Search

At the Earth’s bow shock, most of the solar wind’s kinetic energy is partitioned into wave energy, particle acceleration, and heating. Very recent publications provide strong evidence that current sheets at the shock ramp region and downstream may participate in the thermalization of the solar wind plasma. Their occurrence varies from single to multiple current sheets as well as filamentary structures.We studied multiple bow shock crossings by the MMS spacecraft with its sophisticated instrumentation, characterizing and quantifying the occurrence of filamentary structures, current sheets, the associated magnetic field wave turbulence, and ion acceleration downstream of the shock. At some traversals the shock location is changing due to variable upstream solar wind conditions. During increasing Mach number/dynamic pressure we observe higher wave activity and broader distribution functions with suprathermal tails. Much less suprathermal ions downstream of the shock are observed at shock crossings during decreasing upstream Mach numbers. These MMS observation indicate that current sheets and field gradients are associated with ion acceleration. The associated turbulence is likely a mediator for energy partition. With increasing Mach numbers, the bow shock moves away from the Sun and compresses the magnetosheath that would favour reconnection of currents sheets, stronger electric field gradients and thus ion acceleration. At periods of decreasing upstream Mach numbers, the bow shock moves towards the Sun, becomes blunter, and the sheath region relaxes, making reconnecting current sheets less likely and smoothens field gradients resulting in less acceleration. Other possible acceleration mechanisms will also be discussed in the context of this presentation.

Turbulent plasmas such as the solar wind and the magnetosheath exhibit an energy cascade which is present across a broad range of scales, from the stirring scale at which energy is injected, down to the smallest scales where energy is dissipated through processes such as reconnection and wave-particle interactions. Recent observations of Earth’s bow shock reveal the presence of a disordered or turbulent transition region which exhibits some features of turbulent dissipation, such as reconnecting current sheets. Understanding the variations in the origin and character of these disordered fluctuations addresses open questions such as how disordered or turbulent fluctuations in the bow shock and magnetosheath are related, and how quickly magnetosheath turbulence arises from bow shock processes. Here, we present two case studies of bow shock crossings observed by Magnetospheric Multiscale (MMS), one quasi-perpendicular and one quasi-parallel. Using high-cadence, combined search-coil and fluxgate magnetometer data, we measure changes in correlation lengths of the magnetic field through three regions: the upstream (solar wind), shock transition region, and downstream (magnetosheath). The influence of the discontinuous shock ramp is reduced using high-pass filters with variable cut-off frequencies. We find that correlation lengths are higher on the solar wind side of the shock, reducing to around 20 ion inertial lengths in the magnetosheath for both the quasi-parallel and the quasi-perpendicular shocks. We also discuss implications of the observed evolution of the correlation length to bow shock and magnetosheath processes.

Turbulent plasmas such as the solar wind and magnetosheath exhibit an energy cascade which is present across a broad range of scales, from the stirring scale at which energy is injected, down to the smallest scales where energy is dissipated through processes such as reconnection and wave-particle interactions. Recent observations of Earth’s bow shock reveal a disordered or turbulent transition region which exhibits features of turbulent dissipation, such as reconnecting current sheets. We have used observations from Magnetospheric Multiscale (MMS) over four separate bow shock crossings of varying θ to characterise turbulence in the shock transition region and how it evolves towards the magnetosheath. We observe the magnetic spectrum evolving by fitting power laws over many short intervals and find that the power-law index in the shock transition region is separable from that of the upstream and downstream plasma, for both quasi-perpendicular and quasi-parallel shocks. Across the shock, we see a change in the breakpoint location between inertial and ion power-law slopes. We also observe the evolution of scale-independent kurtosis of magnetic fluctuations across the shock, finding a reduction of high kurtosis intervals downstream of the shock, which is more apparent in the quasi-perpendicular case. Finally, we adapt a method for calculating correlation length to include a high-pass filter, allowing estimates for changes in correlation length across Earth’s bow shock. In a quasi-perpendicular shock, we find the correlation length to be significantly smaller in the magnetosheath than in the solar wind, however the opposite can occur for quasi-parallel shocks.

At the Earth’s bow shock, most of the solar wind’s kinetic energy is partitioned into wave energy, particle acceleration, and heating. Very recent publications provide strong evidence that current sheets at the shock ramp region and downstream participate in the thermalization of the solar wind plasma. Their occurrence varies from single to multiple current sheets. What role do they play in downstream thermalization and ion acceleration?We studied multiple bow shock crossings into the magnetosheath by the MMS spacecraft with its sophisticated instrumentation, characterizing and quantifying the occurrence of these current sheets, the associated magnetic field wave turbulence, and ion acceleration downstream of the shock. Shock traversals during increasing Mach number/dynamic pressure showed higher wave activity and broader distribution functions with suprathermal tails. Much less suprathermal ions downstream of the shock are observed at shock crossings during decreasing Mach numbers. These MMS observations show that current sheets and field gradients are associated with ion acceleration. The associated turbulence is likely a mediator for energy partition. With increasing Mach numbers, the bow shock moves away from the Sun and compresses the magnetosheath that favours reconnection of currents sheets, stronger electric field gradients and thus ion acceleration. With decreasing Mach numbers, the bow shock moves towards the Sun, becomes blunter, and the sheath region relaxes, making reconnecting current sheets less likely and smoothens field gradients resulting in less acceleration.

We investigate the dynamic coupling between the solar wind and Earth’s magnetosphere during northward IMF conditions. The high latitude lobe regions of the magnetosphere during such conditions are generally characterised as containing cool, very low energy plasma populations. However, when the solar wind is directed northward, hot plasma populations can sometimes be observed within the lobes, and transpolar arcs (auroral features which extend from the nightside into the polar cap) can also be present. We discuss three cases in which the Cluster spacecraft observed uncharacteristically energetic plasma populations in the lobe, with the footprint of the spacecraft intersecting a transpolar arc (Fryer et al., 2021). These observations reveal that both the hot plasma populations, and therefore transpolar arcs, are likely to form on closed field lines and are consistent with a mechanism in which magnetotail reconnection builds up closed field lines within the magnetotail, which then become “stuck” in the lobe (Milan et al., 2005).Under certain northward IMF conditions, we find that the Grand Unified Magnetosphere-Ionosphere Coupling Simulation (GUMICS) model reproduces a similar large, closed field line region in the magnetotail. We find similarities between the structures seen within the simulation runs, the results of the in-situ observational study, and the Milan et al. (2005) magnetotail reconnection model, but also note some key differences in the configuration of the magnetotail reproduced in the simulations, compared with the remote and in situ observations. Finally, we note that the SMILE spacecraft will be ideally positioned to observe the coupling between the solar wind and Earth’s magnetosphere during northward IMF conditions, through both high latitude in situ observations and imaging of the auroral response.

Shock waves are common in the heliosphere and beyond. The collisionless nature of most astrophysical plasmas allows for the energy processed by shocks to be partitioned amongst particle sub-populat...

The high latitude, lobe regions of the magnetosphere are often assumed to contain cool, low energy plasma populations. However, during periods of northward Interplanetary Magnetic Field, energetic plasma populations have occasionally been observed. We present three cases when Cluster observed uncharacteristically “hot” plasma populations in the lobe. For two of the three events, we present simultaneous observations of the plasma sheet observed by Double Star. The similarity between the plasma in the lobe and the plasma sheet suggests that the mechanism that produces plasma at high latitudes is likely to be tail reconnection, resulting in a trapped “wedge” of closed flux about the noon-midnight meridian. Complementary images from Imager for Magnetopause to Aurora Global Exploration and DMSP/Special Sensor Ultraviolet Spectrographic Imager show that transpolar arcs, which form in each event in at least one hemisphere, directly intersect the footprint of the Cluster spacecraft in all three events. The intersection of the Cluster footprint with the transpolar arcs is synchronous with the observation of the energetic plasma populations in the lobe. This further supports the conclusion that it is likely this energetic plasma observed in the high latitude lobe regions of magnetosphere is on closed field lines.

While the Earth’s bow shock marks the location at which the solar wind is thermalized, recent publications provided evidence that filamentary structures such as reconnecting current sheets at the shock ramp region may participate in the thermalization process. Small scale filamentary structures are distinct features that are abundant at the shock and inside the magnetosheath. These structures are not limited to current sheets but include electric and magnetic field enhancements. They may consist of a single or multiple filaments. They originate from energy dissipation at and downstream of the bow shock, in particular the parallel bow shock. We have studied several crossings of the magnetosheath made by the MMS spacecraft, characterising and quantifying the occurrence and consequences of current sheets and field enhancements in terms of local plasma heating and ion acceleration far downstream of the shock. These observations suggest that a combination of current sheet formation, and electric field and magnetic field gradients can contribute to local downstream ion acceleration, and heating. The associated turbulence is likely a consequence of solar wind input parameters. These observations provide evidence that under certain plasma conditions these filamentary structures can play a significant role in thermalizing of the magnetosheath plasma as it propagates further downstream toward the magnetopause, thus augmenting the effect due to the bow shock itself.

Actively reconnecting, thin current sheets have been observed both within the transition region of Earth’s bow shock and far downstream into the magnetosheath. Irrespective of whether these structures arise due to shock processes or turbulent dissipation, they are expected to contribute to particle heating and acceleration within their respective regions. In order to assess the integrated impact of the population of thin current sheets on observations of heating and acceleration, we examine shock crossings and extended magnetosheath intervals recorded by the Magnetospheric Multiscale mission (MMS). For each interval we quantify the number density of reconnecting current sheets in the magnetosheath. We estimate the volume associated with each time interval by considering the three-dimensional cone over which Alfvén and magnetoacoustic waves can propagate within the time interval. We then estimate the number of reconnecting sheets within that volume by comparing heating measures observed within individual sheet crossings with the observed change in those properties across the full interval. Given several extended magnetosheath intervals observed by MMS, we perform our analysis for different locations in the magnetosheath and for different solar wind conditions. In this way we determine the dependence of the number density of thin current sheets on shock orientation (i.e. quasi-parallel or quasi-perpendicular), solar wind transients, and incident plasma parameters.

Shock waves in collisionless plasmas rely on kinetic processes to convert the primary incident bulk flow energy into thermal energy. That conversion is initiated within a thin transition layer but may continue well into the downstream region. At the Earth's bow shock, the region downstream of shock locations where the interplanetary magnetic field is nearly parallel to the shock normal is highly turbulent. We study the distribution of thin current events in this magnetosheath. Quantification of the energy dissipation rate made by the Magnetospheric Multiscale spacecraft shows that these isolated intense currents are distributed uniformly throughout the magnetosheath and convert a significant fraction (5%–11%) of the energy flux incident at the bow shock.

International audience; Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasma. Using high-resolution, multispacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent E spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to E over the accessible length scales, which extend to scales smaller than the electron gyroradius at the greatest extent, with the Hall and electron pressure terms dominating at sub-ion scales. The strength of the nonideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial antialignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents in the turbulence. The relative contribution of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping E for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured E agree to within ∼20% across the observable scales. These results both confirm general expectations about the behavior of E in turbulent plasmas and highlight features that should be explored further theoretically. Plain Language Summary Complex turbulent motions are observed in plasmas throughout the Universe and act to transfer energy from large-scale fluctuations to small-scale fluctuations, which can be more easily dissipated into the thermal energy of the particles. Electric fields in these plasmas play a central role in enabling the exchange of energy between the magnetic field and the motion of the charged particles and are, therefore, important for disentangling the complex nonlinear dynamics and dissipative processes. Using cutting-edge, high-resolution, multispacecraft measurements from NASA's Magnetospheric Multiscale mission, we decompose the electric field in Earth's turbulent magnetosheath into the various terms from generalized Ohm's law, which governs the behavior of the electric field across the wide range of length scales in the plasma. The results confirm a number of general expectations about the relative behavior of the terms in Ohm's law, as well as highlight several new features that are significant for understanding the nonlinear behavior and turbulent dissipation at different scales within the plasma.

Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasm...

The particle data delivered by the Fast Plasma Investigation instrument aboard National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission allow for exceptionally high-resolution examination of the electron and ion phase space in the near-Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using Fast Plasma Investigation's Dual Electron Spectrometers, we isolate a high-energy (approximately kiloelectron volt) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H− generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H− density is shown to be approximately 0.2–0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2–0.4% of the solar wind velocity and significantly higher errors in the electron temperature.

Actively reconnecting, thin current sheets have been observed both within the transition region of Earth's bow shock and far downstream into the magnetosheath. Irrespective of whether these structures arise due to shock processes or turbulent dissipation, they are expected to contribute to particle heating and acceleration within their respective regions. In order to assess the prevalence of thin current sheets in the magnetosheath, we examine shock crossings and extended magnetosheath intervals recorded by the magnetospheric multiscale mission (MMS). For each magnetosheath interval, we quantify the prevalence of current sheets in that region of space using: a one-dimensional measure of structures per unit length of observed plasma, a packing factor corresponding to the fraction of time the spacecraft are within current structures, and a three-dimensional measure requiring an estimate of the number of current sheets within an associated volume. We estimate that volume by considering the three-dimensional cone over which Alfvén and magnetoacoustic waves can propagate during each interval. Using 25 extended magnetosheath intervals observed by MMS, we perform our analysis for different locations in the magnetosheath and for different solar wind conditions. We find that the number density of current sheets is higher toward the magnetosheath flanks, that it reduces as a power law with distance from the bow shock, and that it is not strongly influenced by the properties of the upstream bow shock.

We have conducted a comprehensive survey of burst mode observations of Earth's bow shock by the Magnetospheric Multiscale mission to identify and characterize current sheets associated with collisionless shocks, with a focus on those containing fast electron outflows, a likely signature of magnetic reconnection. The survey demonstrates that these thin current sheets are observed within the transition region of approximately 40% of shocks within the burst mode data set of Magnetospheric Multiscale. With only small apparent bias toward quasi‐parallel shock orientations and high Alfvén Mach numbers, the results suggest that reconnection at shocks is a universal process, occurring across all shock orientations and Mach numbers. On examining the distributions of current sheet properties, we find no correlation between distance from the shock, sheet width, or electron jet speed, though the relationship between electron and ion jet speed supports expectations of electron‐only reconnection in the region. Furthermore, we find that robust heating statistics are not separable from background fluctuations, and thus, the primary consequence of reconnection at shocks is in relaxing the topology of the disordered magnetic field in the transition region.

International audience; Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth's magnetosheath revealed a novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ~10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas.

International audience; Using observations of Earth's bow shock by the Magnetospheric Multiscale mission, we show for the first time that active magnetic reconnection is occurring at current sheets embedded within the quasi-parallel shock's transition layer. We observe an electron jet and heating but no ion response, suggesting we have observed an electron-only mode. The lack of ion response is consistent with simulations showing reconnection onset on sub-ion time scales. We also discuss the impact of electron heating in shocks via reconnection.

International audience; Hot Flow Anomalies (HFAs) are transients observed at planetary bow shocks, formed by the shock interaction with a convected interplanetary current sheet. The primary interpretation relies on reflected ions channeled upstream along the current sheet. The short duration of HFAs has made direct observations of this process difficult. We employ high resolution measurements by NASA's Magnetospheric Multiscale Mission to probe the ion microphysics within a HFA. Magnetospheric Multiscale Mission data reveal a smoothly varying internal density and pressure, which increase toward the trailing edge of the HFA, sweeping up particles trapped within the current sheet. We find remnants of reflected or other backstreaming ions traveling along the current sheet, but most of these are not fast enough to out-run the incident current sheet convection. Despite the high level of internal turbulence, incident and backstreaming ions appear to couple gyro-kinetically in a coherent manner. Plain Language Summary Shock waves in space are responsible for energizing particles and diverting supersonic flows around planets and other obstacles. Explosive events known as Hot Flow Anomalies (HFAs) arise when a rapid change in the interplanetary magnetic field arrives at the bow shock formed by, for example, the supersonic solar wind plasma flow from the Sun impinging on the Earth's magnetic environment. HFAs are known to produce impacts all the way to ground level, but the physics responsible for their formation occur too rapidly to be resolved by previous satellite missions. This paper employs NASA's fleet of four Magnetospheric Multiscale satellites to reveal for the first time clear, discreet populations of ions that interact coherently to produce the extreme heating and deflection.

Shock ripples are ion-inertial-scale waves propagating within the front region of magnetized quasi-perpendicular collisionless shocks. The ripples are thought to influence particle dynamics and acceleration at shocks. With the four magnetospheric multiscale (MMS) spacecraft, it is for the first time possible to fully resolve the small scale ripples in space. We use observations of one slow crossing of the Earth’s non-stationary bow shock by MMS. From multi-spacecraft measurements we show that the non-stationarity is due to ripples propagating along the shock surface. We find that the ripples are near linearly polarized waves propagating in the coplanarity plane with a phase speed equal to the local Alfvén speed and have a wavelength close to 5 times the upstream ion inertial length. The dispersive properties of the ripples resemble those of Alfvén ion cyclotron waves in linear theory. Taking advantage of the slow crossing by the four MMS spacecraft, we map the shock-reflected ions as a function of ripple phase and distance from the shock. We find that ions are preferentially reflected in regions of the wave with magnetic field stronger than the average overshoot field, while in the regions of lower magnetic field, ions penetrate the shock to the downstream region. QC 20190710

Systems of multiple current sheets arise in various situations in natural plasmas, such as at the heliospheric current sheet in the solar wind and in the outer heliosphere in the heliosheath. Previous three-dimensional simulations have shown that such systems can develop turbulent-like fluctuations resulting from a forward and inverse cascade in wave vector space. We present a study of the transition to turbulence of such multiple current sheet systems, including the effects of adding a magnetic guide field and velocity shears across the current sheets. Three-dimensional hybrid simulations are performed of systems with eight narrow current sheets in a triply periodic geometry. We carry out a number of different analyses of the evolution of the fluctuations as the initially highly ordered state relaxes to one which resembles turbulence. Despite the evidence of a forward and inverse cascade in the fluctuation power spectra, we find that none of the simulated cases have evidence of intermittency after the initial period of fast reconnection associated with the ion tearing instability at the current sheets. Cancellation analysis confirms that the simulations have not evolved to a state which can be identified as fully developed turbulence. The addition of velocity shears across the current sheets slows the evolution in the properties of the fluctuations, but by the end of the simulation they are broadly similar. However, if the simulation is constrained to be two-dimensional, differences are found, indicating that fully three-dimensional simulations are important when studying the evolution of an ordered equilibrium towards a turbulent-like state.

Supercritical collisionless perpendicular shocks have an average macrostructure determined primarily by the dynamics of ions specularly reflected at the magnetic ramp. Within the overall macrostructure, instabilities, both linear and nonlinear, generate fluctuations and microstructure. To identify the sources of such microstructure, high-resolution two- and three-dimensional simulations have been carried out using the hybrid method, wherein the ions are treated as particles and the electron response is modelled as a massless fluid. We confirm the results of earlier 2-D simulations showing both field-parallel aligned propagating fluctuations and fluctuations carried by the reflected-gyrating ions. In addition, it is shown that, for 2-D simulations of the shock coplanarity plane, the presence of short-wavelength fluctuations in all magnetic components is associated with the ion Weibel instability driven at the upstream edge of the foot by the reflected-gyrating ions. In 3-D simulations we show for the first time that the dominant microstructure is due to a coupling between field-parallel propagating fluctuations in the ramp and the motion of the reflected ions. This results in a pattern of fluctuations counter-propagating across the surface of the shock at an angle inclined to the magnetic field direction, due to a combination of field-parallel motion at the Alfv\'en speed of the ramp, and motion in the sense of gyration of the reflected ions., Comment: Submitted to Journal of Plasma Physics