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  • 251.
    Shukla, Padma Kant
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eliasson, BengtUmeå University, Faculty of Science and Technology, Department of Physics.Stenflo, LennartUmeå University, Faculty of Science and Technology, Department of Physics.
    Frontiers in Modern Plasma Physics: 2008 ICTP International Workshop on the Frontiers of Modern Plasma Physics2008Conference proceedings (editor) (Other academic)
  • 252.
    Shukla, Padma Kant
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Scottish Universities Physics Alliance (SUPA), Department of Physics, University of Strathclyde, Glasgow G4 0NG, Scotland, United Kingdom; Science and Technology Facilities Council (STFC) Centre for Fundamental Physics, Rutherford Appleton Laboratory, Harwell Science and Innovation Centre, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom; GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, 1049-001 Lisboa, Portugal; School of Physics, University of KwaZulu-Natal, 4000 Durban, South Africa.
    Morfill, GE
    Low-frequency electrostatic wave in a metallic electron-hole-ion plasma with nanoparticles2009In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 75, p. 581-585Article in journal (Other academic)
    Abstract [en]

    We investigate die dispersion property of a, low-frequency, electrostatic wave in a dense metallic electron-hole-ion plasma with nanoparticles. The latter are charged due to the Held emission, and hence the metallic can be regarded as charged dust rods surrounded by degenerate electrons and holes and non-degenerate ions. By using a quantum hydrodynamic model for the electrons and holes, we obtain the electron and hole number density perturbations, Mile be In awl dust rod number density perturbations follow the classical expressions. A dispersion relation for the low-frequency electrostatic wave in our multi-species dense metallic plasma 4 derived and analyzed. The possibility of exciting non-thermal electrostatic waves is also discussed.

  • 253. Simon Wedlund, Cyril
    et al.
    Alho, Markku
    Gronoff, Guillaume
    Kallio, Esa
    Gunell, Herbert
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Behar, Etienne
    Swedish Institute of Space Physics, Kiruna.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna.
    Miloch, Wojciech Jacek
    Hybrid modelling of cometary plasma environments: I. Impact of photoionisation, charge-exchange and electron ionisation on bow shock and cometopause at 67P/Churyumov-Gerasimenko2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, article id A73Article in journal (Refereed)
    Abstract [en]

    Context. The ESA/Rosetta mission made it possible to monitor the plasma environment of a comet, from near aphelion to perihelion conditions. To understand the complex dynamics and plasma structures found at the comet, a modelling effort must be carried out in parallel. Aims. Firstly, we present a 3D hybrid model of the cometary plasma environment including photoionisation, solar wind charge exchange, and electron ionisation reactions; this model is used in stationary and dynamic conditions (mimicking the solar wind variations), and is thus especially adapted to a weakly outgassing comet such as 67P/Churyumov-Gerasimenko, the target of the ESA/Rosetta mission. Secondly, we use the model to study the respective effects of ionisation processes on the formation of the dayside macroscopic magnetic and density boundaries upstream of comet 67P in perihelion conditions at 1.3 AU. Thirdly, we explore and discuss the effects of these processes on the magnetic field line draping, ionisation rates, and composition in the context of the Rosetta mission. Methods. We used a new quasi-neutral hybrid model, originally designed for weakly magnetised planetary bodies, such as Venus, Mars, and Titan, and adapted here to comets. Ionisation processes were monitored individually and together following a probabilistic interaction scheme. Three-dimensional paraboloid fits of the bow shock surface, identified for a magnetosonic Mach number equal to 2, and of the cometopause surface, were performed for a more quantitative analysis. Results. We show that charge exchange and electron ionisation play a major role in the formation of a bow shock-like structure far upstream, while photoionisation is the main driver at and below the cometopause boundary, within 1000 km cometocentric distance. Charge exchange contributes to 42% of the total production rate in the simulation box, whereas production rates from electron ionisation and photoionisation reach 33% and 25%, respectively. We also discuss implications for Rosetta's observations, regarding the detection of the bow shock and the cometopause.

  • 254. Slapak, R.
    et al.
    Gunell, H.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Observations of multiharmonic ion cyclotron waves due to inverse ion cyclotron damping in the northern magnetospheric cusp2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 1, p. 22-29Article in journal (Refereed)
    Abstract [en]

    We present a case study of inverse ion cyclotron damping taking place in the northern terrestrial magnetospheric cusp, exciting waves at the ion cyclotron frequency and its harmonics. The ion cyclotron waves are primarily seen as peaks in the magnetic-field spectral densities. The corresponding peaks in the electric-field spectral densities are not as profound, suggesting a background electric field noise or other processes of wave generation causing the electric spectral densities to smoothen out more compared to the magnetic counterpart. The required condition for inverse ion cyclotron damping is a velocity shear in the magnetic field-aligned ion bulk flow, and this condition is often naturally met for magnetosheath influx in the northern magnetospheric cusp, just as in the presented case. We note that some ion cyclotron wave activity is present in a few similar shear events in the southern cusp, which indicates that other mechanisms generating ion cyclotron waves may also be present during such conditions.

  • 255. Slapak, Rikard
    et al.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yamauchi, Masatoshi
    Nilsson, Hans
    Karlsson, Tomas
    Schillings, Audrey
    Quantification of the total ion transport in the near-Earth plasma sheet2017In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 35, no 4, p. 869-877Article in journal (Refereed)
    Abstract [en]

    Recent studies strongly suggest that a majority of the observed O+ cusp outflows will eventually escape into the solar wind, rather than be transported to the plasma sheet. Therefore, an investigation of plasma sheet flows will add to these studies and give a more complete picture of magnetospheric ion dynamics. Specifically, it will provide a greater understanding of atmospheric loss. We have used Cluster spacecraft 4 to quantify the H+ and O+ total transports in the near-Earth plasma sheet, using data covering 2001-2005. The results show that both H+ and O+ have earthward net fluxes of the orders of 1026 and 1024 s(-1), respectively. The O+ plasma sheet return flux is 1 order of magnitude smaller than the O+ outflows observed in the cusps, strengthening the view that most ionospheric O+ outflows do escape. The H+ return flux is approximately the same as the ionospheric outflow, suggesting a stable budget of H+ in the magnetosphere. However, low-energy H+, not detectable by the ion spectrometer, is not considered in our study, leaving the complete magnetospheric H+ circulation an open question. Studying tailward flows separately reveals a total tailward O+ flux of about 0 : 5 x w10(25)s(-1), which can be considered as a lower limit of the nightside auroral region O+ outflow. Lower velocity flows (< 100 km s(-1)) contribute most to the total transports, whereas the high-velocity flows contribute very little, suggesting that bursty bulk flows are not dominant in plasma sheet mass transport.

  • 256.
    Stefan, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Linear and nonlinear wave propagation in weakly relativistic quantum plasmas2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 1, article id 012114Article in journal (Refereed)
    Abstract [en]

    We consider a recently derived kinetic model for weakly relativistic quantum plasmas. We find that that the effects of spin-orbit interaction and Thomas precession may alter the linear dispersion relation for a magnetized plasma in case of high plasma densities and/or strong magnetic fields. Furthermore, the ponderomotive force induced by an electromagnetic pulse is studied for an unmagnetized plasma. It turns out that for this case the spin-orbit interaction always gives a significant contribution to the quantum part of the ponderomotive force.

  • 257.
    Stenberg, Gabriella
    Umeå University, Faculty of Science and Technology, Physics.
    The importance of waves in space plasmas: Examples from the auroral region and the magnetopause2005Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis discusses the reasons for space exploration and space science. Space plasma physics is identified as an essential building block to understand the space environment and it is argued that observation and analysis of space plasma waves is an important approach.

    Space plasma waves are the main actors in many important processes. So-called broadband waves are found responsible for much of the ion heating in the auroral region. We investigate the wave properties of broadband waves and show that they can be described as a mixture of electrostatic wave modes. In small regions void of cold electrons the broadband activity is found to be ion acoustic waves and these regions are also identified as acceleration regions. The identification of the wave modes includes reconstructions of the wave distribution function. The reconstruction technique allow us to determine the wave vector spectrum, which cannot be measured directly. The method is applied to other wave events and it is compared in some detail with a similar method.

    Space plasma wave are also sensitive tools for investigations of both the fine-structure and the dynamics of space plasmas. Studies of whistler mode waves observed in the boundary layer on the magnetospheric side of the magnetopause reveal that the plasma is organized in tube-like structures moving with the plasma drift velocity. The perpendicular dimension of these tubes is of the order of the electron inertial length. We present evidence that each tube is linked to a reconnection site and argue that the high density of tube-like structures indicates patchy reconnection.

  • 258. Stenflo, L.
    et al.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Temperature effects on large amplitude electron plasma oscillations2016In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 23, no 7, article id 074501Article in journal (Refereed)
    Abstract [en]

    We present a very simple model equation that can describe one-dimensional large amplitude electron plasma oscillations. Published by AIP Publishing.

  • 259.
    Stenflo, Lennart
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Decay interactions involving two electrostatic waves and one arbitrary polarized wave in a magnetized plasma2010In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1306, p. 14-17Article in journal (Refereed)
    Abstract [en]

    We demonstrate that previously overlooked formulations of the general theory are highly useful when deriving concrete expressions for specific cases. We also point out that some previous results deduced directly from the basic plasma equations contain inappropriate approximations leading to unphysical results. Finally, the generalization to more elaborate plasma models, containing e.g. kinetic effects, is presented.

  • 260. Stenflo, Lennart
    et al.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rogue waves in the atmosphere2010In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 76, no Part 3-4, p. 293-295Article in journal (Refereed)
    Abstract [en]

    The appearance of rogue waves is well known in oceanographics, optics, and cold matter systems. Here we show a possibility for the existence of atmospheric rogue waves.

  • 261.
    Stenflo, Lennart
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shukla, Padma Kant
    Eliasson, Bengt
    Stimulated scattering instabilities of electromagnetic waves in the Earths mesosphere2004In: Proceedings of the EPS Conference on Plasma Physics, London: European Conference Abstracts ECA, 2004, Vol. 28G, article id P-1.053Conference paper (Other academic)
  • 262.
    Stjernman, Anders
    Umeå University, Faculty of Science and Technology, Rymdfysik.
    Design and development of a microwave multifrequency polarimetric scatterometer for biosphere remote sensing1995Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Microwave radar and radiometer techniques are used to gather crucial information about the earth and its atmosphere. The ERS-1, JERS-1, RadarSAT and NASA’s Mission to Planet Earth projects are designed to study the changing global environment. In all these endeavors, the key instrument is the radar or scatterometer. The advantage of microwave radar is that it is hindered very little by clouds, fog or solar radiation. Polarimetrie sensors like the shuttle-borne SIR-C radar, provides additional information compared to single polarization systems. Correct interpretation of polarimetrie data necessitates proper understanding of the scattering mechanism. Thus theory of polarization synthesis is discussed. Solution to the Kennaugh eigenvalue problem for point targets is derived. Polarimetrie signatures of point targets are shown as surfaces of spherical co-ordinates based on the Poincare sphere. Statistics of the covariance matrix elements for distributed targets are presented.

    The main topic of this research report is the design and development of a multifrequency, polarimetrie scatterometer for biosphere remote sensing. The system was developed using a standard HP network analyzer, a crossed log-periodic dipole antenna and a reflector. The scatterometer functions in a linear polarization basis between the L- and X-bands and gathers full-polarimetric information. The standard S-parameter measurements using the network analyzer were related to surface and volume scattering coefficients of rough surface, snow cover and vegetation media.

    The scatterometer measurements were carried out in the frequency domain to make use of narrow band filters in the receiver chain. The fast Fourier transform was used to convert the frequency domain measurements to the time domain. The range resolution of the system was 20 cm; azimuthal and elevation resolutions are determined by the antenna beam widths. Range side lobes were reduced by making use of appropriate weighting (Kaiser-Bessel window) functions. In the process of receiver design, we developed a number of signal processing techniques which are illustrated using appropriate numerical examples.

    The accuracy of target characterization depends on the quality of scatterometer calibration. A novel technique to estimate the absolute gain and crosstalk of the radar system was developed. Using a distortion matrix approach, the cross-polarization response of the system was improved by 10 to 25 dB. The radar measurements were validated by comparing point target radar observations with the corresponding theoretical values. Also, measurements of fading decorrelation distance and decorrelation bandwidth of rough surfaces were in good agreement with the theory. Backscatter observations of vegetation and snow cover were comparable to earlier published values for a similar environment.

    Based on initial test results and operations capability, we propose to use the present scatterometer for ground-truthing in support of ERS-1 missions. Direct comparisons of electromagnetic backscatter coefficients are possible between the ERS-1 and the present scatterometer. These joint studies are beneficial for developing inverse scattering techniques, designing new experiments and calibrating ERS-1 radar systems for distributed target environments.

  • 263.
    Stude, Joan
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics.
    Advanced Plasma Analyzer for Measurements in the Magnetosphere of Jupiter2016Doctoral thesis, monograph (Other academic)
    Abstract [en]

    The Jupiter Icy Moons Explorer is a planetary exploration mission that aims to study the moons of Jupiter in the planet’s vast magnetosphere. Among the various instruments on board is the Particle Environment Package (PEP), that is led by the Swedish Institute of Space Physics (IRF) in Kiruna. The Jovian plasma Dynamics and Composition analyzer (JDC) is one of six sensors within PEP and focuses on the characterization of positive ions. To be able to measure their three-dimensional distribution and composition, in-situ and in high time resolution, JDC has to cover a large field of view of 2π sr, for the desired energy range, in just a couple of seconds. An electrostatic analyzer within the sensor determines the energy per charge of such particles and a time-of-flight mass spectrometer measures their mass per charge. Constraints on weight and the radiation environment of Jupiter drive the design of the sensor: small and lightweight to allow extra shielding, but still large enough to accomplish measurements in the harsh radiation environment of Jupiter.

    This work focuses on a new type of compact, electrostatic analyzer using spherical wedges and the start signal generation for the time-of-flight measurement using new venetian blind-type surfaces. Simulations on the electrostatic analyzer showed that the most promising design is a hybrid variant, using an inner shell with spherical wedges and a spheroidal outer shell. A prototype sensor was built and tested with successful results.

    A reflectron-type time-of-flight cell measures the time it takes for a particle to pass a linear electric field. The time measurement has to be very accurate and requires that all ions enter the reflectron from the same start position. Commonly this is achieved with thin carbon foils of some nanometer thickness to provide a very accurate start position. Upon impact and after leaving a foil, ions generate secondary electrons that act as start signals for the time measurement. Foils require a substantial pre-acceleration of several kilovolts for the ions to penetrate the foil, thus increasing the size and mass of the instrument.

    When incident ions are reflected at grazing angles from a surface, secondary electrons are released in the same way as with foils. To increase position accuracy during this reflection process, venetian blind-type start surfaces are investigated, where many smaller surfaces replace a large flat surface. The most promising sample was found to be micro pore optics, that were initially designed to focus gamma rays. In several experiments it could be shown that micro pore optics show good reflection properties when used as start surfaces in the time-of-flight measurement.

    Both improvements allow a more compact and lightweight sensor that can be better shielded against the harsh radiation environment in Jupiter’s system. Jupiter hosts the strongest radiation environment in the solar system, that could kill an unprotected human thousand times over.

  • 264.
    Szasz, Csilla
    et al.
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Kero, Johan
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Meisel, David D.
    Suny Geneseo, NY, USA.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Wannberg, Gudmund
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Orbit characteristics of the tristatic EISCAT UHF meteors2008In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 388, no 1, p. 15-25Article in journal (Refereed)
    Abstract [en]

    The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s−1 and a retrograde population peaking at 59 km s−1. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.

  • 265.
    Szasz, Csilla
    et al.
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Kero, Johan
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Meisel, David D.
    SUNY-Geneseo, NY, USA.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Estimated visual magnitudes of the EISCAT UHF meteors2008In: Advances in Meteoroid and Meteor Science, Springer , 2008, p. 373-378Chapter in book (Other (popular science, discussion, etc.))
  • 266.
    Szasz, Csilla
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Meisel, David D.
    SUNY Geneseo, Geneseo, NY, USA.
    Wannberg, Gudmund
    EISCAT Scientific Association, Kiruna, Sweden.
    Westman, Assar
    EISCAT Scientific Association, Kiruna, Sweden.
    Estimated visual magnitudes of the EISCAT UHF meteors2008In: Earth, moon, and planets, ISSN 0167-9295, E-ISSN 1573-0794, Vol. 102, no 1-4, p. 373-378Article in journal (Refereed)
    Abstract [en]

    We have investigated the conditions for simultaneous meteor observations with the EISCAT UHF radar system and telescopic optical devices. The observed characteristics of 410 meteors detected by all three UHF receivers are compared with model simulations and their luminosity is calculated as a part of a meteoroid ablation model using a fifth order Runge–Kutta numerical integration technique. The estimated absolute visual magnitudes are in the range of +9 to +5. The meteors should therefore be observable using intensified CCD or EMCCD (Electron Multiplying CCD) cameras with telephoto lenses. A possible setup of a coordinated radar and optical campaign is suggested.

  • 267. Tamaz, Kaladze
    et al.
    Oleg, Pokhotelov
    Lennart, Stenflo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    H. A., Shah
    G. V., Jandieri
    Electromagnetic inertio-gravity waves in the ionospheric E-layer2007In: Physica Scripta, ISSN 0031-8949, Vol. 76, no 4, p. 343-348Article in journal (Refereed)
    Abstract [en]

    The effect of the Ampére force on inertio-gravity (IG) waves in the partially ionized ionospheric E-layer is considered. Electromagnetic IG waves are then studied. It is shown that the free energy necessary for linear instability of electromagnetic IG waves arises from the field-aligned current. Furthermore, it is found that atmospheric vortex motions can induce substantial variations in the geomagnetic field and field-aligned currents.

  • 268.
    Ugarte, Orlando
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sadek, Jad
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Tsinghua University, Beijing, China.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Critical role of blockage ratio for flame acceleration in channels with tightly spaced obstacles2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 9, article id 093602Article in journal (Refereed)
    Abstract [en]

    A conceptually laminar mechanism of extremely fast flame acceleration in obstructed channels, identified by Bychkov et al. ["Physical mechanism of ultrafast flame acceleration," Phys. Rev. Lett. 101, 164501 (2008)], is further studied by means of analytical endeavors and computational simulations of compressible hydrodynamic and combustion equations. Specifically, it is shown how the obstacles length, distance between the obstacles, channel width, and thermal boundary conditions at the walls modify flamepropagation through a comb-shaped array of parallel thin obstacles. Adiabatic and isothermal (cold and preheated) side walls are considered, obtaining minor difference between these cases, which opposes the unobstructed channel case, where adiabatic and isothermal walls provide qualitatively different regimes offlame propagation. Variations of the obstructed channel width also provide a minor influence on flamepropagation, justifying a scale-invariant nature of this acceleration mechanism. In contrast, the spacing between obstacles has a significant role, although it is weaker than that of the blockage ratio (defined as the fraction of the channel blocked by obstacles), which is the key parameter of the problem. Evolution of the burning velocity and the dependence of the flame acceleration rate on the blockage ratio are quantified. The critical blockage ratio, providing the limitations for the acceleration mechanism in channels with comb-shaped obstacles array, is found analytically and numerically, with good agreement between both approaches. Additionally, this comb-shaped obstacles-driven acceleration is compared to finger flameacceleration and to that produced by wall friction.

  • 269. Ugarte, Orlando
    et al.
    Demir, Sinan
    Demirgok, Berk
    Akkerman, V'yacheslav
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Effect of Wall Boundary Conditions on Flame Propagation in Micro-Chambers2016In: PROCEEDINGS OF THE ASME POWER CONFERENCE, 2015, The american society of mechanical engineers , 2016, article id V001T03A009Conference paper (Refereed)
    Abstract [en]

    Flame dynamics in micro-pipes have been observed to be strongly affected by the wall boundary conditions. In this respect, two mechanisms of flame acceleration are related to the momentum transferred in these regions: 1) that associated with flame stretching produced by wall friction forces; and 2) when obstacles are placed at the walls, as a result of the delayed burning occurring between them, a jet-flow is formed, intensively promoting the flame spreading. Wall thermal conditions have usually been neglected, thus restricting the cases to adiabatic wall conditions. In contrast, in the present work, the effect of the boundary conditions on the flame propagation dynamics is investigated, computationally, with the effect of wall heat losses included in the consideration. In addition, the powerful flame acceleration attained in obstructed pipes is studied in relation to the obstacle size, which determines how different this mechanism is from the wall friction. A parametric study of two-dimensional (2D) channels and cylindrical tubes, of various radiuses, with one end open is performed. The walls are subjected to slip and non-slip, adiabatic and constant temperature conditions, with different fuel mixtures described by varying the thermal expansion coefficients. Results demonstrate that higher wall temperatures promote slower propagation as they reduce the thermal expansion rate, as a result of the post-cooling of the burn matter. In turn, smaller obstacle sizes generate weaker flame acceleration, although the mechanism is noticed to be stronger than the wall friction-driven, even for the smaller sizes considered.

  • 270. Varnik, Fathollah
    et al.
    Mandal, Suvendu
    Chikkadi, Vijaykumar
    Denisov, Dmitry
    Olsson, Peter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vågberg, Daniel
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Raabe, Dierk
    Schall, Peter
    Correlations of plasticity in sheared glasses2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, no 4, p. 040301-Article in journal (Refereed)
    Abstract [en]

    In a recent paper [Mandal et al., Phys. Rev. E 88, 022129 (2013)], the nature of spatial correlations of plasticity in hard-sphere glasses was addressed both via computer simulations and in experiments. It was found that the experimentally obtained correlations obey a power law, whereas the correlations from simulations are better fitted by an exponential decay. We here provide direct evidence-via simulations of a hard-sphere glass in two dimensions (2D)-that this discrepancy is a consequence of the finite system size in the 3D simulations. By extending the study to a 2D soft disk model at zero temperature [Durian, Phys. Rev. Lett. 75, 4780 (1995)], the robustness of the power-law decay in sheared amorphous solids is underlined. Deviations from a power law occur when either reducing the packing fraction towards the supercooled regime in the case of hard spheres or changing the dissipation mechanism from contact dissipation to a mean-field-type drag in the case of soft disks.

  • 271.
    Vaverka, Jakub
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. National I nstitute of Polar Research, Tachikawa, Japan; Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
    Nakamura, Takuji
    Kero, Johan
    Mann, Ingrid
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norberg, Carol
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics,Kiruna, Sweden.
    Lindqvist, Per-Arne
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics,Kiruna, Sweden.
    Comparison of Dust Impact and Solitary Wave Signatures Detected by Multiple Electric Field Antennas Onboard the MMS Spacecraft2018In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 8, p. 6119-6129Article in journal (Refereed)
    Abstract [en]

    Dust impact detection by electric field instruments is a relatively new method. However, the influence of dust impacts on electric field measurements is not completely understood and explained. A better understanding is very important for reliable dust impact identification, especially in environments with low dust impact rate. Using data from Earth-orbiting Magnetospheric Multiscale mission (MMS) spacecraft, we present a study of various pulses detected simultaneously by multiple electric field antennas in the monopole (probe-to-spacecraft potential measurement) and dipole (probe-to-probe potential measurement) configurations. The study includes data obtained during an impact of a millimeter-sized object. We show that the identification of dust impacts by a single antenna is a very challenging issue in environments where solitary waves are commonly present and that some pulses can be easily misinterpreted as dust impacts. We used data from multiple antennas to distinguish between changes in the spacecraft potential (dust impact) and structures in the ambient plasma or electric field. Our results indicate that an impact cloud is in some cases able to influence the potential of the electric field antenna during its expansion.

  • 272.
    Vaverka, Jakub
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics. Arctic University of Norway, Tromsø, Norway.
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norberg, Carol
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Detection of EMPs generated by meteoroid impacts on the MMS spacecraft and problems with signal interpretation2017In: 2017 XXXIInd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS), IEEE, 2017Conference paper (Refereed)
    Abstract [en]

    Signatures of hypervelocity dust impacts detected by electric field instruments are still not completely understood. We have used the electric field instrument onboard one of the MMS spacecraft orbiting the Earth since 2015 to study various pulses in the measured electric field detected simultaneously by multiple antennas. This unique instrument allows a detailed investigation of registered waveforms. The preliminary results shown that the solitary waves can generate similar pulses as dust impacts and detected pulses can easily by misinterpreted when only one antenna is used.

  • 273.
    Vaverka, Jakub
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Mann, Ingrid
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norberg, Carol
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Detection of meteoroid hypervelocity impacts on the Cluster spacecraft: First results2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 6, p. 6485-6494Article in journal (Refereed)
    Abstract [en]

    We present the first study of dust impact events on one of the Earth-orbiting Cluster satellites. The events were identified in the measurements of the wide band data (WBD) instrument on board the satellite operating in monopole configuration. Since 2009 the instrument is operating in this configuration due to the loss of three electric probes and is therefore measuring the potential between the only operating antenna and the spacecraft body. Our study shows that the WBD instrument on Cluster 1 is able to detect pulses generated by dust impacts and discusses four such events. The presence of instrumental effects, intensive natural waves, noncontinuous sampling modes, and the automatic gain control complicates this detection. Due to all these features, we conclude that the Cluster spacecraft are not ideal for dust impact studies. We show that the duration and amplitudes of the pulses recorded by Cluster are similar to pulses detected by STEREO, and the shape of the pulses can be described with the model of the recollection of impact cloud electrons by the positively charged spacecraft. We estimate that the detected impacts were generated by micron-sized grains with velocities in the order of tens of km/s.

  • 274.
    Vaverka, Jakub
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics. Arctic University of Norway, Tromsø, Norway.
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norberg, Carol
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Potential of Earth Orbiting Spacecraft Influenced by Meteoroid Hypervelocity Impacts2017In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 45, no 8, p. 2048-2055Article in journal (Refereed)
    Abstract [en]

    Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth's magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth's magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.

  • 275.
    Vaverka, Jakub
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. e Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics. Arctic University of Norway, Tromsø, Norway.
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norberg, Carol
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Spacecraft potential influenced by meteoroid hypervelocity impacts2016Conference paper (Other academic)
    Abstract [en]

    Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth's magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth's magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.

  • 276.
    Vedin, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Numerical modeling of auroral processes2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    One of the most conspicuous problems in space physics for the last decades has been to theoretically describe how the large parallel electric fields on auroral field lines can be generated. There is strong observational evidence of such electric fields, and stationary theory supports the need for electric fields accelerating electrons to the ionosphere where they generate auroras. However, dynamic models have not been able to reproduce these electric fields. This thesis sheds some light on this incompatibility and shows that the missing ingredient in previous dynamic models is a correct description of the electron temperature. As the electrons accelerate towards the ionosphere, their velocity along the magnetic field line will increase. In the converging magnetic field lines, the mirror force will convert much of the parallel velocity into perpendicular velocity. The result of the acceleration and mirroring will be a velocity distribution with a significantly higher temperature in the auroral acceleration region than above. The enhanced temperature corresponds to strong electron pressure gradients that balance the parallel electric fields. Thus, in regions with electron acceleration along converging magnetic field lines, the electron temperature increase is a fundamental process and must be included in any model that aims to describe the build up of parallel electric fields. The development of such a model has been hampered by the difficulty to describe the temperature variation. This thesis shows that a local equation of state cannot be used, but the electron temperature variations must be descibed as a nonlocal response to the state of the auroral flux tube. The nonlocal response can be accomplished by the particle-fluid model presented in this thesis. This new dynamic model is a combination of a fluid model and a Particle-In-Cell (PIC) model and results in large parallel electric fields consistent with in-situ observations.

  • 277.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Parallel electric fields: variations in space and time on auroral field lines2008In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 74, no 1, p. 53-64Article in journal (Refereed)
    Abstract [en]

    We present results from a particle–fluid simulation of auroral electrons and discuss the distribution of parallel electric fields along auroral field lines and the processes occurring during the build up of these electric fields. Neglecting field-aligned ion dynamics, the main potential drop has a width of about 5000, km and is centered at an altitude of roughly 5000, km. We find that the gradient in the potential becomes steeper and the build up of the potential drop becomes faster if the temperature of the magnetospheric electrons is lower.

  • 278.
    Vorburger, Audrey
    et al.
    Physikalisches Institut, University of Bern, Bern.
    Pfleger, Martin
    Institute for Chemical Engineering and Environmental Technology, Graz.
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lammer, Helmut
    Space Research Institute, Austrian Academy of Sciences, Graz.
    Lichtenegger, Herbert I. M.
    Space Research Institute, Austrian Academy of Sciences, Graz.
    Galli, André
    Rubin, Martin
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern.
    Three-Dimensional Modeling of Callisto's Surface Sputtered Exosphere Environment2019In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402Article in journal (Refereed)
    Abstract [en]

    We study the release of various elements from Callisto's surface into its exosphere by plasma sputtering. The cold Jovian plasma is simulated with a 3‐D plasma‐planetary interaction hybrid model, which produces 2‐D surface precipitation maps for magnetospheric H+, O+, O++, and S++. For the hot Jovian plasma, we assume isotropic precipitation onto the complete spherical surface. Two scenarios are investigated: one where no ionospheric shielding takes place and accordingly full plasma penetration is implemented (no‐ionosphere scenario) and one where an ionosphere lets virtually none of the cold plasma but all of the hot plasma reach Callisto's surface (ionosphere scenario). In the 3‐D exosphere model, neutral particles are sputtered from the surface and followed on their individual trajectories. The 3‐D density profiles show that whereas in the no‐ionosphere scenario the ram direction is favored, the ionosphere scenario produces almost uniform density profiles. In addition, the density profiles in the ionosphere scenario are reduced by a factor of ∼2.5 with respect to the no‐ionosphere scenario. We find that the Neutral Gas and Ion Mass Spectrometer, which is part of the Particle Environment Package on board the JUpiter ICy moons Explorer mission, will be able to detect the different sputter populations from Callisto's icy surface and the major sputter populations from Callisto's nonicy surface. The chemical composition of Callisto's exosphere can be directly linked to the chemical composition of its surface and will offer us information not only on Callisto's formation scenario but also on the building blocks of the Jupiter system.

  • 279. Vågberg, Daniel
    et al.
    Olsson, Peter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Teitel, S.
    Effect of collisional elasticity on the Bagnold rheology of sheared frictionless two-dimensional disks2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 95, no 1, article id 012902Article in journal (Refereed)
    Abstract [en]

    We carry out constant volume simulations of steady-state, shear-driven flow in a simple model of athermal, bidisperse, soft-core, frictionless disks in two dimensions, using a dissipation law that gives rise to Bagnoldian rheology. Focusing on the small strain rate limit, we map out the rheological behavior as a function of particle packing fraction phi and a parameter Q that measures the elasticity of binary particle collisions. We find a Q*(phi) that marks the clear crossover from a region characteristic of strongly inelastic collisions, Q < Q*, to a region characteristic of weakly inelastic collisions, Q > Q*, and give evidence that Q*(phi) diverges as phi -> phi(J), the shear-driven jamming transition. We thus conclude that the jamming transition at any value of Q behaves the same as the strongly inelastic case, provided one is sufficiently close to fJ. We further characterize the differing nature of collisions in the strongly inelastic vs weakly inelastic regions, and recast our results into the constitutive equation form commonly used in discussions of hard granular matter.

  • 280.
    Waara, Martin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    High altitude ion heating observed by the Cluster spacecraft2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis deals with heating of outowing oxygen ions at high altitude above the polar cap using data from the Cluster spacecraft. oInospheric plasma may flow up from the ionosphere but at velocities which are low enough that the ions are still gravitationally bound. For the ions to overcome gravity, further acceleration is needed. The cusp/polar cap is an important source of outowing oxygen ions. In the cusp/polar cap, transverse heating is more common than eld-aligned acceleration through a magnetic eld-aligned electric eld. It is thus believed that transverse heating of ions is important for ion outow and one of the probable explanations for transverse heating is wave-particle interaction.

    A general conclusion from our work on high altitude oxygen ion energization is that ion energization and outow occur in the high altitude cusp and mantle. The particles are often heated perpendicularly to the geomagnetic eld and resonant heating at the gyrofrequency is most of the time intense enough to explain the observed O+ energies measured in the high altitude (8 { 15 Earth radii, R) cusp/mantle region of the terrestrial magnetosphere. The observed average waves can explain the observed average O+ energies. At lower altitude only a few percent of the observed spectral density around the oxygen gyrofrequency needs to be in resonance with the ions to obtain the measured O+ energies. A difference as compared to low altitude measurements is that we must assume that almost all wave activity is due to waves which can interact with the ions, and of these we assume 50 % to be left-hand polarized. We also have shown a clear correlation between temperature and wave intensity at the gyrofrequency at each measurement point. We have described the average wave intensity and corresponding velocity diffusion oeffcients as a function of altitude in a format convenient for modelers.

    Furthermore we have shown that the wave activity observed in this high altitude region is consistent with Alfven waves, and inconsistent with static structures drifting past the spacecraft. We have also shown how large the variability of the observed spectral densities is, and how sporadic the waves typically are. Based on three cases we have found that the regions with enhanced wave activity and increased ion temperature are typically many ion gyro radii in perpendicular extent.

  • 281.
    Walter, Erwin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Field-Aligned Currents and Flow Bursts in the Earth’s Magnetotail2018Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    We use electric and magnetic field data from MMS spacecraft between 2016 and 2017 tostatistically investigate earthward propagating plasma flow bursts and field-aligned currents(FACs) inside the plasma sheet of the geomagnetic tail. We observe that the occurrence rateof flow burst peaks around the midnight region with decreasing trend towards Earth and theplasma sheet flanks. Further, we distinguish between long and short FACs. Long FACs laston average 6 sec and have a magnitude of 5-20 nA/m 2 . Short FACs last on average 10 timesshorter and have an magnitude of 10-50 nA/m 2 . Both, long and short FACs occur on averageone time per flow burst, on minimum 0 times and on maximum 4 times per flow burst. Intotal, 43 % of the observed FACs are located in a flow burst, 40 % before and 17 % right after aflow burst.

  • 282.
    Wannberg, Gudmund
    et al.
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Kero, Johan
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    The EISCAT meteor code2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, p. 2303-2309Article in journal (Refereed)
    Abstract [en]

    Abstract. The EISCAT UHF system has the unique capability to determine meteor vector velocities from the head echo Doppler shifts measured at the three sites. Since even meteors spending a very short time in the common volume produce analysable events, the technique lends itself ideally to mapping the orbits of meteors arriving from arbitrary directions over most of the upper hemisphere.

    A radar mode optimised for this application was developed in 2001/2002. A specially selected low-sidelobe 32-bit pseudo-random binary sequence is used to binary phase shift key (BPSK) the transmitted carrier. The baud-length is 2.4μs and the receiver bandwidth is 1.6MHz to accommodate both the resulting modulation bandwidth and the target Doppler shift. Sampling is at 0.6μs, corresponding to 90-m range resolution. Target range and Doppler velocity are extracted from the raw data in a multi-step matched-filter procedure. For strong (SNR>5) events the Doppler velocity standard deviation is 100–150 m/s. The effective range resolution is about 30 m, allowing very accurate time-of-flight velocity estimates. On average, Doppler and time-of-flight (TOF) velocities agree to within about one part in 103. Two or more targets simultaneously present in the beam can be resolved down to a range separation <300m as long as their Doppler shifts differ by more than a few km/s.

  • 283.
    Wannberg, Gudmund
    et al.
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Meteor head echo polarization at 930MHz studied with the EISCATUHF HPLA radar2011In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 29, p. 1197-1208Article in journal (Refereed)
    Abstract [en]

    The polarization characteristics of 930-MHz meteor head echoes have been studied for the first time, using data obtained in a series of radar measurements carried out with the tristatic EISCAT UHF high power, large aperture (HPLA) radar system in October 2009. An analysis of 44 tri-static head echo events shows that the polarization of the echo signal recorded by the Kiruna receiver often fluctuates strongly on time scales of tens of microseconds, illustrating that the scattering process is essentially stochastic. On longer timescales (> milliseconds), more than 90% of the recorded events show an average polarization signature that is independentof meteor direction of arrival and echo strength and equal to that of an incoherent-scatter return from underdense plasma filling the tristatic observation volume. This shows that the head echo plasma targets scatter isotropically, which in turn implies that they are much smaller than the 33-cm wavelength and close to spherically symmetric, in very good agreement with results from a previous EISCAT UHF study of the head echo RCS/meteor angle-of-incidence relationship.

    Significant polarization is present in only three events withunique target trajectories. These all show a larger effective target cross section transverse to the trajectory than parallelto it. We propose that the observed polarization may be a signature of a transverse charge separation plasma resonance in the region immediately behind the meteor head, similar to the resonance effects previously discussed in connection with meteor trail echoes by Herlofson, Billam and Browne, Jones and Jones and others.

  • 284. Westerberg, Lars G.
    et al.
    Vedin, Jörgen
    Umeå University, Faculty of Science and Technology, Physics.
    Ekenbäck, Anders
    Åkerstedt, Hans o.
    Three-dimensional flow near a reconnection site at the dayside magnetopause: analytical solutions coupled with MHD simulations2007In: Journal of Plasma PhysicsArticle in journal (Refereed)
    Abstract [en]

    We present a coupling between an analytical three-dimensional model covering the plasma flow behaviour through the magnetopause transition layer near a reconnection site, with results from a global MHD simulation describing the plasma flow in the magnetosheath. The structure of the plasma flow near a reconnection site at the dayside terrestrial magnetopause is investigated, together with the development of the magnetopause transition region.

  • 285. Wieser, Gabriella Stenberg
    et al.
    Odelstad, Elias
    Wieser, Martin
    Nilsson, Hans
    Goetz, Charlotte
    Karlsson, Tomas
    Andre, Mats
    Kalla, Leif
    Eriksson, Anders I.
    Nicolaou, Georgios
    Wedlund, Cyril Simon
    Richter, Ingo
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Investigating short-time-scale variations in cometary ions around comet 67P2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S522-S534Article in journal (Refereed)
    Abstract [en]

    The highly varying plasma environment around comet 67P/Churyumov-Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3-82 eV/q and the other featuring a 1 s time resolution in the energy range 13-50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy-time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.

  • 286. Wu, Yegang
    et al.
    Olsson, Peter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Teitel, S
    Search for hyperuniformity in mechanically stable packings of frictionless disks above jamming2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 5, article id 052206Article in journal (Refereed)
    Abstract [en]

    We numerically simulate mechanically stable packings of soft-core, frictionless, bidisperse disks in two dimensions, above the jamming packing fraction phi(J). For configurations with a fixed isotropic global stress tensor, we investigate the fluctuations of the local packing fraction phi(r) to test whether such configurations display the hyperuniformity that has been claimed to exist exactly at phi(J). For our configurations, generated by a rapid quench protocol, we find that hyperuniformity persists only out to a finite length scale and that this length scale appears to remain finite as the system stress decreases towards zero, i.e., towards the jamming transition. Our result suggests that the presence of hyperuniformity at jamming may be sensitive to the specific protocol used to construct the jammed configurations.

  • 287. Xu, J.
    et al.
    Buck, A.
    Chou, S. -W
    Schmid, K.
    Shen, B.
    Tajima, T.
    Kaluza, M. C.
    Veisz, László
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dynamics of electron injection in a laser-wakefield accelerator2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 8, article id 083106Article in journal (Refereed)
    Abstract [en]

    The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way-called shock-front injection-utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r. m. s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages. Published by AIP Publishing.

  • 288.
    Yao, Shutao
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China.
    Wang, X. G.
    Shi, Q. Q.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yao, Z. H.
    Li, Z. Y.
    Ji, X. F.
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Xiao, Y. C.
    Tian, A. M.
    Pu, Z. Y.
    Zong, Q. G.
    Xiao, C. J.
    Fu, S. Y.
    Zhang, H.
    Russell, C. T.
    Giles, B. L.
    Guo, R. L.
    Sun, W. J.
    Li, W. Y.
    Zhou, X. Z.
    Huang, S. Y.
    Vaverka, Jakub
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nowada, M.
    Bai, S. C.
    Wang, M. M.
    Liu, J.
    Observations of kinetic-size magnetic holes in the magnetosheath2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 2, p. 1990-2000Article in journal (Refereed)
    Abstract [en]

    Magnetic holes (MHs), with a scale much greater than ρ(proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), previously called small-size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three-dimensional (3-D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10-20 ρe (electron gyroradii) and lasted 0.1-0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90 degrees pitch angle, the flux of electrons with energy 34-66 eV decreased, while for electrons of energy 109-1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self-consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10-20 ρe.

  • 289.
    Zamanian, Jens
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modelling of spin and other quantum effects in plasmas2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The development of quantum mechanics during the 20th century gave rise to a completely new way of describing physics. The interpretation of quantum theory is inherently difficult: for example, many-body systems are described by a so called density matrix which has no straightforward analogue in classical theory. However, in the 30’s Wigner proposed an alternative way of describing many-body systems, using a quasi-probability distribution function. This made the connection between classical and quantum kinetic theory clearer.

    This thesis is concerned with modelling of quantum effects in plasmas. The focus lies on describing plasmas containing spin-1/2 particles. For this purpose, new models, based on quantum kinetic theory, are derived. This is achieved by starting from the evolution equation for the density matrix and applying a combination of the Wigner transformation for the position degree of freedom and the Q-transformation for the spin. The properties of the resulting kinetic theory are then investigated and it is shown to satisfy basic necessary criteria such as energy conservation. The kinetic equation is then used to derive a fluid theory for spin-1/2 particles.

    In this thesis the kinetic and fluid models are applied to different problems in quantum plasma physics. For example it will be shown that the quantum electrodynamic correction to the electron g-factor can give rise to a wave mode which lacks classical analogue, and that spin may affect the damping rate of Alfvén waves. The models will also be applied to nonlinear problems and it will be shown that they give rise to modifications of the so called spin ponderomotive force.

  • 290.
    Zamanian, Jens
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dynamics of a dusty plasma with intrinsic magnetization2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, no July, p. 073017-Article in journal (Refereed)
    Abstract [en]

    We consider a dusty plasma where dust particles have a magnetic dipole moment. A Hall-MHD type of model, generalized to account for the intrinsic magnetization, is derived. The model is shown to be energy conserving, and the energy density and flux are derived. The general dispersion relation is then derived, and we show that kinetic dust-Alfvén waves exhibit instability for a low dust and ion temperature and high dust density. We discuss the implication of our results.

  • 291.
    Zhang, Lai
    et al.
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics.
    Thygesen, Uffe Høgsbro
    Banerjee, Malay
    Size-dependent diffusion promotes the emergence of spatiotemporal patterns2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 90, no 1, p. 012904-Article in journal (Refereed)
    Abstract [en]

    Spatiotemporal patterns, indicating the spatiotemporal variability of individual abundance, are a pronounced scenario in ecological interactions. Most of the existing models for spatiotemporal patterns treat species as homogeneous groups of individuals with average characteristics by ignoring intraspecific physiological variations at the individual level. Here we explore the impacts of size variation within species resulting from individual ontogeny, on the emergence of spatiotemporal patterns in a fully size-structured population model. We found that size dependency of animal's diffusivity greatly promotes the formation of spatiotemporal patterns, by creating regular spatiotemporal patterns out of temporal chaos. We also found that size-dependent diffusion can substitute large-amplitude base harmonics with spatiotemporal patterns with lower amplitude oscillations but with enriched harmonics. Finally, we found that the single-generation cycle is more likely to drive spatiotemporal patterns compared to predator-prey cycles, meaning that the mechanism of Hopf bifurcation might be more common than hitherto appreciated since the former cycle is more widespread than the latter in case of interacting populations. Due to the ubiquity of individual ontogeny in natural ecosystems we conclude that diffusion variability within populations is a significant driving force for the emergence of spatiotemporal patterns. Our results offer a perspective on self-organized phenomena, and pave a way to understand such phenomena in systems organized as complex ecological networks.

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