H. Nilsson - Academia.edu (original) (raw)

Papers by H. Nilsson

Journal of Geophysical Research: Space Physics, 2013

ABSTRACT Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux ... more ABSTRACT 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 . J < 0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E . 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 18 RE. Our observations are compared to recent simulation results and previous observations.

Planetary and Space Science, 2008

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Expres... more Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Express and data from the Magnetometer/Electron Reflectometer (MAG/ER) on Mars Global Surveyor have been analyzed to determine whether ion beam events (IBEs) are correlated with the direction of the draped interplanetary magnetic field (IMF) or the proximity of strong crustal magnetic fields to the subsolar point. We examined 150 IBEs and found that they are organized by IMF draping direction. However, no clear dependence on the subsolar longitude of the strongest magnetic anomaly is evident, making it uncertain whether crustal magnetic fields have an effect on the formation of the beams. We also examined data from the IMA sensor of the ASPERA-4 instrument suite on Venus Express and found that IBEs are observed at Venus as well, which indicates the morphology of the Martian and Venusian magnetotails are similar. r

Physical Review Letters, 2006

We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnect... more We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnection separatrix region. This region separates two plasmas with significantly different temperatures and densities, at a large distance from the X line. We identify which terms in the generalized Ohm's law balance the observed electric field throughout the separatrix region. The electric field inside a thin approximately c/omega pi Hall layer is balanced by the j x B/ne term while other terms dominate elsewhere. On the low density side of the region we observe a density cavity which forms due to the escape of magnetospheric electrons along the newly opened field lines. The perpendicular electric field inside the cavity constitutes a potential jump of several kV. The observed potential jump and field aligned currents can be responsible for strong aurora.

Journal of Geophysical Research, 2010

1] We report Mars Express (MEX) measurements showing that the regolith-covered Phobos surface bac... more 1] We report Mars Express (MEX) measurements showing that the regolith-covered Phobos surface backscatters 0.5%-10% of the impinging solar wind protons. Backscattered protons with an energy corresponding to 30%-40% of the solar wind energy injected in the flow are picked up by the solar wind and may cause field disturbances and wave activity. Since a similar backscattering of solar wind protons was observed and reported at the Moon, the backscattering of the solar wind particles is a typical physical process for regolith-covered bodies in space, even though the regolith is highly porous and it has been thought that almost 100% of the solar wind protons are implanted.

Journal of Geophysical Research, 2008

1] Auroral images obtained by the Auroral Large Imaging System, an optical facility with extremel... more 1] Auroral images obtained by the Auroral Large Imaging System, an optical facility with extremely low noise level and large dynamical range, have been examined in order to study the scaling properties of spatial variations in auroral luminosity. The images under examination were obtained during substorm conditions and contained active auroral forms of different types. The field of view was ±17°from magnetic zenith, which allowed the investigation of auroral fluctuations at scales from 6.4 to 51.2 km. The narrow field of view is assumed to decrease the contamination from aspect angle broadening which leads to distortions of scaling characteristics in the horizontal plane. We analyzed the logscale diagrams constructed by discrete wavelet transform of the data as well as the standard deviation, kurtosis, and probability density functions of the fluctuations. It is found that the data series for substorm auroral forms often reveal closeness to self-similarity, the observed scaling exponent varying between 0.6 and 0.7. Auroral fluctuations in different emission lines exhibit a very similar scaling behavior. A comparison is performed with fluctuations in an electric field observed by the Dynamics Explorer 2 satellite under substorm conditions. It is shown that the intermittency features are more pronounced for substorm electric fields than for auroral fluctuations.

Journal of Geophysical Research, 2011

ABSTRACT We have applied the Direct Simulation Monte Carlo method to solve the kinetic equation f... more ABSTRACT We have applied the Direct Simulation Monte Carlo method to solve the kinetic equation for the H/H+ transport in the upper Martian atmosphere. We calculate the upward H and H+ fluxes, values that can be measured, and the altitude profile of the energy deposition to be used to understand the energy balance in the Martian atmosphere. The calculations of the upward flux have been made for the Martian atmosphere during solar minimum. We use an energy spectrum of the down moving protons in the altitude range 355–437 km adopted from the Mars Express Analyzer of Space Plasma and Energetic Atoms measurements in the range 700 eV–20 keV. The particle and energy fluxes of the downward moving protons were equal to 3.0 × 10^6 cm−2 s−1 and 1.4 × 10^−2 erg cm−2 s−1. It was found that 22% of particle flux and 12% of the energy flux of the precipitating protons is backscattered by the Martian upper atmosphere, if no induced magnetic field is taken into account in the simulations. If we include a 20 nT horizontal magnetic field, a typical field measured by Mars Global Surveyor in the altitude range of 85–500 km, we find that up to 40%–50% of the energy flux of the precipitating protons is backscattered depending on the velocity distribution of the precipitating protons. We thus conclude that the induced magnetic field plays a crucial role in the transport of charged particles in the upper atmosphere of Mars and, therefore, that it determines the energy deposition of the solar wind.

Journal of Geophysical Research, 2006

Source of wedge-like dispersed sub-keV ring current ions (wedgelike structure) is investigated us... more Source of wedge-like dispersed sub-keV ring current ions (wedgelike structure) is investigated using Cluster CIS data. Statistics from nearly 550 traversals show that the wedge-like structure with upper energy extending to ordinary energy of ring current is found mostly in the morning sector whereas those limited within sub-keV range are found mostly in the noonto-afternoon sector. The former is most likely the original form of the latter. With this knowledge, the Cluster traversal on 21 October 2001 turned out to be a unique observation that reveals the formation of the wedge-like structure. Spacecraft 1 and 4 detected a wedge-like structure of 0.1 ∼ 10 keV protons at 23:50 UT, while spacecraft 3 did not detect it 10 minutes before in the same magnetic flux tube. With the observed electric field of less than 3 mV/m, this fact indicates that the dispersion started within half an hour. Pitch angle distributions of the wedge-like structure is in most cases double conic-like butterfly distributions, but the wedge-like dispersed oxygen ions during the 21 October 2001 event flow mainly from one direction (from northern hemisphere) without a loss cone. With its characteristic energy of 0.05 ∼ 0.3 keV, these oxygen ions originate from the northern hemisphere about

Journal of Geophysical Research, 1996

On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud w... more On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF>50 nT) and high dynamic pressure (>50 nPa). During this interval the Freja satellite detected intense cusp-like plasma injections in four out of six dayside traversals. The first two traversals are carefully studied. During the first traversal the overall morphology of the ion injection is characterized by a ``multiple-injection'' signature over a wide magnetic local time (MLT) range, whereas it is characterized by a ``single-injection'' signature with narrow injection region at 8 MLT in the second traversal. The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversals, the cusp plasma injection is detected by the Søndre Strømfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Søndre Strømfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection). The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high. (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.

Journal of Geophysical Research, 1998

Incoherent scatter radar measurements are used to study the E region and lower F region in the cu... more Incoherent scatter radar measurements are used to study the E region and lower F region in the cusp proper. At the equatorward edge of the cusp a region of enhanced ionization is found, indicating precipitation of either electrons of about 1 keV energy or protons of about 3 keV. The signature is found at the same altitude, around 150 kin, as the maximum of the solar EUV produced ionization, but is clearly recognizable. The position and likely proton energies indicate that this is probably the ionospheric signature of the cusp acceleration region. Regions of low electron densities are also found in the same altitude region, which may be explained by downward field-aligned currents causing depletions in the E and lower F region. First-order estimates show that dense field-aligned currents are needed to cause the observed depletions, possibly as dense as 30/•A/m = . The ionization in the cusp acceleration region, and the depletion caused by downward field-aligned currents both influence the Pedersen conductivity of the ionosphere, and the resulting conductivity gradients strongly influence the small-scale field-aligned currents flowing in the cusp region. 1. Introduction Ionospheric effects of magnetosheath particles precipitating in the cusp region have been described in a number of studies, most based on radar and optical observations. Optical observations can give good temporal and spatial resolution but have a number of limitations. These include that the emission altitude is uncertain and that ground-based optical measurements can be obtained only during mid winter [e.g., Sandholt et al., 1986]. HF radars can study convection and morphology of the cusp precipitation region [e.g., Baker et al., 1995], even though the precise reason for the HF radar spectra from the cusp remains unclear. However, only incoherent scatter radars can give detailed information on the state of the ionosphere. Incoherent scatter (IS) radar can give altitude profiles of electron densities, electron temperatures, ion temperatures, and ion drift velocities. The altitude resolution is important because it means that the energy of precipitating particles causing electron density and tempera-ture enhancements can be estimated [e.g., Kirkwood and Osepian, 1995]. Previous IS radar studies [e.g., Lockwood et al., 1993; Nilsson et al., 1994, 1996] have shown that a clear signature is seen in the ionospheric F region from the 100 eV cusp electrons. The ions cause ionization at about the same altitude as the solar EUV and did not give a distinguishable signature in the study of Nilsson et al. [1994]. However, a careful study of the E region in the data presented by Nilsson et al. [1996] reveals that regions of enhanced E region electron densities can be identified. Most notably, there is a region of such enhancements just at the equatorward edge of the cusp. As will be discussed later, this is the obvious signature of the cusp acceleration region as reported by, for example, Woch and Lundin [1992] and Newell and Meng [1991]. The characteristic charged particle population observed in this region is usually interpreted as the accelerated flows away from the reconnection region predicted by reconnection theories (referred to as the smoking gun evidence of reconnection by among others $onnerup et al. [1995]). Satellite-based measurements of such accelerated flows seem to imply that they are not always present even if reconnection seems to be going on (for a review, see Cowley [1982], and for more recent work, see Phan and Paschmann [1995]). Continuous ground-based observation of the ionospheric signature of this region could shed more light on the nature and persistence of active reconnection. Specifically, 26,721

Journal of Geophysical Research, 1996

Measurements with the Søndre Strømfjord incoherent scatter radar, co-ordinated with the observati... more Measurements with the Søndre Strømfjord incoherent scatter radar, co-ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May-June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp-cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well-defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other dayside precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field-aligned currents.

Geophysical Research Letters, 2010

1] We report on detailed observations by the four Cluster spacecraft of magnetic reconnection and... more 1] We report on detailed observations by the four Cluster spacecraft of magnetic reconnection and a Flux Transfer Event (FTE) at the magnetopause. We detect cold (eV) plasma at the magnetopause with two independent methods. We show that the cold ions can be essential for the electric field normal to the current sheet in the separatrix region at the edge of the FTE and for the associated acceleration of ions from the magnetosphere into the reconnection jet. The cold ions have small enough gyroradii to drift inside the limited separatrix region and the normal electric field can be balanced by this drift, E ≈ −v × B. The separatrix region also includes cold accelerated electrons, as part of the reconnection current circuit. Citation: André, M., A. Vaivads,

Geophysical Research Letters, 2008

Geophysical Research Letters publishes short, concise research letters that present scientific ad... more Geophysical Research Letters publishes short, concise research letters that present scientific advances that are likely to have immediate influence on the research of other investigators. GRL letters can focus on a specific discipline or apply broadly to the geophysical science community ...

Geophysical Research Letters, 2010

We report Cluster observations of oxygen energization by several keV at the boundary between the ... more We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency.

Annales Geophysicae, 2005

A combination of EISCAT incoherent scatter radar measurements, optical and magnetometer data is u... more A combination of EISCAT incoherent scatter radar measurements, optical and magnetometer data is used to study the plasma in and around pre-noon structured precipitation and auroral arcs. Particular attention is paid to regions of comparatively low E-region density observed adjacent to arcs or structured precipitation in the EISCAT Svalbard radar field-aligned measurements. Comparison between luminosity and incoherent scatter electron density measurements shows that the low-density regions occur primarily due to the absence of diffuse precipitation rather than to a cavity formation process. Two cases of arcs and low density/luminosity regions are identified. The first is related to a strong Pc5 pulsation event, and the absence of diffuse precipitation is due to a large-scale modulation of the diffuse precipitation. In the second case the equatormost arc is on a shielding boundary and the low-density region coincides with a strong flow region just poleward of this arc. Regions of high electric field and low luminosity and conductance are observed prior to intensification of the structured precipitation in both cases. The ionospheric current is enhanced in the low conductance region, indicating that the strong electric fields do not result solely from ionospheric polarization electric fields, and thus are mainly driven by magnetospheric processes. The average energy of the precipitating electrons in the arcs and structured precipitation is, according to EIS-CAT measurements, 500 eV and the energy spectra are similar for the pulsation and shielding cases. The average energy is thus significantly less than in the diffuse precipitation region which shows central CPS-like energy spectra. We suggest that the low ionospheric conductance of 0.7 S in the low density regions is favorable for the arc formation process. This is in quantitative agreement with recent simulations of the ionospheric feedback instability.

Annales Geophysicae, 2014

ABSTRACT Energy-latitude dispersed structured sub-keV ions in the inner magnetosphere often show ... more ABSTRACT Energy-latitude dispersed structured sub-keV ions in the inner magnetosphere often show significant development or intensification (by more than factor of 3) within 1-2 h near noon or afternoon where the azimuthal ion drift velocity at the sub-keV range is expected to be near zero. To examine whether such sudden appearances in the dayside can be explained by the drift motion of ions that are formed during substorm-related injections, we numerically simulated two such examples, one at noon and the other in the afternoon, based on the ion drift model. For both cases, the ion drift model with finite duration of proton source in the nightside can explain the observed large inbound-outbound differences in the sub-keV proton population without any new sources. Ion drift motion can thus cause rapid changes of complicated ion populations, at remote places from the source a long time after the substorm activities.

We present the first two-spacecraft simultaneous observations of the Martian bow shock (BS) and m... more We present the first two-spacecraft simultaneous observations of the Martian bow shock (BS) and magnetic pileup boundary (MPB), obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. We compare the positions of these two boundaries with models based on previous measurements by Mars Global Surveyor. The BS is found to

Space Science Reviews, 2007

2-similarities between Mars and the Earth regarding the accelerated electron and ion distribution... more 2-similarities between Mars and the Earth regarding the accelerated electron and ion distributions. Specifically acceleration above the night-side cavity of Mars and acceleration above discrete aurora at the Earth -characterized by nearly monoenergetic downgoing electrons in conjunction with nearly monoenergetic upgoing ions. We describe a number of characteristic features in the accelerated plasma: The "inverted V" energy-time distribution, beam vs temperature distribution, altitude distribution, local time distribution and connection with magnetic anomalies. We also compute the electron energy flux and find that the energy flux is sufficient to cause weak to medium strong aurora at Mars. Monoenergetic counterstreaming accelerated ions and electrons is the signature of field-aligned electric currents and electric field acceleration. The topic is reasonably well understood in terrestrial magnetospheric physics, although some controversy still remains on details and the cause-effect relationships. We present a potential cause-effect relationship leading to auroral plasma acceleration in the nightside cavity of Mars -the downward acceleration of electrons supposedly manifesting itself as discrete aurora above Mars.

Geomagnetism and Aeronomy, 2006

The dynamics of auroral arcs, observed in the prenoon sector during the 2-h period, has been stud... more The dynamics of auroral arcs, observed in the prenoon sector during the 2-h period, has been studied in the context of ionospheric convection. The appearance of an isolated arc, the poleward drift velocity of which pronouncedly exceeded the plasma drift velocity, accompanied the IMF impulse and could result from the Alfvén resonance oscillations of the magnetosphere. Arcs that appeared after the northward turning of the IMF vertical component drifted poleward at a velocity close to the convection velocity. The mechanism of arc generation is related to the flute instability which develops in the region of the large-scale field-aligned current. Flute instability indications are found out in the POLAR satellite data. The study confirms the previously proposed classification criterion for dayside arcs with the source on closed field lines, based on the character of arc drift as compared to convection. PACS numbers: 94.30. Aa

Journal of Geophysical Research: Space Physics, 2013

ABSTRACT Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux ... more ABSTRACT 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 . J < 0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E . 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 18 RE. Our observations are compared to recent simulation results and previous observations.

Planetary and Space Science, 2008

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Expres... more Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Express and data from the Magnetometer/Electron Reflectometer (MAG/ER) on Mars Global Surveyor have been analyzed to determine whether ion beam events (IBEs) are correlated with the direction of the draped interplanetary magnetic field (IMF) or the proximity of strong crustal magnetic fields to the subsolar point. We examined 150 IBEs and found that they are organized by IMF draping direction. However, no clear dependence on the subsolar longitude of the strongest magnetic anomaly is evident, making it uncertain whether crustal magnetic fields have an effect on the formation of the beams. We also examined data from the IMA sensor of the ASPERA-4 instrument suite on Venus Express and found that IBEs are observed at Venus as well, which indicates the morphology of the Martian and Venusian magnetotails are similar. r

Physical Review Letters, 2006

We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnect... more We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnection separatrix region. This region separates two plasmas with significantly different temperatures and densities, at a large distance from the X line. We identify which terms in the generalized Ohm's law balance the observed electric field throughout the separatrix region. The electric field inside a thin approximately c/omega pi Hall layer is balanced by the j x B/ne term while other terms dominate elsewhere. On the low density side of the region we observe a density cavity which forms due to the escape of magnetospheric electrons along the newly opened field lines. The perpendicular electric field inside the cavity constitutes a potential jump of several kV. The observed potential jump and field aligned currents can be responsible for strong aurora.

Journal of Geophysical Research, 2010

1] We report Mars Express (MEX) measurements showing that the regolith-covered Phobos surface bac... more 1] We report Mars Express (MEX) measurements showing that the regolith-covered Phobos surface backscatters 0.5%-10% of the impinging solar wind protons. Backscattered protons with an energy corresponding to 30%-40% of the solar wind energy injected in the flow are picked up by the solar wind and may cause field disturbances and wave activity. Since a similar backscattering of solar wind protons was observed and reported at the Moon, the backscattering of the solar wind particles is a typical physical process for regolith-covered bodies in space, even though the regolith is highly porous and it has been thought that almost 100% of the solar wind protons are implanted.

Journal of Geophysical Research, 2008

1] Auroral images obtained by the Auroral Large Imaging System, an optical facility with extremel... more 1] Auroral images obtained by the Auroral Large Imaging System, an optical facility with extremely low noise level and large dynamical range, have been examined in order to study the scaling properties of spatial variations in auroral luminosity. The images under examination were obtained during substorm conditions and contained active auroral forms of different types. The field of view was ±17°from magnetic zenith, which allowed the investigation of auroral fluctuations at scales from 6.4 to 51.2 km. The narrow field of view is assumed to decrease the contamination from aspect angle broadening which leads to distortions of scaling characteristics in the horizontal plane. We analyzed the logscale diagrams constructed by discrete wavelet transform of the data as well as the standard deviation, kurtosis, and probability density functions of the fluctuations. It is found that the data series for substorm auroral forms often reveal closeness to self-similarity, the observed scaling exponent varying between 0.6 and 0.7. Auroral fluctuations in different emission lines exhibit a very similar scaling behavior. A comparison is performed with fluctuations in an electric field observed by the Dynamics Explorer 2 satellite under substorm conditions. It is shown that the intermittency features are more pronounced for substorm electric fields than for auroral fluctuations.

Journal of Geophysical Research, 2011

ABSTRACT We have applied the Direct Simulation Monte Carlo method to solve the kinetic equation f... more ABSTRACT We have applied the Direct Simulation Monte Carlo method to solve the kinetic equation for the H/H+ transport in the upper Martian atmosphere. We calculate the upward H and H+ fluxes, values that can be measured, and the altitude profile of the energy deposition to be used to understand the energy balance in the Martian atmosphere. The calculations of the upward flux have been made for the Martian atmosphere during solar minimum. We use an energy spectrum of the down moving protons in the altitude range 355–437 km adopted from the Mars Express Analyzer of Space Plasma and Energetic Atoms measurements in the range 700 eV–20 keV. The particle and energy fluxes of the downward moving protons were equal to 3.0 × 10^6 cm−2 s−1 and 1.4 × 10^−2 erg cm−2 s−1. It was found that 22% of particle flux and 12% of the energy flux of the precipitating protons is backscattered by the Martian upper atmosphere, if no induced magnetic field is taken into account in the simulations. If we include a 20 nT horizontal magnetic field, a typical field measured by Mars Global Surveyor in the altitude range of 85–500 km, we find that up to 40%–50% of the energy flux of the precipitating protons is backscattered depending on the velocity distribution of the precipitating protons. We thus conclude that the induced magnetic field plays a crucial role in the transport of charged particles in the upper atmosphere of Mars and, therefore, that it determines the energy deposition of the solar wind.

Journal of Geophysical Research, 2006

Source of wedge-like dispersed sub-keV ring current ions (wedgelike structure) is investigated us... more Source of wedge-like dispersed sub-keV ring current ions (wedgelike structure) is investigated using Cluster CIS data. Statistics from nearly 550 traversals show that the wedge-like structure with upper energy extending to ordinary energy of ring current is found mostly in the morning sector whereas those limited within sub-keV range are found mostly in the noonto-afternoon sector. The former is most likely the original form of the latter. With this knowledge, the Cluster traversal on 21 October 2001 turned out to be a unique observation that reveals the formation of the wedge-like structure. Spacecraft 1 and 4 detected a wedge-like structure of 0.1 ∼ 10 keV protons at 23:50 UT, while spacecraft 3 did not detect it 10 minutes before in the same magnetic flux tube. With the observed electric field of less than 3 mV/m, this fact indicates that the dispersion started within half an hour. Pitch angle distributions of the wedge-like structure is in most cases double conic-like butterfly distributions, but the wedge-like dispersed oxygen ions during the 21 October 2001 event flow mainly from one direction (from northern hemisphere) without a loss cone. With its characteristic energy of 0.05 ∼ 0.3 keV, these oxygen ions originate from the northern hemisphere about

Journal of Geophysical Research, 1996

On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud w... more On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF>50 nT) and high dynamic pressure (>50 nPa). During this interval the Freja satellite detected intense cusp-like plasma injections in four out of six dayside traversals. The first two traversals are carefully studied. During the first traversal the overall morphology of the ion injection is characterized by a ``multiple-injection'' signature over a wide magnetic local time (MLT) range, whereas it is characterized by a ``single-injection'' signature with narrow injection region at 8 MLT in the second traversal. The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversals, the cusp plasma injection is detected by the Søndre Strømfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Søndre Strømfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection). The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high. (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.

Journal of Geophysical Research, 1998

Incoherent scatter radar measurements are used to study the E region and lower F region in the cu... more Incoherent scatter radar measurements are used to study the E region and lower F region in the cusp proper. At the equatorward edge of the cusp a region of enhanced ionization is found, indicating precipitation of either electrons of about 1 keV energy or protons of about 3 keV. The signature is found at the same altitude, around 150 kin, as the maximum of the solar EUV produced ionization, but is clearly recognizable. The position and likely proton energies indicate that this is probably the ionospheric signature of the cusp acceleration region. Regions of low electron densities are also found in the same altitude region, which may be explained by downward field-aligned currents causing depletions in the E and lower F region. First-order estimates show that dense field-aligned currents are needed to cause the observed depletions, possibly as dense as 30/•A/m = . The ionization in the cusp acceleration region, and the depletion caused by downward field-aligned currents both influence the Pedersen conductivity of the ionosphere, and the resulting conductivity gradients strongly influence the small-scale field-aligned currents flowing in the cusp region. 1. Introduction Ionospheric effects of magnetosheath particles precipitating in the cusp region have been described in a number of studies, most based on radar and optical observations. Optical observations can give good temporal and spatial resolution but have a number of limitations. These include that the emission altitude is uncertain and that ground-based optical measurements can be obtained only during mid winter [e.g., Sandholt et al., 1986]. HF radars can study convection and morphology of the cusp precipitation region [e.g., Baker et al., 1995], even though the precise reason for the HF radar spectra from the cusp remains unclear. However, only incoherent scatter radars can give detailed information on the state of the ionosphere. Incoherent scatter (IS) radar can give altitude profiles of electron densities, electron temperatures, ion temperatures, and ion drift velocities. The altitude resolution is important because it means that the energy of precipitating particles causing electron density and tempera-ture enhancements can be estimated [e.g., Kirkwood and Osepian, 1995]. Previous IS radar studies [e.g., Lockwood et al., 1993; Nilsson et al., 1994, 1996] have shown that a clear signature is seen in the ionospheric F region from the 100 eV cusp electrons. The ions cause ionization at about the same altitude as the solar EUV and did not give a distinguishable signature in the study of Nilsson et al. [1994]. However, a careful study of the E region in the data presented by Nilsson et al. [1996] reveals that regions of enhanced E region electron densities can be identified. Most notably, there is a region of such enhancements just at the equatorward edge of the cusp. As will be discussed later, this is the obvious signature of the cusp acceleration region as reported by, for example, Woch and Lundin [1992] and Newell and Meng [1991]. The characteristic charged particle population observed in this region is usually interpreted as the accelerated flows away from the reconnection region predicted by reconnection theories (referred to as the smoking gun evidence of reconnection by among others $onnerup et al. [1995]). Satellite-based measurements of such accelerated flows seem to imply that they are not always present even if reconnection seems to be going on (for a review, see Cowley [1982], and for more recent work, see Phan and Paschmann [1995]). Continuous ground-based observation of the ionospheric signature of this region could shed more light on the nature and persistence of active reconnection. Specifically, 26,721

Journal of Geophysical Research, 1996

Measurements with the Søndre Strømfjord incoherent scatter radar, co-ordinated with the observati... more Measurements with the Søndre Strømfjord incoherent scatter radar, co-ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May-June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp-cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well-defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other dayside precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field-aligned currents.

Geophysical Research Letters, 2010

1] We report on detailed observations by the four Cluster spacecraft of magnetic reconnection and... more 1] We report on detailed observations by the four Cluster spacecraft of magnetic reconnection and a Flux Transfer Event (FTE) at the magnetopause. We detect cold (eV) plasma at the magnetopause with two independent methods. We show that the cold ions can be essential for the electric field normal to the current sheet in the separatrix region at the edge of the FTE and for the associated acceleration of ions from the magnetosphere into the reconnection jet. The cold ions have small enough gyroradii to drift inside the limited separatrix region and the normal electric field can be balanced by this drift, E ≈ −v × B. The separatrix region also includes cold accelerated electrons, as part of the reconnection current circuit. Citation: André, M., A. Vaivads,

Geophysical Research Letters, 2008

Geophysical Research Letters publishes short, concise research letters that present scientific ad... more Geophysical Research Letters publishes short, concise research letters that present scientific advances that are likely to have immediate influence on the research of other investigators. GRL letters can focus on a specific discipline or apply broadly to the geophysical science community ...

Geophysical Research Letters, 2010

We report Cluster observations of oxygen energization by several keV at the boundary between the ... more We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency.

Annales Geophysicae, 2005

A combination of EISCAT incoherent scatter radar measurements, optical and magnetometer data is u... more A combination of EISCAT incoherent scatter radar measurements, optical and magnetometer data is used to study the plasma in and around pre-noon structured precipitation and auroral arcs. Particular attention is paid to regions of comparatively low E-region density observed adjacent to arcs or structured precipitation in the EISCAT Svalbard radar field-aligned measurements. Comparison between luminosity and incoherent scatter electron density measurements shows that the low-density regions occur primarily due to the absence of diffuse precipitation rather than to a cavity formation process. Two cases of arcs and low density/luminosity regions are identified. The first is related to a strong Pc5 pulsation event, and the absence of diffuse precipitation is due to a large-scale modulation of the diffuse precipitation. In the second case the equatormost arc is on a shielding boundary and the low-density region coincides with a strong flow region just poleward of this arc. Regions of high electric field and low luminosity and conductance are observed prior to intensification of the structured precipitation in both cases. The ionospheric current is enhanced in the low conductance region, indicating that the strong electric fields do not result solely from ionospheric polarization electric fields, and thus are mainly driven by magnetospheric processes. The average energy of the precipitating electrons in the arcs and structured precipitation is, according to EIS-CAT measurements, 500 eV and the energy spectra are similar for the pulsation and shielding cases. The average energy is thus significantly less than in the diffuse precipitation region which shows central CPS-like energy spectra. We suggest that the low ionospheric conductance of 0.7 S in the low density regions is favorable for the arc formation process. This is in quantitative agreement with recent simulations of the ionospheric feedback instability.

Annales Geophysicae, 2014

ABSTRACT Energy-latitude dispersed structured sub-keV ions in the inner magnetosphere often show ... more ABSTRACT Energy-latitude dispersed structured sub-keV ions in the inner magnetosphere often show significant development or intensification (by more than factor of 3) within 1-2 h near noon or afternoon where the azimuthal ion drift velocity at the sub-keV range is expected to be near zero. To examine whether such sudden appearances in the dayside can be explained by the drift motion of ions that are formed during substorm-related injections, we numerically simulated two such examples, one at noon and the other in the afternoon, based on the ion drift model. For both cases, the ion drift model with finite duration of proton source in the nightside can explain the observed large inbound-outbound differences in the sub-keV proton population without any new sources. Ion drift motion can thus cause rapid changes of complicated ion populations, at remote places from the source a long time after the substorm activities.

We present the first two-spacecraft simultaneous observations of the Martian bow shock (BS) and m... more We present the first two-spacecraft simultaneous observations of the Martian bow shock (BS) and magnetic pileup boundary (MPB), obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. We compare the positions of these two boundaries with models based on previous measurements by Mars Global Surveyor. The BS is found to

Space Science Reviews, 2007

2-similarities between Mars and the Earth regarding the accelerated electron and ion distribution... more 2-similarities between Mars and the Earth regarding the accelerated electron and ion distributions. Specifically acceleration above the night-side cavity of Mars and acceleration above discrete aurora at the Earth -characterized by nearly monoenergetic downgoing electrons in conjunction with nearly monoenergetic upgoing ions. We describe a number of characteristic features in the accelerated plasma: The "inverted V" energy-time distribution, beam vs temperature distribution, altitude distribution, local time distribution and connection with magnetic anomalies. We also compute the electron energy flux and find that the energy flux is sufficient to cause weak to medium strong aurora at Mars. Monoenergetic counterstreaming accelerated ions and electrons is the signature of field-aligned electric currents and electric field acceleration. The topic is reasonably well understood in terrestrial magnetospheric physics, although some controversy still remains on details and the cause-effect relationships. We present a potential cause-effect relationship leading to auroral plasma acceleration in the nightside cavity of Mars -the downward acceleration of electrons supposedly manifesting itself as discrete aurora above Mars.

Geomagnetism and Aeronomy, 2006

The dynamics of auroral arcs, observed in the prenoon sector during the 2-h period, has been stud... more The dynamics of auroral arcs, observed in the prenoon sector during the 2-h period, has been studied in the context of ionospheric convection. The appearance of an isolated arc, the poleward drift velocity of which pronouncedly exceeded the plasma drift velocity, accompanied the IMF impulse and could result from the Alfvén resonance oscillations of the magnetosphere. Arcs that appeared after the northward turning of the IMF vertical component drifted poleward at a velocity close to the convection velocity. The mechanism of arc generation is related to the flute instability which develops in the region of the large-scale field-aligned current. Flute instability indications are found out in the POLAR satellite data. The study confirms the previously proposed classification criterion for dayside arcs with the source on closed field lines, based on the character of arc drift as compared to convection. PACS numbers: 94.30. Aa