One of the key questions about magnetic reconnection is to understand how energy is partitioned between ions and electrons, especially inside the EDR and in the outflow regions. This requires studying the energy transport terms corresponding to kinetic, thermal and electromagnetic energies respectively, along with the energy conversion terms. Previous studies have shown that ion energy flux dominates close to the EDR in magnetopause reconnection, while the electron energy flux is dominant inside it. However, one must be careful while computing the energy transport terms using MMS data, since the results can be dominated by uncertainty. This is particularly true for magnetotail reconnection, where the plasma is tenuous. Here, we present a detailed analysis of the errors in these energy transport terms, and perform a comparative study between reconnection events observed in the magnetopause, magnetosheath and magnetotail regions.

The plasma properties of the incoming solar wind undergo significant changes as they cross the terrestrial bow shock and traverse the magnetosheath. The solar wind itself can be categorized into different categories depending on their solar origin and linked to large-scale structures like coronal mass ejections (CMEs) or stream interaction regions (SIRs) detected in near-Earth space. Using measurements from THEMIS combined with OMNI data spanning from 2008 onward, we provide a statistical overview of temperature anisotropy-driven plasma instabilities in the dayside magnetosheath. This analysis is conducted under various upstream solar wind conditions and structures, which significantly impact the plasma environment in the magnetosheath. We extend this analysis to transient phenomena such as dynamic pressure enhancements in the magnetosheath (so-called jets) as well. As a consequence of collisionless shock physics, the shocked plasma is expected to display vastly different behaviours in terms of plasma properties and stability when sorted into quasi-parallel and quasi-perpendicular downstream magnetosheath regions. However, this categorization is complicated by the presence of fast solar wind streams originating in solar coronal holes due to the significantly increased ion energy flux of the plasma. Consequently, in our statistical analysis, we emphasize the importance of magnetosheath classification under different solar wind plasma origins and show how stable the magnetosheath plasma is in any given upstream solar wind condition. Combining knowledge of solar wind origins and structures with shock and magnetosheath research can contribute to an improved classification of quasi-perpendicular and quasi-parallel shock conditions across all solar wind origins.

Mercury has an extended exosphere that consists of various species. Based on theoretical considerations, the existence of Lithium (Li) in the exosphere around Mercury is predicted to be less than 5x107 cm-2. Because these density values are well below the detection limits of remote observation instruments on board past missions, Li has never been directly observed. Here we show the first on-site determined altitude-density profile of atomic Li7, derived from in-situ magnetic field observations by the MESSENGER spacecraft. The results suggest that the source of Li at Mercury is most likely meteoritic ablation. The findings will help to interpret the remote observations of Mercury's exosphere that will be realized in the near future by the BepiColombo mission.

Fundamental processes in plasmas act to convert energies into different forms, for example, electromagnetic, kinetic and thermal. Direct derivation from the Vlasov-Maxwell equation yields sets of equations that describe the temporal evolution of magnetic, kinetic and internal energies in either the monofluid or multifluid frameworks. In this work, we focus on the main terms affecting the changes in kinetic energy. These are pressure-gradient-related terms and electromagnetic terms. The former account for plasma acceleration/deceleration from a pressure gradient, while the latter from an electric field. Although limited spatial and temporal deviations are expected, a statistical balance between these terms is fundamental to ensure the overall conservation of energy and momentum. We use in-situ observations from the Magnetospheric MultiScale (MMS) mission to study the relationship between these terms. We perform a statistical analysis of those parameters in the context of magnetic reconnection by focusing on small-scale Electron Diffusion Regions and large-scale Flux Transfer Events. The analysis reveals a correlation between the two terms in the monofluid force balance, and in the ion force and energy balance. However, the expected relationship cannot be verified from electron measurements. Generally, the pressure-gradient-related terms are smaller than their electromagnetic counterparts. We perform an error analysis to quantify the expected underestimation of gradient values as a function of the spacecraft separation compared to the gradient scale. Our findings highlight that MMS is capable of capturing energy and force balance for the ion fluid, but that care should be taken for energy conversion terms based on electron pressure gradients.

Plasma flows with enhanced dynamic pressure, known as magnetosheath jets, are often found downstream of collisionless shocks. As they propagate through the magnetosheath, they interact with the surrounding plasma, shaping its properties, and potentially becoming geoeffective upon reaching the magnetopause. In recent years (since 2016), new research has produced vital results that have significantly enhanced our understanding on many aspects of jets. In this review, we summarise and discuss these findings. Spacecraft and ground-based observations, as well as global and local simulations, have contributed greatly to our understanding of the causes and effects of magnetosheath jets. First, we discuss recent findings on jet occurrence and formation, including in other planetary environments. New insights into jet properties and evolution are then examined using observations and simulations. Finally, we review the impact of jets upon interaction with the magnetopause and subsequent consequences for the magnetosphere-ionosphere system. We conclude with an outlook and assessment on future challenges. This includes an overview on future space missions that may prove crucial in tackling the outstanding open questions on jets in the terrestrial magnetosheath as well as other planetary and shock environments.

We use magnetic field data from the Cluster mission to estimate the value of the Taylor microscale and the effective magnetic Reynolds number in the interplanetary solar wind. Turbulent cascades can be characterized by the spatial scale at which dissipation begins to impact the local energy transfer, estimated by the Taylor microscale, as well as the separation between the injection and dissipation scales, estimated by the effective magnetic Reynolds number. Estimating the Taylor microscale requires measurements of the autocorrelation function at small separations. The Cluster spacecraft have exceptionally sensitive search coil magnetometers with high time resolution, making them ideal for measuring the Taylor microscale. We obtain a value of 430 +/- 20 $430\pm 20$ km; smaller than most previous measurements. We interpret this value as being smaller due to the higher time resolution, enabling the curvature of the autocorrelation function to be measured closer to the origin, giving a more accurate measurement. Combining the Taylor Microscale's computed value with concurrent correlation length measurements, we obtain a value of 150,000 +/- 10,000 $150,000\pm 10,000$ for the effective magnetic Reynolds number, which compares well to other observations. The four spacecraft of Cluster also allow directions transverse to the flow to be surveyed. The small separations (7 km) of Clusters 3 and 4 show that the Taylor microscale may vary as a function of direction to the mean magnetic field direction. The observed differences are small, requiring more observations to confirm this anisotropy.

We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi-scale multipoint analysis methods. In order to study the large-scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X-line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter-rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi-point data from MMS we further show that the local normalized reconnection rate is in the range of R similar to 0.16 to 0.18. We find a highly asymmetric electron in- and outflow structure, consistent with previous simulations on strong guide-field reconnection events. This study shows that magnetic reconnection may not only take place at large-scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field. Magnetic Reconnection is a key energy conversion process, where magnetic energy is converted into kinetic energy of plasma particles. During this process the magnetic field topology changes and the plasma particles decouple from the magnetic field in the so-called diffusion region and get accelerated, forming a fast outflow jet. Over the last decades, hints arise that reconnection can take place at many different places in the magnetosphere and also very locally and intermittently. Fast plasma flows in the Magnetotail, moving toward the Earth, are assumed to be a consequence of magnetic reconnection, and are often accompanied by dipolar-shaped magnetic flux bundles, embedded into them. The leading edges of such flux bundles are called dipolarization fronts (DF). In this work, we investigate a DF event, which hosts a diffusion region. First, we study the large-scale characteristics of the DF, by utilizing data from both the Magnetospheric Multiscale (MMS) and the Cluster mission, that observe different regions of the event almost simultaneously. Second, we performed a 3D magnetic field reconstruction technique and compared the results to MMS data, to investigate the event on small scales. A thin current sheet inside a dipolarization front, embedded in a diverging flow is analyzed using a polynomial reconstruction techniqueTransient reconnection event is detected in a high magnetic shear region, where the magnetic field is deflected due to duskward fast plasma flowThe reconstructed current sheet has a guide field of similar to 1.8 the reconnecting component with normalized reconnection rate between 0.16 and 0.18

The wave telescope technique is used to reconstruct spatial power distributions of space plasmas from multipoint spacecraft missions. This study aims to quantify the impact of uncalibrated uncertainties in the time synchronization and the spatial position on the accuracy of the wave telescope method for observatories with more than four spacecraft, for example, HelioSwarm a nine-spacecraft NASA observatory currently in Phase B. We simulate synthetic data with systemic timing and spatial errors modeled using geometries drawn from HelioSwarm's Design Reference Mission, applying the wave telescope technique to estimate wavevectors for two characteristic ion-scale waves. By carefully selecting optimal polyhedral configurations from the overall geometry, and combining signals from multiple polyhedra, the impact of systematic uncertainties and spatial aliasing can be significantly reduced, leading to more accurate wavevector identification for future multipoint missions.

When the velocity shear between the two plasmas separated by Earth's magnetopause is locally super‐Alfvénic, the Kelvin‐Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non‐thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field‐aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.

Fundamental processes in plasmas act to convert energies into different forms, e.g., electromagnetic, kinetic and thermal. Direct derivation from the Valsov-Maxwell equation yields sets of equations that describe the temporal evolution of the magnetic, kinetic and internal energies in either the monofluid or multifluid frameworks. In this work we focus on the main terms that affect the changes in the kinetic energy. These are pressure gradient-related terms and electromagnetic terms. The former account for plasma acceleration or deceleration from a pressure gradient, while the latter from an electric field. The overall balance between these terms is fundamental to ensure the conservation of energy and momentum. We use in-situ observations from the Magnetospheric MultiScale (MMS) mission to study the relationship between these terms. We perform a statistical analysis of those parameters in the context of magnetic reconnection by focusing on small-scale Electron Diffusion Regions and large-scale Flux Transfer Events. The analysis reveals a correlation between the two terms in the monofluid force balance, and in the ion force and energy balance. However, the expected relationship cannot be verified from electron measurements. Generally, the pressure gradient related terms are smaller than their electromagnetic counterparts. We perform an error analysis to quantify the expected underestimation of gradient values as a function of the spacecraft separation compared to the gradient scale. Our findings highlight that MMS is capable of capturing energy and force balance for the ion fluid, but that care should be taken for energy conversion terms based on electron pressure gradients.

Future in situ space plasma investigations will likely involve spatially distributed observatories comprised of multiple spacecraft, beyond the four and five spacecraft configurations currently in operation. Inferring the magnetic field structure across the observatory, and not simply at the observation points, is a necessary step towards characterizing fundamental plasma processes using these unique multi-point, multi-scale data sets. We propose improvements upon the classic first-order reconstruction method, as well as a second-order method, utilizing magnetometer measurements from a realistic nine-spacecraft observatory. The improved first-order method, which averages over select ensembles of four spacecraft, reconstructs the magnetic field associated with simple current sheets and numerical simulations of turbulence accurately over larger volumes compared to second-order methods or first-order methods using a single regular tetrahedron. Using this averaging method on data sets with fewer than nine measurement points, the volume of accurate reconstruction compared to a known magnetic vector field improves approximately linearly with the number of measurement points.

Magnetic reconnection, a key energy conversion process in the Earth’s magnetosphere, has extensively been studied during the last few decades. Multi-point missions such as Cluster or Magnetospheric Multiscale (MMS) showed that magnetic reconnection takes place not only at large-scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets.Here, we revisit the Dipolarization Front (DF) event, observed by the MMS spacecraft on September 08, 2018/14:51:30 UT in the Earth’s magnetotail. Previous studies reported that this DF shows a strong non-ExB type electron flow and a crescent-shaped distribution function, suggesting that this DF hosts an electron diffusion region (Marshall et al. JGR, 2020).To further characterize this special event, we (1) use conjunction observations of the MMS and Cluster spacecraft to investigate the event in the context of large scales and (2) apply the polynomial magnetic field reconstruction technique by Denton et al. (JGR, 2020) to characterize the embedded electron current sheet including its velocity and the X-line exhaust opening angle.

We use density measurements deduced from spacecraft potential to study the power spectral density (PSD) of compressive fluctuations in the solar wind. Typically, plasma measurements do not have a sufficiently high time resolution to resolve density fluctuations down to ion kinetic scales. However, the calibrated spacecraft potential allows for much higher time resolutions to resolve the spectral break between ion inertial and kinetic ranges. We used fast-survey mode data from Magnetospheric MultiScale when the spacecrafts were in the pristine solar wind. The density spectra’s morphology differs from the trace magnetic field fluctuations, with a flattening often occurring between inertial and kinetic ranges. We find that the spectral break of the trace magnetic field fluctuations occurs near the expected frequency for cyclotron resonance or magnetic reconnection. Meanwhile, the spectral break at the start of the ion kinetic range for density fluctuations is often at a higher frequency when compared to the trace magnetic field. We discuss possible interpretations for these observations.

Solar wind (SW) quantities, referred to as coupling parameters (CPs), are often used in statistical studies devoted to the analysis of SW–magnetosphere–ionosphere couplings. Here, the CPs and their limitations in describing the magnetospheric response are reviewed. We argue that a better understanding of SW magnetospheric interactions could be achieved through estimations of the energy budget in the magnetosheath (MS), which is the interface region between the SW and magnetosphere. The energy budget involves the energy transfer between scales, energy transport between locations, and energy conversions between electromagnetic, kinetic, and thermal energy channels. To achieve consistency with the known multi-scale complexity in the MS, the energy terms have to be complemented with kinetic measures describing some aspects of ion–electron scale physics.

In this study we examine the radial dependence of the inertial and dissipation range indices, as well as the spectral break separating the inertial and dissipation range in power density spectra of interplanetary magnetic field fluctuations using Parker Solar Probe data from the fifth solar encounter between ∼0.1 and ∼0.7 au. The derived break wavenumber compares reasonably well with previous estimates at larger radial distances and is consistent with gyro-resonant damping of Alfvénic fluctuations by thermal protons. We find that the inertial scale power-law index varies between approximately −1.65 and −1.45. This is consistent with either the Kolmogorov (−5/3) or Iroshnikov–Kraichnan (−3/2) values, and has a very weak radial dependence with a possible hint that the spectrum becomes steeper closer to the Sun. The dissipation range power-law index, however, has a clear dependence on radial distance (and turbulence age), decreasing from −3 near 0.7 au (4 days) to −4 [±0.3] at 0.1 au (0.75 days) closer to the Sun.

Magnetosheath jets are dynamic pressure enhancements observed in the terrestrial magnetosheath. Their generation mechanisms are currently debated but the majority of jets can be linked to foreshock processes. Recent results showed that jets are less numerous when coronal mass ejections (CMEs) cross the magnetosheath and more numerous when stream interaction regions (SIRs) cross it. Here, we show for the first time how the pronounced substructures of CMEs and SIRs are related to jet production. We distinguish between compression and magnetic ejecta (ME) regions for the CME as well as compression region associated with the stream interface and high-speed streams (HSSs) for the SIR. Based on THEMIS and OMNI data covering 2008-2021, we show the 2D probability distribution of jet occurrence using the cone angle and Alfvén Mach number. We compare this distribution with the values within each solar wind (SW) structure. We find that both high cone angles and low Alfvén Mach numbers within CME-MEs are unfavorable for jet production as they may inhibit a well-defined foreshock region. 1D histograms of all parameters show, which SW parameters govern jet occurrence in each SW structure. In terms of the considered parameters the most favorable conditions for jet generation are found for HSSs due to their associated low cone angles, low densities, and low magnetic field strengths.

Calculating the pressure-strain terms has recently been performed to quantify energy conversion between the bulk flow energy and the internal energy of plasmas. It has been applied to numerical simulations and satellite data from the Magnetospheric MultiScale Mission. The method requires spatial gradients of the velocity and the use of the full pressure tensor. Here we present a derivation of the errors associated with calculating the pressure-strain terms from multi-spacecraft measurements and apply it to previously studied examples of magnetic reconnection at the magnetopause and the magnetotail. The errors are small in a dense magnetosheath event but much larger in the more tenuous magnetotail. This is likely due to larger counting statistics in the dense plasma at the magnetopause than in the magnetotail. The propagated errors analyzed in this work are important to understand uncertainties of energy conversion measurements in space plasmas and have applications to current and future multi-spacecraft missions.

Exigency: The needs of a Technologically Advanced CivilizationOur society have become heavily reliant on electrical technologies, from power grids to GPS network to wireless communication.Any disruption of these systems will have severe global consequences.One major natural hazard that can cause such disruptions comes from solar wind disturbances that impact the near-Earth environment.Estimates are that a solar storm of the magnitude of the 1859 Carrington Solar Superstorm would cost the modern-day society over $2 trillions to remediate the damages [NRC, 2008].In July 23, 2012, we had a near miss of a solar Superstorm that could have broken the record of largest such storms at Earth [Baker et al., 2013].To enable pre-emptive measures against the hazard of solar storms, developing accurate space weather forecasts is urgent.As much as understanding the underlying physics of atmospheric dynamics is critical to accurate terrestrial weather forecasting, a fundamental understanding of plasma physics is at the core of space weather forecasting.Of particular relevance is plasma turbulence in which distorted flows and magnetic fields transfer energy between scales, from larger injection scales down to smaller scales where dissipation and heating occur.Recent studies have shown the importance of turbulence in processes, such as magnetic reconnection, that are key agents of space weather.A primary challenge in understanding plasma turbulence and its global implications is its multi-scale nature, spanning from electron scales to scales larger than the magnetosphere.This white paper addresses the multi-scale challenge focusing on the contributions that multi-spacecraft measurements can make. Multi-spacecraft measurementsIn-situ probes forming multiple "n-hedrons (n >= 4)" and covering MHD to kinetic scales are key to meeting the multi-scale challenge of characterizing plasma turbulence.Current multi-spacecraft missions with 3D formations, the Magnetospheric Multiscale (MMS) [Burch et al., 2016] and Cluster [Escoubet et al., 2001], have made progress to address plasma turbulence.Yet the limitations of a fixed spacecraft formation size at a given time prohibit probing the multiscale nature as well as the dynamical evolution of turbulence.The recently selected NASA Midsize Explorer mission HelioSwarm, consisting of nine spacecraft, represents a major breakthrough in multi-scale measurements of turbulence.For an in-depth description of the mission, see the White Paper submitted to the 2023 Heliophysics Decadal Survey "HelioSwarm: A Multipoint, Multiscale Mission to Characterize Turbulence" by K. G. Klein et al.. Major achievements and challenges from the Cluster and MMS missionsWe briefly review the major achievements and challenges from the Cluster and MMS missions in the field of shock/foreshock/magnetosheath turbulence to provide a contrast with what can be achieved by a multi-scale mission.The major achievements of the Cluster mission on shock/foreshock/magnetosheath turbulence include: (1) Reconnecting current sheets were discovered in the turbulent magnetosheath [Retino et al., 2007].(2) the dissipation due to reconnecting current sheets is estimated to be about two orders of magnitude higher than that due to wave damping [Sunskvist et al., 2007;Chasapis et al, 2015].(3) The anisotropy of solar wind

We present Magnetospheric Multiscale observations of an electron-scale reconnecting current sheet in the mixing region along the trailing edge of a Kelvin-Helmholtz vortex during southward interplanetary magnetic field conditions. Within this region, we observe intense electrostatic wave activity, consistent with lower-hybrid waves. These waves lead to the transport of high-density magnetosheath plasma across the boundary layer into the magnetosphere and generate a mixing region with highly compressed magnetic field lines, leading to the formation of a thin current sheet associated with electron-scale reconnection signatures. Consistencies between these reconnection signatures and a realistic, local, fully-kinetic simulation modeling this current sheet indicate a temporal evolution of the observed electron-scale reconnection current sheet. The multi-scale and inter-process character of this event can help us understand plasma mixing connected to the Kelvin-Helmholtz instability and the temporal evolution of electron-scale reconnection. Like wind blowing over water, the stream of ionized gas released from the Sun, called the solar wind, can lead to waves and rolled-up vortex structures at the boundary of Earth's magnetosphere, called the magnetopause. These so-called Kelvin-Helmholtz waves have been shown to be connected to various different plasma processes on different scales. This multi-scale and multi-process character makes them an ideal candidate to study the relation between these processes from both spacecraft observations and simulations. By using spacecraft data from the Magnetospheric Multiscale mission, which was designed for the study of small-scale plasma processes in Earth's magnetosphere, we show observations of electron-scale magnetic reconnection, an explosive energy conversion process in plasmas, in a region along the trailing edge of these waves. These observations shed new light on the multi-scale and multi-process character of the Kelvin-Helmholtz instability and the energy conversion processes along its boundary. A reconnecting electron-scale current sheet is observed by Magnetospheric Multiscale (MMS) in mixing plasma along the trailing edge of a Kelvin-Helmholtz vortexRealistic 2.5D fully-kinetic simulation shows reasonable agreement with MMS dataConsistencies between the simulation and MMS indicate a temporal evolution of the reconnecting current sheet