Upstream Interplanetary Magnetic Field Research Articles

Estimating Interplanetary Magnetic Field Conditions at Mercury's Orbit From MESSENGER Magnetosheath Observations Using a Feedforward Neural Network

AbstractMercury's small magnetosphere is embedded in the dynamic and intense solar wind environment characteristic of the inner heliosphere. Both the magnitude and orientation of the interplanetary magnetic field (IMF) significantly influence the solar wind‐magnetospheric interaction at Mercury, driving phenomena such as magnetic reconnection. The MErcury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft provided in‐situ magnetic field measurements of the solar wind, the magnetosheath, and the magnetosphere along each orbit. However, it is a challenge to directly assess the IMF's impact on Mercury's plasma environment due to the temporal separation between observations within the solar wind and the magnetosphere, especially in the absence of an upstream monitor. Here, we present a feedforward neural network (FNN) trained on a subset of magnetosheath observations to estimate the strength and orientation of the IMF upstream of the bow shock. Utilizing magnetosheath magnetic field, cylindrical spatial coordinates, and heliocentric distance measurements, the FNN predicts upstream IMF conditions with an score of 0.70 and mean averaged error of 5.3 nT, thereby greatly decreasing the temporal separation between IMF estimates and magnetospheric measurements throughout the MESSENGER mission. This approach yields IMF estimates for all magnetosheath data measured by MESSENGER, providing a useful tool for future investigations of the IMF impact on Mercury's magnetosphere. This method will be integrable with the dual‐spacecraft BepiColombo magnetosheath measurements, providing useful estimates of upstream IMF conditions particularly during the extended periods in which neither spacecraft sample the solar wind. Our results demonstrate the utility of machine learning techniques on advancing space science research.

Asymmetrical Solar Wind Deflection in the Martian Magnetosheath

AbstractAs incident solar wind encounters the martian upper atmosphere, it undergoes deflection particularly in the magnetosheath. However, the plasma flow exhibits asymmetrical distribution features within this transition region, which is investigated by employing a three‐dimensional Hall magnetohydrodynamic (MHD) model from an energy transfer perspective in this study. Simulation results reveal that solar wind protons transfer momentum to ionospheric heavy ions through motional electric field in the hemisphere where the motional electric field points outward from the planet. In the opposite hemisphere, solar wind flow tends to be effectively accelerated by ambipolar and Hall electric fields. The distinct dynamics of solar wind protons in both hemispheres result in the asymmetrical deflection. Furthermore, the extent of asymmetry grows as the cross‐flow component of the upstream interplanetary magnetic field increases, but diminishes as the density of the solar wind proton increases, contingent upon the energy effectively acquired from ambipolar and Hall electric fields.

Determining the Influence of the IMF and Planetary Magnetic Field Models on Mercury's Magnetosphere Along Spacecraft Trajectories of MESSENGER, BepiColombo and MPO

AbstractMercury's planetary magnetic field models (PMFMs) agree on a majorly dipolar field structure with a northward shift of the magnetic equator. However, due to the northerly biased orbit coverage of past spacecraft missions and different data analyzing methods, the available PMFMs differ in the determined multipole magnitudes for the dipole, quadrupole and octupole moments. While the PMFMs agree well with northern observations, we find that the predicted magnetic field values differ in the unexplored equatorial and southern regions. In the forward modeling approach of this study, we apply three different PMFM representatives, differing in their values of the dipole, quadrupole and octupole moments, and model the resulting solar wind interaction via a global hybrid model with sets of the 4 most common interplanetary magnetic field (IMF) directions under otherwise average solar wind conditions. Extracting our modeled fields along the flybys of MESSENGER and BepiColombo, as well as along four representative orbits of the Mercury Planetary Orbiter (MPO), allows us to estimate local field variations due to IMF and PMFM influence. Our modeled magnetic field components separate significantly by up to 60 nT between the PMFMs in low‐altitude southern regions, leading to displacements of magnetopause, cusps and current sheet locations. We find that MESSENGER flyby observations agree best with modeled field ranges resulting from a quadrupolar PMFM and that certain nightside regions are useful to estimate upstream IMF polarity. We also demonstrate how comparing BepiColombo swingby and MPO orbit phase observations with modeled results can help determine the correct PMFM for Mercury.

Mercury’s Bow Shock and Magnetopause Variations According to MESSENGER Data

Using data from the MESSENGER spacecraft magnetometer that describes the magnetopause and the bow shock crossing points of the Mercury’s magnetosphere, we have calculated the parameters of the paraboloids of revolution approximating the obtained points. For each spacecraft orbit, the subsolar magnetopause and bow shock standoff distances were obtained, based on the paraboloid parameters for each crossing point. The dependences of the magnetopause and bow shock subsolar standoff distances on the Mercury’s position relative to the Sun have been obtained. These profiles agree with decreases of the solar wind plasma dynamic pressure and the interplanetary magnetic field strength with heliocentric distance. The variations of the interplanetary and magnetosheath magnetic field were investigated. The average subsolar magnetosheath thickness and the value of the magnetic field jump at the bow shock during the transition from the upstream interplanetary magnetic field region to the magnetosheath were obtained.

Evidence for Magnetic Reconnection as the Precursor to Discrete Aurora at Mars

AbstractDiscrete aurora at Mars are characterized as localized, short‐lasting ultraviolet emissions on the nightside. They are caused by the precipitation of accelerated electron along open magnetic field lines and their collision with the Martian atmosphere. Discrete auroral emissions detected by the Imaging Ultraviolet Spectrograph (IUVS) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft occur most frequently over the strongest crustal magnetic anomalies at Mars and under particular local time and upstream interplanetary magnetic field (IMF) conditions. These trends suggest that the onset of discrete aurora is controlled by the processes that govern the interaction between the draped IMF and the crustal magnetic anomalies. Here, we analyze MAVEN magnetometer measurements over regions of strong crustal fields during 49 discrete auroral events observed by IUVS. Our results indicate that the draped IMF orientations during each discrete auroral event show a clear tendency to be anti‐parallel to the underlying crustal anomaly magnetic fields near the location of the emission onset. This suggests that magnetic reconnection, a process that favors anti‐parallel magnetic field regimes, between the crustal fields and the draped IMF plays a role in discrete aurora formation. Of the 49 discrete auroral events analyzed, 42 (86%) occurred under draped IMF conditions that are susceptible to reconnection with the underlying crustal anomalies. This investigation produces strong evidence linking discrete aurora to magnetic reconnection at Mars and provides insights into other trends in discrete auroral activation such as local time and upstream IMF conditions.

Magnetic Field Draping in Induced Magnetospheres: Evidence From the MAVEN Mission to Mars

AbstractThe Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has been orbiting Mars since 2014 and now has over 10,000 orbits which we use to characterize Mars' dynamic space environment. Through global field line tracing with MAVEN magnetic field data we find an altitude dependent draping morphology that differs from expectations of induced magnetospheres in the vertical ( Mars Sun‐state, MSO) direction. We quantify this difference from the classical picture of induced magnetospheres with a Bayesian multiple linear regression model to predict the draped field as a function of the upstream interplanetary magnetic field (IMF), remanent crustal fields, and a previously underestimated induced effect. From our model we conclude that unexpected twists in high altitude dayside draping (>800 km) are a result of the IMF component in the MSO direction. We propose that this is a natural outcome of current theories of induced magnetospheres but has been underestimated due to approximations of the IMF as solely directed. We additionally estimate that distortions in low altitude (<800 km) dayside draping along are directly related to remanent crustal fields. We show dayside draping traces down tail and previously reported inner magnetotail twists are likely caused by the crustal field of Mars, while the outer tail morphology is governed by an induced response to the IMF direction. We conclude with an updated understanding of induced magnetospheres which details dayside draping for multiple directions of the incoming IMF and discuss the repercussions of this draping for magnetotail morphology.

The magnetic field clock angle departure in the Venusian magnetosheath and its response to IMF rotation

We investigate the characteristics of interplanetary magnetic field (IMF) draping in the Venusian magnetosheath using both Venus Express (VEX) observations and magnetohydrodynamics simulations. The distributions of magnetosheath field clock angle illustrate the nearly symmetric morphology of draped magnetic field with respect to the solar wind electric field, and the departure of the IMF clock angle is larger at closer distances. Based on VEX data, the sheath field clock angle departures are found to be <45 degrees for 90% of the instances under steady IMF and this parameter can respond almost immediately to the unsteady IMF. We suggest the magnetosheath field just slips around the planet without significant pileup or bending. Our time-dependent simulations indicate that the response time of sheath field to IMF variation is not more than 1 min and it depends on the involved regions of magnetosheath: the timescale in the inner part of magnetosheath adjacent to the induced magnetosphere is longer than that in the outer part. We find this timescale is controlled by the convection velocity in the magnetosheath, emphasizing the magnetohydrodynamic characteristics of the behavior of the sheath field. The finite magnetosheath field clock angle departure and its quick response to IMF variation suggest that the magnetic field clock angle measured within the Venusian magnetosheath can be used as a reasonable proxy for the upstream IMF clock angle.

Energetics of Shock-triggered Supersubstorms (SML < −2500 nT)

The solar wind energy input and dissipation in the magnetospheric–ionospheric systems of 17 supersubstorms (SSSs: SML < −2500 nT) triggered by interplanetary shocks during solar cycles 23 and 24 are studied in detail. The SSS events had durations ranging from ∼42 minutes to ∼6 hr, and SML intensities ranging from −2522 nT to −4143 nT. Shock compression greatly strengthens the upstream interplanetary magnetic field southward component (B s), and thus, through magnetic reconnection at the Earth’s dayside magnetopause, greatly enhances the solar wind energy input into the magnetosphere and ionosphere during the SSS events studied. The additional solar wind magnetic reconnection energy input supplements the ∼1.5 hr precursor (growth-phase) energy input and both supply the necessary energy for the high-intensity, long-duration SSS events. Some of the solar wind energy is immediately deposited in the magnetosphere/ionosphere system, and some is stored in the magnetosphere/magnetotail system. During the SSS events, the major part of the solar wind input energy is dissipated into Joule heating (∼30%), with substantially less energy dissipation in auroral precipitation (∼3%) and ring current energy (∼2%). The remainder of the solar wind energy input is probably lost down the magnetotail. It is found that during the SSS events, the dayside Joule heating is comparable to that of the nightside Joule heating, giving a picture of the global energy dissipation in the magnetospheric/ionospheric system, not simply a nightside-sector substorm effect. Several cases are shown where an SSS is the only substorm that occurs during a magnetic storm, essentially equating the two phenomena for these cases.

A MAVEN Case Study of Radial IMF at Mars: Impacts on the Dayside Ionosphere

AbstractThe solar wind interaction with Mars controls the transfer of energy and momentum from the solar wind into the magnetosphere, ionosphere and atmosphere, driving structure, and dynamics within each. This interaction is highly dependent on the upstream Interplanetary Magnetic Field (IMF) orientation. We use in‐situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to identify several prominent features that arise when the IMF is aligned approximately parallel or antiparallel to solar wind flow (conditions known as “radial IMF”). In particular, solar wind protons and alphas are observed to directly penetrate down to periapsis altitudes, while the magnetic barrier forms deep within the dayside ionosphere. The MAVEN observations are consistent with either an ionopause‐like boundary or diamagnetic cavity forming beneath the barrier, as a consequence of the dense cold ionosphere and the absence of significant crustal magnetic fields at this periapsis location. The planetary ions above the magnetic barrier are exposed to solar wind flow and subsequent mass‐loading. The (convective electric field or “ion pickup”) force is weak and highly variable during radial IMF. While wave particle interactions and subsequent wave heating contribute to incorporating the heavy planetary ions into the solar wind flow, the solar wind momentum is not fully deflected around the obstacle and is delivered into the collisional atmosphere. Significant ion heating is observed deep within the dayside ionosphere, and observed ionospheric density and temperature profiles demonstrate that these ion energization mechanisms drive significant erosion and likely escape to space.

Global Three‐Dimensional Draping of Magnetic Field Lines in Earth’s Magnetosheath From In‐Situ Spacecraft Measurements

AbstractMagnetic field draping occurs when the magnetic field lines frozen in a plasma flow wrap around a body or plasma environment. The draping of the interplanetary magnetic field (IMF) around the Earth’s magnetosphere has been confirmed in the early days of space exploration. However, its global and three‐dimensional structure is known from modeling only, mostly numerical. Here, this structure in the dayside of the Earth’s magnetosheath is determined as a function of the upstream IMF orientation purely from in‐situ spacecraft observations. We show the draping structure can be organized in three regimes depending on how radial the upstream IMF is. Quantitative analysis demonstrates how the draping pattern results from the magnetic field being frozen in the magnetosheath flow, deflected around the magnetopause. The role of the flow is emphasized by a comparison of the draping structure to that predicted to a magnetostatic draping.

The dependence of Martian ion escape on solar EUV irradiance as observed by MAVEN

We analyze the planetary ion, solar wind, interplanetary magnetic field (IMF), and solar extreme ultraviolet (EUV) irradiance data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to quantify the variation in ion escape with solar EUV irradiance. With relatively strict constraints on the upstream solar wind and IMF conditions, we divide the planetary ion data into 4 subsets with different solar EUV conditions to estimate ion escape rates. The results show that the total ion escape rate may increase nonlinearly with solar EUV irradiance and eventually approach an asymptotic limit as EUV increases. Further analysis on different ion species, escape channels, and energy ranges show that the dependence on solar EUV varies between these different ion populations. We also find that the spatial distributions of ion density and velocity vary between high and low solar EUV conditions, which is likely caused by the variations of electromagnetic field distributions. These results suggest that as solar EUV controls the ion production and affects the ion distributions near Mars, it will in turn influence the electromagnetic field distributions and thus affect the acceleration of escaping ions. This feedback mechanism may limit the total ion escape rate as solar EUV increases under constrained solar wind and IMF conditions.

On the Occurrence of Magnetic Reconnection Along the Terrestrial Magnetopause, Using Magnetospheric Multiscale (MMS) Observations in Proximity to the Reconnection Site

AbstractObservations at the reconnection site from the Magnetospheric Multiscale mission are used to examine the occurrence of magnetic reconnection along the magnetopause in relation to both the upstream interplanetary magnetic field (IMF) orientation and local magnetosheath variations. While there is a statistically larger number of reconnection intervals observed downstream of the quasi‐parallel bow shock than is accounted for based on the histogram of overall IMF orientations, it is surmised that increased fluctuations downstream of the quasi‐parallel bow shock cause greater large‐scale motion of the magnetopause surface; but do not preferentially enhance the occurrence of steady magnetic reconnection. An examination of the locations of observed reconnection sites relative to model predictions also suggest that enhanced magnetosheath fluctuations do not result in steady reconnection occurring at random locations at the magnetopause.

The Dependence of the Venusian Induced Magnetosphere on the Interplanetary Magnetic Field: An MHD Study

The influences of the interplanetary magnetic field (IMF) on the induced magnetosphere of Venus are investigated using a global multispecies magnetohydrodynamics (MHD) model. The simulation results show that the induced magnetosphere is controlled by the IMF components perpendicular to the solar wind velocity (B Y and B Z in the Venus Solar Orbital coordinate), rather than the IMF magnitude (∣B∣). With the increase of , the induced magnetosphere becomes stronger in field strength and thicker in spatial scale, and the bow shock locates farther from the planet. The parallel IMF component (B X ) has relatively small impacts on the magnetic barrier and the magnetotail, regardless of the various IMF magnitudes and orientations caused by different B X . The responses of the Venusian induced magnetosphere to the change of upstream IMF are also studied. The time-dependent MHD calculations show that the dayside magnetosphere responds quickly with a timescale of 10 s–10 minutes, depending on the considered magnetospheric region. For comparison, the timescale required for the adjustment of magnetotail as driven by an IMF rotation is derived to be ∼10–20 minutes.

Cross‐Shock Electrostatic Potentials at Mars Inferred From MAVEN Measurements

AbstractA bow shock is generated by the interaction of the solar wind with the planetary global dipole field (e.g., Earth), or with (mainly) the planetary ionosphere (e.g., Mars). The cross‐shock potential has been well studied at Earth but not yet for Mars. We infer and approximate the peak in the frame invariant de Hoffmann‐Teller shock potential profile at Mars (ϕ) with data from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. We find that ϕ and its ratio to the solar wind ram ion energy (Eram) vary similarly against solar zenith angle (SZA, a proxy for the angle between the solar wind flow and the shock normal) and magnetic latitudes. Our results also reveal no significant dependence of the shock potential on parameters such as the angle between the upstream interplanetary magnetic field (IMF) and the shock normal and plasma beta of upstream solar wind. There is a somewhat positive correlation with the magnetosonic Mach number and the magnetic amplification ratio across the shock. We also find a solar cycle effect on the shock location, closer to the planet near the solar minimum, as expected. Lastly, similarities and differences of cross‐shock potentials at Mars and Earth are discussed. Characterizing electron energization and high‐altitude ion loss at Mars is influenced by the bow shock and thereby the work here.

Effects of the IMF Direction on Atmospheric Escape From a Mars‐like Planet Under Weak Intrinsic Magnetic Field Conditions

AbstractDirection of the upstream interplanetary magnetic field (IMF) significantly changes the magnetospheric configuration, influencing the atmospheric escape mechanism. This paper investigates effects of IMF on the ion escape mechanism from a Mars‐like planet that has a weak dipole magnetic field directing northward on the equatorial surface. The northward (parallel to the dipole at subsolar), southward (antiparallel), and Parker‐spiral IMFs under present solar wind conditions are compared based on multispecies magnetohydrodynamics simulations. In the northward IMF case, molecular ions escape from the high‐latitude lobe reconnection region, where ionospheric ions are transported upward along open field lines. Atomic oxygen ions originating either in the ionosphere or oxygen corona escape through a broader ring‐shaped region. In the southward IMF case, the escape flux of heavy ions increases significantly and has peaks around the equatorial dawn and dusk flanks. The draped IMF can penetrate into the subsolar ionosphere by erosion, and the IMF becomes mass‐loaded as it is transported through the dayside ionosphere. The mass‐loaded draped IMF is carried to the tail, contributing to ion escape. The escape channels in the northward and southward IMF cases are different from those in the Parker‐spiral IMF case. The escape rate is the lowest in the northward IMF case and comparable in the Parker‐spiral and southward IMF cases. In the northward IMF case, a weak intrinsic dipole forms a magnetosphere configuration similar to that of Earth, quenching the escape rate, while the Parker‐spiral and southward IMFs cause reconnection and erosion, promoting ion escape from the upper atmosphere.

Variability of the Solar Wind Flow Asymmetry in the Martian Magnetosheath Observed by MAVEN

AbstractWe perform the first statistical analysis of effects that different external conditions have on a solar wind (SW) flow asymmetry observed in the Martian magnetosheath due to mass loading, making use of ∼5 years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. We find that the difference between the mean magnetosheath SW velocity component along the SW convective electric field direction in regions in the positive and negative hemispheres displays a strong linear correlation with the ratio between the upstream Interplanetary Magnetic Field (IMF) cross‐flow component (By) and the SW proton density (nsw). The asymmetry is maximized (∼25–35 km s−1) for low nsw (∼1 cm−3) and large IMF By (∼2 nT). These results suggest that the SW flow asymmetry variability is due to a force arising from the differential streaming between SW protons and oxygen ions, with boundary conditions partly defined by the pristine SW properties.

Direct Observations of a Polar Cap Patch Formation Associated With Dayside Reconnection Driven Fast Flow

AbstractDayside solar‐produced concentrated F region plasma can be transported from the midlatitude region into the polar cap during geomagnetically disturbed period, creating plasma density irregularities like polar cap patches, which can cause scintillation and degrade performance of satellite communication and navigation at polar latitudes. In this paper, we observed and investigated a dynamic formation process of a polar cap patch during the 13 October 2016 intense geomagnetic storm. During the storm main phase, storm‐enhanced density (SED) was formed within an extended period of strong southward interplanetary magnetic field (IMF) Bz condition. Total electron content (TEC) map shows that a polar cap patch was segmented from the SED plume. The Sondrestrom Incoherent Scatter Radar (ISR) was right underneath the segmentation region and captured the dynamic process. It shows that the patch segmentation was related with a sudden northeastward flow enhancement reaching ~2 km/s near the dayside cusp inflow region. The flow surge was observed along with abrupt E region electron temperature increase, F region ion temperature increase, and density decrease. The upstream solar wind and IMF observations suggest that the flow enhancement was associated with dayside magnetic reconnection triggered by a sudden and short period of IMF By negative excursion. Quantitative estimation suggests that plasma density loss due to enhanced frictional heating was insufficient for the patch segmentation because the elevated F region density peaking at ~500 km made dissociative recombination inefficient. Instead, the patch was segmented from the SED by low‐density plasma transported by the fast flow channel from earlier local time.

Comparison of Observed and Modeled Magnetic Fields in the Earth's Magnetosheath

AbstractObservations of the magnetic field for two passages through the Earth's magnetosheath are used for comparison with two simple models of the magnetosheath field that describe it as a potential field. We apply the Kobel and Flückiger (1994, https://doi.org/10.1029/94JA01778) model and the Romashets and Vandas (2019, https://doi.org/10.1029/2006JA012072) model complemented with the Jelínek et al. (2012, https://doi.org/10.1029/2011JA017252) empirical determination of the bow shock and magnetopause shapes and positions. The models yield a satisfactory match to observations when the positions and shapes of the bow shock and the magnetopause are controlled by the instantaneous solar wind upstream dynamic pressure and the instantaneous upstream interplanetary magnetic field vector is taken into account.

Statistics on the Magnetosheath Properties Related to Magnetopause Magnetic Reconnection

Abstract Magnetosheath properties, particularly those related to magnetopause magnetic reconnection (MR), are investigated in this study. (1) Asymmetries are found to exist in the distributions of plasma and magnetic field parameters in the magnetosheath. These asymmetries are related to the interplanetary magnetic field (IMF) orientation, and they are produced either on the bow shock or inside the magnetosheath. Thus, one must be very cautious in directly using the upstream solar wind and IMF properties as the magnetopause MR initiation conditions, since the magnetosheath parameters are not the same everywhere. (2) A unique method is introduced to estimate how much IMF magnetic flux passes through the magnetosphere via MR on either the low-latitude or the high-latitude magnetopause. This flux mainly varies with three independent parameters: the IMF clock angle θ CL, the magnetosheath plasma β, and the solar wind sound Mach number M S . Surprisingly, the magnetic fluxes passing through the magnetosphere are comparable under the southward and northward IMF conditions. (3) The dipole tilt angle, the property from the inside of the magnetosphere, also controls the magnetosheath parameters. As the Earth’s dipole tilt angle varies, the plasma pressure ridge shifts its location but remains near the magnetic equator. The stagnation point of the magnetosheath flow on the magnetopause, however, remains at the subsolar point no matter how large the dipole tilt angle is. These behaviors may be determinative of the locations of MR and the generation of flux transfer events on the magnetopause.

Magnetic Field in the Martian Magnetosheath and the Application as an IMF Clock Angle Proxy

AbstractWe present a study of the magnetic field in the Martian magnetosheath using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and provide a proxy for the upstream interplanetary magnetic field (IMF) clock angles based on the sheath field directions. The magnetic field data from MAVEN taken in the sheath region are organized in the Mars Solar Electric field coordinate system and sorted by the IMF polarity. The sheath magnetic field distributions clearly illustrate the morphology of draped IMF around the planetary obstacle and show strong asymmetries between the quasi‐perpendicular and quasi‐parallel bow shock quadrants as well as a slight asymmetry between the +E and –E quadrants. We have also studied the effects of different drivers on the strengths and clock angles of the sheath magnetic field and find that the sheath field is strongly affected by upstream IMF and solar wind and that Martian crustal fields also have small contributions to the sheath magnetic field strength. Based on the morphology of the draped field in the sheath, we developed a simple IMF clock angle proxy as the clock angle of the orbital‐averaged sheath magnetic field, which works well under steady IMF conditions. This proxy has an error &lt;44° for 75% of all instances. The uncertainty of the proxy depends on upstream conditions and spacecraft orbit geometry. We also provide estimates of the proxy uncertainty based on sheath magnetic field variability and proxy clock angle.