Terrestrial Bow Shock Research Articles

Bow Shock Ripples and Their Modulation of Whistler Wave Packets: MMS Observations

AbstractThe terrestrial bow shock is the boundary where supersonic solar wind slows down abruptly near the magnetopause. The shock front geometry could be modulated by surface waves to form rippled structures, which impact the acceleration process of the solar wind particles. However, the rippled structures are hard to be identified unambiguously due to the similar signatures in single‐spacecraft observations between rippled shocks and reforming shocks. Here, we utilize the four‐spacecraft observations from the MMS mission to investigate an event of quasi‐perpendicular bow shock crossing. The periodic oscillations of shock normal directions and normal velocities support the scenario of surface wave propagation in the tangential direction. We also reconstruct the shock profile along the normal direction, and its monotonic shape further excludes the occurrence of shock reformation. These ripples are found to modulate the reflected ions and whistler wave packets, which adds to the complexity of the bow shock plasma environments.

Statistical Study of the Sub-ion-scale Magnetic Holes in the Lunar Space Environment

Sub-ion-scale magnetic holes play a significant role in electron transportation and energy dissipation. In the upstream region of the terrestrial bow shock, they are expected to originate from the upstream solar wind as well as the foreshock. The Moon can move into the solar wind; whether it can affect the observation of the sub-ion-scale magnetic holes is unclear. Here, we statistically investigate 268 sub-ion-scale magnetic holes in the lunar space environment based on observations of the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun mission. The median duration of these magnetic holes is ∼0.31 s, and the median size of their cross sections is ∼0.5 ρ i or ∼38.9 ρ e. We regard an isolated or a train of magnetic holes as an event; thus, these magnetic holes belong to 207 events. The data at X SSE < 0 and |Y SSE| < 2 R M (lunar radii) account for ∼21.7% of all observed data, but ∼89.4% of the events are observed in this region, suggesting that they are more likely to occur in the lunar wake. Furthermore, their occurrence rates in the lunar wake are much larger in the region close to its boundary than in other wake regions. And the occurrence rates in the lunar wake near the boundary at X SSE > −3 R M are larger in the dawnside than that in the duskside. These observations suggest that the region in the lunar wake close to its boundary is a possible source of sub-ion-scale magnetic holes.

Energy Repartition and Entropy Generation across the Earth’s Bow Shock: MMS Observations

The evolution of plasma entropy and the process of plasma energy redistribution at the collisionless plasma shock front are evaluated based on the high temporal resolution data from the four Magnetospheric Multiscale spacecraft during the crossing of the terrestrial bow shock. The ion distribution function has been separated into the populations with different characteristic behaviors in the vicinity of the shock: the upstream core population, the reflected ions, the gyrating ions, the ions trapped in the vicinity of the shock, and the downstream core population. The values of ion and electron moments (density, bulk velocity, and temperature) have been determined separately for these populations. It is shown that the solar wind core population bulk velocity slows down mainly in the ramp with the electrostatic potential increase but not in the foot region as it was supposed. The reflected ion population determines the foot region properties, so the proton temperature peak in the foot region is an effect of the relative motion of the different ion populations, rather than an actual increase in the thermal speed of any of the ion population. The ion entropy evaluated showed a significant increase across the shock: the enhancement of the ion entropy occurs in the foot of the shock front and at the ramp, where the reflected ions are emerging in addition to the upstream solar wind ions, the anisotropy growing to generate the bursts of ion-scale electrostatic waves. The entropy of electrons across the shock does not show a significant change: electron heating goes almost adiabatically.

Interplanetary Magnetic Field Effect on the Location of the Martian Bow Shock: MAVEN Observations

The Martian bow shock (BS) is generated with the mass-loading and magnetic pileup processes when the solar wind interacts with the Martian ionosphere. In this vein, the interplanetary magnetic field (IMF) frozen in the solar wind can affect the location of the Martian BS, which is less reported. Based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we manually identify 10,283 BS crossings during a period of the gradually declining solar cycle phase (2014 October–2020 December) and investigate the effects of the intensity and orientation of the IMF on the Martian BS. In the Mars Solar Orbital coordinate system, our results show the following: (1) The Martian BS, including the subsolar and flank regions, linearly moves away from Mars when the IMF intensity increases, which confirms the theoretical and the MHD simulation results. (2) Under the radial IMF condition, we first demonstrate that the subsolar and flank regions of the Martian BS are situated closer to Mars compared to other IMF situations. This might be caused by the weaker magnetic pileup process and the “low-pressure magnetosheath” model under the radial IMF condition. (3) Moreover, the cross section of the Martian BS is elongated in the north–south direction when the Y component of the IMF is dominant, which is on account of the fast magnetosonic speed effect and verifies the elongation phenomenon of the terrestrial BS. The IMF intensity and orientation effects cannot be ignored and should be considered in future models of the Martian BS.

Ion Acceleration at the Quasi‐Parallel Shock: The Source Distributions of the Diffuse Ions

AbstractThe terrestrial bow shock is the boundary that slows and diverts the supermagnetosonic solar wind around the terrestrial magnetosphere by converting the kinetic energy of the solar wind into thermal and magnetic energy. Shock fronts are an important acceleration site for ions and electrons in collisionless plasmas, and are responsible for much of the particle acceleration in solar, planetary, and astrophysical regions. One of the fundamental outstanding questions of ion acceleration at shocks for which the upstream magnetic field is nearly aligned with the shock normal (i.e., quasi‐parallel shocks) is which portion of the incoming solar wind ion distribution ultimately becomes the seed population that is subsequently accelerated to high energies. This study discusses distribution functions of protons and alpha particles observed by the HPCA and FPI instruments onboard the MMS satellites during a crossing of the quasi‐parallel bow shock. The bow shock transition from the downstream region into the upstream solar wind shows the occasional presence of reflected ions and a population of 90° pitch angle ions in the shock ramp consistent with shock drift accelerated ions. Both populations contribute to the seed population of the shock accelerated ions known as the diffuse ion population.

Observations of Rapidly Growing Whistler Waves in Front of Space Plasma Shock due to Resonance Interaction between Fluctuating Electron Velocity Distributions and Electromagnetic Fields

The whistler-mode wave extending from the fast-magnetosonic wave branch is a fundamental perturbation of electromagnetic fields and plasmas in various environments including planetary space, laboratory, and astrophysics. The origin and evolution of the waves is a long-standing question due to the limited instrumental capability in resolving highly variable plasma and electromagnetic fields. Here, we analyze observational data with a high time resolution from the Magnetospheric Multiscale spacecraft in front of the terrestrial bow shock (e.g., foreshock). We develop a novel approach to extract the three-dimensional fluctuating electron velocity distributions (δf e( V )) from their background (f e0( V )), and have successfully captured the coherent resonance between fluctuating electrons (δf e( V )) and wavelike electromagnetic fields (δ B , δ E ) at an unprecedentedly high frequency (>1 Hz) for investigating wave–particle interactions. We provide that the unstable whistler wave grows rapidly over a timescale that is much shorter than the proton gyro-period. Regarding the energy origin for the waves, we find the ion distributions consisting of the solar wind ion flows and the ion beams reflected from the shock play crucial roles in providing the free energy and determining the eigenmode disturbances of fields and electrons. The quantification of wave growth rate and the characterization of wave–particle interactions for the instability driver can significantly advance the understandings of wave evolution and energy conversion between multisource multispecies particles and wave electromagnetic fields.

Properties of a Supercritical Quasi-perpendicular Interplanetary Shock Propagating in the Terrestrial Foreshock Region

We investigate the properties of an interplanetary (IP) shock (M f = 2.1, θ = 80°) that was detected by Wind on 1999 September 12 and was magnetically connected to the terrestrial bow shock. Key results are obtained concerning how the IP shock modifies the terrestrial foreshock electron and ion dynamics: (1) Intensive Langmuir waves were detected downstream of the IP shock due to the enhanced penetrating terrestrial foreshock electron beams. (2) Whistler heat flux instabilities associated with the oblique precursor whistler pitch-angle scatter the suprathermal electrons, together with the normal betatron acceleration that occurs across the IP shock. (3) The IP shock interacts with the antisunward propagating Alfvén waves/fluctuations, and the shocked plasma shows both Alfvénic and mirror-mode features where Alfvén waves were separated into two parts propagating in opposite directions. (4) Intense specularly reflected gyrating ions were detected around the IP shock, which indicates that the energy dissipation effects at the IP shock depend on both reflected ions and the waves intrinsic to the shock. These results provide new insights into the interaction between an IP shock and the terrestrial foreshock.

Dynamics of Earth's bow shock under near-radial interplanetary magnetic field conditions

We investigate the dynamics of Earth's quasi-parallel terrestrial bow shock based on measurements from the Magnetospheric MultiScale (MMS) spacecraft constellation during a period of near-radial interplanetary magnetic conditions, when the interplanetary magnetic field and the solar wind (SW) velocity are nearly anti-parallel. High-speed earthward ion flows with properties that are similar to those of the pristine SW are observed to be embedded within the magnetosheath-like plasma. These flows are accompanied by Interplanetary Magnetic Field (IMF) intensity of less than about 10 nT, compared to nearby magnetosheath intensities of generally greater than 10 nT. The high-speed flow intervals are bounded at their leading and trailing edges by intense fluxes of more energetic ions and large amplitude quasi-sinusoidal magnetic oscillations, similar to ultra-low frequency waves known to steepen and pileup on approach toward Earth to form the quasi-parallel bow shock. The MMS string-of-pearls configuration is aligned with the outbound trajectory and provides inter-spacecraft separations of several hundred km along its near 103 length, allowing sequential observation of the plasma and magnetic field signatures during the event by the four spacecraft. The SW-like interval is most distinct at the outer-most MMS-2 and sequentially less distinct at each of the trailing MMS spacecraft. We discuss the interpretation of this event alternatively as MMS having observed a quasi-rigid bow shock contraction/expansion cycle, ripples or undulations propagating on the bow shock surface, or a more spatially local evolution in the context of either a deeply deformed shock surface or a porous shock surface, as in the three-dimensional patchwork concept of the quasi-parallel bow shock, under the extant near-radial IMF condition.

Magnetic Discontinuities in the Solar Wind and Magnetosheath: Magnetospheric Multiscale Mission (MMS) Observations

We perform a statistical investigation of the geometric features of interplanetary discontinuities (IDs) in the near-Earth solar wind and magnetosheath, by utilizing 14 months of Magnetospheric Multiscale mission data. 117,669 IDs are collected, including 108,049 events in the solar wind and 6399 events in the magnetosheath, with the remnant in the magnetosphere or near the bow shock/magnetopause. We find the following: (1) the ID occurrence rate is 17.0 events hr−1 in the solar wind and 5.5 events hr−1 in the magnetosheath, (2) the field rotation angles during ID crossings in the magnetosheath exhibit a two-exponential distribution with a breakpoint at 50°, which is not observed for IDs in the solar wind, (3) the magnetosheath IDs with small field rotation angles tend to be clustered, (4) by classifying the IDs into rotational discontinuities (RDs), tangential discontinuities (TDs), either TDs or RDs (EDs), and neither TDs nor RDs (NDs), we estimate RD:TD:ED:ND = 68%:5%:20%:7% in the solar wind, and RD:TD:ED:ND = 15%:44%:18%:23% in the magnetosheath, (5) the occurrence rates of RDs and TDs are, respectively 7.95 and 0.58 events hr−1 in the solar wind, and 0.57 and 1.60 events hr−1 in the magnetosheath, (6) RDs are more likely to propagate antisunward in the plasma rest frame, especially in the magnetosheath, and (7) the average thicknesses of the RDs and TDs are estimated, respectively, as 10.4 and 8.1 proton gyroradii (r p ) in the solar wind, and 17.4 and 5.0 r p in the magnetosheath. This work can improve our understanding of IDs’ interaction with the terrestrial bow shock.

Analysis of multiscale structures at the quasi-perpendicular Venus bow shock

Context.Solar Orbiter is a European Space Agency mission with a suite of in situ and remote sensing instruments to investigate the physical processes across the inner heliosphere. During the mission, the spacecraft is expected to perform multiple Venus gravity assist maneuvers while providing measurements of the Venusian plasma environment. The first of these occurred on 27 December 2020, in which the spacecraft measured the regions such as the distant and near Venus magnetotail, magnetosheath, and bow shock.Aims.This study aims to investigate the outbound Venus bow shock crossing measured by Solar Orbiter during the first flyby. We study the complex features of the bow shock traversal in which multiple large amplitude magnetic field and density structures were observed as well as higher frequency waves. Our aim is to understand the physical mechanisms responsible for these high amplitude structures, characterize the higher frequency waves, determine the source of the waves, and put these results into context with terrestrial bow shock observations.Methods.High cadence magnetic field, electric field, and electron density measurements were employed to characterize the properties of the large amplitude structures and identify the relevant physical process. Minimum variance analysis, theoretical shock descriptions, coherency analysis, and singular value decomposition were used to study the properties of the higher frequency waves to compare and identify the wave mode.Results.The non-planar features of the bow shock are consistent with shock rippling and/or large amplitude whistler waves. Higher frequency waves are identified as whistler-mode waves, but their properties across the shock imply they may be generated by electron beams and temperature anisotropies.Conclusions.The Venus bow shock at a moderately high Mach number (∼5) in the quasi-perpendicular regime exhibits complex features similar to the Earth’s bow shock at comparable Mach numbers. The study highlights the need to be able to distinguish between large amplitude waves and spatial structures such as shock rippling. The simultaneous high frequency observations also demonstrate the complex nature of energy dissipation at the shock and the important question of understanding cross-scale coupling in these complex regions. These observations will be important to interpreting future planetary missions and additional gravity assist maneuvers.

Statistical Properties of Electron-scale Magnetic Peaks in the Solar Wind at 1 au

Abstract Energy cascade from magnetohydrodynamic to kinetic scales can create many coherent structures in the turbulent astrophysical plasma environment, such as magnetic holes and magnetic peaks. Knowing the properties of each coherent structure is critical to better understand the process of the energy cascade. Recently, electron-scale magnetic peaks (ESMPs) are revealed to exist in the solar wind at 1 au. Here, we investigate the properties of the ESMPs upstream of the terrestrial bow shock based on observations of the Magnetospheric Multiscale spacecraft. We regard an isolated ESMP or a train of ESMPs as an ESMP event, and 204 ESMPs or 32 ESMP events are found. Both the durations and cross-section sizes of the ESMPs obey log-normal distributions. The median duration and cross-section size are ∼0.25 s and ∼0.33 ion gyroradius, respectively. The ESMP event with an average occurrence rate of ∼8.8 events per day tends to occur during the weak interplanetary magnetic field strength or the slow solar wind. We also find that the ion foreshock is an important source of the ESMP events, and a small part of the ESMP events originates from the upstream pristine solar wind. Although only 12 out of 204 ESMPs have bipolar electron velocities, we suggest that the electron vortex is an essential feature for the stable ESMP. The generation mechanism of the ESMPs is unclear; nevertheless, finding out the origin of the electron vortex in the ion foreshock might help to reveal their generation mechanism.

Evidence of the nonstationarity of the terrestrial bow shock from multi-spacecraft observations: methodology, results, and quantitative comparison with particle-in-cell (PIC) simulations

Abstract. The nonstationarity of the terrestrial bow shock is analyzed in detail from in situ magnetic field measurements issued from the fluxgate magnetometer (FGM) experiment of the Cluster mission. Attention is focused on statistical analysis of quasi-perpendicular supercritical shock crossings. The present analysis stresses for the first time the importance of a careful and accurate methodology in the data processing, which can be a source of confusion and misunderstanding if not treated properly. The analysis performed using 96 shock front crossings shows evidence of a strong variability of the microstructures of the shock front (foot and ramp), which are analyzed in great detail. The main results are that (i) most statistics clearly show that the ramp thickness is very narrow and can be as low as a few c/ωpe (electron inertia length); (ii) the width is narrower when the angle θBn (between the shock normal and the upstream magnetic field) approaches 90∘; (iii) the foot thickness strongly varies, but its variation has an upper limit provided by theoretical estimates given in previous studies (e.g., Schwartz et al., 1983; Gosling and Thomsen, 1985; Gosling and Robson, 1985); and (iv) the presence of foot and overshoot, as shown in all front profiles, confirms the importance of dissipative effects. Present results indicate that these features can be signatures of the shock front self-reformation among a few mechanisms of nonstationarity identified from numerical simulation and theoretical studies. A comparison with 2D particle-in-cell (PIC) simulation for a perpendicular supercritical shock (used as reference) has been performed and shows the following: (a) the ramp thickness varies only slightly in time over a large fraction of the reformation cycle and reaches a lower-bound value on the order of a few electron inertial length; (b) in contrast, the foot width strongly varies during a self-reformation cycle but always stays lower than an upper-bound value in agreement with the value given by Woods (1971); and (c) as a consequence, the time variability of the whole shock front is depending on both ramp and foot variations. Moreover, a detailed comparative analysis shows that many elements of analysis were missing in previous reported studies concerning both (i) the important criteria used in the data selection and (ii) the different and careful steps of the methodology used in the data processing itself. The absence of these precise elements of analysis makes the comparison with the present work difficult; worse, it makes some final results and conclusive statements quite questionable at the present time. At least, looking for a precise estimate of the shock transition thickness presents nowadays a restricted interest, since recent results show that the terrestrial shock is rather nonstationary, and one unique typical spatial scaling of the microstructures of the front (ramp, foot) must be replaced by some “variation ranges” (with lower-bound and upper-bound values) within which the spatial scales of the fine structures can extend.

Electron-scale Magnetic Peaks Upstream of the Terrestrial Bow Shock Observed by the Magnetospheric Multiscale Mission

Abstract The solar wind has rich wave activity and various magnetic structures. Here we report on a new type of magnetic structure in the solar wind using the unprecedented high temporal resolution data from the Magnetospheric Multiscale Mission. We find that a train of magnetic peaks with a size less than 1 ion inertial length exists upstream of the terrestrial bow shock. The electron number density and the perpendicular electron temperature have a slight decrease inside the magnetic peaks, leading to a decrease of the electron thermal pressure in the structure. These structures are pressure-balanced, and they are approximately stationary in the ambient electron flow. These electron-scale magnetic peaks are identified as magnetic bottle–like, and their cross sections are roughly circular. The electron velocity has a bipolar feature relative to the ambient flow in the cross section, indicating the existence of an electron vortex. The current density is mainly contributed by electrons. The peaks occur in a marginally mirror-stable environment; thus they are not locally generated by mirror instabilities. We suggest that the origin of the electron vortex might help to shed light on the formation of electron-scale magnetic peaks in the solar wind.

Laboratory Observations of Ultra-Low Frequency Analogue Waves Driven by the Right-Hand Resonant Ion Beam Instability.

The Right-Hand Resonant Instability (RHI) is one of several electromagnetic ion/ion beam instabilities responsible for the formation of parallel magnetized collisionless shocks and the generation of ultra-low frequency (ULF) waves in their foreshocks. This instability has been observed for the first time under foreshock-relevant conditions in the laboratory through the repeatable interaction of a preformed magnetized background plasma and a super-Alfvénic laser-produced plasma. This platform has enabled unprecedented volumetric measurements of waves generated by the RHI, revealing filamentary current structures in the transverse plane. These measurements are made in the plasma rest frame with both high spatial and temporal resolution, providing a perspective that is complementary to spacecraft observations. Direct comparison of data from both the experiment and the Wind spacecraft to 2D hybrid simulations demonstrates that the waves produced are analogous to the ULF waves observed upstream of the terrestrial bow shock.

Turbulence‐Driven Magnetic Reconnection in the Magnetosheath Downstream of a Quasi‐Parallel Shock: A Three‐Dimensional Global Hybrid Simulation

AbstractSatellite observations with high‐resolution measurements have demonstrated the existence of intermittent current sheets and occurrence of magnetic reconnection in a quasi‐parallel magnetosheath behind the terrestrial bow shock. In this letter, by performing a three‐dimensional global hybrid simulation, we investigated the characteristics of the quasi‐parallel magnetosheath of the bow shock, which is formed due to the interaction of the solar wind with the Earth's magnetosphere. Current sheets with widths of several ion inertial lengths are found to be produced in the magnetosheath after the upstream large‐amplitude electromagnetic waves penetrate through the shock and are then compressed in the downstream. Magnetic reconnection consequently occurs in these current sheets, where high‐speed ion flow jets are identified in the outflow region. Simultaneously, flux ropes with the extension (along the y direction) of about several Earth's radii are also observed. Our simulation shed new insight on the mechanism for the occurrence of magnetic reconnection in the quasi‐parallel shocked magnetosheath.

Magnetized Dust Clouds Penetrating the Terrestrial Bow Shock Detected by Multiple Spacecraft

AbstractClouds of charged nanometer dust, produced in intra‐meteoroid collisions and accelerated by the magnetized solar wind, twist the interplanetary magnetic field as they are accelerated. Depending on the cloud size, minutes to hours‐long perturbations are observed by interplanetary spacecraft and have been termed interplanetary field enhancements (IFEs). This dust‐solar wind interaction hypothesis is supported by reconstructed magnetic field geometry using IFEs detected nearly simultaneously by multiple spacecraft. On 16 January 2018, an IFE was observed by eight spacecraft in the solar wind and later by four spacecraft in the magnetosheath, after the IFE crossed the bow shock. The post‐shock magnetic field geometry differed from that in the solar wind in a way compatible with the dust cloud having interacted with the shocked solar wind. These observations of dust clouds surfing in the solar wind show us how submicron‐sized interplanetary dust is transported and ultimately expelled from the solar system.

Evidence of Magnetic Nulls in the Reconnection at Bow Shock

AbstractMagnetic nulls are believed to play important roles in the energy dissipation during reconnection. Such nulls have been observed in reconnection at the magnetopause, magnetosheath, and magnetotail but have never been observed in reconnection at the bow shock. Recently, four reconnection events were reported at the terrestrial bow shock, by utilizing Magnetospheric Multiscale (MMS) data. We examine whether the magnetic nulls exist in these events. We successfully find radial nulls in three of the events, meaning that spacecraft were close to X points in these events; we, however, cannot find radial nulls in another event, which was actually a crossing of the reconnection exhaust region. The minimum distance between radial nulls and spacecraft is 8 km, about 0.3 ion inertial length. We reconstruct topologies of these nulls and subsequently resolve the reconnection rates in these events. We find that the resolved reconnection rates are comparable with those at the magnetopause and magnetotail. Surprisingly, we do not find electron heating at the radial nulls. Our results are useful to understand the reconnection at bow shock.

Comment on “Cavitons and spontaneous hot flow anomalies in a hybrid-Vlasov global magnetospheric simulation” by Blanco-Cano et al. (2018)

Blanco-Cano et al. (2018) intended to find a type of transient event in the solar wind before the terrestrial bow shock using a special type of simulation. However, the simulation results cannot reproduce the main features of the event. Based on the remarks described below, I am sure that the features in the simulations are not those type of events. The VLASIATOR code simulated proto-SHFAs.

Solar Wind Proton Deceleration in Front of the Terrestrial Bow Shock

AbstractThe terrestrial foreshock as a region permeated by different types of plasma waves, various particle populations, and strong wave activity is a subject of intensive investigations. Our statistical study of the solar wind proton velocity deceleration in the foreshock uses multipoint observations of the THEMIS mission and compare them with the Wind solar wind monitor with a motivation to estimate the factors influencing evolution/modification of the solar wind speed. In order to follow changes of the solar wind proton speed, our moment calculations do not include the reflected particles as well as heavier ions. We have found a systematic deceleration of the solar wind protons with a decreasing distance to the bow shock that is correlated with the flux of reflected and accelerated particles, with the level of magnetic field fluctuations in the ultralow frequency range, and with their compressibility. We can conclude that the reflected particles excite waves of large amplitudes and, as a consequence, modify median values of the velocity measured in an unperturbed solar wind. The deceleration is as high as 3% in front of the quasi‐parallel bow shock and decreases with the distance being observable up to 50 RE. We found similar effects in front of the Moon and attributed them to an influence of the solar wind ions reflected from the lunar surface.

A Fast Fermi Acceleration at Mars Bow Shock

AbstractWe report, for the first time, strong evidences that a fast Fermi mechanism is taking place at the Mars bow shock. The MAVEN spacecraft observations from the Solar Wind Electron Analyzer instrument show electron flux spikes with energies up to ∼1.5 keV. These spikes are associated with sunward propagating electrons and appear when the interplanetary field line threading the spacecraft is connected near the Martian bow shock tangency point. The observed loss cone distribution is a salient feature of these backstreaming electrons as the phase space density peaks on a ring centered along the magnetic field direction. Moreover, the data show no evidence of any effect due to a hypothetical cross‐shock electric potential on the observed angular distributions. Although similar distributions are seen at the terrestrial bow shock, the quantitative analysis of the measurements strongly indicates that the electrons are produced at the shock foot and escape upstream before exploring the entire shock structure.