Solar Wind Conditions Research Articles

MHD study of the planetary magnetospheric response during extreme solar wind conditions: Earth and exoplanet magnetospheres applications

Context.The stellar wind and the interplanetary magnetic field modify the topology of planetary magnetospheres. Consequently, the hazardous effect of the direct exposition to the stellar wind, for example, regarding the integrity of satellites orbiting the Earth or the habitability of exoplanets, depends upon the space weather conditions.Aims.The aim of the study is to analyze the response of an Earth-like magnetosphere for various space weather conditions and interplanetary coronal mass ejections. The magnetopause standoff distance, the open-close field line boundary, and plasma flows toward the planet surface are calculated.Methods.We used the magnetohydrodynamics code PLUTO in spherical coordinates to perform a parametric study of the dynamic pressure and temperature of the stellar wind as well as of the interplanetary magnetic field intensity and orientation. The range of the parameters we analyzed extends from regular to extreme space weather conditions, which is consistent with coronal mass ejections at the Earth orbit for the present and early periods of the solar main sequence. In addition, implications of sub-Afvénic solar wind configurations for the Earth and exoplanet magnetospheres were analyzed.Results.The direct precipitation of the solar wind at the Earth dayside in equatorial latitudes is extremely unlikely even during super coronal mass ejections. On the other hand, for early evolution phases during the solar main sequence, when the solar rotation rate was at least five times faster (<440 Myr), the Earth surface was directly exposed to the solar wind during coronal mass ejections. Today, satellites at high, geosynchronous, and medium orbits are directly exposed to the solar wind during coronal mass ejections because part of the orbit at the Earth dayside is beyond the nose of the bow shock.

Possible Ionospheric Influence on Substorm Onset Location

AbstractAuroral substorm onset locations are highly unpredictable. Previous studies have shown that the By component of the interplanetary magnetic field (IMF) explains ∼5% of the variation in onset magnetic local time (MLT), while solar wind conditions and the other IMF components have even less explanatory power. In this study, we show that the level of geomagnetic activity before substorm onset, as indicated by the AL index, explains an additional ∼5% of the variation in onset MLT. We discuss our results with regard to recent modeling studies, which show that gradients in the ionospheric Hall conductance can lead to a duskward shift of the magnetotail dynamics. Our findings suggest that this magnetosphere‐ionosphere coupling effect may also influence the location of substorm onsets.

Space Weather Effects on the Earth’s Upper Atmosphere: Short Report on Ionospheric Storm Effects at Middle Latitudes

During geomagnetic storm events, the highly variable solar wind energy input in the magnetosphere significantly alters the structure of the Earth’s upper atmosphere through the interaction of the ionospheric plasma with atmospheric neutrals. A key element of the ionospheric storm-time response is considered to be the large-scale increases and decreases in the peak electron density that are observed globally to formulate the so-called positive and negative ionospheric storms, respectively. Mainly due to their significant impact on the reliable performance of technological systems, ionospheric storms have been extensively studied in recent decades, and cumulated knowledge and experience have been assigned to their understanding. Nevertheless, ionospheric storms constitute an important link in the complex chain of solar-terrestrial relations. In this respect, any new challenge introduced in the field by a better understanding of the geospace environment, new modeling and monitoring capabilities and/or new technologies and requirements also introduces new challenges for the interpretation of ionospheric storms. This paper attempts a brief survey of present knowledge on the fundamental aspects of large-scale ionospheric storm time response at middle latitudes. Further attention is paid to the results obtained regarding the critical role that solar wind conditions which trigger disturbances may play on the morphology and the occurrence of ionospheric storm effects.

Natural Orthogonal Component Analysis of Daily Magnetic Variations at the Martian Surface: InSight Observations

AbstractDistinguishing different sources of magnetic field variations at InSight is important to understand dynamic processes in the Martian ionosphere as well as the coupling between the solar wind and the Martian induced magnetosphere. Recent studies based on magnetic field measurements from InSight have suggested that the daily and seasonal variations in the magnetic field at the Martian surface are at least partially the result of neutral wind‐driven ionospheric dynamo currents and their seasonal variations. However, the sources of the daily variations with different time scales need be further investigated. In this paper, magnetic field variations in a sol as well as during nearly a whole Martian year from InSight observations were decomposed into their natural orthogonal components. We found that the first eigenmode shows the previously identified early to midmorning peak, and varies with season. This corresponds to the solar quiet variations. The second and higher eigenmodes manifest the quasi‐Carrington and sub‐Carrington rotation periodicity represent disturbed components that may be stimulated by variations in the draped interplanetary magnetic field and/or the Martian ionospheric electron density. Different from their counterparts at the Earth, the amplitude of the first eigenmode is comparable with the sum of second to fifth ones, showing that the quiet and disturbed diurnal variations contribute similarly to the total diurnal variation. Decomposition of Martian surface magnetic field variations could provide monitoring of the Martian ionospheric current system as well as the solar wind conditions in the near‐Mars space, which will be greatly enhanced when combined with Zhurong Martian surface field measurements in the future.

Modeling Ion Beams, Kinetic Instabilities, and Waves Observed by the Parker Solar Probe near Perihelia

Abstract Recent in situ observations from the Parker Solar Probe (PSP) mission in the inner heliosphere near perihelia show evidence of ion beams, temperature anisotropies, and kinetic wave activity, which are likely associated with kinetic heating and acceleration processes of the solar wind. In particular, the proton beams were detected by PSP/Solar Probe Analyzers-Ion (SPAN-I) and related magnetic fluctuation spectra associated with ion-scale waves were observed by the FIELDS instrument. We present the ion velocity distribution functions (VDFs) from SPAN-I and the results of 2.5D and 3D hybrid-particle-in-cell models of proton and α particle super-Alfvénic beams that drive ion kinetic instabilities and waves in the inner heliospheric solar wind. We model the evolution of the ion VDFs with beams, and obtain the ion relative drifts speeds, and ion temperature anisotropies for solar wind conditions near PSP perihelia. We calculate the partition of energies between the particles (ions) along and perpendicular and parallel to the magnetic field, as well as the evolution of magnetic energy, and compare to observationally deduced values. We conclude that the ion beam driven kinetic instabilities in the solar wind plasma near perihelia are important components in the cascade of energy from fluid to kinetic scales, an important component in the solar wind plasma heating process.

Ensemble Numerical Simulations of Realistic SEP Events and the Inspiration for Space Weather Awareness

The solar energetic particle (SEP) event is a kind of hazardous space weather phenomena, so its quantitative forecast is of great importance from the aspect of space environmental situation awareness. We present here a set of SEP forecast tools, which consists of three components : (1) a simple polytropic solar wind model to estimate the background solar wind conditions at the inner boundary of 0.1 AU (about 20 R ⊙); (2) an ice-cream-cone model to estimate the erupted coronal mass ejection (CME) parameters; and (3) the improved Particle Acceleration and Transport in the Heliosphere (iPATH) model to calculate particle fluxes and energy spectra. By utilizing the above models, we have simulated six realistic SEP events from 2010 August 14 to 2014 September 10, and compared the simulated results to the Geostationary Operational Environmental Satellites (GOES) spacecraft observations. The results show that the simulated fluxes of >10 MeV particles agree with the observations while the simulated fluxes of >100 MeV particles are higher than the observed data. One of the possible reasons is that we have adopted a simple method in the model to calculate the injection rate of energetic particles. Furthermore, we have conducted the ensemble numerical simulations over these events and investigated the effects of different background solar wind conditions at the inner boundary on SEP events. The results imply that the initial CME density plays an important role in determining the power spectrum, while the effect of varying background solar wind temperature is not significant. Naturally, we have examined the influence of CME initial density on the numerical prediction results for virtual SEP cases with different CME ejection speeds. The result shows that the effect of initial CME density variation is inversely associated with CME speed.

Cross‐Terminator Variations of the Photoelectron Energy Distribution in the Martian Ionosphere

AbstractPhotoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X‐ray ionization and are also frequently observed on the nightside as a result of field‐aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross‐terminator variations, in that the He II peak becomes less pronounced and the 10 − 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of “atmospheric absorption” via photoelectron collisions with ambient neutrals during day‐to‐night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X‐ray radiation, all using the 10–50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross‐terminator spectral reshaping is proposed as the outcome of varying day‐to‐night magnetic connectivity which naturally implies varying field‐aligned atmospheric column for “absorption.” Crude estimates suggest that an “absorbing” atmospheric column of ≈1.5 × 1015 cm−2 is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross‐terminator magnetic field lines, as complementary to the day‐to‐night magnetic connectivity inferred from the occurrence of nightside photoelectrons.

Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS

AbstractForeshock transients such as foreshock bubbles (FBs), hot flow anomalies (HFAs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that some cores contain local density enhancements which can be studied to better understand the evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the higher resolution data from the Magnetospheric Multiscale mission (MMS). We identify 164 FB‐like, HFA‐like, and SHFA events from two MMS dayside phases for a statistical study to investigate their solar wind conditions, properties, and substructure properties. Occurrence rates of the three event types are higher for lower magnetic field strengths, higher solar wind speeds and Mach numbers, and quasi‐parallel bow shocks. Events usually span up to 3 RE along the bow shock surface and extend up to 6 RE upstream from the bow shock. Though events with and without substructures exhibit similar solar wind conditions, events with substructures are more likely to have longer core durations and larger sizes. Substructure densities display a positive correlation with bulk flows and a negative correlation with temperatures. Substructure sizes vary between 4 and 24 ion inertial lengths, indicating multiple formation mechanisms. Substructures could be the boundary between two foreshock transient events that have merged into a single event, fast‐mode variations, generated by slow or mirror mode instabilities, or produced from instabilities due to parameter gradients at the compressional boundaries or shocks.

Estimating ion escape from unmagnetized planets

Abstract. We propose a new method to estimate ion escape from unmagnetized planets that combines observations and models. Assuming that upstream solar wind conditions are known, a computer model of the interaction between the solar wind and the planet is executed for different ionospheric ion production rates. This results in different amounts of mass loading of the solar wind. We then obtain the ion escape rate from the model run that best fits observations of the bow shock location. As an example of the method, we estimate the heavy-ion escape from Mars on 1 March 2015 to be 2×1024 ions s−1, using a hybrid plasma model and observations by the Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX) missions. This method enables studies on how escape depends on different parameters as well as studies on escape rates during extreme solar wind conditions; moreover, the technique is applicable to studies of escape in the early solar system and at exoplanets.

A Statistical Study of the Solar Wind Dependence of Multi‐Harmonic Toroidal ULF Waves Observed by the Arase Satellite

AbstractToroidal standing Alfvén wave is one of the ultra‐low frequency waves that are frequently observed in the terrestrial magnetosphere. They sometimes exhibit multi‐harmonic frequency spectra, indicating wide energy range input in the magnetosphere. However, their energy source has not been fully understood due to the lack of statistical studies. Here we used the data of the Arase satellite observations for ∼3.5 years and conducted a statistical analysis of the solar wind dependence of the occurrence rate, wave power, and frequency of the multi‐harmonic toroidal waves. We automatically detected the multi‐harmonic waves and categorized them into four groups according to the solar wind velocity and the cone angle of the interplanetary magnetic field. We found that the occurrence rate and wave power of the multi‐harmonic waves increase with the solar wind velocity on the flank sides. In the noon sector, the occurrence rate of the multi‐harmonic waves increases with the decrease of the cone angle. The median frequency of the multi‐harmonic waves on the dayside is positively correlated with the upstream wave frequency predicted by the theory of the ion beam instability for a small cone angle. The occurrence rate also increases with the solar wind dynamic pressure fluctuations. Therefore, we suggest that the Kelvin‐Helmholtz instability, the upstream waves, and the dynamic pressure fluctuations are possible sources of the multi‐harmonic waves. This study sheds light on the activity of the multi‐harmonic waves which can affect radiation belt electrons under various solar wind conditions.

Transport Path of Cold‐Dense Plasmas in the Dusk Magnetotail Plasma Sheet: MMS Observations

AbstractThe near‐Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold‐dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold‐dense plasma inside the magnetotail has not been understood yet. Here, we present a case study of the CDPS in the dusk magnetotail by magnetospheric multiscale (MMS) spacecraft under strongly northward IMF and high‐density solar wind conditions. The ion distribution function consists of high‐ and low‐energy components, and the low‐energy one intermittently shows energy dispersion in the directions parallel and antiparallel to the local magnetic field. The time‐of‐flight analysis of the energy‐dispersed low‐energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch‐angle dispersion analysis gives consistent results on the traveling time and path length of the energy‐dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy‐dispersed structure of the low‐energy ions during the northward IMF.

S-shaped dependence of the sound pressure level in outdoor propagation on the effective sound speed gradient

The effective sound speed gradient is used to describe the meteorological conditions during sound measurements at roads and railways. Meteorological parameters were assessed up to a height of 10 m. The sound level differences between a reference point close to the passing vehicles and at distances of 100–500 m from motorways and railway tracks were determined. These differences were found to correlate well with the effective sound speed gradient determined from the measured temperature and wind speed gradients which follow the day/night cycle as a result of the reversing air temperature gradient, incoming solar radiation and wind conditions. The correlation with sound level differences can be approximated by an S-shaped function which is constant for large positive or negative gradients of the effective sound speed. These threshold values are a consequence of the local meteorological and attenuation conditions. The analysis shows that large effective sound speed gradients are mainly a result of the wind conditions whereas gradients without a substantial wind speed contribution are low and a result of the temperature gradient. In the distance range between 150 m and 250 m, the meteorological influences cause a level increase of 3–4 dB(A) for downward refracted sound rays (favourable sound propagation) compared to a situation without meteorological influence (effective sound speed gradient of zero). In the same distance range, meteorological conditions cause a maximum sound level attenuation of 5–10 dB(A) for upward curved sound rays (unfavourable sound propagation).

Modeling the Lunar Wake Response to a CME Using a Hybrid PIC Model

Abstract In the solar wind, a low-density wake region forms downstream of the nightside lunar surface. In this study, we use a series of 3D hybrid particle-in-cell simulations to model the response of the lunar wake to a passing coronal mass ejection (CME). Average plasma parameters are derived from the Wind spacecraft located at 1 au during three distinct phases of a passing halo (Earth-directed) CME on 2015 June 22. Each set of plasma parameters, representing the shock/plasma sheath, a magnetic cloud, and plasma conditions we call the mid-CME phase, are used as the time-static upstream boundary conditions for three separate simulations. These simulation results are then compared with results that use nominal solar wind conditions. Results show a shortened plasma void compared to nominal conditions and a distinctive rarefaction cone originating from the terminator during the CME’s plasma sheath phase, while a highly elongated plasma void reforms during the magnetic cloud and mid-CME phases. Developments of electric and magnetic field intensification are also observed during the plasma sheath phase along the central wake, while electrostatic turbulence dominates along the plasma void boundaries and 2–3 lunar radii R M downstream in the central wake during the magnetic cloud and mid-CME phases. The simulations demonstrate that the lunar wake responds in a dynamic way with the changes in the upstream solar wind during a CME.

Study of Mars Magnetosheath Fluctuations using the Kurtosis Technique: Mars Express Observations

Planetary magnetosheaths are characterized by high plasma wave and turbulence activity. The Martian magnetosheath is no exception; both upstream and locally generated plasma waves have been observed in the region between its bow shock and magnetic boundary layer, its induced magnetosphere. This statistical study of wave activity in the Martian magnetosheath is based on 12 years (2005–2016) of observations made during Mars Express (MEX) crossings of the planet’s magnetosheath — in particular, data on electron density and temperature data collected by the electron spectrometer (ELS) of the plasma analyzer (ASPERA-3) experiment on board the MEX spacecraft. A kurtosis parameter has been calculated for these plasma parameters. This value indicates intermittent behavior in the data when it is higher than 3 (the value for a normal or Gaussian distribution). The variation of wave activity occurrence has been analyzed in relation to solar cycle, Martian orbit, and distance to the bow shock. Non-Gaussian properties are observed in the magnetosheath of Mars on all analyzed scales, especially in those near the proton gyrofrequency in the upstream region of the Martian magnetosphere. We also report that non-Gaussian behavior is most prominent at the smaller scales (higher frequencies). A significant influence of the solar cycle was also observed; the kurtosis parameter is higher during declining and solar maximum phases, when the presence of disturbed solar wind conditions, caused by large scale solar wind structures, increases. The kurtosis decreases with increasing distance from the bow shock, which indicates that the intermittence level is higher near the bow shock. In the electron temperature data the kurtosis is higher near the perihelion due to the higher incidence of EUV when the planet is closer to the Sun, which causes a more extended exosphere, and consequently increases the wave activity in the magnetosheath and its upstream region. The extended exosphere seems to play a lower effect in the electron density data.

Dynamics of variable dusk–dawn flow associated with magnetotail current sheet flapping

Abstract. We present Cluster spacecraft observations from 12 October 2006 of convective plasma flows in the Earth's magnetotail. Earthward flow bursts with a dawnward v⊥y component, observed by Cluster 1 (C1), are inconsistent with the duskward flow that might be expected at the pre-midnight location of the spacecraft. Previous observations have suggested that the dusk–dawn sense of the flow can be governed by the interplanetary magnetic field (IMF) By conditions, with the related “untwisting hypothesis” of magnetotail dynamics commonly invoked to explain this dependence, in terms of a large-scale magnetospheric asymmetry. In the current study, observations of the upstream solar wind conditions from OMNI, magnetic field observations by Cluster and ionospheric convection data using SuperDARN indicate a large-scale magnetospheric morphology consistent with positive IMF By penetration into the magnetotail. At the pre-midnight location of Cluster, however, the dawnward flow observed below the neutral sheet by C1 could only be explained by the untwisting hypothesis in a negative IMF By scenario. The Cluster magnetic field data also reveal a flapping of the magnetotail current sheet, a phenomenon known to influence dusk–dawn flow. Results from the curlometer analysis technique suggest that the dusk–dawn sense of the J×B force was consistent with localised kinks in the magnetic field and the flapping associated with the transient perturbations to the dusk–dawn flow observed by C1. We therefore suggest that the flapping overcame the dusk–dawn sense of the large-scale convection which we would expect to have been net duskward in this case. We conclude that invocation of the untwisting hypothesis may be inappropriate when interpreting intervals of dynamic magnetotail behaviour such as during current sheet flapping, particularly at locations where magnetotail flaring becomes dominant.

Magnetotail reconnection asymmetries in an ion-scale, Earth-like magnetosphere

Abstract. We use a newly developed global Hall magnetohydrodynamic (MHD) code to investigate how reconnection drives magnetotail asymmetries in small, ion-scale magnetospheres. Here, we consider a magnetosphere with a similar aspect ratio to Earth but with the ion inertial length (δi) artificially inflated by a factor of 70: δi is set to the length of the planetary radius. This results in a magnetotail width on the order of 30 δi, slightly smaller than Mercury's tail and much smaller than Earth's with respect to δi. At this small size, we find that the Hall effect has significant impact on the global flow pattern, changing from a symmetric, Dungey-like convection under resistive MHD to an asymmetric pattern similar to that found in previous Hall MHD simulations of Ganymede's subsonic magnetosphere as well as other simulations of Mercury's using multi-fluid or embedded kinetic physics. We demonstrate that the Hall effect is sufficient to induce a dawnward asymmetry in observed dipolarization front locations and find quasi-periodic global-scale dipolarizations under steady, southward solar wind conditions. On average, we find a thinner current sheet dawnward; however, the measured thickness oscillates with the dipolarization cycle. During the flux-pileup stage, the dawnward current sheet can be thicker than the duskward sheet. This could be an explanation for recent observations that suggest Mercury's current sheet is actually thicker on the duskside: a sampling bias due to a longer lasting “thick” state in the sheet.

Kelvin-Helmholtz Instability Associated With Reconnection and Ultra Low Frequency Waves at the Ground: A Case Study

The Kelvin-Helmholtz instability (KHI) and its effects relating to the transfer of energy and mass from the solar wind into the magnetosphere remain an important focus of magnetospheric physics. One such effect is the generation of Pc4-Pc5 ultra low frequency (ULF) waves (periods of 45–600 s). On July 3, 2007 at ∼ 0500 magnetic local time the Cluster space mission encountered Pc4 frequency Kelvin-Helmholtz waves (KHWs) at the high latitude magnetopause with signatures of persistent vortices. Such signatures included bipolar fluctuations of the magnetic field normal component associated with a total pressure increase and rapid change in density at vortex edges; oscillations of magnetosheath and magnetospheric plasma populations; existence of fast-moving, low-density, mixed plasma; quasi-periodic oscillations of the boundary normal and an anti-phase relation between the normal and parallel components of the boundary velocity. The event occurred during a period of southward polarity of the interplanetary magnetic field according to the OMNI data and THEMIS observations at the subsolar point. Several of the KHI vortices were associated with reconnection indicated by the Walén relation, the presence of deHoffman-Teller frames, field-aligned ion beams observed together with bipolar fluctuations in the normal magnetic field component, and crescent ion distributions. Global magnetohydrodynamic simulation of the event also resulted in KHWs at the magnetopause. The observed KHWs associated with reconnection coincided with recorded ULF waves at the ground whose properties suggest that they were driven by those waves. Such properties were the location of Cluster’s magnetic foot point, the Pc4 frequency, and the solar wind conditions.

Relativistic Electron Model in the Outer Radiation Belt Using a Neural Network Approach

AbstractWe present a machine‐learning‐based model of relativistic electron fluxes >1.8 MeV using a neural network approach in the Earth's outer radiation belt. The Outer RadIation belt Electron Neural net model for Relativistic electrons (ORIENT‐R) uses only solar wind conditions and geomagnetic indices as input. For the first time, we show that the state of the outer radiation belt can be determined using only solar wind conditions and geomagnetic indices, without any initial and boundary conditions. The most important features for determining outer radiation belt dynamics are found to be AL, solar wind flow speed and density, and SYM‐H indices. ORIENT‐R reproduces out‐of‐sample relativistic electron fluxes with a correlation coefficient of 0.95 and an uncertainty factor of ∼2. ORIENT‐R reproduces radiation belt dynamics during an out‐of‐sample geomagnetic storm with good agreement to the observations. In addition, ORIENT‐R was run for a completely out‐of‐sample period between March 2018 and October 2019 when the AL index ended and was replaced with the predicted AL index (lasp.colorado.edu/home/personnel/xinlin.li). It reproduces electron fluxes with a correlation coefficient of 0.92 and an out‐of‐sample uncertainty factor of ∼3. Furthermore, ORIENT‐R captured the trend in the electron fluxes from low‐earth‐orbit (LEO) SAMPEX, which is a completely out‐of‐sample data set both temporally and spatially. In sum, the ORIENT‐R model can reproduce transport, acceleration, decay, and dropouts of the outer radiation belt anywhere from short timescales (i.e., geomagnetic storms) and very long timescales (i.e., solar cycle) variations.

Energetic Neutral Atom Distribution on the Lunar Surface and Its Relationship with Solar Wind Conditions

Abstract The Advanced Small Analyzer for Neutrals (ASAN) on board the Chang’E-4 Yutu-2 rover first detected energetic neutral atoms (ENAs) originating from the lunar surface at various lunar local times on the lunar farside. In this work, we examine the ENA energy spectra, obtained in the first 23 lunar days from 2019 January 11 to 2020 October 12, and find a higher ENA differential flux on the lunar dawnside than on the duskside. Combined with Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) data, we analyze the correlation between the ENA differential flux and solar wind parameters, such as flux, density, dynamic pressure, and velocity, for each ASAN energy channel on the dawnside and duskside. The results show that ENA differential flux is positively correlated with solar wind flux, density, and dynamic pressure and relatively lower on the duskside than on the dawnside. To determine the relationship between solar wind energy and ENA energy, we analyze the correlation between solar wind energy and ENA cutoff energy and temperature on the dawnside and duskside. The results show that the ENA cutoff energy and temperature are lower on the duskside than on the dawnside at the same solar wind energy. The difference between the ENA–solar wind observation on the dawnside and duskside is possibly caused by solar wind deflection and deceleration on the duskside, which can be attributed to the interaction between solar wind and the lunar magnetic anomalies located nearby in the northwestern direction of the Chang’E-4 landing site.

Effects of the Solar Wind Dynamic Pressure on the Martian Topside Ion Distribution: Implications on the Variability of Bulk Ion Outflow

Abstract With the aid of the ion densities measured by the Neutral Gas and Ion Mass Spectrometer and the solar wind dynamic pressures measured by the Solar Wind Ion Analyzer on board the Mars Atmosphere and Volatile EvolutioN, we investigate the modulation of a sequence of ion species in the Martian topside ionosphere by the upstream solar wind condition. Almost all ion species, except for CO 2 + and OCOH+, are very sensitive to the variation of the solar wind condition, and their densities decrease with increasing solar wind dynamic pressure. The response of the topside ion distribution to the variation of the solar wind condition is also found to be remarkably related to the magnetic field orientation, in that the solar wind modulation occurs mainly over regions with near-horizontal field lines. These observations imply substantially enhanced outflow velocities for all ion species under high solar wind dynamic pressures when the ambient magnetic fields are near-horizontal.