Electron heating and acceleration at collisionless shocks in the solar wind

The electron heating at collisionless shocks in near‐Earth space is normally found to be relatively small. We report here on two sets of bow shock crossings observed by the ISEE 1 and ISEE 2 spacecraft in which very large electron temperature increases were found. The two sets of shocks are part of a larger set of 52 bow shock crossings compiled from the ISEE data set. When the shocks with the large electron heating are compared to the rest of the shocks in the compiled set, it is found that the only upstream parameter which is out of the ordinary is the upstream solar wind flow speed Vu. Indeed, for the entire shock set the only upstream parameters which correlate well with the electron heating are the solar wind speed and quantities derived from it. The best correlation among those tested is between the temperature increase (Ted ‐ Teu) and the difference in the square of the bulk flow speed (Vu² ‐ Vd²) across the shock, which is proportional to the total amount of bulk flow energy dissipated by the shock. A subsequent search for bow shock crossings which occur during intervals of high solar wind speed confirms that the electron heating is very large under high‐speed conditions. The first‐order dependence of the electron temperature increase on the available bulk flow energy is consistent with a heating process which is dominated by the macroscopic cross‐shock electrostatic potential jump. This study suggests that the temperature difference is the appropriate measure of electron heating at shocks (rather than the ratio Ted/Teu) and that the first‐order dependence on (Vu² ‐ Vd²) should be normalized out in future studies of electron heating at space shocks.

<p>Recent observational and theoretical work on solar wind turbulence and dissipation suggests that kinetic-scale fluctuations are both heating and isotropizing the solar wind during transit to 1 AU.  The nature of these fluctuations and associated heating processes are poorly understood. Whatever the dissipative process that links the fields and particles - Landau damping, cyclotron damping, stochastic heating, or energization through coherent structures - heating and acceleration of ions and electrons occurs because of electric field fluctuations. The dissipation due to the fluctuations depends intimately upon the temporal and spatial variations of those fluctuations in the plasma frame.<span>  </span>In order to derive that distribution in the plasma frame, one must also use magnetic field and density fluctuations, in addition to electric field fluctuations, as measured in the spacecraft frame (s/c) to help constrain the type of fluctuation and dissipation mechanisms that are at play.</p><p>We present here an analysis of electromagnetic fluctuations in the solar wind from MHD scales down to electron scales based on data from the Artemis spacecraft at 1 AU. We focus on a few time intervals of pristine solar wind, covering a reasonable range of solar wind properties (temperature ratios and anisotropies; plasma beta; and solar wind speed). We analyze magnetic, electric field, and density fluctuations from the 0.01 Hz (well in the inertial range) up to 1 kHz. We compute parameters such as the electric to magnetic field ratio, the magnetic compressibility, magnetic helicity, compressibility and other relevant quantities in order to diagnose the nature of the fluctuations at those scales between the ion and electron cyclotron frequencies, extracting information on the dominant modes composing the fluctuations. We also use the linear Vlasov-Maxwell solver, PLUME, to determine the various relevant modes of the plasma with parameters from the observed solar wind intervals. We discuss the results and the relevant modes as well as the major differences between our results in the solar wind and results in the magnetosheath.</p>

The Martian bow shock represents the main interface between the upstream interplanetary space and the downstream planetary obstacle, where the solar-wind plasma and the frozen-in interplanetary magnetic field (IMF) begin to be perturbed. However, the physical characteristics of the Martian bow shock layer and the influence of the external solar wind drivers on them remain unclear. By employing a three-dimensional Hall magneto-hydrodynamic (MHD) model, this study aims to reveal the physical characteristics of the Martian bow-shock layer extracted from the maximum radially inward gradient of the solar-wind velocity (VS W), including the magnetic field, current density, electric fields, and the energy transfer between the fields and solar wind protons, as well as the influence of the VS W and the IMF on these features. Simulation results indicate that the IMF has initiated the processes of piling-up, draping, bending, and slipping at the Martian bow shock, inducing an associate current to flow from the +ZMS E pole to the −ZMS E pole along the bow-shock layer, with the strongest being located at the subsolar position. Furthermore, the total electric field at the Martian bow shock is constituted by the motional electric field (EM) with the +ZMS E direction around the ±ZMS E flanks and the outward ambipolar (EA) and Hall (EH) electric fields around the lower solar zenith angles; through these, the solar wind transfers its kinetic energy to the electromagnetic fields. A higher VS W gives rise to an enhanced magnetic field, current density, and electric fields at the Martian bow shock, thereby leading to an increase in the corresponding energy-transfer rates. A greater magnitude of the IMF cross-flow component tends to result in an intensified magnetic field, current densities, EM, and EH; while it causes a decreased EA and associated energy-transfer rate at the bow shock layer. If the Parker spiral angle of the IMF is not restricted to 90°, a portion of the quasi-parallel bow-shock layer will be formed, within which the magnitudes of the magnetic field, current density, EH, and the corresponding energy-transfer rate through EH are all lower than those of the quasi-perpendicular bow-shock layer. The results of this study provide valuable insights into the physical properties at the bow-shock layer that emerge during the Mars-solar-wind interactions.

Modification of small- and middle-scale solar wind structures by the bow shock and magnetosheath: Correlation analysis

Heliospheric shocks associated with interplanetary coronal mass ejections (ICMEs) were observed by Wind, and DSCOVR at L1, STEREO spacecraft at ~1AU, and recently by the Parker Solar Probe in the inner heliosphere. The magnetic structure and the downstream magnetic oscillations were detected by Wind with 10.9 samples/s and DSCOVR with 50 samples/s. However, the velocity distributions of the protons are available at much lower cadence, and the potentially important interaction between  the alpha particles and  the heliospheric shocks are difficult to obtain directly from present data. Since the alpha particles in the solar wind are the second most abundant ion that can carry significant energy, momentum and mass flux of the solar wind, the alphas can significantly affect the propagation of these shocks. Recently, using hybrid-(PIC) models we studied the effects of alpha particles on the structure and magnetic oscillations of oblique high Mach number heliospheric shocks, and found that the magnetic and density structures of these shocks are significantly affected by the alpha particles with typical solar wind relative abundances. Here, we extend the study and report the results of new hybrid models of oblique shocks guided by observations. We investigate the typical observed relative solar wind abundances of alphas, Mach numbers, and shock normal directions, and compare the results for the various shock parameters. We model the effects of alpha particles properties on the shock ramp, wake, and downstream oscillations and study the properties of proton and alpha particle velocity distribution functions (VDFs) and the kinetic waves downstream of the shocks in the inner heliosphere. We expand the model and study for the first time the effects of relative streaming of proton-alpha ion populations as well as the ion anisotropies on the shock propagation. We investigate the effects of the ion kinetic properties on the heliospheric shock structures and discuss how the modeling results can improve the interpretation of spacecraft observations of these shocks. 

Plasma measurements were obtained with the Galileo spacecraft during an approximately 3.5-hour interval in the vicinity of Venus on 10 February 1990. Several crossings of the bow shock in the local dawn sector were recorded before the spacecraft passed into the solar wind upstream from this planet. Although observations of ions of the solar wind and the postshock magnetosheath plasmas were not possible owing to the presence of a sunshade for thermal protection of the instrument, solar wind densities and bulk speeds were determined from the electron velocity distributions. A magnetic field-aligned distribution of hotter electrons or ;;strahl'' was also found in the solar wind. Ions streaming into the solar wind from the bow shock were detected. Electron heating at the bow shock, </=20%, was notably small, with substantial density increases by factors of 2 to 3 at the day side of the shock that decrease for shock crossings further downstream from the planet. A search for pickup ions from the hot hydrogen and oxygen planetary coronas yielded an upper limit for these densities in the range of 10(-3) ion per cubic centimeter, which is consistent with densities expected from current models of neutral gas densities.

Context. The Parker Solar Probe (PSP) measures solar wind protons and electrons near the Sun. To study the thermodynamic properties of electrons and protons, we include electron effects, such as distributed turbulent heating between protons and electrons, Coulomb collisions between protons and electrons, and heat conduction of electrons. Aims. We develop a general theoretical model of nearly incompressible magnetohydrodynamic (NI MHD) turbulence coupled with a solar wind model that includes electron pressure and heat flux. Methods. It is important to note that 60% of the turbulence energy is assigned to proton heating and 40% to electron heating. We use an empirical expression for the electron heat flux. We derived a nonlinear dissipation term for the residual energy that includes both the Alfvén effect and the turbulent small-scale dynamo effect. Similarly, we obtained the NI/slab time-scale in an NI MHD phenomenology to use in the derivation of the nonlinear term that incorporates the Alfvén effect. Results. A detailed comparison between the theoretical model solutions and the fast solar wind measured by PSP and Helios 2 shows that they are consistent. The results show that the nearly incompressible NI/slab turbulence component describes observations of the fast solar wind periods when the solar wind flow is aligned or antialigned with the magnetic field.

The transport of typical ions from the surface of the Moon to the vicinity of Earth was calculated using a test particle approach. It was assumed that the ions were sputtered from the surface by the solar wind, with fluxes in the range determined experimentally by Elphic et al. (1991), and were accelerated initially to 10 eV by the potential of the Moon on its sunlit side. Si+ and Ca+ ions were selected for this transport analysis because their masses are within two prominent ion mass groups that have high sputtering yields. In the solar wind the ion trajectories were traced in the following superimposed fields: (1) a steady magnetic field B0 at an angle of 45° to the solar wind velocity Vsw, (2) the motional electric field E0 = −Vsw × B0, and (3) turbulent magnetic and electric fields generated by hydromagnetic waves with a k‐space power spectrum of |k|−5/3 propagating along both directions of the magnetic field B0. Interactions with Earth's bow shock and magnetosphere were included. Case histories of the ions were recorded in the XGSM, YGSM plane and in various planes perpendicular to the E0 × B0 drift direction of the ions between the Moon and Earth. The number density, energy and angular distributions, and directional and omnidirectional fluxes of the ions were constructed from the case histories. It was found that the diffusion of the ions increases rapidly as the amplitude of the turbulence δBrms increases beyond the value 0.04B0. The directional fluxes of Si+ and Ca+ in the solar wind, upstream from the bow shock, were found to be of the order of tens of ions cm−2 s−1 sr−1 keV−1, with energies 35‐70 keV, for δBrms = 0.04B0, B0 = 5 nT, and Vsw = 400 km/s. The fluxes of Si+ and Ca+ ions that originate in the Moon's atmosphere were estimated to be smaller than the corresponding sputtered‐ion fluxes by about an order of magnitude. Recent measurements of lunar ions upstream of the bow shock by Hilchenbach et al. (1992) generally confirm the predicted behavior of the ions.

The Earth’s bow shock is the best-known collisionless shock in space. Although much is known about the bow shock, the mechanisms of heating and thermalization processes still remain poorly understood. Collisionless shocks are different from ordinary fluid shocks, because a fraction of the incident solar wind is reflected from the bow shock and the transmitted particles are not immediately thermalized. The reflected particles interact with the incident solar wind producing waves and instabilities that can heat and accelerate particles to high energies. Some of the waves can grow to large amplitudes such as Short Large Amplitude Magnetic Structures. Other upstream nonlinear structures include hot flow anomalies and density holes. The upstream nonlinear structures subsequently convect Earthward with the SW and could impact the structure and dynamics of the bow shock. These observations have clearly indicated that the upstream dynamics are an integral part of the bow shock system. Although much has been learned about the behavior of Earth’s bow shock dynamics from the existing data, many fundamental questions remain not answered. This article will review observations of ion dynamics of Earth’s bow shock system, what we have learned from recent and past observations. We provide new perspectives from multi-spacecraft Cluster observations about the spatial and temporal variations including the fundamental shock heating, acceleration, and entropy generation processes.

We present detailed quantitative results from a theoretical model for fundamental (fp) and second harmonic (2fp) radio emission generated in Earth's foreshock by beam‐driven Langmuir waves. The model is the first to self‐consistently incorporate all key aspects of foreshock physics: reflection and acceleration of incident solar wind electrons at the bow shock, generation of unstable electron beams in the foreshock, stochastic growth of Langmuir waves and their conversion to electromagnetic waves via nonlinear processes. The physical properties of the foreshock electron beams, Langmuir waves and electromagnetic emissivities are calculated as a function of position for nominal solar wind conditions. Variations in the bow shock's location and characteristics with changes in the solar wind parameters are included. The predicted fluxes of fp and 2fp radiation are explored over a wide range of solar wind conditions. We find that the 2fp flux is most sensitive to the electron and ion temperatures and the solar wind speed and that the radio fluxes of both fp and 2fp radiation also vary significantly with the solar wind electron number density, and proportion of nonthermal electrons. Our macroscopic model provides a powerful and unique diagnostic tool with which to interpret observational data from various spacecraft under a range of solar wind conditions.

Adiabatic heating of solar wind electrons at the Earth's bow shock and its foreshock region produces transversely anisotropic hot electrons that, in turn, generate intense high-frequency whistler-mode waves. These waves are often detected by spacecraft as narrow-band, electromagnetic emissions in the frequency range of [0.1, 0.5] of the local electron gyrofrequency. Resonant interactions between these waves and electrons may cause electron acceleration and pitch-angle scattering, which can be important for creating the electron population that seeds shock drift acceleration. The high intensity and coherence of the observed whistler-mode waves prohibit the use of quasi-linear theory to describe their interaction with electrons. In this paper, we aim to develop a new theoretical approach to describe this interaction, which incorporates nonlinear resonant interactions, gradients of the background density and magnetic field, and the fine structure of the waveforms that usually consist of short, intense wave-packet trains. This is the first of two accompanying papers. It outlines a probabilistic approach to describe the wave–particle interaction. We demonstrate how the wave-packet size affects electron nonlinear resonance at the bow shock and foreshock regions, and how to evaluate electron distribution dynamics in such a system that is frequented by short, intense whistler-mode wave-packets. In the Paper II, this probabilistic approach is merged with a mapping technique, which allows us to model systems containing short and long wave-packets.

On the role of the magnetic field in the solar wind interaction with venus: Expectations versus observations

The Earth’s magnetosphere is permanently influenced by the solar wind. When supersonic and superalfvenic plasma flow interacts with the magnetosphere, the magnetosheath region is formed, which is filled with shocked turbulent plasma. Varying SW parameters influence the mechanisms of formation of this boundary layer, including the dynamics of turbulence behind the bow shock. The effect of the solar wind on the development of turbulence in the magnetosheath was demonstrated recently based on broad statistics of spacecraft measurements. The present study considers the multipoint observations of turbulent fluctuations in the solar wind, in the dayside magnetosheath and at the flanks, to analyze the evolution of the turbulent cascade while the solar wind plasma enters the magnetosheath. Observations of the magnetosheath behind the quasi-perpendicular bow shock are analyzed to exclude the influence of the bow shock topology from consideration. Three basic types of solar wind flows are considered: slow undisturbed solar wind, compressed regions, and interplanetary manifestations of coronal mass ejections. The results show surviving Kolmogorov scaling behind the bow shock for steady solar wind flow and amplification of the compressive fluctuations at the kinetic scales at the magnetosheath flanks for the solar wind associated with compressed plasma streams. During interplanetary manifestations of the coronal mass ejection, the spectra in the dayside magnetosheath substantially deviate from those observed in the solar wind (including the absence of Kolmogorov scaling and steepening at the kinetic scales) and restore at the flanks.

The problem of thundercloud electrification is one of the most difficult ones in atmospheric physics. The structure of electric fields in clouds escapes from the detailed in situ measurements; few balloon flights reveal these rather complicated structures. To gain insight into the problem of the charge structure of a thundercloud, we use new key evidence---the fluxes of particles from a thundercloud, the so-called thunderstorm ground enhancements---TGEs. TGEs originate from electron acceleration and multiplication processes in the strong electric fields in the thundercloud, and the intensity and energy spectra of electrons and gamma rays as observed on the Earth's surface are directly connected with the atmospheric electric field. Discovery of long-lasing TGEs poses new challenges for revealing structures in the thundercloud responsible for hours-extending gamma ray fluxes. In the presented paper, we demonstrate that experimentally measured intensities and energy spectra of the ``thundercloud particles'' give clues for understanding charge structures embedded in the atmosphere. A rather short ``runaway'' process above the detector site, which is consistent with the tripole structure of the cloud electrification, is changing to a much less energetic emission that lasts for hours. Measurements of enhanced particle fluxes are accompanied by the simulation experiments with corsika and geant4 codes.

The Martian bow shock stands as the first defense against the solar wind and shapes the Martian magnetosphere. Previous studies showed the correlation between the Martian bow shock location and solar wind parameters. Here we present direct evidence of solar wind effects on the Martian bow shock by analyzing Tianwen‐1 and MAVEN data. We examined three cases where Tianwen‐1 data show rapid oscillations of the bow shock, while MAVEN data record changes in solar wind plasma and magnetic field. The results indicate that the bow shock is rapidly compressed and then expanded during the dynamic pressure pulse in the solar wind, and is also oscillated during the IMF rotation. The superposition of variations in multiple solar wind parameters leads to more intensive bow shock oscillation. This study emphasizes the importance of joint observations by Tianwen‐1 and MAVEN for studying the real‐time response of the Martian magnetosphere to the solar wind.