Solar Wind Motional Electric Field Research Articles

Two‐Spacecraft Observations of Asymmetric Martian Bow Shock: Conjunctions of Tianwen‐1 and MAVEN

AbstractThe Martian bow shock has been extensively studied through magnetic field and plasma instrument observations from various Mars space missions. However, prior investigations primarily involve statistical analyses based on single spacecraft crossings, leaving the asymmetry of the Martian bow shock unstudied through simultaneous two‐spacecraft observations. In this study, utilizing simultaneous observations from Tianwen‐1 and MAVEN, we examine the instantaneous asymmetry of the Martian bow shock. We present the asymmetry of the Martian bow shock in the Mars‐Solar‐Electric and Mars‐Solar‐Orbital reference frames, possibly influenced by the solar wind motional electric field and Martian crustal magnetic field, respectively. Moreover, we suggest that the bow shock exhibits increased asymmetry under stronger solar wind motional electric field conditions. This study highlights how a two‐point observation approach offers valuable insights into the dynamic behavior of the Martian induced magnetosphere.

Asymmetrical Looping Magnetic Fields and Marsward Flows on the Nightside of Mars

AbstractAs the interplanetary magnetic field (IMF) carried by the solar wind encounters the martian atmosphere, it tends to pile up and drape around the planet, forming looping magnetic fields and inducing marsward ion flows on the nightside. Previous statistical observations revealed asymmetrical distribution features within this morphology; however, the underlying physical mechanism remains unclear. In this study, utilizing a three‐dimensional multi‐fluid magnetohydrodynamic simulation model, we successfully reproduce the asymmetrical distributions of the looping magnetic fields and corresponding marsward flows on the martian nightside. Analyzing the magnetic forces resulting from the bending of the IMF over the polar area, we find that the asymmetry is guided by the orientation of the solar wind motional electric field (ESW). A higher solar wind velocity leads to enhanced magnetic forces, resulting in more tightly wrapped magnetic fields with an increased efficiency in accelerating flows as they approach closer to Mars.

Structure of the Martian Dayside Magnetosphere: Two Types

A set of scientific instruments capable of high temporal resolution measurements onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made it possible to study the structure and properties of the dayside Martian magnetosphere. The Mars plasma envelope on the dayside of the planet was discovered based on observations on the first orbiters of Mars (Vaisberg et al., 1976; Gringauz et al., 1976). The high temporal and mass resolution of MAVEN instruments made it possible to study the structure and processes in the plasma of the dayside magnetosphere, the plasma layer that exists between the solar wind heated by the shock wave and the ionosphere (Vaisberg et al., 2017). It is shown that, under different external conditions, two different types of plasma layers are formed between the ionosphere and magnetosheath on the dayside of the planet: (1) mixture of heated ionospheric ions and picked-up exospheric ions and (2) a layer of ionospheric ions accelerated by the solar wind motional electric field called “plume” (Dong et al., 2015). The first type of the Martian daytime magnetosphere is formed as a result of the interaction of oxygen exospheric ions captured and accelerated by the solar wind with the upper part of the Martian ionosphere (Vaisberg and Shuvalov, 2021). The second type of the Martian dayside magnetosphere is formed by the acceleration of an ion beam from the outer ionosphere, which then passes into a more energetic plume beam in the magnetosheath. This article discusses typical plasma populations, their properties and conditions that lead to the formation of the daytime magnetosphere from the material of the upper ionosphere.

Structure and Variability of the Martian Ion Composition Boundary Layer

AbstractA complex boundary layer with a variety of charged particle and electromagnetic field signatures, including a transition between plasma predominantly of solar wind origin and plasma of planetary origin, lies between the Martian bow shock and the ionosphere. In this paper, we develop and utilize algorithms to autonomously identify and characterize this ion composition boundary (ICB), using data from the Mars Atmosphere and Volatile EvolutioN mission. We find an asymmetric ICB with a larger average thickness, lower altitude, and lower velocity shear in the hemisphere where the solar wind motional electric field points outward, as a result of the asymmetry of the mass loading process. The ICB thickness scales with the magnetosheath proton gyroradius at the top of the boundary layer but does not clearly vary with external drivers. The ICB location varies with solar wind ram pressure and crustal magnetic field strength, but does not clearly respond to solar wind Mach number or extreme ultraviolet irradiance. The ICB represents a distinct boundary for ion density and flow speed, but the magnetic field strength and direction typically do not vary significantly across the ICB. The plasma density and flow speed at the ICB vary seasonally, likely in response to variations in the neutral exosphere and/or atmosphere. However, the ICB on average remains at or below the altitude where pressure balance is achieved between the piled up magnetic field and the solar wind ram pressure, regardless of season or crustal magnetic field strength.

Solar Wind Deflection by Mass Loading in the Martian Magnetosheath Based on MAVEN Observations

AbstractMars Atmosphere and Volatile EvolutioN observations at Mars show clear signatures of the shocked solar wind interaction with the extended oxygen atmosphere and hot corona displayed in a lateral deflection of the magnetosheath flow in the direction opposite to the direction of the solar wind motional electric field. The value of the velocity deflection reaches ∼50 km/s. The occurrence of such deflection is caused by the “Lorentz‐type” force due to a differential streaming of the solar wind protons and oxygen ions originating from the extended oxygen corona. The value of the total deceleration of the magnetosheath flow due to mass loading is estimated as ∼40 km/s.

The Effect of Solar Wind Variations on the Escape of Oxygen Ions From Mars Through Different Channels: MAVEN Observations

AbstractWe present multi‐instrument observations of the effects of solar wind on ion escape fluxes on Mars based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) data from 1 November 2014 to 15 May 2016. Losses of oxygen ions through different channels (plasma sheet, magnetic lobes, boundary layer, and ion plume) as a function of the solar wind and the interplanetary magnetic field variations were studied. We have utilized the modified Mars Solar Electric (MSE) coordinate system for separation of the different escape routes. Fluxes of the low‐energy (≤30 eV) and high‐energy (≥30 eV) ions reveal different trends with changes in the solar wind dynamic pressure, the solar wind flux, and the motional electric field. Major oxygen fluxes occur through the tail of the induced magnetosphere. The solar wind motional electric field produces an asymmetry in the ion fluxes and leads to different relations between ion fluxes supplying the tail from the different hemispheres and the solar wind dynamic pressure (or flux) and the motional electric field. The main driver for escape of the high‐energy oxygen ions is the solar wind flux (or dynamic pressure). On the other hand, the low‐energy ion component shows the opposite trend: ion flux decreases with increasing solar wind flux. As a result, the averaged total oxygen ion fluxes reveal a low variability with the solar wind strength. The large standard deviations from the averages values of the escape fluxes indicate the existence of mechanisms which can enhance or suppress the efficiency of the ion escape. It is shown that the Martian magnetosphere possesses the properties of a combined magnetosphere which contains different classes of field lines. The existence of the closed magnetic field lines in the near‐Mars tail might be responsible for suppression of the ion escape fluxes.

Analysis of dayside magnetosphere of Mars: High mass loading case as observed on MAVEN spacecraft

MAVEN spacecraft provides new opportunities for analysis of Martian environment and physical process in near-Mars space. One of interesting regions of near-Mars space is the Martian magnetosphere that is formed from mass-loaded magnetic flux tubes. There is quite detailed knowledge of the night-side magnetosphere of Mars, however the number of publications on the dayside magnetosphere are quite limited. We analyze the plasma and magnetic structure and properties of Martian magnetosphere at strong mass-loading conditions as observed on MAVEN at Mars at the solar-zenith angle of ∼80° on January 4, 2015. This strong mass loading of upstream flow was apparently associated with plume ions ejected from upper part of Martian magnetosphere by the solar wind motional electric field. The magnetosphere is defined by two current layers separating it from the magnetosheath at higher altitudes and from ionosphere plasma at lower altitudes. It is characterized by dominance of planetary ions which number density increases by two orders of magnitude from upper boundary to lower one. There is approximate equipartition between magnetic, ion thermal and kinetic energies through magnetosphere. The data suggest that the boundary of the magnetosphere is in pressure equilibrium with magnetosheath flow. The total energy of ion flow above (in the magnetosheath) and below (in the region of accelerated ionospheric ions) magnetosphere exceeds the magnetic energy. The upper boundary of magnetosphere was located at the place where the ratio of heavy ions and protons number densities reached ∼0.4. Within magnetosphere this ratio continued to rise and increased by about 2 orders of magnitude at the inner boundary of magnetosphere. The heavy ion number density profile within magnetosphere suggests that it was formed by the solar wind magnetic flux tubes that reached Mars in a narrow region near the subsolar point, and then drifted around Mars to the terminator region, mass-loaded by UV-ionized upper atmosphere neutrals during this drift.

Effects of solar irradiance on the upper ionosphere and oxygen ion escape at Mars: MAVEN observations

AbstractWe present multi‐instrument observations of the effects of solar irradiance on the upper Martian ionosphere and escape fluxes based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) data from November 2014 to February 2016. It is shown that fluxes of oxygen ions with E > 30 eV both inside and outside of the Martian magnetosphere are nonsensitive to EUV variations. In contrast, the fluxes of ions with lower energies extracted from the upper ionosphere increase with solar irradiance. Such an enhancement is nonlinear with the EUV variations and exhibits a growth by almost 1 order of magnitude when the EUV (0.1–50 nm) radiation increases to ≥0.1 W/m2 implying an enhancement of total ion losses of the low‐energy component to ∼1.8·1025 s−1. The flow of cold ions in the near‐Mars tail occurs very asymmetrical shifting in the direction opposite to the direction of the solar wind motional electric field. Fluxes of the low‐energy (E≤30 eV) ion component are also nonsensitive to the variations in solar wind dynamic pressure.

MAVEN measured oxygen and hydrogen pickup ions: Probing the Martian exosphere and neutral escape

AbstractSoon after the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft started orbiting Mars, the SEP (Solar Energetic Particle), SWIA (Solar Wind Ion Analyzer), and STATIC (Supra‐Thermal and Thermal Ion Composition) instruments on board the spacecraft detected planetary pickup ions. SEP can measure energetic (>60 keV) oxygen pickup ions, the source of which is the extended hot oxygen exosphere of Mars. Model results show that these pickup ions originate from tens of Martian radii upstream of Mars and are energized by the solar wind motional electric field as they gyrate back toward Mars. SWIA and STATIC can detect both pickup oxygen and pickup hydrogen with energies below ~30 keV and created closer to Mars. In this study, data from the SEP, SWIA, and STATIC instruments containing pickup ion signatures are provided and model‐data comparisons are shown. During the times when MAVEN is outside the Martian bow shock and in the upstream undisturbed solar wind, the solar wind velocity measured by SWIA and the solar wind (or interplanetary) magnetic field measured by the MAG (magnetometer) instrument can be used to model pickup oxygen and hydrogen fluxes. By comparing measured pickup ion fluxes with model results, the Martian thermal hydrogen and hot oxygen neutral densities can be probed outside the bow shock, providing a helpful tool in constraining estimates of neutral oxygen and hydrogen escape rates. Our analysis reveals an order of magnitude density change with Mars season in the hydrogen exosphere, whereas the hot oxygen exosphere was found to remain steadier.

Relativity and the Solar Wind: The Maxwell-Equation Origins of the Solar-Wind Motional Electric Field

The motional electric field of the solar wind as seen by the Earth is examined theoretically and with spacecraft measurements. As it flows outward from the sun, the solar-wind plasma carries a spatially structured magnetic field with it. To calculate the motional electric field of the solar wind the spatially structured magnetic field is Lorentz transformed; for a full physical understanding, it is also necessary to Lorentz transform the current densities and charge densities in the solar wind. Referring to Maxwell’s equations, two related questions are asked: 1) Is the source of the solar-wind motional electric field charge density in the solar wind, time derivatives of current densities in the solar wind, or both? 2) Is the solar-wind motional electric field at Earth an electrostatic field, an induction field, or a superposition of the two? A Helmholtz decomposition of the motional electric field of the solar wind is made into a divergence-origin (electrostatic) and a curl-origin (induction) electric field. The global electric field associated with the outward advection of the global Parker-spiral magnetic field is found to be electrostatic with its origin being a distributed charge density in the solar-wind plasma. The electrostatic versus induction nature of the time-varying electric field associated with the advection of mesoscale magnetic structure varies with time as differently shaped magnetic structures in the solar-wind plasma pass the Earth; the mesoscale structure of the solar-wind plasma contains sheets of space charge and sheets wherein the current density has nonzero time derivatives.

Mars Express observations of high altitude planetary ion beams and their relation to the “energetic plume” loss channel

AbstractThis study presents observational evidence of high‐energy (ions >2 keV) beams of planetary ions above Mars' induced magnetospheric boundary (IMB) and relates them with the energetic plume loss channel calculated from numerical models. A systematic search of the Mars Express (MEX) ion data using an orbit filtering criteria is described, using magnetometer data from Mars Global Surveyor (MGS) to determine the solar wind motional electric field (Esw) direction. Two levels of statistical survey are presented, one focused on times when the MEX orbit was directly in line with the Esw and another for all angles between the MEX location and the Esw. For the first study, within the 3 year overlap of MGS and MEX, nine brief intervals were found with clear and unambiguous high‐energy O+ observations consistent with the energetic plume loss channel. The second survey used a point‐by‐point determination of MEX relative to the E‐field and contained many thousands of 192 s measurements. This study yielded only a weak indication for an Esw‐aligned plume. Furthermore, the y‐z components of the weighted average velocities in the bins of this y‐z spatial domain survey do not systematically point in the Esw direction. The first survey implies the existence of this plume and shows that its characteristics are seemingly consistent with the expected energy and flight direction from numerical studies; the second study softens the finding and demonstrates that there are many planetary ions beyond the IMB moving in unexpected directions. Several possible explanations for this discrepancy are discussed.

Toroidal and poloidal magnetic fields at Venus. Venus Express observations

Magnetic field and plasma measurements carried out onboard Venus Express during solar minimum conditions suggest the existence of two kinds of magnetic field configuration in the Venusian ionosphere. We interpret these as the manifestation of two different types of generation mechanisms for the induced magnetosphere. A different magnetic field topology (toroidal and poloidal) arises if the induced currents are driven either by the solar wind motional electric field or by the Faraday electric field—a conducting ionosphere sees the magnetic field carried by solar wind as a time-varying field. At the dayside, both driving agents produce a similar draping pattern of the magnetic field. However, different magnetic field signatures inherent to both induction mechanisms appear at lower altitudes in the terminator region. The conditions at low solar EUV flux when the ionosphere of Venus becomes magnetized seem to be favorable to distinguish between two different types of the induced fields. We present cases of both types of the magnetic field topology. The cases when the effects of the Faraday induction become well noticeable are especially interesting since they provide us with an example of solar wind interaction with a tiny induced dipole field immersed into the ionosphere. Another interesting case when poloidal magnetic fields are evidently displayed is observed when the IMF vector is almost aligned with the solar wind velocity. In general case, both mechanisms of induction probably complement each other.

Proton cyclotron wave generation mechanisms upstream of Venus

[1] Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 RV upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 RV toward the Sun from a sufficiently dense Venus hydrogen exosphere.

Conditions for the formation of hot flow anomalies at Earth's bow shock

Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earth's bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional electric field pointing toward the current sheet on at least one side and that the current sheet must be a tangential discontinuity. Recent reports of a rapid displacement of the magnetopause by 5 Re as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HFAs. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most HFAs have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar‐terrestrial dynamics.

Pioneer Venus Orbiter magnetic field and plasma observations in the Venus magnetotail

This study uses Pioneer Venus orbiter (PVO) magnetometer and plasma analyzer measurements to investigate the draped‐field tail of Venus with an emphasis on determining the magnetic field and plasma conditions within the various tail regions and their dependence upon interplanetary magnetic field (IMF) orientation. For this purpose PVO orbits during which the spacecraft's high inclination trajectory took it through the central magnetotail were identified. The criteria used to select these orbits were the existence of relatively complete magnetic field and plasma data during crossings of the magnetotail which lasted more than 3 hours. Examination of observations from four PVO tail seasons in 1981–1983 produced 12 orbits meeting these requirements. Analysis of the observations taken during those orbits indicates that the distribution of plasma within the magnetotail is highly asymmetric and controlled by the orientation of the IMF. In the plasma sheet and adjacent lobe regions downstream of the Venus hemisphere over which the solar wind motional electric field, Esw = −Vsw × Bsw, is directed away from the planet, PVO observed increasing fluxes of H+ and O+ as the spacecraft moves away from the tail axis toward the outer boundary of the tail. The greatest concentration of these O+ ions was found in the vicinity of the cross‐tail current layer downstream of this Venus hemisphere. No O+ ions were observed outside of the magnetotail based upon the magnetic field data and the definitions adopted in this study. However, only slow, <310 km/s, antisolar directed O+ is detectable by the PVO plasma analyzer and this factor may be the reason for the frequent observation of O+ in the cross‐tail current and the absence of measurable O+ outside of the magnetotail where the bulk flow speeds are significantly slower than in the distant ionosheath. Downstream of the Venus hemisphere over which the solar wind motional electric field is directed in toward the planet, PVO does not usually observe significant fluxes of E/Q = 0–8 kV ions, except sometimes directly adjacent to the outer boundary of the tail. The outer boundary of the Venus magnetotail downstream of this hemisphere is typically a well‐defined magnetopauselike current layer. The magnetotail‐ionosheath interface downstream of Venus hemisphere over which the solar wind motional electric field is outward, however, is very broad and resembles a slow mode expansion fan with field strength slowly decreasing and plasma density gradually increasing as PVO moves outward and enters the ionosheath. These results are interpreted as being due to the more efficient pick‐up of newly ionized atmospheric neutrals over the Venus hemisphere where the initial gyromotion takes the newly created ions away from the dense, lower atmosphere where they might be lost due to scattering (Cloutier et al., 1974). The implications of these findings for the formation and maintenance of the Venus magnetotail are discussed.

Solar wind control of the low‐latitude asymmetric magnetic disturbance field

The technique of empirical linear prediction filters is used to investigate the extent to which the low latitude dawn‐dusk magnetic asymmetry is controlled by the dawn‐dusk solar wind motional electric field VBs and/or by substorm processes measured by the westward auroral electrojet index AL. The dawn‐dusk asymmetry is measured by a new index defined as the difference between dawn and dusk deviations in the X (geomagnetic Northward) magnetic field component. The empirically determined filters obtained from this analysis provide quantitative information which characterizes the coupling processes. For example, the VBs to AL filter is a delayed pulse beginning after a delay of about 15 minutes, peaking at 60 minutes and returning to zero at 120 min. The filter has the characteristics of a low pass filter with a cutoff frequency at 10−4 Hz. The VBs to ASYM filter is also a delayed pulse with similar constants. The VBs to ASYM filter, however, also has a long tail which gradually decays reaching zero near 5 or 6 hours lag time. The AL to ASYM filter is acausal, being nonzero at future lag times. The peak of the filter occurs at a delay of about 10 min. and decays exponentially becoming nearly zero after 4 hours lag. Our results indicate that some currents are directly driven by the solar wind‐magnetosphere interaction and that their magnetic perturbations contribute to both the AL and ASYM indices. A portion of the AL index that is uncorrelated with VBs is, however, correlated with ASYM suggesting that internal magnetospheric processes contribute to AL and ASYM as well.

Sheath-limited unipolar induction in the solar wind

The collection of charged particles by electrodes in plasmas is controlled by the currentvoltage characteristics of the plasma sheath which forms at the electrode surface. This principle is applied to the steady-state electromagnetic interaction of the solar wind with moon-like bodies, or ‘solid-body’ interactions. In some cases the unipolar dynamo response of an electrically conducting body in the solar wind motional electric field can be controlled by sheath effects. This occurs for highly conducting bodies when the body radiusR is less than a critical valueR c , with the result that no induced bow shock wave can form.