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• 1. Aikio, Anita T.
EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary2008Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, s. 87-105Artikel i tidskrift (Refereegranskat)
• 2.
Department of Physical Sciences, University of Oulu, Finland.
Department of Physical Sciences, University of Oulu, Finland. CETP/UVSQ, Velizy, France. CESR/CNRS, Toulouse, France. Finnish meteorological Institute, Helsinki, Finland. Department of Physical Sciences, University of Oulu, Finland; Sodankylä Geophysical Observatory, Sodankylä, Finland. Swedish Institute of Space Physics, Ångströmlaboratoriet, Uppsala, Sweden. Mullard Space Science Laboratory, University College, London, UK.
EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary2008Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, s. 87-105Artikel i tidskrift (Refereegranskat)

The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6 degree cgmLat) and Longyearbyen (75.2 degree cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5 degree cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm.

During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4RE mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H+ and O+ ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations.

The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2 degree during ˜ 5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse.

• 3. Aikio, Anita T.
IMF effect on the polar cap contraction and expansion during a period of substorms2013Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 31, s. 1021-1034Artikel i tidskrift (Refereegranskat)
• 4.
Oulu, Finland.
Oulu, Finland. Helsinki, Finland. Helsinki, Finland. Helsinki, Finland.
IMF effect on the polar cap contraction and expansion during a period of substorms2013Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 31, s. 1021-1034Artikel i tidskrift (Refereegranskat)

The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase.

In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

• 5. Aikio, Anita T.
Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event2006Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, s. 1905-1917Artikel i tidskrift (Refereegranskat)
• 6. Cresswell-Moorcock, Kathy
A reexamination of latitudinal limits of substorm-produced energetic electron precipitation2013Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 6694-6705Artikel i tidskrift (Refereegranskat)
• 7.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function2019Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 4, s. 2736-2754Artikel i tidskrift (Refereegranskat)

Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2-D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0 degrees to 180 degrees in the Northern Hemisphere and from 180 degrees to 0 degrees in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time-of-flight effect from which we estimate that the source of particle is located similar to 5-25R(E) and similar to 40-107R(E) tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain similar to 21-46 R-E for the Southern Hemisphere event and tailward of X similar to - 65R(E) (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.

• 8.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics and Technology, The Arctic University of Norway, Tromsø, Norway. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Low-frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 7, s. 7042-7056Artikel i tidskrift (Refereegranskat)

Using plasma sheet data from Cluster 1 spacecraft from 2001 till 2011, we statistically investigate oscillatory signatures in the plasma bulk flow. These periodic oscillations are compared to high-speed and quiet flows. Periodic oscillations are observed approximately 8% of the time, while high-speed flows and quiet flows are observed around 0.5% and 12% of the time, respectively. We remark that periodic oscillations can roughly occur everywhere for x(gsm) < -10 R-E and |y(gsm)| < 10 RE, while quiet flows mainly occur toward the flanks of this region and toward x = -10 R-E. The relation between the geomagnetic and solar activity and the occurrence of periodic oscillations is investigated and reveal that periodic oscillations occur for most Kp values and solar activity, while quiet flows are more common during low magnetospheric and solar activity. We find that the median oscillation frequency of periodic oscillations is 1.7 mHz and the median duration of the oscillation events is 41 min. We also observe that their associated Poynting vectors show a tendency to be earthward (S-x >= 0). Finally, the distribution of high-speed flows and periodic oscillations as a function of the velocity is investigated and reveals that thresholds lower than 200 km/s should not be used to identify high-speed flows as it could result in misinterpreting a periodic oscillations for a high-speed flow.

• 9.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
The use of the power density for identifying reconnection regions2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 10, s. 8644-8662Artikel i tidskrift (Refereegranskat)

In the vicinity of magnetic reconnection, magnetic energy is transferred into kinetic energy. A reconnection region hence corresponds to a load, and it should manifest itself as large and positive values of the power density, E·J ≫ 0, where E is the electric field and J the current density. In this article we analyze Cluster plasma sheet data from 2001–2004 to investigate the use of the power density for identifying possible magnetic reconnection events from large sets of observed data. From theoretical arguments we show that an event with   pW/m3 in the Earth's magnetotail observed by the Cluster instruments (X <− 10RE and  ) is likely to be associated with reconnection. The power density can be used as a primary indicator of potential reconnection regions, but selected events must be reviewed separately to confirm any possible reconnection signatures by looking for other signatures such as Hall electric and magnetic fields and reconnection jets. The power density can be computed from multispacecraft data, and we argue that the power density can be used as a tool for identifying possible reconnection events from large sets of data, e.g., from the Cluster and the Magnetospheric Multiscale missions.

• 10.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
The evolution of flux pileup regions in the plasma sheet: Cluster observations2013Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, nr 10, s. 6279-6290Artikel i tidskrift (Refereegranskat)

Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, E<bold></bold>J<0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E<bold></bold>J>0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R(E). Our observations are compared to recent simulation results and previous observations.

• 11.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Evidence for the braking of flow bursts as they propagate toward the Earth2014Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 11, s. 9004-9018Artikel i tidskrift (Refereegranskat)

In this article we use energy conversion arguments to investigate the possible braking of flow bursts as they propagate toward the Earth. By using EJ data (E and J are the electric field and the current density) observed by Cluster in the magnetotail plasma sheet, we find indications of a plasma deceleration in the region -20 R-E < X < - 15 R-E. Our results suggest a braking mechanism where compressed magnetic flux tubes in so-called dipolarization fronts (DFs) can decelerate incoming flow bursts. Our results also show that energy conversion arguments can be used for studying flow braking and that the position of the flow velocity peak with respect to the DF can be used as a single-spacecraft proxy when determining energy conversion properties. Such a single-spacecraft proxy is invaluable whenever multispacecraft data are not available. In a superposed epoch study, we find that a flow burst with the velocity peak behind the DF is likely to decelerate and transfer energy from the particles to the fields. For flow bursts with the peak flow at or ahead of the DF we see no indications of braking, but instead we find an energy transfer from the fields to the particles. From our results we obtain an estimate of the magnitude of the deceleration of the flow bursts, and we find that it is consistent with previous investigations.

• 12. Hubert, Benoit
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)
• 13.
Liège, Belgium.
Oulu, Finland. Helsinki, Finland. Oulu, Finland. Helsinki, Finland. Leicester, UK. Leicester, UK. 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)

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.

• 14. Hølland, Vidar
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)
• 15.
Bergen, Norway.
Oulu, Finland. Copenhagen, Denmark; Helsinki, Finland. Bergen, Norway. Bergen, Norway. Bergen, Norway. Bergen, Norway. Sodankylä, Finland. Oulu, Finland. Sodankylä, Finland. 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)

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.

• 16. Juusola, Liisa
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)
• 17. Karlsson, Tomas
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Magnetic forces associated with bursty bulk flows in Earth's magnetotail2015Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, nr 9, s. 3122-3128Artikel i tidskrift (Refereegranskat)

We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 R-E geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 R-E, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near-Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere.

• 18.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium. Swedish Institute of Space Physics. University of Oslo, Department of Physics, Oslo, Norway. Aalto University, Department of Electronics and Nanoengineering, Espoo, Finland. University of Tromsø, Department of Physics and Technology, Tromsø, Norway. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. 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)

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.

• 19. Maggiolo, R.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
The Delayed Time Response of Geomagnetic Activity to the Solar Wind2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 11, s. 109-127Artikel i tidskrift (Refereegranskat)

We investigate the lagged correlation between a selection of geomagnetic indices and solar wind parameters for a complete solar cycle, from 2000 to 2011. We first discuss the mathematical assumptions required for such a correlation analysis. The solar wind parameters and geomagnetic indices have inherent timescales that smooth the variations of the correlation coefficients with time lag. Furthermore, the solar wind structure associated with corotating interaction regions and coronal mass ejections, and the compression regions ahead of them, strongly impacts the lagged correlation analysis results. This work shows that such bias must be taken into account in a correct interpretation of correlations. We then evidence that the magnetospheric response time to solar wind parameters involves multiple timescales. The simultaneous and quick response of the PC and AE indices to solar wind dynamic pressure with a delay of similar to 5 min suggests that magnetospheric compression by solar wind can trigger substorm activity. We find that the PC and AE indices respond to interplanetary magnetic field (IMF) B-Z with a response time of respectively similar to 20 and similar to 35 min. The response of the SYM-H index takes longer (similar to 80 min) and is less sharp, SYM-H being statistically significantly correlated to the IMF B-Z observed up to more than similar to 10 h before. Our results suggest that the solar wind velocity's dominant impact on geomagnetic activity is caused by the compression regions at the interface of fast/slow solar wind regimes, which are very geo-effective as they are associated with high solar wind pressure and strong interplanetary magnetic field.

• 20. Nilsson, Hans
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Oxygen ion response to proton bursty bulk flows2016Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 8, s. 7535-7546Artikel i tidskrift (Refereegranskat)

We have used Cluster spacecraft data from the years 2001 to 2005 to study how oxygen ions respond to bursty bulk flows (BBFs) as identified from proton data. We here define bursty bulk flows as periods of proton perpendicular velocities more than 100 km/s and a peak perpendicular velocity in the structure of more than 200 km/s, observed in a region with plasma beta above 1 in the near-Earth central tail region. We find that during proton BBFs only a minor increase in the O+ velocity is seen. The different behavior of the two ion species is further shown by statistics of H+ and O+ flow also outside BBFs: For perpendicular earthward velocities of H+ above about 100 km/s, the O+ perpendicular velocity is consistently lower, most commonly being a few tens of kilometers per second earthward. In summary, O+ ions in the plasma sheet experience less acceleration than H+ ions and are not fully frozen in to the magnetic field. Therefore, H+ and O+ motion is decoupled, and O+ ions have a slower earthward motion. This is particularly clear during BBFs. This may add further to the increased relative abundance of O+ ions in the plasma sheet during magnetic storms. The data indicate that O+ is typically less accelerated in association with plasma sheet X lines as compared to H+.

• 21. Pitkänen, Timo
Conjugate EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere2010Ingår i: URSI Finnish XXXII Convention on Radio Science and Electromagnetism 2010 meeting, Oulu, Finland, 26 August 2010, 2010Konferensbidrag (Övrigt vetenskapligt)
• 22. Pitkänen, Timo
EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere2011Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 29, s. 299-319Artikel i tidskrift (Refereegranskat)
• 23. Pitkänen, Timo
Observations of polar cap flow channel and plasma sheet flow bursts during substorm expansion2013Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 774-784Artikel i tidskrift (Refereegranskat)
• 24. Pitkänen, Timo
Corrigendum to “Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm” published in Ann. Geophys., 27, 2157-2171, 20092009Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, s. 3007-3007Artikel i tidskrift (Refereegranskat)
• 25. Pitkänen, Timo
Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm2009Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, s. 2157-2171Artikel i tidskrift (Refereegranskat)
• 26.
Department of Physical Sciences, University of Oulu, Finland.
Department of Physical Sciences, University of Oulu, Finland. Sodankylä Geophysical Observatory, Sodankylä, Finla. Finnish Meteorological Institute, Helsinki, Finland.
Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm2009Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, s. 2157-2171Artikel i tidskrift (Refereegranskat)

The dynamics of the polar cap and the auroral oval are examined in the evening sector during a substorm period on 25 November 2000 by using measurements of the EISCAT incoherent scatter radars, the north-south chain of the MIRACLE magnetometer network, and the Polar UV Imager.

The location of the polar cap boundary (PCB) is estimated from electron temperature measurements by the mainland low-elevation EISCAT VHF radar and the 42m antenna of the EISCAT Svalbard radar. A comparison to the poleward auroral emission (PAE) boundary by the Polar UV Imager shows that in this event the PAE boundary is typically located 0.7 of magnetic latitude poleward of the PCB by EISCAT. The convection reversal boundary (CRB) is determined from the 2-D plasma drift velocity extracted from the dual-beam VHF data. The CRB is located 0.5–1 equatorward of the PCB indicating the existence of viscous-driven antisunward convection on closed field lines.

East-west equivalent electrojets are calculated from the MIRACLE magnetometer data by the 1-D upward continuation method. In the substorm growth phase, electrojets together with the polar cap boundary move gradually equatorwards. During the substorm expansion phase, the Harang discontinuity (HD) region expands to the MLT sector of EISCAT. In the recovery phase the PCB follows the poleward edge of the westward electrojet.

The local ionospheric reconnection electric field is calculated by using the measured plasma velocities in the vicinity of the polar cap boundary. During the substorm growth phase, values between 0 and 10 mV/m are found. During the late expansion and recovery phase, the reconnection electric field has temporal variations with periods of 7–27 min and values from 0 to 40 mV/m. It is shown quantitatively, for the first time to our knowledge, that intensifications in the local reconnection electric field correlate with appearance of auroral poleward boundary intensifications (PBIs) in the same MLT sector. The results suggest that PBIs (typically 1.5 h MLT wide) are a consequence of temporarily enhanced longitudinally localized magnetic flux closure in the magnetotail.

• 27.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Space and Plasma Physics, EES, KTH, Stockholm, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Space and Plasma Physics, EES, KTH, Stockholm, Sweden.
On IMF By-induced dawn-dusk asymmetries in earthward convective fast flows2017Ingår i: Dawn-dusk asymmetries in planetary plasma environments / [ed] Stein Haaland, Andrei Runov, Colin Forsyth, American Geophysical Union (AGU), 2017, 1st, s. 95-106Kapitel i bok, del av antologi (Refereegranskat)

Studies of earthward plasma and magnetic field transport in the Earth’s magnetotail plasma sheet have shown that, on the average, Earthward ion flows in the premidnight and midnight sectors exhibit a duskward component while flows in the postmidnight sector are dawnward. The flow pattern is more pronounced for slower flows (<100 km/s) and alters gradually to a more symmetric one with respect to midnight for increasing flow speeds. However, recent ionospheric and magnetospheric studies have suggested that a nonzero By component in the interplanetary magnetic field (IMF) may significantly influence the earthward transport, creating previously unnoticed dawn‐dusk asymmetries between the hemispheres. In this article, we give a short overview of the present understanding of the topic, present new results, and briefly discuss the importance of the IMF By component for the Earthward transport processes in the magnetotail plasma sheet.

• 28.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Response of magnetotail twisting to variations in IMF B-y: a THEMIS case study 1-2 January 20092016Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, nr 15, s. 7822-7830Artikel i tidskrift (Refereegranskat)

Theoretical considerations, observations, and simulations have shown that the B-y component of the interplanetary magnetic field (IMF) may cause twisting of the magnetotail. However, the fundamental issues, the temporal and spatial responses of the magnetotail in the twisting process, are still unresolved. We report unique multipoint observations of the response of the magnetotail to the variations in IMF B-y on 1-2 January 2009. For the first time, estimates of the tail twisting response time at different (Time History of Events and Macroscale Interactions during Substorms, THEMIS) distances in the same event are inferred. Using cross-correlation and timing analyses, we find that the tail twisting propagates from farther out toward the Earth and the response time increases significantly to the inner magnetosphere.

• 29.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, Stockholm, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden.
IMF dependence of the azimuthal direction of earthward magnetotail fast flows2013Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, nr 21, s. 5598-5604Artikel i tidskrift (Refereegranskat)

Cluster magnetotail data together with ACE solar wind data from 2001 to 2009 are used to investigate the dependence of the azimuthal flow direction of earthward magnetotail fast flows on the interplanetary magnetic field (IMF). We find an indication that fast flows have favored azimuthal directions that have dependence on the IMF. Our results suggest that for positive IMF By, the favored azimuthal direction of the fast flows is dawnward in the northern plasma sheet and duskward in the southern plasma sheet. For negative IMF By, an opposite situation takes place, the favored azimuthal flow directions are then duskward and dawnward in the northern and southern plasma sheet, respectively. As a possible explanation for the results, it is suggested that the untwisting reconnected magnetic field lines may direct the fast flows in the magnetotail, the field line twist itself being dependent on the IMF.

• 30.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. KTH, Stockholm. IRF-K, Kiruna. KTH, Stockholm. University of Leicester, UK. University of Leicester, UK.
Azimuthal velocity shear within an Earthward fast flow: further evidence for magnetotail untwisting?2015Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 33, s. 245-255Artikel i tidskrift (Refereegranskat)

It is well known that nonzero interplanetary magnetic field By conditions lead to a twisted magnetotail configuration. The plasma sheet is rotated around its axis and tail magnetic field lines are twisted, which causes an azimuthal displacementof their ionospheric footprints. According to the untwisting hypothesis, the untwisting of twisted field lines is suggested to influence the azimuthal direction of convective fast flows in the nightside geospace. However, there is a lack of in situ magnetospheric observations, which show actual signatures of the possible untwisting process. In this paper, we report detailed Cluster observations of an azimuthal flow shear across the neutral sheet associated with an Earthward fast flow on 5 September 2001. The observations show a flow shear velocity pattern with a Vperpy sign change, near the neutral sheet (Bx $\sim$ 0) within a fast flow during the neutral sheet flapping motion over the spacecraft. Firstly, this implies that convective fast flows may not generally be unidirectional across the neutral sheet, but may have a more complex structure. Secondly, in this event tail By and the flow shear are as expected by the untwisting hypothesis. The analysis of the flow shear reveals a linear dependence between Bx and Vperpy close to the neutral sheet and suggests that Cluster crossed the neutral sheet in the dawnward part of the fast flow channel. The magnetospheric observations are supported by the semi-empirical T96 and TF04 models. Furthermore, the ionospheric SuperDARN convection maps support the satellite observations proposing that the azimuthal component of the magnetospheric flows is enforced by a magnetic field untwisting. In summary,the observations give strong supportive evidence to the tail untwisting hypothesis. However, the T96 ionospheric mapping demonstrates the limitations of the model in mapping from a twisted tail.

• 31.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Space Sciences, Shandong University, Weihai, China.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Convection electric field and plasma convection in a twisted magnetotail: t THEMIS case study 1-2 January 20092018Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, nr 9, s. 7486-7497Artikel i tidskrift (Refereegranskat)

We investigate THEMIS satellite measurements made in a tail-aligned constellation during a time interval on 1-2 January 2009, which has previously been attributed to an interval of an interplanetary magnetic fieldB(y)-driven magnetotail twisting. We find evidence for that the orientation of the convection electric field in the tail is twist-mode dependent. For earthward flow and a negative twist (induced tail B-y < 0), the electric field is found to have northward E-z and tailward E-x components. During a positive twist (induced tail B-y > 0), the directions of E-z and E-x are reversed. The E-y component shows the expected dawn-to-dusk direction for earthward flow. The electric field components preserve their orientation across the neutral sheet, and a quasi-collinear field is observed irrespective to the tail distance. The electric field associated with the tailward flow has an opposite direction compared to the earthward flow for the negative twist. For the positive twist, the results are less clear. The corresponding plasma convection and thus the magnetic flux transport have an opposite dawn-dusk direction above and below the neutral sheet. The directions depend on the tail twist mode. The hemispherically asymmetric earthward plasma flows are suggested to be a manifestation of an asymmetric Dungey cycle in a twisted magnetotail. The role of tailward flows deserve further investigation.

• 32.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Arctic University of Norway, Tromsø, Norway. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Detection of EMPs generated by meteoroid impacts on the MMS spacecraft and problems with signal interpretation2017Ingår i: 2017 XXXIInd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS), IEEE, 2017Konferensbidrag (Refereegranskat)

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.

• 33.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Detection of meteoroid hypervelocity impacts on the Cluster spacecraft: First results2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 6, s. 6485-6494Artikel i tidskrift (Refereegranskat)

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.

• 34.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Arctic University of Norway, Tromsø, Norway. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Potential of Earth Orbiting Spacecraft Influenced by Meteoroid Hypervelocity Impacts2017Ingår i: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 45, nr 8, s. 2048-2055Artikel i tidskrift (Refereegranskat)

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.

• 35.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. e Swedish Institute of Space Physics, Kiruna, Sweden. Swedish Institute of Space Physics, Kiruna, Sweden. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Arctic University of Norway, Tromsø, Norway. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Spacecraft potential influenced by meteoroid hypervelocity impacts2016Konferensbidrag (Övrigt vetenskapligt)

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.

• 36. Yao, S. T.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Propagation of small size magnetic holes in the magnetospheric plasma sheet2016Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 6, s. 5510-5519Artikel i tidskrift (Refereegranskat)

Magnetic holes (MHs), characteristic structures where the magnetic field magnitude decreases significantly, have been frequently observed in space plasmas. Particularly, small size magnetic holes (SSMHs) which the scale is less than or close to the proton gyroradius are recently detected in the magnetospheric plasma sheet. In this study of Cluster observations, by the timing method, the minimum directional difference (MDD) method, and the spatiotemporal difference (STD) method, we obtain the propagation velocity of SSMHs in the plasma flow frame. Furthermore, based on electron magnetohydrodynamics (EMHD) theory we calculate the velocity, width, and depth of the electron solitary wave and compare it to SSMH observations. The result shows a good accord between the theory and the observation.

• 37.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Observations of kinetic-size magnetic holes in the magnetosheath2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 2, s. 1990-2000Artikel i tidskrift (Refereegranskat)

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.

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