Magnetosheath Plasma Research Articles

Waves in Kinetic‐Scale Magnetic Dips: MMS Observations in the Magnetosheath

AbstractKinetic‐scale magnetic dips (KSMDs), with a significant depression in magnetic field strength, and scale length close to and less than one proton gyroradius, were reported in the turbulent plasmas both in recent observation and numerical simulation studies. These KSMDs likely play important roles in energy conversion and dissipation. In this study, we present observations of the KSMDs that are labeled whistler mode waves, electrostatic solitary waves, and electron cyclotron waves in the magnetosheath. The observations suggest that electron temperature anisotropy or beams within KSMD structures provide free energy to generate these waves. In addition, the occurrence rates of the waves are higher in the center of the magnetic dips than at their edges, implying that the KSMDs might be the origin of various kinds of waves. We suggest that the KSMDs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas.

Plasma flow patterns in and around magnetosheath jets

Abstract. The magnetosheath is commonly permeated by localized high-speed jets downstream of the quasi-parallel bow shock. These jets are much faster than the ambient magnetosheath plasma, thus raising the question of how that latter plasma reacts to incoming jets. We have performed a statistical analysis based on 662 cases of one THEMIS spacecraft observing a jet and another (second) THEMIS spacecraft providing context observations of nearby plasma to uncover the flow patterns in and around jets. The following results are found: along the jet's path, slower plasma is accelerated and pushed aside ahead of the fastest core jet plasma. Behind the jet core, plasma flows into the path to fill the wake. This evasive plasma motion affects the ambient magnetosheath, close to the jet's path. Diverging and converging plasma flows ahead and behind the jet are complemented by plasma flows opposite to the jet's propagation direction, in the vicinity of the jet. This vortical plasma motion results in a deceleration of ambient plasma when a jet passes nearby. Keywords. Magnetospheric physics (magnetosheath; MHD waves and instabilities; solar wind–magnetosphere interactions)

Magnetosheath Propagation Time of Solar Wind Directional Discontinuities

AbstractObserved delays in the ground response to solar wind directional discontinuities have been explained as the result of larger than expected magnetosheath propagation times. Recently, Samsonov et al. (2017, https://doi.org/10.1002/2017GL075020) showed that the typical time for a southward interplanetary magnetic field (IMF) turning to propagate across the magnetosheath is 14 min. Here by using a combination of magnetohydrodynamic simulations, spacecraft observations, and analytic calculations, we study the dependence of the propagation time on solar wind parameters and near‐magnetopause cutoff speed. Increases in the solar wind speed result in greater magnetosheath plasma flow velocities, decreases in the magnetosheath thickness and, as a result, decreases in the propagation time. Increases in the IMF strength result in increases in the magnetosheath thickness and increases in the propagation time. Both magnetohydrodynamic simulations and observations suggest that propagation times are slightly smaller for northward IMF turnings. Magnetosheath flow deceleration must be taken into account when predicting the arrival times of solar wind structures at the dayside magnetopause.

Neutralized solar wind ahead of the Earth's magnetopause as contribution to non-thermal exospheric hydrogen

Abstract. In a most recent paper by Qin and Waldrop (2016), it had been found that the scale height of hydrogen in the upper exosphere of the Earth, especially during solar minimum conditions, appears to be surprisingly large. This indicates that during minimum conditions when exobasic temperatures should be small, large exospheric H-scale heights predominate. They thus seem to indicate the presence of a non-thermal hydrogen component in the upper exosphere. In the following parts of the paper we shall investigate what fraction of such expected hot hydrogen atoms could have their origin from protons of the shocked solar wind ahead of the magnetopause converted into energetic neutral atoms (ENAs) via charge-exchange processes with normal atmospheric, i.e., exospheric hydrogen atoms that in the first step evaporate from the exobase into the magnetosheath plasma region. We shall show that, dependent on the sunward location of the magnetopause, the density of these types of non-thermal hydrogen atoms (H-ENAs) becomes progressively comparable with the density of exobasic hydrogen with increasing altitude. At low exobasic heights, however, their contribution is negligible. At the end of this paper, we finally study the question of whether the H-ENA population could even be understood as a self-consistency phenomenon of the H-ENA population, especially during solar activity minimum conditions, i.e., H-ENAs leaving the exosphere being replaced by H-ENAs injected into the exosphere. Keywords. Magnetospheric physics (plasmasphere; solar wind-magnetosphere interactions) – solar physics, astrophysics, and astronomy (energetic particles)

Survey of Magnetosheath Plasma Properties at Saturn and Inference of Upstream Flow Conditions

AbstractA new Cassini magnetosheath data set is introduced that is based on a comprehensive survey of intervals in which the observed magnetosheath flow was encompassed within the plasma analyzer field of view and for which the computed numerical moments are therefore expected to be accurate. The data extend from 2004 day 299 to 2012 day 151 and comprise 19,155 416 s measurements. In addition to the plasma ion moments (density, temperature, and flow velocity), merged values of the plasma electron density and temperature, the energetic particle pressure, and the magnetic field vector are included in the data set. Statistical properties of various magnetosheath parameters, including dependence on local time, are presented. The magnetosheath field and flow are found to be only weakly aligned, primarily because of a relatively large z component of the magnetic field, attributable to the field being pulled out of the equatorial orientation by flows at higher latitudes. A new procedure for using magnetosheath properties to estimate the upstream solar wind speed is proposed and used to determine that the amount of electron heating at Saturn's high Mach‐number bow shock is ~4% of the dissipated flow energy. The data set is available as supporting information to this paper.

Intermittent turbulence in the heliosheath and the magnetosheath plasmas based on Voyager and THEMIS data

Abstract. Turbulence is complex behavior that is ubiquitous in space, including the environments of the heliosphere and the magnetosphere. Our studies on solar wind turbulence including the heliosheath, and even at the heliospheric boundaries, also beyond the ecliptic plane, have shown that turbulence is intermittent in the entire heliosphere. As is known, turbulence in space plasmas often exhibits substantial deviations from normal Gaussian distributions. Therefore, we analyze the fluctuations of plasma and magnetic field parameters also in the magnetosheath behind the Earth's bow shock. Based on THEMIS observations, we have already suggested that turbulence behind the quasi-perpendicular shock is more intermittent with larger kurtosis than that behind the quasi-parallel shocks. Following this study, we would like to present a detailed analysis of intermittent anisotropic turbulence in the magnetosheath depending on various characteristics of plasma behind the bow shock and now also near the magnetopause. In particular, for very high Alfvénic Mach numbers and high plasma beta we have clear non-Gaussian statistics in the directions perpendicular to the magnetic field. On the other hand, for directions parallel to this field the kurtosis is small and the plasma is close to equilibrium. However, the level of intermittency for the outgoing fluctuations seems to be similar to that for the ingoing fluctuations, which is consistent with approximate equipartition of energy between the oppositely propagating Alfvén waves. We hope that the difference in characteristic behavior of these fluctuations in various regions of space plasmas can help to detect some complex structures in space missions in the near future.

The SMILE Soft X-ray Imager (SXI) CCD design and development

SMILE, the Solar wind Magnetosphere Ionosphere Link Explorer, is a joint science mission between the European Space Agency and the Chinese Academy of Sciences. The spacecraft will be uniquely equipped to study the interaction between the Earth's magnetosphere-ionosphere system and the solar wind on a global scale. SMILE's instruments will explore this science through imaging of the solar wind charge exchange soft X-ray emission from the dayside magnetosheath, simultaneous imaging of the UV northern aurora and in-situ monitoring of the solar wind and magnetosheath plasma and magnetic field conditions. The Soft X-ray Imager (SXI) is the instrument being designed to observe X-ray photons emitted by the solar wind charge exchange process at photon energies between 200 eV and 2000 eV . X-rays will be collected using a focal plane array of two custom-designed CCDs, each consisting of 18 μm square pixels in a 4510 by 4510 array. SMILE will be placed in a highly elliptical polar orbit, passing in and out of the Earth's radiation belts every 48 hours. Radiation damage accumulated in the CCDs during the mission's nominal 3-year lifetime will degrade their performance (such as through decreases in charge transfer efficiency), negatively impacting the instrument's ability to detect low energy X-rays incident on the regions of the CCD image area furthest from the detector outputs. The design of the SMILE-SXI CCDs is presented here, including features and operating methods for mitigating the effects of radiation damage and expected end of life CCD performance. Measurements with a PLATO device that has not been designed for soft X-ray signal levels indicate a temperature-dependent transfer efficiency performance varying between 5×10−5 and 9×10−4 at expected End of Life for 5.9 keV photons, giving an initial set of measurements from which to extrapolate the performance of the SXI CCDs.

Negative Potential Solitary Structures in the Magnetosheath With Large Parallel Width

AbstractWe report multispacecraft observations by the Magnetospheric MultiScale (MMS) mission of large (80–155 λDe parallel to B) electrostatic solitary waves (ESWs). Observations of the same ESWs at different positions and times enable unprecedented accuracy in determining the size, velocity, and evolution of these nonlinear structures. This particular event is a short series of negative potential solitary waves in the magnetosheath. The observed structures differ in amplitude and speed, merging or colliding with one another. A Vlasov simulation supports initial wave growth by a cold feature in the electron distribution function. Such cold electrons are likely the result of mixing between the cold magnetospheric and warm magnetosheath plasma along a reconnection separatrix. ESW speed and perpendicular size are inconsistent with ion phase space holes. Solving Poisson's equation for the measured potential, we find a reduction in electron density rather than an enhancement expected from an electron soliton. These ESWs have an unusual combination of properties on both ion and electron scales, likely supported by a complex electron distribution in the nonlinear state.

The Role of Solar Wind Density in Cross Polar Cap Potential Saturation Under Northward Interplanetary Magnetic Field

AbstractThe role of solar wind density in the cross polar cap potential (CPCP) response under northward interplanetary magnetic field is investigated with observation‐based global simulations. A rare event was reported by Clauer et al. (2016) during which the ionospheric electric field EISP does not saturate under extreme interplanetary electric field (IEF) of ∼15 mV/m. While commonly utilized coupling functions based on IEF fail to provide an unambiguous explanation for the linear response, the Lyon‐Fedder‐Mobarry‐Magnetosphere‐Ionosphere Coupler/Solver model is used to explore the mechanisms in this study. The model first reproduces the observed linear features of the EISP. The simulated CPCP also responds linearly to IEF variations. A controlled simulation is designed with solar wind density artificially reduced to 10% of the observed value while all other parameters such as the IEF are kept the same. The controlled simulation shows saturation of the EISP as well as the CPCP. Further analysis shows the difference in the magnetosheath plasma β, implying the distinct dominant forces between the two simulations. The Lopez magnetosheath force balance theory is used to explain the CPCP responses under different solar wind densities. This comparison study highlights the role of solar wind density in determining the magnetosphere‐ionosphere response to extreme interplanetary drivings.

Properties of the Equatorial Magnetotail Flanks ∼50–200 RE Downtail

AbstractIn space, thin boundaries separating plasmas with different properties serve as a free energy source for various plasma instabilities and determine the global dynamics of large‐scale systems. In planetary magnetopauses and shock waves, classical examples of such boundaries, the magnetic field makes a significant contribution to the pressure balance and plasma dynamics. The configuration and properties of such boundaries have been well investigated and modeled. However, much less is known about boundaries that form between demagnetized plasmas where the magnetic field is not important for pressure balance. The most accessible example of such a plasma boundary is the equatorial boundary layer of the Earth's distant magnetotail. Rather, limited measurements since its first encounter in the late 1970s by the International Sun‐Earth Explorer‐3 spacecraft revealed the basic properties of this boundary, but its statistical properties and structure have not been studied to date. In this study, we use Geotail and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) missions to investigate the equatorial boundary layer from lunar orbit (∼55 Earth radii, RE, downtail) to as far downtail as ∼200 RE. Although the magnetic field has almost no effect on the structure of the boundary layer, the layer separates well the hot, rarefied plasma sheet from dense cold magnetosheath plasmas. We suggest that the most important role in plasma separation is played by polarization electric fields, which modify the efficiency of magnetosheath ion penetration into the plasma sheet. We also show that the total energies (bulk flow plus thermal) of plasma sheet ions and magnetosheath ions are very similar; that is, magnetosheath ion thermalization (e.g., via ion scattering by magnetic field fluctuations) is sufficient to produce hot plasma sheet ions without any additional acceleration.

Impact of interplanetary shock on parameters of plasma turbulence in the Earth’s magnetosheath

Data from the BMSW spectrometer, which measures the ion flux value and sometimes plasma parameters with a time resolution of 31 ms, allow the study of the parameters of turbulence of the solar wind and magnetosheath plasma on kinetic scales. In this work, the frequency spectra of the ion flux fluctuations before and after recording the interplanetary shock front in the Earth’s magnetosheath are compared based on these data. It is shown that, in contrast to the solar wind, where the exponential decay of the spectrum often occurs after the shock front on the kinetic scales, no such phenomenon is observed in the magnetosheath: the spectrum on these scales can be approximated by a power function in all the cases considered. In half of these cases, the spectrum slope on the kinetic scales does not change during the interplanetary shock propagation. The results indicate a weak impact of interplanetary shock waves on the parameters of the plasma turbulence. In addition, it is shown that an interplanetary shock does not change the level of intermittency of the ion flux in the magnetosheath at both low and high level before the front.

Global Three‐Dimensional Simulation of Earth's Dayside Reconnection Using a Two‐Way Coupled Magnetohydrodynamics With Embedded Particle‐in‐Cell Model: Initial Results

AbstractWe perform a three‐dimensional (3‐D) global simulation of Earth's magnetosphere with kinetic reconnection physics to study the flux transfer events (FTEs) and dayside magnetic reconnection with the recently developed magnetohydrodynamics with embedded particle‐in‐cell model. During the 1 h long simulation, the FTEs are generated quasi‐periodically near the subsolar point and move toward the poles. We find that the magnetic field signature of FTEs at their early formation stage is similar to a “crater FTE,” which is characterized by a magnetic field strength dip at the FTE center. After the FTE core field grows to a significant value, it becomes an FTE with typical flux rope structure. When an FTE moves across the cusp, reconnection between the FTE field lines and the cusp field lines can dissipate the FTE. The kinetic features are also captured by our model. A crescent electron phase space distribution is found near the reconnection site. A similar distribution is found for ions at the location where the Larmor electric field appears. The lower hybrid drift instability (LHDI) along the current sheet direction also arises at the interface of magnetosheath and magnetosphere plasma. The LHDI electric field is about 8 mV/m, and its dominant wavelength relative to the electron gyroradius agrees reasonably with Magnetospheric Multiscale (MMS) observations.

Influence of velocity fluctuations on the Kelvin‐Helmholtz instability and its associated mass transport

AbstractKelvin‐Helmholtz instability (KHI) and associated magnetic reconnection and diffusion processes provide plasma transport from solar wind into the magnetosphere. The efficiency of this transport depends on the magnetosheath and magnetospheric plasma and field properties at the vicinity of the magnetopause. Our recent statistical study using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft indicates that the amplitude of the magnetosheath velocity fluctuations perpendicular to the magnetopause can be substantial. We have performed a series of local macroscale 2.5‐dimensional magnetohydrodynamic simulations of the KHI during strongly northward interplanetary magnetic field and with the initial plasma parameters typical to the dayside magnetopause by perturbing the initial equilibrium with time‐dependent perpendicular velocity field fluctuations. The effect of the single‐mode and multimode seed spectrums at different frequencies and amplitudes is studied. The plasma transport in Kelvin‐Helmholtz vortices is quantified. The results show that when large‐amplitude, low‐frequency seed velocity fluctuations exist in the magnetosheath, the resulting KH waves grow faster, get larger in size, and can transport more plasma through magnetic boundary, resulting in diffusion coefficient of the order 109 m2/s. The relevance of these findings to the solar wind‐magnetosphere coupling is discussed.

Large‐scale solar wind flow around Saturn's nonaxisymmetric magnetosphere

AbstractThe interaction between the solar wind and a magnetosphere is central to the dynamics of a planetary system. Here we address fundamental questions on the large‐scale magnetosheath flow around Saturn using a 3‐D magnetohydrodynamic (MHD) simulation. We find Saturn's polar‐flattened magnetosphere to channel ~20% more flow over the poles than around the flanks at the terminator. Further, we decompose the MHD forces responsible for accelerating the magnetosheath plasma to find the plasma pressure gradient as the dominant driver. This is by virtue of a high‐β magnetosheath and, in turn, the high‐MA bow shock. Together with long‐term magnetosheath data by the Cassini spacecraft, we present evidence of how nonaxisymmetry substantially alters the conditions further downstream at the magnetopause, crucial for understanding solar wind‐magnetosphere interactions such as reconnection and shear flow‐driven instabilities. We anticipate our results to provide a more accurate insight into the global conditions upstream of Saturn and the outer planets.

Dual E × B flow responses in the dayside ionosphere to a sudden IMF By rotation

AbstractWe report for the first time a dual transition state in the dayside ionosphere following a sudden rotation of the interplanetary magnetic field (IMF) in the upstream magnetosheath from IMF By < 0 to By > 0 during Bz < 0. E × B drifts respond with different time delays in the dayside auroral zone and high‐latitude polar cap with an initial 11 min transition state of oppositely directed E × B drifts coexisting at these locations. This is followed by a 6–8 min rotation of lower latitude E × B flow from dusk to dawn. We propose that this sequence of events is consistent with two separate X lines coexisting on the subsolar and lobe magnetopause. Time delays are proposed for merged flux of the draped preceding IMF to exit the subsolar region before the new IMF may be processed along a newly reconfigured component reconnection X line. Finally, a strong direct correlation is observed between magnetosheath plasma density and auroral zone E × B speeds.

Mars plasma system response to solar wind disturbances during solar minimum

AbstractThis paper is a phenomenological description of the ionospheric plasma and induced magnetospheric boundary (IMB) response to two different types of upstream solar wind events impacting Mars in March 2008, at the solar minimum. A total of 16 Mars Express orbits corresponding to five consecutive days is evaluated. Solar TErrestrial RElations Observatory‐B (STEREO‐B) at 1 AU and Mars Express and Mars Odyssey at 1.644 AU detected the arrival of a small transient interplanetary coronal mass ejection (ICME‐like) on the 6 and 7 of March, respectively. This is the first time that this kind of small solar structure is reported at Mars's distance. In both cases, it was followed by a large increase in solar wind velocity that lasted for ~10 days. This scenario is simulated with the Wang‐Sheeley‐Arge (WSA) ‐ ENLIL + Cone solar solar wind model. At Mars, the ICME‐like event caused a strong compression of the magnetosheath and ionosphere, and the recovery lasted for ~3 orbits (~20 h). After that, the fast stream affected the upper ionosphere and the IMB, which radial and tangential motions in regions close to the subsolar point are analyzed. Moreover, a compression in the Martian plasma system is also observed, although weaker than after the ICME‐like impact, and several magnetosheath plasma blobs in the upper ionosphere are detected by Mars Express. We conclude that, during solar minimum and at aphelion, small solar wind structures can create larger perturbations than previously expected in the Martian system.

Magnetospheric balance of solar wind dynamic pressure

AbstractThe magnetopause is the boundary established by pressure balance between the solar wind flow in the magnetosheath and the magnetosphere. Generally, this pressure balance is represented to be between the solar wind, the dynamic pressure, and the magnetic pressure of Earth's dipole field. The plasma actually in contact with the magnetosphere is the slowed, compressed, and heated solar wind downstream of the shock. The force exerted on the magnetosheath plasma is the J × B force produced by the Chapman‐Ferraro current that flows on the magnetopause. Under typical solar wind conditions of relatively high magnetosonic Mach number flow (>6), this simple picture is a reasonable description of the situation. However, under conditions of low solar wind magnetosonic Mach number flow (~2) the force on the solar wind plasma is not exerted at the magnetopause and must be exerted at the bow shock by currents that connect to the Region 1 currents. In this paper we present observations from two magnetopause crossings observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft to compare and contrast the force balance with the solar wind for two situations with very different solar wind magnetosonic Mach numbers.

Magnetospheric Multiscale Observations of Electron Vortex Magnetic Hole in the Turbulent Magnetosheath Plasma

Abstract We report on the observations of an electron vortex magnetic hole corresponding to a new type of coherent structure in the turbulent magnetosheath plasma using the Magnetospheric Multiscale mission data. The magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the core region and a peak in the outer region of the magnetic hole. The estimated size of the magnetic hole is about 0.23 ρ i (∼30 ρ e) in the quasi-circular cross-section perpendicular to its axis, where ρ i and ρ e are respectively the proton and electron gyroradius. There are no clear enhancements seen in high-energy electron fluxes. However, there is an enhancement in the perpendicular electron fluxes at 90° pitch angle inside the magnetic hole, implying that the electrons are trapped within it. The variations of the electron velocity components V em and V en suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the cross-section in the M–N plane. These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations.

Evolution of a typical ion‐scale magnetic flux rope caused by thermal pressure enhancement

AbstractWith high time‐resolution field and plasma measurements by the Magnetospheric Multiscale spacecraft, interior fine structures of two ion‐scale magnetic flux ropes (~5 and ~11 ion inertial length radius) separated by ~14 s are resolved. These two ion‐scale flux ropes (FR1 and FR2) show non‐frozen‐in ion behavior and consist of a strong axial magnetic field at the reversal of the negative‐then‐positive bipolar field component. The negative bipolar field component of the FR2 is found to be depressed, where magnetic pressure and total pressure decrease, but ion and electron thermal pressures increase, a feature akin to a crater‐like flux rope. The pressure enhancement is due to the magnetosheath plasma feeding into the flux rope along the field lines. Magnetic field draping and energetic electrons are also observed in the trailing part of the FR2. The ratio of perpendicular and parallel currents indicates that the FR1 appears force‐free but the FR2 seems not. Moreover, the FR2 is time‐dependent as a result of a low correlation coefficient (CC = 0.75) for the derivation of the deHoffmann‐Teller frame using the direct measured electric fields, while the FR1 is in quasi‐steady conditions (CC = 0.94). It is concluded that the crater formation within the FR2 can be interpreted by the analytical flux rope simulation as the evolution of typical flux rope to crater‐like one due to the thermal pressure enhancement, which could be induced by the depression of transverse magnetic fields of the flux rope.

Plasma fluctuations at the flanks of the Earth's magnetosheath at ion kinetic scales

Abstract. We present a statistical study of the magnetosheath plasma fluctuation spectra at a high-frequency range (with frequencies from 0.01 to 10 Hz). Variations of ion flux value and its direction are considered. The direction of ion flux is characterized by a polar angle – the deviation of the ion flux vector from the Sun–Earth line. We consider 290 Fourier's spectra that can be described by two power laws with a break, i.e., a change of slope. The ion flux fluctuation spectra are shown to have breaks at higher frequencies compared to the polar angle spectra. We compare the frequency of the break with the gyrostructure frequency for a number of cases. We show the polar angle break frequency to usually be smaller than the gyrostructure frequency. The dependencies of spectrum parameters such as the slopes and the break frequency on plasma parameters are also considered.