface-scattering is a strong source of ENAs at Mercury (up to similar to 10(8) cm(-2) s(-1)), which can be very useful for remote-sensing of the plasma conditions at the surface. Protons scattered from the surface back into space are also expected with high fluxes up to similar to 10(7) cm(-2) s(-1), and may be important for wave generation and the filling in of the loss cone of mirroring and quasi-trapped populations. Scattered protons at the cusp region (of similar to 10(6) cm(-2) s(-1)) can potentially be detected by orbiters as outflowing protons within the loss cone.

  • 135.
    Lundberg, Petter
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Lindh, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Tang, Shi
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Edman, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Toward Efficient and Metal-Free Emissive Devices: A Solution Processed Host Guest Light-Emitting Electrochemical Cell Featuring Thermally Activated Delayed Fluorescence2017Ingår i: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, nr 34, s. 28810-28816Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The next generation of emissive devices should preferably be efficient, low-cost, and environmentally sustainable, and as such utilize all electrically generated excitons (both singlets and triplets) for the light emission, while being free from rare metals such as iridium. Here, we report on a step toward this vision through the design, fabrication, and operation of a host guest light-emitting electrochemical cell (LEC) featuring an organic thermally activated delayed fluorescence (TADF) guest that harvests both singlet and triplet excitons for the emission. The rare-metal-free active material also consists of a polymeric electrolyte and a polymeric compatibilizer for the facilitation of a cost-efficient and scalable solution-based fabrication, and for the use of air-stable electrodes. We report that such TADF-LEC devices can deliver uniform green light emission with a maximum luminance of 228 cd m(-2) when driven by a constant-current density of 770 A m(-2), and 760 cd m(-2) during a voltage ramp, which represents a one-order-of-magnitude improvement in comparison to previous TADF-emitting LECs.

  • 136.
    Lundin, Joakim
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    QED and collective effects in vacuum and plasmas2010Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The theory of quantum electrodynamics (QED) was born out of an attempt to merge Einsteins theory of special relativity and quantum mechanics. Einsteins energy/mass equivalence together with Heisenberg's uncertainty principle allows for particle pairs to be spontaneously created and annihilated in vacuum. These spontaneous fluctuations gives the quantum vacuum properties analogous to that of a nonlinear medium. Although these fluctuations in general does not give note of themselves, effects due to their presence can be stimulated or enhanced through external means, such as boundary conditions or electromagnetic fields. Whereas QED has been very well tested in the high-energy, low-intensity regime using particle accelerators, the opposite regime where the photon energy is low but instead the intensity is high is still to a large degree not investigated. This is expected to change with the rapid progress of modern high-power laser-systems.

    In this thesis we begin by studying the QED effect of photon-photon scattering. This process has so far not been successfully verified experimentally, but we show that this may change already with present day laser powers. We also study QED effects due to strong magnetic fields. In particular, we obtain an analytical description for vacuum birefringence valid at arbitrary field strengths. Astrophysics already offer environments where QED processes may be influential, e.g. in neutron star and magnetar environments. For astrophysical purposes we investigate how effects of QED can be implemented in plasma models. In particular, we study QED dispersive effects due to weak rapidly oscillating fields, nonlinear effects due to slowly varying strong fields, as well as QED effects in strongly magnetized plasmas. Effects of quantum dispersion and the electron spin has also been included in an extended plasma description, of particular interest for dense and/or strongly magnetized systems.

  • 137.
    Lundin, Joakim
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Zamanian, Jens
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marklund, Mattias
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Short wavelength electromagnetic propagation in magnetized quantum plasmas2007Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, nr 6, s. 2112-7 sidorArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    The quantum electrodynamical (QED) short wavelength correction on plasma wave propagation for a nonrelativistic quantum plasma is investigated. A general dispersion relation for a thermal multicomponent quantum plasma is derived. It is found that the classical dispersion relation for any wave mode can be modified to include quantum and short wavelength QED effects by simple substitutions of the thermal velocity and the plasma frequency. Furthermore, the dispersion relation has been modified to include QED effects of strong magnetic fields. It is found that strong magnetic fields together with the short wavelength QED correction will induce dispersion both in vacuum and in otherwise nondispersive plasma modes. Applications to laboratory and astrophysical systems are discussed.

  • 138. Ma, Guangjin
    et al.
    Dallari, William
    Borot, Antonin
    Krausz, Ferenc
    Yu, Wei
    Tsakiris, George D.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Intense isolated attosecond pulse generation from relativistic laser plasmas using few-cycle laser pulses2015Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, nr 3, artikel-id 033105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have performed a systematic study through particle-in-cell simulations to investigate the generation of attosecond pulse from relativistic laser plasmas when laser pulse duration approaches the few-cycle regime. A significant enhancement of attosecond pulse energy has been found to depend on laser pulse duration, carrier envelope phase, and plasma scale length. Based on the results obtained in this work, the potential of attaining isolated attosecond pulses with ∼100 μJ energy for photons >16 eV using state-of-the-art laser technology appears to be within reach.

  • 139. Madsen, B.
    et al.
    Wedlund, C. Simon
    Eriksson, A.
    Goetz, C.
    Karlsson, T.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Spicher, A.
    Henri, P.
    Vallieres, X.
    Miloch, W. J.
    Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko2018Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, nr 9, s. 3854-3864Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The European Space Agency/Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual-probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well-studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low-frequency waves within two cavities. These low-frequency electrostatic waves are likely triggered by lower-hybrid waves observed in the surrounding magnetized plasma. Plain Language Summary As sunlight heats a comet nucleus, frozen volatile gases sublimate are ionized and interact with the solar wind and its embedded magnetic field, inducing a dynamical plasma environment around the comet. With the cornerstone European mission Rosetta and its 2years of near-continuous orbiting of comet 67P/Churyumov-Gerasimenko, the origin, structure, and evolution of this environment are only starting to be unveiled. Exciting are the numerous crossings of the diamagnetic cavity, the innermost plasma region from which the solar wind magnetic field is excluded. Whilst the magnetic field structure of the cavity crossings is well studied, the related electric field activity remains until now unexplored. Studying the electric field with the Langmuir probes onboard Rosetta, we find that whereas the large-scale electric field structure agrees well with the observed magnetic field behavior during cavity crossings, unexpected short-lived low-frequency electric field signals manifest themselves within the cavity. We interpret these as electrostatic waves triggered by a modulating of the cavity boundary caused by observed electrostatic waves at the same frequency in the surrounding magnetized plasma. This unravels a new aspect of the electromagnetic activity in the inner cometary environment, which is crucial for our understanding of the comet-solar wind-induced plasma environment.

  • 140.
    Mann, Ingrid
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. EISCAT Scientific Association.
    Dust dynamic pressure and magnetopause displacement: reasons for non-detection2013Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 31, s. 39-44Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In a recent paper, Treumann and Baumjohann (2011) propose that the contribution of dust particles to the solar wind dynamic pressure can cause large compressions of the Earth's magnetopause and suggest that this occurs when Earth encounters meteoroid streams. In this paper we estimate the contribution from charged dust particles to the solar wind dynamical pressure, and we exclude that the dust associated to meteoroid streams can influence the extension of the magnetopause according to the proposed model. A sufficient coupling to the solar wind is only expected for so-called nanodust. However, the dynamic pressure of the nanodust is orders of magnitudes below that of the solar wind, making it unlikely that its variation can be observed in displacements of the magnetopause. We also discuss the equation that the authors use for estimating the extension of the Earth's magnetopause, and conclude that this is not applicable due to the large gyroradius of the nanodust. We finally note that an influence of dust on the extension of a magnetosphere might be quite possible in other astrophysical systems and based on other processes.

  • 141.
    Mann, Ingrid
    et al.
    Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Murad, Edmond
    AFRL retired, MA, USA.
    Popova, Olga
    Institute for Dynamics of Geospheres, Russian Academy of Science, Moscow, Russia.
    Meyer-Vernet, Nicole
    LESIA, CNRS, UPMC, Univ. Paris Diderot, Observatoire de Paris, Meudon, France.
    Rosenberg, Marlene
    Dept. of Electrical & Computer Engineering, Univ. of California, San Diego, USA.
    Mukai, Tadashi
    Kobe University, Kobe, Japan.
    Czechowski, Andrzej
    Polish Space Research Institute, Warsaw, Poland.
    Mukai, Sonoyo
    Kinki University, Higashi Osaka, Japan.
    Safrankova, Jana
    Charles University, Prague, Czech Republic.
    Nemecek, zdenek
    Charles University, Prague, Czech Republic.
    Dusty plasma effects in near earth space and interplanetary medium2011Ingår i: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 161, nr 1-4, s. 1-47Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We review dust and meteoroid fluxes and their dusty plasma effects in the interplanetary medium near Earth orbit and in the Earth’s ionosphere. Aside from in-situ measurements from sounding rockets and spacecraft, experimental data cover radar and optical observations of meteors. Dust plasma interactions in the interplanetary medium are observed by the detection of charged dust particles, by the detection of dust that is accelerated in the solar wind and by the detection of ions and neutrals that are released from the dust. These interactions are not well understood and lack quantitative description. There is still a huge discrepancy in the estimates of meteoroid mass deposition into the atmosphere. The radar meteor observations are of particular interest for determining this number. Dust measurements from spacecraft require a better understanding of the dust impact ionization process,as well as of the dust charging processes. The latter are also important for further studying nanodust trajectories in the solar wind. Moreover understanding of the complex dependencies that cause the variation of nanodust fluxes is still a challenge.

  • 142. Marghitu, Octav
    et al.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Klecker, Berndt
    Vaivads, Andris
    McFadden, Jim
    Buchert, Stephan
    Kistler, Lynn M
    Dandouras, Iannis
    André, Mats
    Rème, Henri
    Experimental investigation of auroral generator regions with conjugate Cluster and FAST data2006Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, s. 619-635Artikel i tidskrift (Refereegranskat)
  • 143. Markidis, Stefano
    et al.
    Henri, Pierre
    Lapenta, Giovanni
    Rönnmark, Kjell
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Meliani, Zakaria
    Laure, Erwin
    The fluid-kinetic particle-in-cell method for plasma simulations2014Ingår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 271, s. 415-429Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A method that solves concurrently the multi-fluid and Maxwell's equations has been developed for plasma simulations. By calculating the stress tensor in the multi-fluid momentum equation by means of computational particles moving in a self-consistent electromagnetic field, the kinetic effects are retained while solving the multi-fluid equations. The Maxwell's and multi-fluid equations are discretized implicitly in time enabling kinetic simulations over time scales typical of the fluid simulations. The Fluid-Kinetic Particle-in-Cell method has been implemented in a three-dimensional electromagnetic code, and tested against the two-stream instability, the Weibel instability, the ion cyclotron resonance and magnetic reconnection problems. The method is a promising approach for coupling fluid and kinetic methods in a unified framework.

  • 144.
    Marklund, Mattias
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden.
    Liu, Chuan S
    New quantum limits in plasmonic devices2008Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 84, nr 1, artikel-id 17006Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Surface plasmon polaritons (SPPs) have recently been recognized as an important future technique for microelectronics. Such SPPs have been studied using classical theory. However, current state-of-the-art experiments are rapidly approaching nanoscales, and quantum effects can then become important. Here we study the properties of quantum SPPs at the interface between an electron quantum plasma and a dielectric material. It is shown that the effect of quantum broadening of the transition layer is most important. In particular, the damping of SPPs does not vanish even in the absence of collisional dissipation, thus posing a fundamental size limit for plasmonic devices. Consequences and applications of our results are pointed out.

  • 145.
    Marklund, Mattias
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Morrison, P. J.
    Gauge-free Hamiltonian structure of the spin Maxwell-Vlasov equations2011Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 375, nr 24, s. 2362-2365Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We derive the gauge-free Hamiltonian structure of an extended kinetic theory, for which the intrinsic spin of the particles is taken into account. Such a semi-classical theory can be of interest for describing, e.g., strongly magnetized plasma systems. We find that it is possible to construct a generalized noncanonical Poisson bracket on the extended phase space, and discuss the implications of our findings, including stability of monotonic equilibria. (C) 2011 Elsevier B.V. All rights reserved.

  • 146.
    Marklund, Mattias
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire OX11 0QX, United Kingdom .
    Stenflo, Lennart
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Shukla, Padma Kant
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire OX11 0QX, United Kingdom .
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire OX11 0QX, United Kingdom .
    Quantum electrodynamical effects in dusty plasmas2005Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, nr 7, artikel-id 072111Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new nonlinear electromagnetic wave mode in a magnetized dusty plasma is predicted. Its existence depends on the interaction of an intense circularly polarized electromagnetic wave with a dusty plasma, where quantum electrodynamical photon-photon scattering is taken into account. Specifically, we consider a dusty electron-positron-ion plasma and show that the propagation of the new mode is admitted. It could be of significance for the physics of supernova remnants and in neutron star formation.

  • 147. Masood, W.
    et al.
    Eliasson, Bengt
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Electrostatic solitary waves in a quantum plasma with relativistically degenerate electrons2011Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 18, nr 3, artikel-id 034503Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A model for nonlinear ion waves in an unmagnetized plasma with relativistically degenerate electrons and cold fluid ions is presented here. The inertia is given here by the ion mass while the restoring force is provided by the relativistic electron degeneracy pressure, and the dispersion is due to the deviation from charge neutrality. A nonlinear Korteweg-de Vries equation is derived for small but finite amplitude waves and is used to study the properties of localized ion acoustic solitons for parameters relevant for dense astrophysical objects such as white dwarf stars. Different degrees of relativistic electron degeneracy are discussed and compared.

  • 148. McCrea, Ian
    et al.
    Aikio, Anita
    Alfonsi, Lucilla
    Belova, Evgenia
    Buchert, Stephan
    Clilverd, Mark
    Engler, Norbert
    Gustavsson, Bjorn
    Heinselman, Craig
    Kero, Johan
    Kosch, Mike
    Lamy, Herve
    Leyser, Thomas
    Ogawa, Yasunobu
    Oksavik, Kjellmar
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics (IRF), Kiruna, Sweden.
    Pitout, Frederic
    Rapp, Markus
    Stanislawska, Iwona
    Vierinen, Juha
    The science case for the EISCAT_3D radar2015Ingår i: Progress in Earth and Planetary Science, ISSN 2197-4284, Vol. 2, nr 1Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005-2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010-2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.

  • 149. McLachlan, Robert I.
    et al.
    Modin, Klas
    Verdier, Olivier
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Symplectic integrators for spin systems2014Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, nr 6, s. 061301-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a symplectic integrator, based on the implicit midpoint method, for classical spin systems where each spin is a unit vector in R-3. Unlike splittingmethods, it is defined for all Hamiltonians and is O(3)-equivariant, i.e., coordinate-independent. It is a rare example of a generating function for symplectic maps of a noncanonical phase space. It yields a new integrable discretization of the spinning top.

  • 150.
    Meisel, David D.
    et al.
    SUNY Geneseo, NY, USA.
    Kero, Johan
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik. Institutet för rymdfysik.
    Sidorov, Vladimir
    Kazan State University, Russia.
    Briczinski, Stan
    Penn State University, State College, PA, USA.
    Physical Characteristics of Kazan Minor Showers as Determined by Correlations with the Arecibo UHF radar2008Ingår i: Advances in Meteoroid and Meteor Science, 2008Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    In the northern hemisphere, the month of February is characterized by a lack of major meteor shower activity yet a number of weak minor showers are present as seen by

    the Kazan radar. Using the Feller transformation to obtain the distribution of true meteor velocities from the distribution of radial velocities enables the angle of incidence to be obtained for the single beam AO (Arecibo Observatory) data. Thus the loci of AO radiants become beam-centered circles on the sky and one can, with simple search routines, find where these circles intersect on radiants determined by other means. Including geocentric velocity as an additional search criterion, we have examined a set of February radiants obtained at Kazan for coincidence in position and velocity. Although some may be chance associations, only those events with probabilities of association[0.5 have been kept. Roughly 90 of the Kazan showers have been verified in this way with mass, radius and

    density histograms derived from the AO results. By comparing these histograms with those of the ‘‘background’’ in which the minor showers are found, a qualitative scale of dynamical minor shower age can be formulated. Most of the showers are found outside the usual ‘‘apex’’ sporadic source areas where it is easiest to detect discrete showers with less confusion from the background.

123456 101 - 150 av 291
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf