Earth's Bow Shock Research Articles

Hybrid simulations of the decay of reconnected structures downstream of the bow shock

Observations by Magnetospheric Multiscale have demonstrated that magnetic reconnection occurs at Earth's bow shock, typically at thin current sheets arising from plasma instabilities and turbulence in the shock transition region. Observational surveys of both the shock transition and the magnetosheath downstream suggest that the number of current sheets in these regions may not be strongly dependent on the shock Mach number MA or the angle between the upstream magnetic field and shock normal (θBn). This result is somewhat surprising given that quasi-parallel and high Mach number shocks tend to have a more disordered and non-stationary structure. In order to investigate how shock reconnection manifests across different parameters, we perform a series of hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a range of Mach numbers and orientations. Given that hybrid simulations cannot resolve electron-scale current sheets and reconnection, these simulations isolate an ion-scale mechanism for shock reconnection driven by an ion–ion beam instability in the foot. We find that this mechanism is strongly constrained to quasi-parallel shocks across all simulated Mach numbers. By quantifying reconnection using the area occupied by plasma on closed magnetic field lines, we find the number of reconnecting structures and closed field area increase with MA and decrease with θBn in the upstream and ramp regions. Downstream of the shock, however, we find a similar result to observational surveys: within the subset of quasi-parallel shocks, the decay rate of the closed field area (and hence thin current sheets) is not strongly dependent on upstream shock parameters.

Double Layers in the Earth's Bow Shock

AbstractWe present Magnetospheric Multiscale observations of electrostatic double layers in quasi‐perpendicular Earth's bow shock. These double layers have predominantly parallel electric field with amplitudes up to 100 mV/m, spatial widths of 50–700 m, and plasma frame speeds within 100 km/s. The potential drop across a single double layer is 2%–7% of the cross‐shock potential in the de Hoffmann‐Teller frame and occurs over the spatial scale of 10 Debye lengths or one tenth of electron inertial length. Some double layers can have spatial width of 70 Debye lengths and potential drop up to 30% of the cross‐shock potential. The electron temperature variation observed across double layers is roughly consistent with their potential drop. While electron heating in the Earth's bow shock occurs predominantly due to the quasi‐static electric field in the de Hoffmann‐Teller frame, these observations show that electron temperature can also increase across Debye‐scale electrostatic structures.

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.

Solar Wind Impact on ENAs From Earth's Subsolar Magnetosheath

AbstractAs the solar wind (SW) crosses Earth's bow shock, it is heated and decelerated, resulting in a hot and dense magnetosheath ion population. This increases the probability of charge exchange between a magnetosheath proton and a geocoronal Hydrogen atom, which produces energetic neutral atoms (ENAs). These ENAs propagate away from the magnetosheath carrying information about the inherent properties of the progenitor plasma. In this work, ENA fluxes from the subsolar magnetosheath, observed by the Interstellar Boundary Explorer, are compared to SW conditions. These comparisons reveal that the passband‐weighted average differential flux of ENAs is strongly influenced by the SW density, and that ENA spectra flatten with increasing SW speed and temperature, and steepen with increasing SW density. In addition, there is evidence from ENA spectra for different interplanetary magnetic field configurations that most of the ENAs observed by Interstellar Boundary Explorer come from the specularly reflected proton population in the magnetosheath.

On Magnetosheath Jet Kinetic Structure and Plasma Properties

AbstractHigh‐speed plasma jets downstream of Earth's bow shock are high velocity streams associated with a variety of shock and magnetospheric phenomena. In this work, using the Magnetosphere Multiscale mission, we study the properties of a jet found downstream of the Quasi‐parallel bow shock using high‐resolution (burst) data. By doing so, we demonstrate how the jet is an inherently kinetic structure described by highly variable velocity distributions. The observed distributions show the presence of two plasma population, a cold/fast jet and a hotter/slower background population. We derive partial moments for the jet population to isolate its properties. The resulting partial moments appear different from the full ones which are typically used in similar studies. These discrepancies show how jets are more similar to upstream solar wind beams compared to what was previously believed. Finally, we explore the consequences of our results and methodology regarding the characterization, origin, and evolution of jets.

Energy Partition at Collisionless Supercritical Quasi‐Perpendicular Shocks

AbstractCollisionless shocks in astrophysical plasmas are important thermalizers, converting some of the incident flow energy into thermal energy, and non‐thermalizers, partitioning that energy in unequal ways to different particle species, subpopulations thereof, and field components. This partition problem, or equivalently the shock equation of state, lies at the heart of shock physics. Here, we employ systematically a framework to capture all the incident and downstream energy fluxes at two example traversals of the Earth's bow shock by the Magnetospheric Multiscale Mission. Here and traditionally, such data has to be augmented by information from other spacecraft, for example, to provide more accurate measurements of the cold solar wind beam. With some care and fortuitous choices, the total energy flux is conserved, including instantaneous measurements through the shock layer. The dominant incident proton ram energy is converted primarily into downstream proton enthalpy flux, the majority of which is actually carried by a small fraction of suprathermal protons. Fluctuations include both real and instrumental effects. Separating these, resolving the solar wind beam, and other considerations point the way to a dedicated mission to solve this energy partition problem across a full range of plasma and shock conditions.

Dynamics of He++ Ions at Interplanetary and Earth’s Bow Shocks

Experimental investigations of the fine plasma structure of interplanetary shocks are extremely difficult to conduct due to their small thickness and high speed relative to the spacecraft. We studied the variations in the parameters of twice-ionized helium ions (4He++ ions or α-particles) in the solar wind plasma during the passage of interplanetary shocks and Earth’s bow shock. We used data with high time resolution gathered by the BMSW (Bright Monitor of Solar Wind) instrument installed on the SPEKTR-R satellite, which operated between August 2011 and 2019. The MHD parameters of He++ ions (the bulk velocity Vα, temperature Tα, absolute density Nα, and helium abundance Nα/Np) are analyzed for 20 interplanetary shocks and compared with similar parameters for 25 Earth bow shock crossings. Measurements from the WIND, Cluster, and THEMIS satellites were also analyzed. The correlations in the changes in helium abundance Nα/Np with the parameters βi, θBn, and MMS were investigated. The following correlation between Nα/Np and the angle θBn was found: the lower the value of θBn, the greater the drop in helium abundance (Nα/Np) falls behind the IP shock front. For Earth’s bow shock crossings, we found a significant increase in the helium abundance (Nα/Np) in quasi-perpendicular events.

Variations of He++ Ion Parameters at Interplanetary and Earth’s Bow Shock Fronts

Experimental investigations of the fine plasma structure of interplanetary shocks are extremely difficult to conduct due to their small thickness and high speed relative to the spacecraft. We study the variations in the parameters of twice-ionized helium ions (4He++ ions or {\alpha}-particles) in the solar wind plasma during the passage of interplanetary shocks and Earth's bow shock. We use data with high time resolution gathered by the BMSW (Bright Monitor of Solar Wind) instrument installed on the SPEKTR-R satellite, which operated between August 2011 and 2019. The MHD parameters of He++ ions (the bulk velocity Va, temperature Ta, absolute density Na, and helium abundance Na/Np) are analyzed for 20 interplanetary shocks and compared with similar parameters for 25 Earth's bow shock crossings. Measurements from the WIND, Cluster and THEMIS satellites were also analyzed. The correlations in the changes in helium abundance Na/Np with the parameters {\beta}_{i}, {\theta}_{Bn} and M_{ms} were investigated. A correlation between Na/Np and the angle {\theta}_{Bn} was found: the lower the value of {\theta}_{Bn}, the greater the drop in helium abundance Na/Np falls behind the IP shock front. For Earth's bow shock crossings, we found a significant increase in the helium abundance Na/Np in quasi-perpendicular events.

Slow electron holes in the Earth's bow shock

We present analysis of about one hundred bipolar structures of positive polarity identified in ten quasi-perpendicular crossings of the Earth's bow shock by the Magnetospheric Multiscale spacecraft. The bipolar structures have amplitudes up to a few tenths of local electron temperature, spatial scales of a few local Debye lengths, and plasma frame speeds of the order of local ion-acoustic speed. We argue that the bipolar structures of positive polarity are slow electron holes, rather than ion-acoustic solitons. The electron holes are typically above the transverse instability threshold, which we argue is due to high values of the ratio ωpe/ωce between electron plasma and cyclotron frequencies. We speculate that the transverse instability can strongly limit the lifetime of the electron holes, whose amplitude is above a certain threshold, which is only a few mV/m in the Earth's bow shock. We suggest that electron surfing acceleration by large-amplitude electron holes reported in numerical simulations of high-Mach number shocks might not be as efficient in realistic shocks, because the transverse instability strongly limits the lifetime of large-amplitude electron holes at ωpe/ωce values typical of collisionless shocks in nature.

Inetic Alfven waves generation ahead of the Earth bow shock

inetic Alfven waves generation ahead of the Earth bow shock

Quantifying electron scattering by electrostatic solitary waves in the Earth's bow shock

The electrostatic fluctuations are always present in the Earth's bow shock at frequencies above about 100 Hz, but the effects of this wave activity on electron dynamics have not been quantified yet. In this paper, we quantify electron pitch-angle scattering by electrostatic solitary waves, which make up a substantial part of the electrostatic fluctuations in the Earth's bow shock and were recently shown to be predominantly ion holes. We present analytical estimates and test-particle simulations of electron pitch-angle scattering by ion holes typical of the Earth's bow shock and conclude that this scattering can be rather well quantified within the quasi-linear theory. We use the observed distributions of ion hole parameters to estimate pitch-angle scattering rates by the ensemble of ion holes typical of the Earth's bow shock. We use the recently proposed theory of stochastic shock drift acceleration to show that pitch-angle scattering of electrons by the electrostatic fluctuations can keep electrons in the shock transition region long enough to support acceleration of thermal electrons by a factor of a few tens, that is up to a few hundred eV. Importantly, the electrostatic fluctuations can be more efficient in pitch-angle scattering of ≲1 keV electrons, than typically observed whistler waves.

Ion‐Acoustic Waves in a Quasi‐Perpendicular Earth's Bow Shock

AbstractWe present analysis of electrostatic waves around the ramp of a quasi‐perpendicular Earth's bow shock observed by the Magnetospheric Multiscale spacecraft. The electrostatic waves have amplitudes up to 800 mV/m, which is the largest value ever reported in the Earth's bow shock. In contrast to previous studies, the electrostatic waves have large amplitudes of the electrostatic potential, up to 20 V or 20% of local electron temperature. The wavelengths are from 150 m to 3 km, that is from 15 to 300 Debye lengths and typically from 0.4 to 1.5 thermal electron gyroradii. Importantly, these waves can propagate not only quasi‐parallel or oblique, but also almost perpendicular to local magnetic field. The electrostatic waves are interpreted in terms of ion‐acoustic waves, although the presence of electron cyclotron harmonic waves cannot be entirely ruled out. These results suggest that electrostatic waves may strongly affect the dynamics of electrons in collisionless shocks.

Asymmetric Interaction of a Solar Wind Reconnecting Current Sheet and Its Magnetic Hole With Earth's Bow Shock and Magnetopause

AbstractWe report results of our multi‐spacecraft analysis of a solar wind reconnecting current sheet (RCS) and its solar wind magnetic hole (SWMH) observed on November 20, 2018. In the solar wind, the normal vector to the current sheet plane makes an angle of 32° with the Sun‐Earth line. A combination of tilted current sheet plane and foreshock effects cause an asymmetric interaction with the bow shock, in which the structure arrives at the quasi‐perpendicular side of the bow shock before the quasi‐parallel side. The magnetic field strength inside the magnetic hole decreases by ∼69 percent in the solar wind, with a similar depression rate observed inside the magnetosheath due to this structure. The solar wind flow slowdown and deflection during the bow shock crossing significantly disrupt the reconnection exhausts within the RCS. The interaction of the RCS and SWMH with the bow shock creates enhanced fluxes of accelerated electrons and ions. Plasma flow deflection in the magnetosheath also increases with the passage of the RCS. The ion density and temperature both increase within the current sheet to form a roughly pressure balanced structure. Field rotation and change in the dynamic pressure during this event modify the reconnection zones at the magnetopause and cause asymmetric inward motions in portions of the bow shock and the magnetopause boundaries (i.e., deformation). Unlike localized magnetosheath jets, an RCS and its associated SWMH in the solar wind have a global impact on the bow shock and the magnetopause.

Strong reconnection electric fields in shock-driven turbulence

Turbulent magnetic reconnection in a quasi-parallel shock under parameters relevant to the Earth's bow shock is investigated by means of a two-dimensional particle-in-cell simulation. The addressed aspects include the reconnection electric field, the reconnection rate, and the electron and the ion outflow speeds. In the shock transition region, many current sheets are generated in shock-driven turbulence, and electron-only reconnection and reconnection where both ions and electrons are involved can occur in those current sheets. The electron outflow speed in electron-only reconnection shows a positive correlation with the theoretical speed, which is close to the local electron Alfvén speed, and a strong convection electric field is generated by the large electron outflow. As a result, the reconnection electric field becomes much larger than those in the standard magnetopause or magnetotail reconnection. In shock-driven reconnection that involves ion dynamics, both electron outflows and ion outflows can reach of the order of 10 times the Alfvén speed in the X-line rest frame, leading to a reconnection electric field the same order as that in electron-only reconnection. An electron-only reconnection event observed by the magnetospheric multiscale mission downstream of a quasi-parallel shock is qualitatively similar to those in the simulation and shows that the outflow speed reaches approximately half the local electron Alfvén speed, supporting the simulation prediction.

Statistical Study of Foreshock Density Holes

AbstractDensity holes (DHs), characterized by the correlating depletion of plasma density and magnetic field strength, are one of the transient structures frequently observed upstream of Earth's bow shock. The low dynamic pressure of DHs may disturb the bow shock and magnetopause, and have potential geoeffects. In this paper, we perform a statistical study of 411 density hole events, identified by the Magnetospheric Multiscale Mission. The average occurrence rate of DHs is ∼5.4 events/day. All DHs are associated with foreshock ions. DHs tend to occur on the dawn side than on the dusk side. The occurrence rate is higher for faster solar wind, larger magnetic shear angle across DHs, and lower magnetic field strength. The spatial scale of DHs is several RE. Within DHs, electron heating and density depletion are correlated. Most DHs do not have significant flow deflections. Most DHs with discontinuities have the convection electric field pointing toward the discontinuity on at least one side. About two thirds of the 411 events show different characteristics compared to other types of foreshock transients that also have correlated density and field strength depletion.

Determining the Near‐Instantaneous Curvature of Earth's Bow Shock Using Simultaneous IBEX and MMS Observations

AbstractThe Interstellar Boundary Explorer (IBEX) mission provides a unique opportunity to study Earth's magnetosphere on a global scale via energetic neutral atom imaging. Due to the orbital configuration, IBEX is in a side‐viewing vantage point relative to the magnetosphere for a few months every year. This allows the Earth's magnetosphere to be imaged in the X‐Z geocentric solar ecliptic (GSE) plane. Using IBEX observations and near‐simultaneous in‐situ observations of Earth's bow shock (BS) from the Magnetospheric Multiscale mission, we determine the curvature of Earth's BS in the X‐Z GSE plane for 22 instances. We then compare the curvature to different upstream solar wind parameters in order to identify correlations with the contemporary physical conditions. We find that the solar wind dynamic pressure, as well as the interplanetary magnetic field (IMF) strength, X‐Y, and Y‐Z components are all linearly related to the curvature. We then investigate the time scales over which these correlations persist using variable time windows over which the upstream parameters are averaged. Lastly, we investigate the asymmetry of the BS in the X‐Z plane by comparing an asymmetry measure with different solar wind parameters. We find that the magnetic tension force from the IMF deflects the BS, and thus the underlying magnetopause obstacle. The results presented here provide the first near‐instantaneous observations of Earth's BS on a global scale, show the importance of IMF on generating asymmetries, and provide a basis for future modeling and observational studies of the BS.

Global Asymmetries of Hot Flow Anomalies

AbstractA three‐dimensional global hybrid simulation is conducted for the interaction of the Earth's bow shock with an interplanetary directional tangential discontinuity (TD) whose normal lies in the equatorial plane. Although the convection electric field points into the TD from its trailing side both north and south of the equator, the interaction only generates a hot flow anomaly (HFA) in the southern hemisphere. HFA generation results from bow shock‐reflected ions energized by the inward‐pointing electric field on one side of the TD and inward ion gyration on the other side. The latter only occurs south of the equator due to the global bow shock geometry. The global HFA asymmetry is driven by a reversal in the north‐south component of reflected ion velocities in the two hemispheres, such that ions gyrate into (away from) the TD in the southern (northern) hemisphere. Our results indicate HFAs must generally exhibit north‐south and dawn‐dusk asymmetries.

Electron Resonant Interaction With Whistler Waves Around Foreshock Transients and the Bow Shock Behind the Terminator

AbstractInvestigation of electron energization at (and around) the Earth's bow shock is critical to our understanding of space weather and astrophysical phenomena. The traditional adiabatic mechanisms for such energization compete with transient wave‐particle interactions there. One of the most intense wave modes resonating with electrons is the high‐frequency whistler mode, which is widely observed at (and around) the Earth's bow shock. Here, we examine these interactions in the context of the strong magnetic field gradients often found near the bow shock and at foreshock transients. Using THEMIS and ARTEMIS wave measurements, we quantify the nonlinear effects of resonant interactions between ≥100 eV electrons and intense coherent whistler waves. Such nonlinear interactions include the electron phase trapping by waves. As a result, the trapped electrons gain an energy up to several hundreds of eV. We estimate the main characteristics of the proposed acceleration mechanism and discuss its applicability to realistic plasma and magnetic field distributions.

Near-Earth Sub-Alfvénic Solar Winds: Interplanetary Origins and Geomagnetic Impacts

Abstract The near-Earth solar wind is in general super-Alfvénic and supermagnetosonic. Using all available near-Earth solar wind measurements between 1973 and 2020, we identified 30 intervals with sub-Alfvénic solar winds. The majority (83%) of the events occurred within interplanetary coronal mass ejection magnetic clouds (MCs)/driver gases. These MC sub-Alfvénic events are characterized by exceptionally low plasma densities (N sw) of ∼0.04–1.20 cm−3, low temperatures (T sw) of ∼0.08 × 105 K to 12.46 × 105 K, enhanced magnetic field intensities (B 0) of ∼8.3–53.9 nT, and speeds (V sw) of ∼328–949 km s−1. The resultant high Alfvén wave speeds (V A) ranged from ∼410 to 1471 km s−1. This is consistent with a mechanism of the MC expansions as they propagate radially outward, causing small pockets of sub-Alfvénic wind regions within the MCs. The remainder of the sub-Alfvénic intervals (17%) occurred within the extreme trailing portions of solar wind high-speed streams (HSSs). These HSS sub-Alfvénic winds had low N sw of ∼0.04–0.97 cm−3, low T sw of ∼0.06 × 105 K to 0.46 × 105 K, B 0 of ∼6.3–18.2 nT, V sw of ∼234–388 km s−1, and a V A range of ∼364–626 km s−1. This is consistent with a mechanism of solar wind super-radial expansions in the trailing HSS regions. During sub-Alfvénic solar wind intervals, Earth's bow shock nose exhibited rapid evanescence, and the estimated geocentric magnetopause distance increased by ∼33%–86%. The inner magnetosphere was more or less unaffected by the sub-Alfvénic solar winds. No significant impact was observed in the outer radiation belt relativistic electrons, and no geomagnetic storms or substorms were triggered during the sub-Alfvénic solar wind events.

Quasi-Parallel Shock Reformation Seen by Magnetospheric Multiscale and Ion-Kinetic Simulations.

Shock waves in collisionless plasmas are among the most efficient particle accelerators in space. Shock reformation is a process important to plasma heating and acceleration, but direct observations of reformation at quasi‐parallel shocks have been lacking. Here, we investigate Earth's quasi‐parallel bow shock with observations by the four Magnetospheric Multiscale spacecraft. The multi‐spacecraft observations provide evidence of short large‐amplitude magnetic structures (SLAMS) causing reformation of the quasi‐parallel shock. We perform an ion‐kinetic Vlasiator simulation of the bow shock and show that SLAMS reforming the bow shock recreates the multi‐spacecraft measurements. This provides a method for identifying shock reformation in the future.