G. Stenberg - Academia.edu (original) (raw)
Papers by G. Stenberg
Whistler waves inside the magnetopause are found to be generated in thin sheets moving with the p... more Whistler waves inside the magnetopause are found to be generated in thin sheets moving with the plasma drift velocity. In the same region we also observe time-dependent dispersive whistler emissions. The duration of each emission is about 1 second and the frequency ranges from 50 to 150 Hz. Using electric and magnetic wave field observations from the four Cluster satellites
ABSTRACT We use the multi-spacecraft mission Cluster to make observational estimates of the local... more ABSTRACT We use the multi-spacecraft mission Cluster to make observational estimates of the local energy transfer across the dayside high-latitude magnetopause during an interval of continuous reconnection. The instantaneous power input is estimated from jxB observations from the FGM instrument and CIS magnetosheath velocity observations and is compared to E·j measurements with the electric field deduced from the EFW instrument. The good agreement between these two independent measurements validates the quality of the technique. The energy flux is estimated during ten complete magnetopause crossings under steady southward and dawnward interplanetary magnetic field conditions (IMF). In most cases we find that there is no or little flow of energy across the magnetopause boundary as expected on the dayside magnetopause where the magnetic tension is mainly perpendicular to the solar wind flow. However, at some crossings when Cluster observes high-speed plasma jets energy it is found to be transferred from the magnetic field to the particles as a result of the magnetic reconnection process. Furthermore, it is possible to make estimations of the inflow reconnection velocity from the magnitude of the local energy estimate. This was found to be in reasonable agreement with the results from standard techniques. However, the inflow velocity as well as the energy transfer rate varies considerably during the course of the ten crossings. This may support the previous theory that the reconnection rate is continuously active but its rate is modulated.
ABSTRACT Downstream a parallel bow-shock the magnetosheath is one of the most turbulent regions i... more ABSTRACT Downstream a parallel bow-shock the magnetosheath is one of the most turbulent regions in the magnetosphere. In this environment thin current sheets are formed, and Retinó et al. (2007) found evidence that magnetic reconnection occurs in these layers. The width of a typical current sheet is of the order of an ion gyroradius but the reconnection signatures are the same as for large-scale reconnection at the magnetopause or in the solar wind. We perform a small statistical study to determine the probability that reconnection actually takes place given a thin current sheet in a turbulent plasma. If reconnection turns out to be common in the magnetosheath, we expect it to be a common process in turbulent plasmas throughout the universe.
Annales Geophysicae, 2006
We use Freja satellite data to investigate irregular small-scale density variations. The observat... more We use Freja satellite data to investigate irregular small-scale density variations. The observations are made in the auroral region at about 1000-1700 km. The density variations are a few percent, and the structures are found to be spatial down to a scale length of a few ion gyroradii. Irregular density variations are often found in an environment of whistler mode/lower hybrid waves and we show that at the density gradients both the electric and magnetic wave fields are enhanced.
We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions ... more We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions (O +) in the high altitude cusp and mantle. The ion energies considered are of the order 1000 eV and higher, observed above an altitude of 8 earth radii together with high wave power at the O + gyrofrequency. We show that heating by waves can explain the observed high perpendicular energy of O + ions, using a simple gyroresonance model and 25-45% of the observed wave spectral density at the gyrofrequency. This is in contrast to a recently published study where the wave intensity was too low to explain the observed high altitude ion energies. Long lasting cases (>10 min) of high perpendicularto-parallel temperature ratios are sometimes associated with low wave activity, suggesting that high perpendicular-toparallel temperature ratio is not a good indicator of local heating. Using multiple spacecraft, we show that the regions of enhanced wave activity are at least one order of magnitude larger than the gyroradius of the heated ions.
Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science
... Gurnett, DA, and LA Frank, ELF Noise Bands Associated with Auroral Electron Precipitation, J.... more ... Gurnett, DA, and LA Frank, ELF Noise Bands Associated with Auroral Electron Precipitation, J. Geophys. Res., 77, 3411, 1972. T. Oscarsson et al.: Wave Mode Identification Lefeuvre, F., JL Rauch, D. Lagoutte, JJ Berthelier, and JC Cerisier, Propagation characteristics of ...
Journal of Geophysical Research: Space Physics
Due to the small size of the Martian magnetic pileup region, especially at the subsolar point, he... more Due to the small size of the Martian magnetic pileup region, especially at the subsolar point, heated protons with high enough energy can penetrate the induced magnetosphere boundary without being backscattered, i.e., they precipitate. We present a statistical study of the downgoing~keV proton fluxes measured in the Martian ionosphere by the Analyzer of Space Plasma and Energetic Atoms experiment onboard the Mars Express spacecraft. We find that on the dayside, the events of proton penetration occur during 3% of the observation time; the precipitation is an intermittent phenomenon. The proton events carry on average~0.2% of the incident solar wind flux. Therefore, the induced magnetosphere is an effective shield against the magnetosheath protons. The events are more frequent during fast solar wind conditions than during slow solar wind conditions. The sporadic proton penetration is thought to be caused by transient increases in the magnetosheath temperature. The precipitating flux is higher on the dayside than on the nightside, and its spatial deposition is controlled by the solar wind convective electric field. The largest crustal magnetic anomalies tend to decrease the proton precipitation in the southern hemisphere. The particle and energy fluxes vary in the range 10 4-10 6 cm-2 s-1 and 10 7-10 9 eVcm-2 s-1 , respectively. The corresponding heating for the dayside atmosphere is on average negligible compared to the solar extreme ultraviolet heating, although the intermittent penetration may cause local ionization. The net precipitating proton particle flux input to the dayside ionosphere is estimated as 1.2 Á 10 21 s-1 .
Journal of Geophysical Research: Space Physics
Journal of Geophysical Research: Space Physics
We performed a statistical study of downward moving protons and alpha particles of keV energy (as... more We performed a statistical study of downward moving protons and alpha particles of keV energy (assumed to be of solar wind origin) inside the Martian induced magnetosphere from July 2006 to July 2010. Ion and electron data are from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) package on board Mars Express. We investigated the solar wind ion entry into the ionosphere, excluding intervals of low-altitude magnetosheath encounters. The study compares periods of quiet solar wind conditions and periods of solar wind pressure pulses, including interplanetary coronal mass ejections and corotating interaction regions. The solar wind ion precipitation appears localized and/or intermittent, consistent with previous measurements. Precipitation events are less frequent, and the precipitating fluxes do not increase during pressure pulse encounters. During pressure pulses, the occurrence frequency of observed proton precipitation events is reduced by a factor of~3, and for He 2+ events the occurrence frequency is reduced by a factor of~2. One explanation is that during pressure pulse periods, the mass loading of the solar wind plasma increases due to a deeper penetration of the interplanetary magnetic flux tubes into the ionosphere. The associated decrease of the solar wind speed thus increases the pileup of the interplanetary magnetic field on the dayside of the planet. The magnetic barrier becomes thicker in terms of solar wind ion gyroradii, causing the observed reduction of H + /He 2+ precipitations.
Journal of Geophysical Research: Space Physics, 2013
Solar wind helium may be a significant source of neutral helium in the Martian atmosphere. The pr... more Solar wind helium may be a significant source of neutral helium in the Martian atmosphere. The precipitating particles also transfer mass, energy, and momentum. To investigate the transport of He 2+ in the upper atmosphere of Mars, we have applied the direct simulation Monte Carlo method to solve the kinetic equation. We calculate the upward He, He + , and He 2+ fluxes, resulting from energy spectra of the downgoing He 2+ observed below 500 km altitude by the Analyzer of Space Plasmas and Energetic Atoms 3 instrument onboard Mars Express. The particle flux of the downward moving He 2+ ions was 1-2 Â 10 6 cm-2 s-1 , and the energy flux is equal to 9-10 Â 10-3 erg cm-2 s-1. The calculations of the upward flux have been made for the Martian atmosphere during solar minimum. It was found, that if the induced magnetic field is not introduced in the simulations the precipitating He 2+ ions are not backscattered at all by the Martian upper atmosphere. 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 30%-40% of the energy flux of the precipitating He 2+ ions is backscattered depending on the velocity distribution of the precipitating particles. 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.
We study emissions of whistler waves observed on the magnetospheric side of the magnetopause. Ele... more We study emissions of whistler waves observed on the magnetospheric side of the magnetopause. Electric and magnetic field observations from the four Cluster spacecraft are used to determine the wave vectors. The results show that the semi-continuous emissions actually are composed of several individual bursts. Some of the bursts can be seen by two or more satellites while other are
Journal of Geophysical Research: Space Physics, 2014
ABSTRACT Knowledge of the magnetic field morphology in the near-Venus wake is essential to the st... more ABSTRACT Knowledge of the magnetic field morphology in the near-Venus wake is essential to the studies of magnetotail dynamics and the planetary plasma escape. In this study we use the magnetic field measurements made by Venus Express during the period of April 2006 ~ December 2012, to investigate the global magnetic field morphology in the near-Venus magnetotail (0 ~ 3 Venus radii, RV, down tail) in the frame of solar wind electric field coordinates. The hemisphere with electric field pointing towards /away is indicated as –/+E-hemisphere. It has been reported that the cross-tail field component has a hemispheric asymmetry in the Venusian magnetotail. We report here that this asymmetry should have been formed at the terminator and would transport tailwards. In addition, we find that the draped magnetic field lines near both hemispheric flanks are directed equatorward in the region 0 ~ 1.5 RV down tail as it looks like “sinking” into Venus umbra. We estimate the thickness of the magnetotail current sheet and the current density at the sheet center. We find that the average half-thickness of central current sheet near + E-hemispheric flank (~460 km) is almost twice as thick as that near magnetic equatorial plane (~200 km), but the corresponding current densities at the sheet center are comparable (~6.0 nA/m2). As a result, the larger cross-tail field component found near the + E-hemispheric flank suggests a stronger tailward j × B force, i.e. the more efficient tailward acceleration of plasma in this region, showing the agreement with previous observations of heavy ion outflow from Venus. In contrast, the average magnetic field structure near ‒E-hemispheric flank is irregular, which suggests that dynamic activities, such as magnetic reconnection and magnetic field turbulence preferentially appear there.
This thesis discusses the reasons for space exploration and space science. Space plasma physics i... more This thesis discusses the reasons for space exploration and space science. Space plasma physics is identified as an essential building block to understand the space environment and it is argued that observation and analysis of space plasma waves is an important approach. Space plasma waves are the main actors in many important processes. So-called broadband waves are found responsible for much of the ion heating in the auroral region. We investigate the wave properties of broadband waves and show that they can be described as a mixture of electrostatic wave modes. In small regions void of cold electrons the broadband activity is found to be ion acoustic waves and these regions are also identified as acceleration regions. The identification of the wave modes includes reconstructions of the wave distribution function. The reconstruction technique allow us to determine the wave vector spectrum, which cannot be measured directly. The method is applied to other wave events and it is compared in some detail with a similar method. Space plasma wave are also sensitive tools for investigations of both the fine-structure and the dynamics of space plasmas. Studies of whistler mode waves observed in the boundary layer on the magnetospheric side of the magnetopause reveal that the plasma is organized in tube-like structures moving with the plasma drift velocity. The perpendicular dimension of these tubes is of the order of the electron inertial length. We present evidence that each tube is linked to a reconnection site and argue that the high density of tube-like structures indicates patchy reconnection.
ABSTRACT We have found that solar wind particles frequently precipitate onto the atmosphere of Ma... more ABSTRACT We have found that solar wind particles frequently precipitate onto the atmosphere of Mars [1,2]. The precipitating particles contribute to the energy and matter flux into the ionosphere. We use ion data from the ASPERA-3 instrument onboard Mars Express to investigate the precipitation patterns, processes and the total transfer of energy and matter from the solar wind to the atmosphere. The main reason for the proton and alpha particle precipitation is likely the large gyroradii of hot particles compared to the size of the induced magnetosphere/magnetic barrier. We find that the particle penetration depends on the direction of the convection electric field in the solar wind but that the crustal magnetic fields have very little influence. The total energy flux is low compared to the solar radiation heating on the dayside, but a significant energy source on the nightside. We also believe that the solar wind alphaparticles precipitating into the atmosphere is an important source of the neutral helium in the Martian atmosphere. We combine our observations with computer modeling [3,4]. We have applied a Direct Simulation Monte Carlo method to solve the kinetic equation for the H/H+ transport in the upper Martian atmosphere including CO2, N2 and O. We conclude that the induced magnetic field around Mars plays the crucial role in the transport of charged particles in the upper atmosphere, and it determines the energy deposition of the solar wind.
Physics of Plasmas, 2012
Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presen... more Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%-0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux. V
Whistler waves inside the magnetopause are found to be generated in thin sheets moving with the p... more Whistler waves inside the magnetopause are found to be generated in thin sheets moving with the plasma drift velocity. In the same region we also observe time-dependent dispersive whistler emissions. The duration of each emission is about 1 second and the frequency ranges from 50 to 150 Hz. Using electric and magnetic wave field observations from the four Cluster satellites
ABSTRACT We use the multi-spacecraft mission Cluster to make observational estimates of the local... more ABSTRACT We use the multi-spacecraft mission Cluster to make observational estimates of the local energy transfer across the dayside high-latitude magnetopause during an interval of continuous reconnection. The instantaneous power input is estimated from jxB observations from the FGM instrument and CIS magnetosheath velocity observations and is compared to E·j measurements with the electric field deduced from the EFW instrument. The good agreement between these two independent measurements validates the quality of the technique. The energy flux is estimated during ten complete magnetopause crossings under steady southward and dawnward interplanetary magnetic field conditions (IMF). In most cases we find that there is no or little flow of energy across the magnetopause boundary as expected on the dayside magnetopause where the magnetic tension is mainly perpendicular to the solar wind flow. However, at some crossings when Cluster observes high-speed plasma jets energy it is found to be transferred from the magnetic field to the particles as a result of the magnetic reconnection process. Furthermore, it is possible to make estimations of the inflow reconnection velocity from the magnitude of the local energy estimate. This was found to be in reasonable agreement with the results from standard techniques. However, the inflow velocity as well as the energy transfer rate varies considerably during the course of the ten crossings. This may support the previous theory that the reconnection rate is continuously active but its rate is modulated.
ABSTRACT Downstream a parallel bow-shock the magnetosheath is one of the most turbulent regions i... more ABSTRACT Downstream a parallel bow-shock the magnetosheath is one of the most turbulent regions in the magnetosphere. In this environment thin current sheets are formed, and Retinó et al. (2007) found evidence that magnetic reconnection occurs in these layers. The width of a typical current sheet is of the order of an ion gyroradius but the reconnection signatures are the same as for large-scale reconnection at the magnetopause or in the solar wind. We perform a small statistical study to determine the probability that reconnection actually takes place given a thin current sheet in a turbulent plasma. If reconnection turns out to be common in the magnetosheath, we expect it to be a common process in turbulent plasmas throughout the universe.
Annales Geophysicae, 2006
We use Freja satellite data to investigate irregular small-scale density variations. The observat... more We use Freja satellite data to investigate irregular small-scale density variations. The observations are made in the auroral region at about 1000-1700 km. The density variations are a few percent, and the structures are found to be spatial down to a scale length of a few ion gyroradii. Irregular density variations are often found in an environment of whistler mode/lower hybrid waves and we show that at the density gradients both the electric and magnetic wave fields are enhanced.
We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions ... more We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions (O +) in the high altitude cusp and mantle. The ion energies considered are of the order 1000 eV and higher, observed above an altitude of 8 earth radii together with high wave power at the O + gyrofrequency. We show that heating by waves can explain the observed high perpendicular energy of O + ions, using a simple gyroresonance model and 25-45% of the observed wave spectral density at the gyrofrequency. This is in contrast to a recently published study where the wave intensity was too low to explain the observed high altitude ion energies. Long lasting cases (>10 min) of high perpendicularto-parallel temperature ratios are sometimes associated with low wave activity, suggesting that high perpendicular-toparallel temperature ratio is not a good indicator of local heating. Using multiple spacecraft, we show that the regions of enhanced wave activity are at least one order of magnitude larger than the gyroradius of the heated ions.
Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science
... Gurnett, DA, and LA Frank, ELF Noise Bands Associated with Auroral Electron Precipitation, J.... more ... Gurnett, DA, and LA Frank, ELF Noise Bands Associated with Auroral Electron Precipitation, J. Geophys. Res., 77, 3411, 1972. T. Oscarsson et al.: Wave Mode Identification Lefeuvre, F., JL Rauch, D. Lagoutte, JJ Berthelier, and JC Cerisier, Propagation characteristics of ...
Journal of Geophysical Research: Space Physics
Due to the small size of the Martian magnetic pileup region, especially at the subsolar point, he... more Due to the small size of the Martian magnetic pileup region, especially at the subsolar point, heated protons with high enough energy can penetrate the induced magnetosphere boundary without being backscattered, i.e., they precipitate. We present a statistical study of the downgoing~keV proton fluxes measured in the Martian ionosphere by the Analyzer of Space Plasma and Energetic Atoms experiment onboard the Mars Express spacecraft. We find that on the dayside, the events of proton penetration occur during 3% of the observation time; the precipitation is an intermittent phenomenon. The proton events carry on average~0.2% of the incident solar wind flux. Therefore, the induced magnetosphere is an effective shield against the magnetosheath protons. The events are more frequent during fast solar wind conditions than during slow solar wind conditions. The sporadic proton penetration is thought to be caused by transient increases in the magnetosheath temperature. The precipitating flux is higher on the dayside than on the nightside, and its spatial deposition is controlled by the solar wind convective electric field. The largest crustal magnetic anomalies tend to decrease the proton precipitation in the southern hemisphere. The particle and energy fluxes vary in the range 10 4-10 6 cm-2 s-1 and 10 7-10 9 eVcm-2 s-1 , respectively. The corresponding heating for the dayside atmosphere is on average negligible compared to the solar extreme ultraviolet heating, although the intermittent penetration may cause local ionization. The net precipitating proton particle flux input to the dayside ionosphere is estimated as 1.2 Á 10 21 s-1 .
Journal of Geophysical Research: Space Physics
Journal of Geophysical Research: Space Physics
We performed a statistical study of downward moving protons and alpha particles of keV energy (as... more We performed a statistical study of downward moving protons and alpha particles of keV energy (assumed to be of solar wind origin) inside the Martian induced magnetosphere from July 2006 to July 2010. Ion and electron data are from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) package on board Mars Express. We investigated the solar wind ion entry into the ionosphere, excluding intervals of low-altitude magnetosheath encounters. The study compares periods of quiet solar wind conditions and periods of solar wind pressure pulses, including interplanetary coronal mass ejections and corotating interaction regions. The solar wind ion precipitation appears localized and/or intermittent, consistent with previous measurements. Precipitation events are less frequent, and the precipitating fluxes do not increase during pressure pulse encounters. During pressure pulses, the occurrence frequency of observed proton precipitation events is reduced by a factor of~3, and for He 2+ events the occurrence frequency is reduced by a factor of~2. One explanation is that during pressure pulse periods, the mass loading of the solar wind plasma increases due to a deeper penetration of the interplanetary magnetic flux tubes into the ionosphere. The associated decrease of the solar wind speed thus increases the pileup of the interplanetary magnetic field on the dayside of the planet. The magnetic barrier becomes thicker in terms of solar wind ion gyroradii, causing the observed reduction of H + /He 2+ precipitations.
Journal of Geophysical Research: Space Physics, 2013
Solar wind helium may be a significant source of neutral helium in the Martian atmosphere. The pr... more Solar wind helium may be a significant source of neutral helium in the Martian atmosphere. The precipitating particles also transfer mass, energy, and momentum. To investigate the transport of He 2+ in the upper atmosphere of Mars, we have applied the direct simulation Monte Carlo method to solve the kinetic equation. We calculate the upward He, He + , and He 2+ fluxes, resulting from energy spectra of the downgoing He 2+ observed below 500 km altitude by the Analyzer of Space Plasmas and Energetic Atoms 3 instrument onboard Mars Express. The particle flux of the downward moving He 2+ ions was 1-2 Â 10 6 cm-2 s-1 , and the energy flux is equal to 9-10 Â 10-3 erg cm-2 s-1. The calculations of the upward flux have been made for the Martian atmosphere during solar minimum. It was found, that if the induced magnetic field is not introduced in the simulations the precipitating He 2+ ions are not backscattered at all by the Martian upper atmosphere. 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 30%-40% of the energy flux of the precipitating He 2+ ions is backscattered depending on the velocity distribution of the precipitating particles. 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.
We study emissions of whistler waves observed on the magnetospheric side of the magnetopause. Ele... more We study emissions of whistler waves observed on the magnetospheric side of the magnetopause. Electric and magnetic field observations from the four Cluster spacecraft are used to determine the wave vectors. The results show that the semi-continuous emissions actually are composed of several individual bursts. Some of the bursts can be seen by two or more satellites while other are
Journal of Geophysical Research: Space Physics, 2014
ABSTRACT Knowledge of the magnetic field morphology in the near-Venus wake is essential to the st... more ABSTRACT Knowledge of the magnetic field morphology in the near-Venus wake is essential to the studies of magnetotail dynamics and the planetary plasma escape. In this study we use the magnetic field measurements made by Venus Express during the period of April 2006 ~ December 2012, to investigate the global magnetic field morphology in the near-Venus magnetotail (0 ~ 3 Venus radii, RV, down tail) in the frame of solar wind electric field coordinates. The hemisphere with electric field pointing towards /away is indicated as –/+E-hemisphere. It has been reported that the cross-tail field component has a hemispheric asymmetry in the Venusian magnetotail. We report here that this asymmetry should have been formed at the terminator and would transport tailwards. In addition, we find that the draped magnetic field lines near both hemispheric flanks are directed equatorward in the region 0 ~ 1.5 RV down tail as it looks like “sinking” into Venus umbra. We estimate the thickness of the magnetotail current sheet and the current density at the sheet center. We find that the average half-thickness of central current sheet near + E-hemispheric flank (~460 km) is almost twice as thick as that near magnetic equatorial plane (~200 km), but the corresponding current densities at the sheet center are comparable (~6.0 nA/m2). As a result, the larger cross-tail field component found near the + E-hemispheric flank suggests a stronger tailward j × B force, i.e. the more efficient tailward acceleration of plasma in this region, showing the agreement with previous observations of heavy ion outflow from Venus. In contrast, the average magnetic field structure near ‒E-hemispheric flank is irregular, which suggests that dynamic activities, such as magnetic reconnection and magnetic field turbulence preferentially appear there.
This thesis discusses the reasons for space exploration and space science. Space plasma physics i... more This thesis discusses the reasons for space exploration and space science. Space plasma physics is identified as an essential building block to understand the space environment and it is argued that observation and analysis of space plasma waves is an important approach. Space plasma waves are the main actors in many important processes. So-called broadband waves are found responsible for much of the ion heating in the auroral region. We investigate the wave properties of broadband waves and show that they can be described as a mixture of electrostatic wave modes. In small regions void of cold electrons the broadband activity is found to be ion acoustic waves and these regions are also identified as acceleration regions. The identification of the wave modes includes reconstructions of the wave distribution function. The reconstruction technique allow us to determine the wave vector spectrum, which cannot be measured directly. The method is applied to other wave events and it is compared in some detail with a similar method. Space plasma wave are also sensitive tools for investigations of both the fine-structure and the dynamics of space plasmas. Studies of whistler mode waves observed in the boundary layer on the magnetospheric side of the magnetopause reveal that the plasma is organized in tube-like structures moving with the plasma drift velocity. The perpendicular dimension of these tubes is of the order of the electron inertial length. We present evidence that each tube is linked to a reconnection site and argue that the high density of tube-like structures indicates patchy reconnection.
ABSTRACT We have found that solar wind particles frequently precipitate onto the atmosphere of Ma... more ABSTRACT We have found that solar wind particles frequently precipitate onto the atmosphere of Mars [1,2]. The precipitating particles contribute to the energy and matter flux into the ionosphere. We use ion data from the ASPERA-3 instrument onboard Mars Express to investigate the precipitation patterns, processes and the total transfer of energy and matter from the solar wind to the atmosphere. The main reason for the proton and alpha particle precipitation is likely the large gyroradii of hot particles compared to the size of the induced magnetosphere/magnetic barrier. We find that the particle penetration depends on the direction of the convection electric field in the solar wind but that the crustal magnetic fields have very little influence. The total energy flux is low compared to the solar radiation heating on the dayside, but a significant energy source on the nightside. We also believe that the solar wind alphaparticles precipitating into the atmosphere is an important source of the neutral helium in the Martian atmosphere. We combine our observations with computer modeling [3,4]. We have applied a Direct Simulation Monte Carlo method to solve the kinetic equation for the H/H+ transport in the upper Martian atmosphere including CO2, N2 and O. We conclude that the induced magnetic field around Mars plays the crucial role in the transport of charged particles in the upper atmosphere, and it determines the energy deposition of the solar wind.
Physics of Plasmas, 2012
Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presen... more Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%-0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux. V