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We use a large set of nearly 15,000 subsolar magnetopause crossings identified in the THEMIS A-E, Magion 4, Geotail, and Interball-1 satellite data to analyze the effect of interplanetary magnetic field (IMF) on the location of the magnetopause. Differences between the observed and empirical model magnetopause distances are used to account for the magnetopause distance variations due to the changes in the solar wind dynamic pressure. It is shown that not only the IMF Bz component but also the IMF clock angle has a significant effect on the magnetopause location, which is not included in traditional empirical models. Additionally, IMF By component can cause considerable dawn-dusk asymmetry in the shape of the magnetopause at times of very low Alfvén Mach numbers (MA

Since September 2014, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has been measuring the in-situ ionospheric constituents of Mars. Recently, MAVEN detected the presence of large-scale plasma depletions (at least ten-fold) within the Martian ionosphere, also known as Plasma Depletion Events (PDEs). Geometrically, the Martian PDEs appear to be bubble-like plasma structures. Although the origin and formation of PDEs are not entirely understood, they are known to occur primarily on the nightside and in regions with stronger crustal magnetic fields.In this study, we analyze the variation of magnetic field magnitude and direction and electric field power (2–100 Hz) associated with PDEs. We show that, in most cases, the magnetic fields do not considerably change within the plasma-depleted region. Conversely, the low-frequency electric field wave power is enhanced by up to two orders of magnitude at the peak depletion. Both ions and electrons within PDEs are highly magnetized. We present possible formation mechanisms of PDEs supported by recent findings.

We use conjugate observations of magnetospheric whistler mode electromagnetic waves at frequencies up to 16 kHz to determine their typical spatial scales and propagation to the ground. For this purpose, we use data obtained by the DEMETER spacecraft at an altitude of about 700 km and by the ground-based Kannuslehto station in Finland. The overlap between the two data sets corresponds to more than 500 DEMETER half-orbits between November 2006 and March 2008. Two different approaches are used. First, specific wave events observed simultaneously by both the spacecraft and the ground station are analyzed in detail. Second, the correlations of the power spectral densities of measured signals are calculated as a function of the frequency and geomagnetic longitude/L-shell separation. These are used to determine typical longitudinal/L-shell correlation lengths and to discuss wave propagation to the ground.

We use electron flux data measured by the Energetic Particle Telescope (EPT) onboard the Proba-V satellite in a Low Earth Orbit (LEO) to investigate the radiation belt response to the interplanetary shock arrival. Altogether, as many as 31 interplanetary shocks selected from the OMNI data during 2013-2018 are investigated. While the radiation belt fluxes are nearly unaffected by the shock arrival in some cases, other events reveal a sudden drop of energetic electron fluxes spanning over a broad range of L-shells. Electron flux changes at various L-shells and energies are evaluated and compared with the solar wind dynamic pressure change across the shock front, magnetopause location, and z-component of the interplanetary magnetic field. The aim is to identify parameters governing the radiation belt response to the interplanetary shock passage and to understand the strikingly different responses to the seemingly similar solar wind variations.

The Radio Occultation Science Experiment (ROSE; Withers et al., 2018, 2020) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite has produced over 400 electron density profiles from July 2016 to November 2019. These Ne(h) profiles occurred over a large range of solar zenith angles (54o- 130o) and solar flux conditions (24–54 solar flux units at Mars). One of the goals of the MAVEN mission is to characterize the status of the topside ionosphere at Mars as a reservoir of possible escaping plasma. Here we evaluate how ROSE topside ionospheric measurements, made predominantly under solar minimum conditions, compare with the only empirical model of the topside ionosphere (Němec et al., 2019). To assess congruence between the model and the observations, a deviation factor (DF) is calculated for each predicted versus observed Ne(h) profile. Diurnally, low DFs (and thus higher agreement) occur for occultations with solar zenith angles (SZA)

Magnetopause is a critical boundary dividing the space controlled by the Earth magnetic field from the solar wind and interplanetary magnetic field. Its position is controlled mainly by the solar wind dynamic pressure and north-south IMF component and these quantities are included in a variety of empirical magnetopause models. Comparison of observed magnetopause locations with model predictions can serve as a proof of our understanding of the interaction between solar wind and Earth magnetic field. Since the corresponding upstream conditions are usually derived from observation at L1, our knowledge on solar wind propagation and evolution on short scales are tested as well. We have collected about 40 000 of dayside magnetopause crossings observed by THEMIS, Cluster and Geotail spacecraft in course of 2007–2019 years and compared the observed magnetopause position with prediction of several empirical magnetopause models using OMNI upstream parameters. The difference between observed and predicted magnetopause radial distance, Robs - Rmod was used for quantification of the model-observation agreement. We have found that the median values of Robs – Rmod are well predicted by the tested models till Robs≈12 Re for all models but large positive deviations were found for larger magnetopause distances. A detailed analysis of such events revealed that they are connected with transient magnetopause displacements caused by magnetosheath perturbations of large amplitude and we are searching for their sources.

Observations of proton density fluctuations of the solar wind at 1 au have shown the presence of a decade-long transition region of the density spectrum above sub-ion scales, characterized by a flattening of the spectral slope. We use the proton density fluctuations data collected by the BMSW instrument on-board the Spektr-R satellite in order to delimit the plasma parameters under which the transition region can be observed. Under similar plasma conditions to those in observations, we carry out 3D compressible magnetohydrodynamics (MHD) and Hall-MHD numerical simulations and find that Hall physics is necessary to generate the transition region. The analysis of the kω power spectrum in the Hall-MHD simulation indicates that the flattening of the density spectrum is associated with fluctuations having frequencies smaller than the ion cyclotron frequency.

We use more than 10years of the Martian topside ionospheric data measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft to der ...

International audience; Very low frequency wave intensity measurements provided by the French low‐altitude DEMETER spacecraft are studied using the principal component analysis (PCA). We focus on both the physical interpretation of the first two principal components and their application to real physical problems. Variations of the first principal component (PC1) coefficients due to the geomagnetic activity and seasonal/longitudinal changes are studied. It is shown that their distribution corresponds to the wave intensity dependences obtained in previous studies. Moreover, the variations of PC1 coefficients around interplanetary shock arrivals are analyzed. The study is performed for fast forward (FF), fast reverse, slow forward, and slow reverse shocks separately. It shows that the most significant effect on the wave intensity is displayed in the FF case. Furthermore, it turns out that the wave intensity variations depend on the wave intensity detected before the shock arrival. Finally, the shock strength and interplanetary magnetic field orientation are also important. The performed analysis shows that PCA can be successfully applied to characterize large data sets of spacecraft measurements by limited sets of numbers—principal component coefficients (typically first one or two are enough), which still maintain a sufficient amount of information.

Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind and thus the turbulent behavior of these quantities is intensively studied. However, the temperature is also highly fluctuating quantity but behavior of its variations is studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of the temperature variations requires knowledge of the full velocity distribution with a time resolution and such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board the Spektr-R spacecraft uses the Maxwellian approximation and provides the thermal velocity with 32 ms time resolution. We use these measurements and complement them with 10 Hz magnetic field observations from the Wind spacecraft propagated to the Spektr-R location and analyze factors influencing the shape of the temperature power spectral density. A special attention is devoted to mutual relations of power spectral densities of different quantities like parallel and perpendicular temperature, magnetic field and velocity fluctuations and their evolution in course of solar wind expansion.

The solar wind is a unique laboratory to study the turbulent processes occurring in a collisionless plasma with high Reynolds numbers. A turbulent cascade—the process that transfers the free energy contained within the large scale fluctuations into the smaller ones—is believed to be one of the most important mechanisms responsible for heating of the solar corona and solar wind. The paper analyzes power spectra of solar wind velocity, density and magnetic field fluctuations that are computed in the frequency range around the break between inertial and kinetic scales. The study uses measurements of the Bright Monitor of the Solar Wind (BMSW) on board the Spektr-R spacecraft with a time resolution of 32 ms complemented with 10 Hz magnetic field observations from the Wind spacecraft propagated to the Spektr-R location. The statistics based on more than 42,000 individual spectra show that: (1) the spectra of both quantities can be fitted by two (three in the case of the density) power-law segments; (2) the median slopes of parallel and perpendicular fluctuation velocity and magnetic field components are different; (3) the break between MHD and kinetic scales as well as the slopes are mainly controlled by the ion beta parameter. These experimental results are compared with high-resolution 2D hybrid particle-in-cell simulations, where the electrons are considered to be a massless, charge-neutralizing fluid with a constant temperature, whereas the ions are described as macroparticles representing portions of their distribution function. In spite of several limitations (lack of the electron kinetics, lower dimensionality), the model results agree well with the experimental findings. Finally, we discuss differences between observations and simulations in relation to the role of important physical parameters in determining the properties of the turbulent cascade.

We analyze variations of energetic particle fluxes measured by low altitude spacecraft after interplanetary shock arrivals and around the times of significant geomagnetic storms. Data from two different spacecraft and energetic particle detectors are used and compared. First, we use data measured by the energetic particle detector (IDP) onboard the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft. The spacecraft operated between 2004 and 2010 on a circular Sun-synchronous orbit at an altitude of initially 710 km, which was decreased to 660 km in December 2005. The IDP instrument measured electron flux close to the loss cone at energies between about 70 keV and 2.3 MeV (128 energy channels). Second, we use data measured by the Space Application of Timepix Radiation Monitor (SATRAM) onboard the Proba-V satellite operating since May 2013 on a circular Sun-synchronous orbit at an altitude of 820 km. The semi-conductor based pixelated radiation detector called Timepix is capable of detecting all charged particles and X-rays with sufficiently high energies. Electron energies higher than about 2 MeV and proton energies higher than about 20 MeV are detected. We identify the times of interplanetary shock arrivals and significant (Dst < –100 nT) geomagnetic storms during the mission durations. Then we perform a superposed epoch analysis to reveal characteristic particle flux variations around these times at different energies and L-shells. Although the used satellite missions do not overlap in time, we aim to compare the revealed flux variation signatures between these two independent data sets.

Wave intensity measured in the very low frequency (VLF) range (up to 20 kHz) is typically represented using frequency-time spectrograms. Since the characterization of spectrogram main features and/or their direct comparison is a challenging task, we transform the measurements of the low-altitude DEMETER spacecraft using the principal component analysis (PCA). The present study is focused on both the physical interpretation of the first two principal components and their application to real physical problems. To understand the physical meaning of the first principal components, their scatter plot is constructed and discussed. Moreover, the dependence of the first principal component (PC1) coefficients on the geomagnetic activity and their seasonal/longitudinal variations are analyzed. The obtained distributions are well comparable with those obtained by previous studies for average wave intensities, indicating that the PC1 coefficients are directly related to the overall wave intensity. Furthermore, the variations of PC1 coefficients around interplanetary (IP) shock arrivals are analyzed, suggesting that the fast forward shock occurrence has the most significant effect. It is shown that the wave intensity variations depend on the wave intensity detected before the shock arrival. The shock strength and interplanetary magnetic field orientation are also important. To further demonstrate the adaptability of PCA, we use a similar method to analyze also ground-based VLF measurements performed by the Kannuslehto station located in northern Finland.

International audience; Electromagnetic waves observed in the inner magnetosphere at frequencies between about 0.5 and 4 kHz sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity with modulation periods from tens of seconds up to a few minutes. Such waves are typically termed QP emissions and their origin is still not fully understood. We use a large set of more than 2,000 of these events identified in the low-altitude DEMETER spacecraft data to check for energetic electron flux variations matching the individual QP wave elements. Altogether, 7 such events are identified and their detailed analysis is performed. Energetic electron fluxes are found to be modulated primarily at energies lower than about 250 keV. While the waves may propagate unducted across L-shells, the energetic particles follow magnetic field lines from the interaction region down to the observation point. This is used to estimate the locations of anticipated generation regions to L-shells between about 4 and 6, and the respective source radial dimensions to about 0.6-1.2 Earth radii. The frequencies of the events are confined below half of the equatorial electron gyrofrequency in the determined source regions. Finally, it is shown that individual QP elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited., We study the seasonal and geographical variability of the peak electron density and the altitude of the main ionospheric peak at Mars. For this purpose, we use the data obtained by the ESA Mars Express mission, namely by the radar MARSIS and the radio occultation experiment MaRS. The accumulation of data during the long lifetime of Mars Express provides for the first time an almost complete seasonal and geographical coverage. We first remove the dominant variability factors affecting the main ionospheric peak, namely the effect of changes in the solar zenith angle (SZA), and the changes in the solar ultraviolet radiation output at the Sun. When averaging results obtained at all latitudes, we find that the seasonal variation of both the peak density and the peak altitude can be well reproduced by sinusoidal functions with amplitudes about 8%–9% of the annually averaged peak density, and between 8 and 9.5 km for the peak altitude. We also find elevated peak electron densities in the region of strong crustal fields and latitudinal asymmetries in both the peak density and altitude. Comparing the seasonal evolution of the peak altitude during Mars Year 28, a year with a global dust storm, and the rest of the years, we find that the global dust storm raised the altitude of the ionospheric peak by about 10–15 km. © 2021. The Authors., F. Gonzalez-Galindo is funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades, the Agencia Estatal de Investigacion and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). F. Nemec was supported by MSMT Grant LTAUSA17070. The Mars Express Radio Science Experiment (MaRS) is funded by the German Space Agency (DLR) under Grant 50QM1802. Support for Mars Express Radio Science at Stanford University is provided by NASA through JPL Contract 1217744. Support for the Multimission Radio Science Support Team is provided by NASA/JPL. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We thank everyone involved in the Mars Express projects at ESTEC, ESOC, ESAC, JPL, and the ESTRACK and DSN ground stations for their continuous support.

International audience; NWC is an extremely powerful very low frequency (19.8 kHz) transmitter located in the north‐west Australia. Although the transmitter typically operates continuously, it was off during the second half of 2007. This allows for a direct comparison of an ionospheric situation at the times when the transmitter is on with the times when the transmitter is off. We use electromagnetic wave and plasma measurements performed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft at an altitude of about 660 km. Given that the transmitter signal is significantly attenuated in lower ionospheric layers during the daytime and, moreover, the dayside ionosphere is controlled primarily by the solar radiation, concealing possible transmitter‐related effects, we focus exclusively on the nightside. We show that although the NWC transmitter signal does not significantly change the mean plasma density and only slightly increases the electron temperature, it causes significant perturbations of both these quantities at distances up to about 200 km. The wave intensity is considerably enhanced in the same spatial region close to the transmitter in a large range of frequencies above about 14 kHz. Finally, clear signatures of transmitter induced electron precipitation are detected to the east of the transmitter at somewhat larger L‐shells, consistent with a gyroresonance condition.

Introduction The Martian dayside ionosphere features a well-defined maximum in the electron density profile placed at about 130 km from the surface and produced by the interaction of the incoming UV solar radiation with the upper atmosphere. The ionosphere of Mars is thus strongly affected by changes in the underlying neutral atmosphere and in the amount of solar radiation getting to the planet. Being one of the main reservoirs for atmospheric escape to space, it is important to characterize the variability of the ionosphere and its potential implications for the long-term evolution of the Martian climate. The major variability factor affecting the ionospheric main peak region is the change in the solar zenith angle (SZA), which produces an increase in the peak altitude and a decrease in the peak electron density when approaching the terminator (Withers, 2009). It is also well known that an increase in the solar activity would produce an increase in the peak electron density due to the enhanced ionization. Less attention has been devoted to the ionospheric seasonal variability. The significant eccentricity of the Martian orbit is expected to induce changes in the ionosphere. The changing Sun-Mars distance during the Martian year affects the amount of solar radiation getting to the planet, and thus is expected to modify the peak electron density. The changes in the temperature of the lower/middle atmosphere driven by the orbital eccentricity are also expected to affect the altitude of the peak. While previous works have confirmed these expectations (e.g. Zou et al., 2005; Morgan et al., 2008; Němec et al., 2011), a complete characterization of the seasonal variability has not yet been performed due to the limited seasonal coverage of the existing datasets. In this work we take advantage of the large ionospheric dataset collected by two instruments on Mars Express, the radar MARSIS on its Active Ionospheric Sounding (AIS) mode, and the radio-occultation experiment MaRS, during the more than 16 years (and still running) of the mission. Results In order to isolate the seasonal variability, we first remove the main variability factors, the change in the SZA and the change in the intrinsic solar activity, by using the well-known and thoroughly tested expressions derived from Chapman theory. Then we average the obtained corrected peak electron densities and peak altitudes in bins of 5 degrees of Ls. Finally we fit a sinusoidal function to the obtained seasonal variability. The main results we obtain are: 1. The seasonal variability of both the peak electron density and the peak altitude can be well reproduced by sinusoidal functions maximizing around the date of the perihelion. The fitted amplitudes are, for the peak electron density, about 9% of the annually-averaged value, and for the peak altitude about 9 km. 2. We find hints of latitudinal differences in the seasonal evolution. However the large spread of the data does not allow for a detailed study of these latitudinal effects. 3. By separating the variability obtained during Mars Year 28 (MY28) from the rest of the years we find that the global dust storm during MY28 produced an increase in the peak altitude of about 10-15 km, in line with previous works (Girazian et al., 2020) 4. We do not find an increase in the peak electron density around Ls=30-70, such as previously found for the TEC (Sánchez-Cano et al., 2018) Fig. 1. Seasonal variability of the latitudinally-averaged, SZA and solar activity corrected peak electron density Fig. 2. Seasonal variability of the latitudinally-averaged, SZA corrected peak altitude References Girazian, Z., et al., Variations in the Ionospheric Peak Altitude at Mars in Response to Dust Storms: 13 Years of Observations From the Mars Express Radar Sounder. Journal of Geophysical Research (Planets), 125 (5), e06092. doi: 10.1029/2019JE006092, 2020 Morgan, D. D., et al., Variation of the Martian ionospheric electron density from Mars Express radar soundings. Journal of Geophysical Research (Space Physics), 113 , 9303. doi: 10.1029/2008JA013313, 2008 Němec, F., et al., Dayside ionosphere of Mars: Empirical model based on data from the MARSIS instrument. Journal of Geophysical Research (Planets), 116 (E7), E07003. doi:10.1029/2010JE003789, 2011 Sánchez-Cano, B., et al., Spatial, Seasonal, and Solar Cycle Variations of the Martian Total Electron Content (TEC): Is the TEC a Good Tracer for Atmospheric Cycles? Journal of Geophysical Research (Planets), 123 (7), 1746-1759. doi: 10.1029/2018JE005626, 2018JE005626, 2018 Withers, P.. A review of observed variability in the dayside ionosphere of Mars. Advances in Space Research, 44 , 277-307. doi: 10.1016/j.asr.2009.04.027, 2009 Acknowledgements F.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). F. Němec was supported by MSMT Grant LTAUSA17070

Two-dimensional hybrid particle-in-cell (PIC) simulations are carried out on a constant L-shell (or drift shell) surface of the dipole magnetic field to investigate the generation process of near-e...

International audience; Quasiperiodic emissions are magnetospheric whistler mode waves at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity. We use large data sets of events observed by the Van Allen Probes in the equatorial region at larger radial distances and by the low-altitude DEMETER spacecraft. While Van Allen Probes observe the events at all local times and longitudes, DEMETER observations are limited nearly exclusively to the daytime and significantly less frequent at the longitudes of the South Atlantic Anomaly. Further, while the events observed by Van Allen Probes are smoothly distributed over seasons with only mild maxima in spring/autumn, DEMETER occurrence rate has a single pronounced minimum in July. The apparent inconsistency is explained by considering a nondipolar Earth's magnetic field and significant background wave intensities which in these cases prevent the quasiperiodic events from being identified in DEMETER data.

The Space Application of Timepix Radiation Monitor (SATRAM) on board the Proba-V satellite of the European Space Agency (ESA) was launched in May 2013 into a sun-synchronous orbit with an altitude of about 820 km. This technology demonstration payload is based on the Timepix technology developed by the CERN-based Medipix2 Collaboration. It is equipped with a 300 um thick silicon sensor with a pixel pitch of 55 um in a 256 x 256 pixel matrix. The device is sensitive to X-rays and all charged particles. A Monte Carlo simulation was conducted to determine the detector response to electrons (0.5–7 MeV) and protons (10–400 MeV) taking into account the shielding of the detector housing and the satellite. With the help of the simulation, a strategy was developed to estimate omnidirectional electron, proton, and ion fluxes around Earth using stopping power, maximum energy deposition per pixel of the particle track, and the shape of the particle tracks in the sensor. Presented are typical overall dose rates as well as fluxes of individual particle species. A superposed epoch analysis is used to analyze variations of particle fluxes related to geomagnetic storms and interplanetary shock arrivals as a function of time and L-shell.

Similarly to the power density spectrum of magnetic field fluctuations in the solar wind, the spectrum of density fluctuations also shows multiple spectral slopes. Both of them present a spectral index varying between –3/2 and –5/3 in the inertial range and close to –2.8 between the proton and electron gyrofrequencies.Despite these similarities, the spectrum of density fluctuations has a significant difference with respect to the magnetic and velocity fluctuations spectra: it shows a transition region between the inertial and the kinetic ranges with spectral index typically around –1.We have combined the results of compressible Hall-MHD numerical simulations and measurements of the BMSW instrument onboard Spektr-R satellite to study the possible causes of the flattening in the density spectrum. Both numerical and experimental approaches point towards an important role played by Kinetic Alfvén Waves.

Electric field data obtained by the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft are used to identify signals related to dust impacts on the spacecraft body and/or on the instrument probes. The analyzed waveforms snapshots are 62.5 ms long (4,096 points sampled at 65,536 Hz). An automatic procedure to identify short electric field pulses with signatures corresponding to those expected for the dust impacts has been developed and applied to available data in years 2014–2018, resulting in about 40,000 of events. Each of the identified pulses is characterized by several quantitative parameters (polarity, magnitude, relaxation time, magnitude of a possible pre-spike). The event occurrence and respective quantitative parameters of detected pulses are then analyzed as a function of local plasma conditions in the Martian ionosphere (electron density and temperature), the spacecraft location, and the spacecraft potential. The obtained results are compared with a simple scheme of the signal formation upon a dust impact.

The analysis of magnetic field and velocity fluctuations in corresponding minimum variance frames revealed that: (1) Minimum variance and mean magnetic field directions would be similar but these two directions are often perpendicular, especially in the high-beta environment, and a number of perpendicular cases decreases with the scale length; (2) Compressibility computed in the minimum variance frame generally increases with frequency but the increase is not monotonic; it exhibits two breaks observed for the magnetic field as well as for velocity fluctuations with approximately the same break frequencies. (3) We suggest that the first break can be connected with a change of pure Alfven to kinetic Alfven modes and the second break approximately coincides with the transition from the inertial to kinetic scales.

International audience; Various solar wind ion species move with different speeds and theoretical considerations as well as limited observations in a region close to the Sun show that heavy solar wind ions tend to flow faster than protons, at least in less-aged fast solar wind streams. The solar wind flow carries the frozen-in interplanetary magnetic field (IMF) and this situation evokes three related questions: (I) what is the proper solar wind speed, (II) is this speed equal to the speed of the dominant component, whatever that may be, and (III) what is the speed of the magnetic field? We show that simple theoretical considerations based on the MHD approximation as well as on the dynamics of charged particles in electric and magnetic fields suggest that the IMF velocity of motion (de Hoffmann-Teller (HT) velocity) would be deliberated as the velocity appropriate for solar wind studies. Our analysis based on the Wind, Helios, ACE, and SOHO observations of differential streaming of solar wind populations shows that their energy is conserved in the HT frame. On the other hand, the noise and temporal resolution of the data do not allow us to decide whether the total momentum is also conserved in this frame.

We investigate the influence of lightning‐generated whistlers on the overall intensity of electromagnetic waves measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft (2004–2010, quasi Sun‐synchronous polar orbit with an altitude of about 700 km) at frequencies below 18 kHz. Whistler occurrence rate evaluated using an onboard neural network designed for automated whistler detection is used to distinguish periods of high and low whistler occurrence rates. It is shown that especially during the night and particularly in the frequency‐geomagnetic latitude intervals with a low average wave intensity, contribution of lightning‐generated whistlers to the overall wave intensity is significant. At frequencies below 1 kHz, where all six electromagnetic wave components were measured during specific intervals, the study is accompanied by analysis of wave propagation directions. When we limit the analysis only to fractional‐hop whistlers, which propagate away from the Earth, we find a reasonable agreement with results obtained from the whole data set. This also confirms the validity of the whistler occurrence rate analysis at higher frequencies.

Magnetic fields in the inner magnetosphere can be obtained as vector sums of the Earth’s own internal magnetic field and magnetic fields stemming from currents flowing in the space plasma. While the Earth’s internal magnetic field is accurately described by the International Geomagnetic Reference Field (IGRF) model, the characterization of the external magnetic fields is significantly more complicated, as they are highly variable and dependent on the actual level of the geomagnetic activity. Tsyganenko family magnetic field models (T89, T96, T01, TA15B, TA15N), parameterized by the geomagnetic activity level and solar wind parameters, are often used by the involved community to describe these fields. In the present paper, we use a large dataset (2001–2018) of magnetospheric magnetic field measurements obtained by the four Cluster spacecraft to assess the accuracy of these models. We show that, while the newer models (T01, TA15B, TA15N) perform significantly better than the old ones (T89, T96), there remain some systematic deviations, in particular at larger latitudes. Moreover, we compare the locations of the min-B equator determined using the four-point Cluster spacecraft measurements with the locations determined using the magnetic field models. We demonstrate that, despite the newer models being comparatively slightly more accurate, an uncertainty of about one degree in the latitude of the min-B equator remains.

Quasiperiodic (QP) electromagnetic emissions are whistler mode waves at typical frequencies of a few kHz characterized by a periodic time modulation of their intensity. The DEMETER spacecraft observed events where the QP emissions exhibit a sudden change in the wave vector and Poynting vector directions. The change happens in a short interval of latitudes. We explain this behavior by ionospheric reflection and present a ray tracing simulation which matches resulting wave vector directions. We also attempt to locate the source region of these emissions and conclude that they are most probably generated at the inner boundary of the plasmapause which also acts as a guide during the propagation of the QP emissions.

Coronal mass ejections (CMEs) are responsible for most severe space weather events, such as solar energetic particle events and geomagnetic storms at Earth. Type II radio bursts are slow drifting emissions produced by beams of suprathermal electrons accelerated at CME-driven shock waves propagating through the corona and interplanetary medium. Here, we report a statistical study of 153 interplanetary type II radio bursts observed by the two STEREO spacecraft between 2008 March and 2014 August. The shock associated radio emission was compared with CME parameters included in the Heliospheric Cataloguing, Analysis and Techniques Service catalog. We found that faster CMEs are statistically more likely to be associated with the interplanetary type II radio bursts. We correlate frequency drifts of interplanetary type II bursts with white-light observations to localize radio sources with respect to CMEs. Our results suggest that interplanetary type II bursts are more likely to have a source region situated closer to CME flanks than CME leading edge regions.