For cold and heavy magnetospheric ion populations that reach the dayside magnetopause, how those populations evolve across magnetopause separatrices into the reconnection exhaust, and how the populations may affect or be affected by reconnection, are still not well understood. Observations from the Magnetospheric Multiscale (MMS) mission from January 2019 are analyzed for a series of magnetopause crossings during a time period with a “string-of-pearls” configuration of the MMS constellation. With inter-spacecraft separations of ~100-300 km, this configuration allows for simultaneous measurements of the cold ion populations in different regions of the magnetopause boundary and current layers. For several magnetopause crossings on 2019-01-25, while magnetospheric heavy ions (He+ and O+) are not observable, a significant amount of cold (temperatures of ~1’s-10 eV) magnetospheric H+ is present in the outer magnetosphere. This cold H+ population is accelerated by the E×B drift near the magnetopause, but remains as a cold beam (temperatures of 10’s eV) well into the boundary layers and reconnection exhaust. While wave modes are present that could potentially contribute to ion heating, temperature changes are small and occur primarily at the edge of the boundary layer, and so more likely related to the initial acceleration by the normal electric field than wave-particle interactions. The lack of heating for the magnetopause crossings on 2019-01-25 differs from that observed in previous work where MMS was farther away from the X-line, pointing to the highly spatially structured nature of reconnection sites along the separatrices and the importance of the relative density of the cold ion population reaching the magnetopause.

Magnetospheric Multiscale (MMS) observations have revealed that reconnection occurs along the flanks of Earth's magnetopause, near and past the terminator when the interplanetary magnetic field is southward. However, the question remains how common this reconnection is. Previous research has indicated that reconnection may be suppressed in the far-flank regions due to the high velocity shear between the plasma in the magnetosheath and the plasma in the magnetosphere. This study investigates far-flank reconnection by looking for evidence of reconnection in crossings of the far-flank magnetopause. Forty-one magnetopause crossing events were found that satisfied certain criteria and 35 of these events showed identifiable signs of reconnection. Based on these findings, reconnection is not suppressed along the magnetospheric flanks due to the velocity shear during southward IMF.

We present a case study of large amplitude, compressional Pc5 waves observed by GOES 13 and 15 at geosynchronous orbit near noon, driven by two consecutive solar wind dynamic pressure spikes on September 26th, 2011.We present clear evidence that suggests that the first pressure spike gave rise to enhanced pressure anisotropy to drive the drift-mirror mode, whilst the second pressure, associated with a large and rapid southward turning of the Interplanetary Magnetic Field (IMF), spike allowed this anisotropy to increase in spatial extent. Magnetopause oscillations at a similar frequency were observed by the THEMIS spacecraft in the post-noon magnetosphere, however without clear signatures of compressional waves.Additionally, we analysed the ground response near the GOES foot-points, located close to the Churchill line of magnetometer stations. We observed an increase in wave power following each spike, although with different polarisations.Finally, following the second pressure spike’s arrival at the magnetosphere, we found evidence of Field line Resonances (FLR) in the Northward component of the magnetic field, which suggests coupling with the compressional waves observed at GOES.

Recent multi-point measurements, in particular from the Magnetospheric Multiscale (MMS) spacecraft, have advanced the understanding of micro-scale aspects of magnetic reconnection. In addition, the MMS mission, as part of the Heliospheric System Observatory, combined with recent advances in global magnetospheric modeling, have furthered the understanding of meso- and global-scale structure and consequences of reconnection. Magnetic reconnection at the dayside magnetopause and in the magnetotail are the drivers of the global Dungey cycle, a classical picture of global magnetospheric circulation. Some recent advances in the global structure and consequences of reconnection that are addressed here include a detailed understanding of the location and steadiness of reconnection at the dayside magnetopause, the importance of multiple plasma sources in the global circulation, and reconnection consequences in the magnetotail. These advances notwithstanding, there are important questions about global reconnection that remain. These questions focus on how multiple reconnection and reconnection variability fit into and complicate the Dungey Cycle picture of global magnetospheric circulation.

There is ample evidence for magnetic reconnection in the solar system, but it is a nontrivial task to visualize, to determine the proper approaches and frames to study, and in turn to elucidate the physical processes at work in reconnection regions from in-situ measurements of plasma particles and electromagnetic fields. Here an overview is given of a variety of single- and multi-spacecraft data analysis techniques that are key to revealing the context of in-situ observations of magnetic reconnection in space and for detecting and analyzing the diffusion regions where ions and/or electrons are demagnetized. We focus on recent advances in the era of the Magnetospheric Multiscale mission, which has made electron-scale, multi-point measurements of magnetic reconnection in and around Earth's magnetosphere.

Kelvin Helmholtz Instabilities (KHI) result from interactions between the shocked solar wind and the Earth's magnetosphere. These are formed due to the velocity shear between the plasma in the magnetosphere and magnetosheath. The role of KHI in bringing in the shocked solar wind into the terrestrial magnetosphere has been studied extensively using MHD, Hall-MHD, hybrid and PIC simulations. Such simulations oftentimes make simplifying assumptions of the boundary layer in the magnetopause. To experimentally study the effects of KHI on the boundary layer and its effectiveness in bringing in solar wind, we analyze 43 KHI events. All these events have quasi-constant IMF orientation during its interval, thereby mitigating the effects of variation of IMF in the ongoing transient magnetopause process. In this statistical study of KHIs, we demonstrate that there is a preexisting boundary layer before KHIs begin to develop. As KHI develops to its non-linear state, the ions in the magnetosphere, magnetopause, and magnetosheath are mixed, which is demonstrated using the alpha-to-proton density ratio. As a result of this mixing, the well-defined preexisting boundary layer is replaced by a much more uniformly mixed boundary layer. Kelvin-Helmholtz Instabilities (KHI) occur due to differences in plasma velocity on either side of the magnetopause, the boundary between Earth's magnetic field and the solar wind. These instabilities can allow solar wind to enter Earth's magnetosphere. To understand KHI better, we studied 43 events where the interplanetary magnetic field (IMF) was steady, reducing its influence on the results. Our findings indicate that a boundary layer exists before KHI begins. As KHI develops, it mixes particles from Earth's magnetosphere with those from the solar wind, creating a more uniform boundary layer. We can observe this mixing by tracking the ratio of the number density of doubly charged Helium ions particles and protons. 43 K Helmholtz Instability (KHI) events have been analyzed under quasi-constant IMF conditions The non-linear KHI events show evidence for plasma mixing, seen as a smooth transition in the alpha-to-proton ratio in the boundary layer We propose using Alpha-To-Proton density ratio as a diagnostic tool to identify the stage of KHI development

This article presents a statistical analysis of overlapping double ion-energy dispersion events in the northern cusp (“double dispersion”). Double dispersion in either cusp is a phenomenon associated with multiple reconnections occurring on the dayside magnetosphere as a result of its constant interaction with the variable solar wind. Using observations from a low Earth orbiting (LEO) Defense Meteorological Satellite Program (DMSP) satellite, we analyze 138 dayside events selected by the automatic algorithm extended from our previous work. We conducted a correlation study between the number of detected double dispersion events and 1) the month of the year to analyze the seasonal response of the cusp, and 2) solar wind interplanetary magnetic field (IMF) components and clock/cone angles to investigate its relationship with magnetic reconnection. We found that dispersion events occur more frequently during the northern summer months (i.e., when the dipole is tilted Sunward) and when the By component of IMF is positive. In addition, we provide a machine-readable list of the events and the code used to automatically detect the events.

Two sounding rockets, designated TRICE-2, were launched on 8 December 2018 into the northern cusp region. The two rockets were designated the high- and low-flyers, respectively, and launched 2 min apart to investigate cusp structures, specifically their spatial or temporal nature. 2 hr prior to the cusp encounter by the TRICE-2 rockets, the MMS satellites, located in the magnetopause boundary layer, observed switching ion beams under very similar IMF conditions as later observed by TRICE-2. The observed ion beam switch in the boundary layer defined the location of the primary dayside X-line. Both, TRICE-2 and MMS, also observed the signatures of multiple X-lines at the magnetopause, overlapping ion-energy dispersions in the cusp and counterstreaming ion beams in the magnetopause boundary layer, respectively. In addition to the TRICE-2 cusp observations, ionospheric convection patterns from the SuperDARN radar are used to explain the vastly different cusp ion signatures observed by the TRICE-2 rockets. While the high-flyer rocket progressed north through the center of the cusp, the low-flyer rocket drifted off to the east and crossed into the dusk convection cell, traveling perpendicular to the ionospheric convection direction before reaching the poleward oriented section of the convection cell also observed by the high-flyer counterpart. TRICE-2 cusp ion dispersions are explained using the different magnetic foot points of the rockets through the ionospheric convection cells TRICE-2 cusp crossing occurred 2 hr after an MMS magnetopause crossing during similar IMF conditions Overlapping cusp ion energy dispersions result from multiple magnetopause reconnection locations in agreement with MMS observations

AbstractTwo sounding rockets, designated TRICE‐2, were launched on 8 December 2018 into the northern cusp region. The two rockets were designated the high‐ and low‐flyers, respectively, and launched 2 min apart to investigate cusp structures, specifically their spatial or temporal nature. 2 hr prior to the cusp encounter by the TRICE‐2 rockets, the MMS satellites, located in the magnetopause boundary layer, observed switching ion beams under very similar IMF conditions as later observed by TRICE‐2. The observed ion beam switch in the boundary layer defined the location of the primary dayside X‐line. Both, TRICE‐2 and MMS, also observed the signatures of multiple X‐lines at the magnetopause, overlapping ion‐energy dispersions in the cusp and counterstreaming ion beams in the magnetopause boundary layer, respectively. In addition to the TRICE‐2 cusp observations, ionospheric convection patterns from the SuperDARN radar are used to explain the vastly different cusp ion signatures observed by the TRICE‐2 rockets. While the high‐flyer rocket progressed north through the center of the cusp, the low‐flyer rocket drifted off to the east and crossed into the dusk convection cell, traveling perpendicular to the ionospheric convection direction before reaching the poleward oriented section of the convection cell also observed by the high‐flyer counterpart.

Cometary comae are a mixture of gas and ice-covered dust. Processing on the surface and in the coma change the composition of ice on dust grains relative to that of the nucleus. As the ice on dust grains sublimates, the local coma composition changes. Rosetta observations of 67P/Churyumov-Gerasimenko previously reported one of the highest D/H values for a comet. However, reanalysis of more than 4000 water isotope measurements over the full mission shows that dust markedly increases local D/H. The isotope ratio measured at a distance from the nucleus where the gas is well mixed is close to terrestrial, like that of other Jupiter family comets. This lower D/H has implications for understanding comet formation and the role of comets in delivering water to Earth.

Observations by Magnetospheric Multiscale have demonstrated that magnetic reconnection occurs at Earth's bow shock, typically at thin current sheets arising from plasma instabilities and turbulence in the shock transition region. Observational surveys of both the shock transition and the magnetosheath downstream suggest that the number of current sheets in these regions may not be strongly dependent on the shock Mach number MA or the angle between the upstream magnetic field and shock normal (θBn). This result is somewhat surprising given that quasi-parallel and high Mach number shocks tend to have a more disordered and non-stationary structure. In order to investigate how shock reconnection manifests across different parameters, we perform a series of hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a range of Mach numbers and orientations. Given that hybrid simulations cannot resolve electron-scale current sheets and reconnection, these simulations isolate an ion-scale mechanism for shock reconnection driven by an ion–ion beam instability in the foot. We find that this mechanism is strongly constrained to quasi-parallel shocks across all simulated Mach numbers. By quantifying reconnection using the area occupied by plasma on closed magnetic field lines, we find the number of reconnecting structures and closed field area increase with MA and decrease with θBn in the upstream and ramp regions. Downstream of the shock, however, we find a similar result to observational surveys: within the subset of quasi-parallel shocks, the decay rate of the closed field area (and hence thin current sheets) is not strongly dependent on upstream shock parameters.

Abstract Combined in situ ion measurements and remote sensing of energetic neutral atoms are used to determine the geocoronal Hydrogen density at large (∼10 RE) distances from the Earth. This method for determining the geocoronal density requires global magnetospheric modeling. Observations in the Earth's subsolar magnetosheath from the Magnetospheric Multiscale mission are used to determine the accuracy of using global models to predict the geocoronal density. On average, gas dynamic and magnetohydrodynamic (MHD) models and observations are in reasonable agreement, with differences <25%. In addition, the MHD model subsolar magnetopause is about 0.5 RE sunward of the observed location. However, variations around averages are large (up to a factor of 2), indicating that global models introduce relatively large uncertainties in geocoronal density estimates. Finally, the critical ion flux in the Interstellar Boundary Explorer IBEX‐Hi energy range is often minimally affected by fluctuations of a factor of 2 in the density.

The launch of the Twin Rockets to Investigate Cusp Electrodynamics-2 (TRICE-2) took place on the morning of 08 December 2018. The two rockets of this campaign each sampled the low-altitude Northern Hemisphere cusp region at approximately the apex of each rocket's trajectory (1,042 and 757 km for the High flyer and Low flyer, respectively). Ion and electron electrostatic analyzers (ESAs) on board each rocket measured in situ particle populations throughout the flights. Energy-flux spectrograms of ions and electrons from each ESA clearly show the passage of each rocket through the cusp region. This work examines the locations of these entrance/exit points in relation to cusp models as provided in and compiled from the published literature, along with a discussion of model variables that have been optimized to best fit the cusp region boundary sampling locations by TRICE-2. The results of this study set the stage for understanding the upcoming NASA Small Explorer Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites orbital plane intersections with (i.e., "trajectory cuts " through) the cusp region.

We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts.

Observations of Earth’s bow shock and magnetosheath have shown that magnetic reconnection occurs within these regions at thin current sheets, typically arising from turbulence and plasma instabilities in the shock transition layer. Broad observational surveys of these regions have shown that, somewhat surprisingly, the prevalence of reconnecting current structures may not be strongly dependent on the shock Mach number or the angle between the upstream magnetic field and shock normal (θBn), despite quasi-parallel shocks typically exhibiting more disordered and non-stationary structure. To investigate how shock reconnection manifests across different parameters, we perform a series of two- and three-dimensional hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a broad range of Mach numbers and orientations. These simulations isolate ion-scale mechanisms for reconnection in the shock, principally those driven by ion-ion beam instabilities in the foot and foreshock. For 2D simulations, we show that reconnection via these ion-driven mechanisms is strongly constrained to quasi-parallel shocks. However, downstream of quasi-parallel shocks, we find that the decay rate of closed-field regions, and hence thin current sheets, is not strongly dependent on upstream shock parameters. We also explore the differences that arise in shock structure, the generation of reconnecting current structures, and their decay rates for three-dimensional simulations.

High-resolution global magnetohydrodynamics (MHD) simulations include both meso- and global-scale processes occurring at the magnetopause, which interact to determine the time-dependent orientation of the day-side x-line (DXL). This study demonstrates that the global orientation of the DXL in GAMERA global MHD simulations varies on a time scale of minutes during steady southward interplanetary magnetic field conditions. This behavior manifests in observational data when reconnection outflows indicate that the direction to the x-line is opposite to the prediction from a steady-state model of the reconnection location. Because steady-state models of the DXL do not capture dynamics that are independent of solar wind variations, particularly surface waves and flux transfer events, they represent a time-averaged state of the system.

We demonstrate that any plasmaspheric/cold ions accelerated in the vicinity of the magnetopause boundary, can proxy the local magnetopause motion over many minutes. The timeseries of this motion capture local structures such as waves on the boundary. We determine cold ion velocities normal to full magnetopause boundary crossings for three events with varying distances to the predicted reconnection X‐line, thus, providing a proof‐of‐concept study demonstrating the potential for using cold ion velocities to track magnetopause motion over a long period of time. Obtaining the time history of the (local) motion of the magnetopause relative to the spacecraft is determined by integrating the bulk (<100 eV for H + ) ion velocities normal to the boundary. Timeseries of these tracked cold ion accelerations may be used to investigate boundary layer thicknesses, potential wave structures on the magnetopause, and their evolution beyond the boundary crossing. This method generally tracks magnetopause motion out to distances of ∼1–2R E away from the spacecraft during quasi‐steady space weather conditions.

&lt;p&gt;At the Earth&amp;#8217;s bow shock, most of the solar wind&amp;#8217;s kinetic energy is partitioned into wave energy, particle acceleration, and heating. Very recent publications provide strong evidence that current sheets at the shock ramp region and downstream may participate in the thermalization of the solar wind plasma. Their occurrence varies from single to multiple current sheets as well as filamentary structures.&lt;/p&gt; &lt;p&gt;We studied multiple bow shock crossings by the MMS spacecraft with its sophisticated instrumentation, characterizing and quantifying the occurrence of filamentary structures, current sheets, the associated magnetic field wave turbulence, and ion acceleration downstream of the shock. At some traversals the shock location is changing due to variable upstream solar wind conditions. During increasing Mach number/dynamic pressure we observe higher wave activity and broader distribution functions with suprathermal tails. Much less suprathermal ions downstream of the shock are observed at shock crossings during decreasing upstream Mach numbers. These MMS observation indicate that current sheets and field gradients are associated with ion acceleration. The associated turbulence is likely a mediator for energy partition. With increasing Mach numbers, the bow shock moves away from the Sun and compresses the magnetosheath that would favour reconnection of currents sheets, stronger electric field gradients and thus ion acceleration. At periods of decreasing upstream Mach numbers, the bow shock moves towards the Sun, becomes blunter, and the sheath region relaxes, making reconnecting current sheets less likely and smoothens field gradients resulting in less acceleration. Other possible acceleration mechanisms will also be discussed in the context of this presentation.&lt;/p&gt;

We analyze a magnetotail reconnection onset event on 3 July 2017 that was observed under otherwise quiescent magnetospheric conditions by a fortuitous conjunction of six space and ground-based observatories. The study investigates the large-scale coupling of the solar wind - magnetosphere system that precipitated the onset of the magnetotail reconnection, focusing on the processes that thinned and stretched the cross-tail current layer in the absence of significant flux loading during a two-hour-long preconditioning phase. It is demonstrated with data in the (1) upstream solar wind, (2) at the low-latitude magnetopause, (3) in the high-latitude polar cap, and (4) in the magnetotail that the typical picture of solar wind-driven current sheet thinning via flux loading does not appear relevant for this particular event. We find that the current sheet thinning was, instead, initiated by a transient solar wind pressure pulse and that the current sheet thinning continued even as the magnetotail and solar wind pressures decreased. We suggest that field line curvature induced scattering (observed by Magnetospheric Multiscale (MMS)) and precipitation (observed by Defense Meteorological Satellite Program (DMSP)) of high-energy thermal protons may have evacuated plasma sheet thermal energy, which may require a thinning of the plasma sheet to preserve pressure equilibrium with the solar wind.