Magnetic reconnection is a fundamental physical process that can drive rapid conversion of magnetic energy into plasma bulk flows, thermal heating, and particle acceleration in space and astrophysical plasmas. Classical reconnection theory predicts that the Alfv & eacute;nic reconnection exhausts are bounded by pairs of slow-mode shocks. However, identifying and characterizing these shocks through in situ spacecraft observations remains a challenge. Here, we report Parker Solar Probe observations of a reconnection exhaust embedded in the heliospheric current sheet at a heliocentric distance of 12.2 R & xodot; . The reconnection exhaust is bounded on both boundaries by compound magnetic structures rather than a pair of pure slow shocks. Each boundary consists of a rapidly evolving, steep, inner slow shock, whose Mach numbers and shock-normal angles change significantly within several minutes, and an outer, gradual compound structure that comprises a slow shock and a rotational discontinuity. These slow shocks are quasi-perpendicular and are accompanied by enhanced proton perpendicular heating. Deep within the reconnection exhaust, high perpendicular temperature together with large plasma beta trigger mirror instability and generate mirror-mode structures. These observations provide new insights into the structure of reconnection exhaust boundaries and their role in energy conversion in the near-Sun plasma.

We utilize plasma turbulence in interplanetary coronal mass ejections as a natural laboratory to analyze more than 4600 current sheets observed over a broad range of electron and proton beta values, 10-2 less than or similar to beta e less than or similar to 10 and 10-3 less than or similar to beta p less than or similar to 10. The analysis shows that the shear angle Delta theta and the thickness lambda CS of the current sheets depend on both electron and proton beta. Specifically, the dependencies on electron beta are Delta theta approximate to 12.6 degrees beta e0.41 and lambda CS/lambda p approximate to 1.9 beta e0.26 , where lambda p is the local proton inertial length. We argue that the beta dependence of the shear angle is an intrinsic feature of solar wind turbulence arising from the natural correlation between turbulence intensity and plasma beta. In contrast, the beta dependence of current sheet scales is likely a general characteristic of plasma turbulence. According to recent theory, current sheets formed in turbulence are disrupted by the electron tearing instability once their thickness is smaller than a critical scale. We demonstrate that normalizing the thickness of current sheets by this critical scale eliminates the dependence on beta across the entire range of plasma beta. This observation not only supports the theory suggesting that current sheets mediate the transition from the inertial to the kinetic cascade through the electron tearing instability, but also demonstrates the plausibility of similar processes in astrophysical plasma. The presented results also indicate that the theory may be applicable beyond its formal limits, me/mi << beta << 1.

The solar wind is filled with intense current sheets (magnetic field discontinuities) that host multiple instabilities and may undergo magnetic reconnection, a basic process for plasma heating and charged particle acceleration. Although numerous studies have examined current sheets, most statistical analyses cover relatively narrow heliocentric ranges, leaving essential aspects of their radial dependence only partially resolved. In order to investigate how current sheet properties evolve as the solar wind expands, we analyze three intervals when the Parker Solar Probe (PSP) was near perihelion and compare its observations with near-Earth measurements from ARTEMIS and WIND. We identify and characterize more than 45,000 current sheets across the three intervals, including approximately 17,000 from PSP. We find that current sheets broaden with increasing heliocentric distance, while their characteristic current density decreases proportionally to their Alfv & eacute;n current density (JA = eNVA, where symbols have their usual definitions). Most current sheets exhibit only small variations in magnetic field magnitude. Their thickness and current density are strongly anti-correlated, indicating that thinner current sheets carry stronger currents, consistent with intermittent turbulence. Distinct distributions of magnetic field jumps and rotation angles across different missions suggest structural evolution as the solar wind expands. We also quantify their departure from the Wal & eacute;n relation, which describes the expected correspondence between velocity and magnetic-field jumps for an ideal rotational discontinuity. Significant deviations persist even after accounting for plasma pressure anisotropy. Finally, the Alfv & eacute;nicity of the current sheets correlates strongly with the cross helicity and residual energy of the surrounding solar wind.

Magnetic reconnection is a fundamental and ubiquitous process in astrophysical plasmas that converts magnetic energy into plasma kinetic and thermal energy. Throughout the heliosphere, the solar wind is permeated with current sheets (CSs), providing a natural laboratory for investigating this process. Using measurements from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we report the first direct observations of magnetic reconnection occurring within the solar wind CSs near Mars. Specifically, MAVEN observed the classic Petschek-type reconnection exhaust regions, evidenced by bifurcated magnetic field signatures and Alfv & eacute;nic ion outflows. Notably, the observed exhaust region appears to be large scale, significantly exceeding the typical thickness of solar wind CSs near Mars. This suggests that magnetic reconnection may significantly broaden the CS. Our results underscore the ubiquity of magnetic reconnection across heliocentric distances and may provide new insights into the large-scale evolution of the solar wind and the development of turbulence within it.

Magnetopause boundary layers (BLs) play an important role mediating plasma and energy exchange between the solar wind/magnetosheath and Earth's magnetosphere. Energy exchange across the magnetopause is enhanced during storms, yet little work has been done investigating BLs during storms. In this letter, using MMS and THEMIS observations, we investigate the structure and formation of magnetopause BLs during a large coronal mass ejection (CME) driven storm containing 20 hours of low Alfv & eacute;n Mach number solar wind (M A ). Separated by Earth radii (), MMS and THEMIS observe a low latitude boundary layer (LLBL), magnetosheath boundary layer (MSBL), and formation of a plasma depletion layer after a northward interplanetary magnetic field turning. MMS observations indicate lobe reconnection drives the MSBL and LLBL formation. Observations of CME ions on closed field lines demonstrate dual reconnection can trap solar wind plasma under sub-Alfvenic solar wind conditions and constrain the trapped populations' dwell time.

Magnetic reconnection is a fundamental process in astrophysical plasma, as it enables the dissipation of energy at kinetic scales as well as large-scale reconfiguration of the magnetic topology. In the solar wind, its quantitative role in plasma dynamics and particle energization remains an open question that is starting to come into focus as more missions now probe the inner heliosphere. To more efficiently detect magnetic reconnection in-situ using automated and modern methods is one of the challenges that can bring us closer to understanding the impact of magnetic reconnection on its surrounding magnetized environment. In this presentation, we make use of existing databases to focus on the evolution of magnetic reconnection properties through the heliosphere, using several space missions such as Parker Solar Probe (PSP), Solar Orbiter and Wind. We investigate the properties of small-scale reconnecting current sheets found in the turbulent solar wind as a function of radial distance and plasma source. In parallel, we also make use of PSP-Solar Orbiter alignments to study how the large-scale and high-shear reconnection occurring at the heliospheric current sheet evolves as it propagates in the solar wind. Finally, we emphasize how reconnection has a high impact on coherent structure evolution such as coronal mass ejection erosion or merging. Collectively, these results show that magnetic reconnection is ubiquitous in the solar wind and occurs in a wide variety of settings, with a high impact on its surrounding environment. We discuss how the recent growth of available in-situ spacecraft mission data inside the Earth orbit promises further substantial progress in our understanding of magnetic reconnection occurrence, properties and impact in the solar wind.

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP encounters 4–11 (2020 January–2022 March), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus nonreconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvénic wind streams. We compare with fast, slow non-Alfvénic, and slow Alfvénic streams, finding that the growth rate is suppressed in highly Alfvénic fast and slow wind, and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvénic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvénic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

Magnetic reconnection is a universal process that allows for the transfer and release of energy previously stored in a magnetic field configuration. At Earth, this process can occur in the boundary layer between Earth's magnetic field and the solar wind, called the magnetopause. This allows the magnetopause to act as the primary “entry gate” for the solar wind's energy into the Earth's magnetosphere and upper atmosphere. Along the magnetopause, magnetic reconnection is triggered inside diffusion regions: an electron diffusion region (EDR) embedded inside a larger ion diffusion region (IDR). Inside these diffusion regions, plasma decouples from the magnetic field, allowing the release of magnetic energy into the plasma. We identify a hitherto undefined region in the diffusion regions, which results from the interaction of demagnetized electrons in the EDR's reconnection current sheet and remagnetizing electrons in the IDR. We call this region the Diffusion Interaction Region. Using in situ Magnetospheric Multiscale plasma data and 2.5D particle‐in‐cell simulations, we isolate its defining features, which includes a significant parallel current structure and regions of parallel energy conversion. Defining this important and underexplored region of magnetic reconnection helps us determine how the local diffusion regions may couple with the large‐scale current structures within which they are embedded.

Magnetic reconnection is an explosive energy release event. It plays an important role in accelerating particles to high non‐thermal energies. These particles often exhibit energy spectra characterized by a power‐law distribution. However, the partitioning of energy between thermal and non‐thermal components, and between ions and electrons, remains unclear. This study provides estimates of energy partition based on a statistical analysis of magnetic reconnection events in Earth's magnetotail using data from the Magnetospheric Multiscale mission. Ions are up to 10 times more energetic than electrons but have softer spectra. We found for both ions and electrons that, as the average energy of particles (temperature) increases, their energy spectra become softer (steeper) and thus, the fraction of energy carried by the non‐thermal components decreases. These results challenge existing theories of particle acceleration through magnetotail reconnection.

Electrons are accelerated to high, nonthermal energies during explosive energy-release events in space, such as magnetic reconnection. However, the properties and acceleration mechanisms of relativistic electrons directly associated with the reconnection X-line are not well understood. This study utilizes Magnetospheric Multiscale (MMS) measurements to analyze the flux and spectral features of subrelativistic to relativistic (∼80–560 keV) electrons during a magnetic reconnection event in Earth’s magnetotail. This event provided a unique opportunity to measure the electrons directly energized by the X-line as MMS stayed in the separatrix layer, where the magnetic field directly connects to the X-line, for approximately half of the observation period. Our analysis revealed that the fluxes of relativistic electrons were clearly enhanced within the separatrix layer, and the highest flux was directed away from the X-line, which suggested that these electrons originated directly from the X-line. Spectral analysis showed that these relativistic electrons deviated from the main plasma sheet population and exhibited an “ankle” feature similar to that observed in galactic cosmic rays. The contribution of “ankle” electrons to the total electron energy density increased from 0.1% to 1% in the separatrix layer though the spectral slopes did not exhibit clear variations. Further analysis indicated that while these relativistic electrons originated from the X-line, they experienced a nonnegligible degree of scattering during transport. These findings provide clear evidence that magnetic reconnection in Earth’s magnetotail can efficiently energize relativistic electrons directly at the X-line, providing new insights into the complex processes governing electron dynamics during magnetic reconnection.

Magnetic reconnection has long been thought to play an important role in turbulent plasmas – with the nonlinear dynamics of turbulent systems being well known to self-consistently generate intense current structures and associated magnetic shears that can be sites where so-called turbulence-driven magnetic reconnection can occur. However, complex three-dimensional magnetic topologies and the small-scale nature of these magnetic reconnection events have traditionally made it challenging to assess the role of magnetic reconnection in the turbulent dynamics from either a numerical or observational perspective. Recent high-resolution observations from NASA’s Magnetospheric Multiscale (MMS) mission have provided an unprecedented new opportunity to systematically examine turbulence-driven reconnection in the region of shock-driven turbulence within Earth’s magnetosheath. These observations have provided new insight into the nature of magnetic reconnection within turbulent plasmas, revealing that under the right conditions so-called electron-only magnetic reconnection, in which ion jets are not accelerated by the newly reconnected magnetic fields, can occur. In this talk, we explore how to observationally constrain the contribution turbulence-driven magnetic reconnection makes to the energy dissipation rate of the turbulence. We then directly compare estimates of the dissipation rate associated with reconnection events observed by MMS to estimates of the turbulent energy cascade rate for the specific intervals of magnetosheath turbulence that the reconnection events are observe within. The potential implications of traditional ion-coupled reconnection versus electron-only reconnection for the energy budget of turbulent dissipation and the magnetosheath overall are then discussed in detail.

Magnetic reconnection is a ubiquitous plasma process that transforms magnetic energy into particle energy during eruptive events throughout the universe. Reconnection not only converts energy during solar flares and geomagnetic substorms that drive space weather near Earth, but it may also play critical roles in the high energy emissions from the magnetospheres of neutron stars and black holes. In this review article, we focus on collisionless plasmas that are most relevant to reconnection in many space and astrophysical plasmas. Guided by first-principles kinetic simulations and spaceborne in-situ observations, we highlight the most recent progress in understanding this fundamental plasma process. We start by discussing the non-ideal electric field in the generalized Ohm’s law that breaks the frozen-in flux condition in ideal magnetohydrodynamics and allows magnetic reconnection to occur. We point out that this same reconnection electric field also plays an important role in sustaining the current and pressure in the current sheet and then discuss the determination of its magnitude (i.e., the reconnection rate), based on force balance and energy conservation. This approach to determining the reconnection rate is applied to kinetic current sheets with a wide variety of magnetic geometries, parameters, and background conditions. We also briefly review the key diagnostics and modeling of energy conversion around the reconnection diffusion region, seeking insights from recently developed theories. Finally, future prospects and open questions are discussed.

We report observations of direct evidence of energetic protons being accelerated above ∼400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA’s Parker Solar Probe (PSP) at a distance of ∼16.25 solar radii ( R _s ) from the Sun. Inside the exhaust, both the reconnection-generated plasma jet and the accelerated protons up to ∼400 keV propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located antisunward of PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ∼400 keV, which is ≈1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power law with spectral index of ∼−5. Supporting simulations using the kglobal model suggest that the trapping and acceleration of protons up to ∼400 keV in the reconnection exhaust are likely facilitated by merging magnetic islands with a guide field between ∼0.2 and 0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP’s proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. Our findings of in situ particle acceleration via magnetic reconnection at the HCS provide valuable insights into this fundamental process, which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the Sun’s atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.

Magnetic reconnection is a fundamental mechanism for the transport of mass and energy in planetary magnetospheres and astrospheres. While the process of reconnection is itself ubiquitous across a multitude of systems, the techniques used for its analysis can vary across scientific disciplines. Here we frame the latest understanding of reconnection theory by missions such as NASA’s Magnetospheric Multiscale (MMS) mission for use throughout the solar system and beyond. We discuss how reconnection can couple magnetized obstacles to both sub- and super-magnetosonic upstream flows. In addition, we address the need to model sheath plasmas and field-line draping around an obstacle to accurately parameterize the possibility for reconnection to occur. We conclude with a discussion of how reconnection energy conversion rates scale throughout the solar system. The results presented are not only applicable to within our solar system but also to astrospheres and exoplanets, such as the first recently detected exoplanet magnetosphere of HAT-11-1b.

Electromagnetic Ion Cyclotron (EMIC) waves mediate energy transfer from the solar wind to the magnetosphere, relativistic electron precipitation, or thermalization of the ring current population, to name a few. How these processes take place depends on the wave properties, such as the wavevector and polarization. However, inferring the wavevector from in-situ measurements is problematic since one needs to disentangle spatial and time variations. Using 8 years of Magnetospheric Multiscale (MMS) mission observations in the dayside magnetosphere, we present an algorithm to detect proton-band EMIC waves in the Earth's dayside magnetosphere, and find that they are present roughly 15% of the time. Their normalized frequency presents a dawn-dusk asymmetry, with waves in the dawn flank magnetosphere having larger frequency than in the dusk, subsolar, and dawn near subsolar region. It is shown that the observations are unstable to the ion cyclotron instability. We obtain the wave polarization and wavevector by comparing Single Value Decomposition and Ampere methods. We observe that for most waves the perpendicular wavenumber (k(perpendicular to)) is larger than the inverse of the proton gyroradius (rho(i)), that is, k(perpendicular to)rho(i) > 1, while the parallel wavenumber is smaller than the inverse of the ion gyroradius, that is, k(& Vert;)rho(i) < 1. Left-hand polarized waves are associated with small wave normal angles (theta(Bk) < 30 degrees), while linearly polarized waves are associated with large wave normal angles (theta(Bk) > 30 degrees). This work constitutes, to our knowledge, the first attempt to statistically infer the full wavevector of proton-band EMIC waves observed in the outer magnetosphere.

This study investigates the escape of Mercury's sodium-group ions (Na+-group, including ions with m/q from 21 to 30 amu/e) and their dependence on true anomaly angle (TAA), i.e., Mercury's orbital phase around the Sun, using measurements from MESSENGER. The measurements are categorized into solar wind, magnetosheath, and magnetosphere, and further divided into four TAA intervals. Na+-group ions form escape plumes in the solar wind and magnetosheath, with higher fluxes along the solar wind's motional electric field. The total escape rates vary from 0.2 to 1 times 10^{25} atoms/s with the magnetosheath being the main escaping region. These rates exhibit a TAA dependence, peaking near the perihelion and similar during Mercury's remaining orbit. Despite Mercury's tenuous exosphere, Na+-group ions escape rate is comparable to other inner planets. This can be attributed to several processes, including that Na+-group ions may include several ion species, efficient photoionization frequency for elements within Na+-group, etc.

Magnetic reconnection and plasma turbulence are ubiquitous and key processes in the Universe. These two processes are suggested to be intrinsically related: magnetic reconnection can develop turbulence, and, in turn, turbulence can influence or excite magnetic reconnection. In this study, we report a rare and unique electron diffusion region (EDR) observed by the Magnetospheric Multiscale mission in the Earth’s magnetotail with significantly enhanced energetic particle fluxes. The EDR is in a region of strong turbulence within which the plasma density is dramatically depleted. We present three salient features. (1) Despite the turbulence, the EDR behaves nearly the same as that in 2D quasi-planar reconnection; the observations suggest that magnetic reconnection continues for several minutes. (2) The observed reconnection electric field and inferred energy transport are exceptionally large. However, the aspect ratio of the EDR (one definition of reconnection rate) is fairly typical. Instead, extraordinarily large-amplitude Hall electric fields appear to enable the strong energy transport. (3) We hypothesize that the high-energy transport rate, density depletion, and the strong particle acceleration are related to a near-runaway effect, which is due to the combination of low-plasma-density inflow (from lobes) and possible positive feedback between turbulence and reconnection. The detailed study on this EDR gives insight into the interplay between reconnection and turbulence, and the possible near-runaway effect, which may play an important role in other particle acceleration in astrophysical plasma.

We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-β and high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfvén speeds (800–4000 km s−1) and inflow ion β (0.002–1) covered by this study is in a plasma regime that could be applicable to the solar corona and flare environments. We find that the observed ion heating increases with increasing inflow (upstream) Alfvén speed, V A, based on the reconnecting magnetic field and the upstream plasma density. However, ion heating does not increase linearly as a function of available magnetic energy per particle, m i V A 2 . Instead, the heating increases progressively less as m i V A 2 rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfvén-speed and low-β regime scales linearly with m i V A 2 , with a scaling factor nearly identical to that found for the low-V A and high-β magnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstream V A, suggesting that the energy partition between ions and electrons in reconnection exhausts could be a function of the available magnetic energy per particle. Finally, we find that the observed difference in ion and electron heating scaling may be consistent with the predicted effects of a trapping potential in the exhaust, which enhances electron heating, but reduces ion heating.

Dayside magnetic reconnection allows for the transfer of the solar wind's energy into Earth's magnetosphere. This process takes place in electron diffusion regions (EDRs) embedded in ion diffusion regions (IDRs), which form in the magnetopause boundary's current sheet. A significant out-of-plane parallel current contribution in the diffusion regions was reported in Beedle et al. 2023. In order to understand the underlying structure of this parallel current, we compared EDR statistical results with a 2.5D Particle-In Cell (PIC) simulation. From this comparison, we identified out-of-plane parallel current signatures as defining features of the outer EDR and IDR. This significant out-of-plane parallel current indicates the interaction of the IDR and EDR systems, and provides implications for not only understanding energy dissipation in the diffusion regions, but also determining the location of the outer EDR.

We present the analysis of 1,831 current sheets (CS) observed aboard four Cluster spacecraft in a pristine solar wind. Four-spacecraft estimates of the CS normal and propagation velocity are compared with different single-spacecraft estimates. The Minimum Variance Analysis (MVA) of the magnetic field is shown to be highly inaccurate in estimating the normal. The MVA normal often differs by more than 60 degrees from the normal obtained by multi-spacecraft timing method, likely due to ambient turbulent fluctuations. In contrast, the cross-product of magnetic fields at the CS boundaries delivers the normal with an uncertainty of less than 15 degrees at the confidence level of 90%. The CSs are essentially frozen into plasma flow, since their propagation velocity is consistent with local ion flow velocity within 20% at the confidence level of 90%. The single-spacecraft methodology based on the cross-product method and frozen-in assumption delivers the CS thickness and current density amplitude within 20% of their actual values at the confidence level of 90%. The CSs are kinetic-scale structures with half-thickness lambda from a few tenths to tens of local proton inertial length lambda p and scale-dependent shear angle and current density amplitude, Delta theta proportional to lambda/lambda p0.5 ${\Delta }\theta \propto {\left(\lambda /{\lambda }_{p}\right)}<^>{0.5}$ and J0 proportional to lambda/lambda p-0.5 ${J}_{0}\propto {\left(\lambda /{\lambda }_{p}\right)}<^>{-0.5}$. The classification of the CSs in terms of tangential and rotational discontinuities remains a challenge, because even the four-spacecraft normal has too large uncertainties to reveal the actual normal magnetic field component. The presented results will be valuable for the analysis of solar wind CSs, when only single-spacecraft measurements are available. Current sheets (CS) are sharp magnetic field rotations that could play a role in solar wind heating. These locally planar one-dimensional structures are highly likely produced by turbulence cascade, though some may originate in solar corona. Most of the studies of solar wind CSs were carried out using single-spacecraft measurements, the CS normal was typically computed using Minimum Variance Analysis (MVA), and the other CS properties were estimated using that normal. In this study we present multi-spacecraft analysis of 1,831 CSs observed aboard four Cluster spacecraft and determine the CS normal using four-spacecraft observations. We demonstrate that MVA is highly inaccurate in estimating the CS normal, while the cross-product of magnetic fields at the CS boundaries delivers accurate estimates of the normal. We show that the single-spacecraft methodology based on the cross-product normal and the assumption that solar wind CSs are frozen into local plasma flow provides accurate estimates of the CS thickness and current density. This single-spacecraft methodology assumes zero normal component of the magnetic field and does not allow classifying CSs in terms of tangential and rotational discontinuities, but we show that because of methodology uncertainties four-spacecraft observations do not allow carrying out such a classification either. Four-spacecraft observations are used to estimate the normal and propagation velocity for more than 1,800 current sheets at 1 AU MVA is highly inaccurate in estimating the current sheet normal, while the cross-product method delivers accurate estimates Single-spacecraft methodology based on the cross-product normal and frozen-in assumption is accurate in estimating current sheet properties