MMS Observations of Plasma Heating Associated With FTE Growth

Upon formation, flux transfer events (FTEs) in the subsolar magnetosheath have been observed to grow in diameter, λ, while convecting along the magnetopause. Plasma pressure has also been found to decrease subâ adiabatically with increasing λ, indicating the presence of internal plasma acceleration and heating processes. Here, the Magnetospheric Multiscale (MMS) fields and plasma measurements are used to determine the relative roles of parallel electric fields, betatron, and Fermi processes in plasma heating inside an ensemble of 55 subsolar FTEs. Plasma heating is shown asymmetric inside FTEs. Parallel electric fields dominate (>75%) ion and electron heating at the leading edge of FTEs. At the trailing edge, betatron and Fermi processes overtake (>50%), resulting in ion cooling and electron heating, respectively. The observed strong net heatings inside FTEs are proportional to λâ 1/2. It is concluded that reconnectionâ driven heating continues inside FTEs far from the subsolar electron and ion diffusion regions.Plain Language SummaryEnergetic charged particles are observed in many space and astrophysical environments, including our solar system. However, the acceleration and heating mechanisms responsible for generating these energetic charged particles remain to be discovered. Simulations and in situ observations have shown that magnetic reconnection, a process through which magnetic field lines â reconnectâ and release magnetic energy, plays a major role in generating energetic charged particles. The primary sites for magnetic energy transfer to charged particle acceleration and heating are the twin exhaust regions that emanate from the reconnection Xâ line. However, the amount of kinetic energy gained by charged particles in the exhaust regions represents only a small fraction of the total energy released by magnetic reconnection. Here, the Magnetospheric Multiscale (MMS) multipoint fields and plasma measurements are used to determine the contributions of acceleration mechanisms operating inside flux transfer events (FTEs), which are formed in the reconnection exhaust regions. We observe that acceleration mechanisms contribute to the charged particles’ energy gain inside FTEs. We further reveal that while acceleration mechanisms are most significant inside smaller FTEs, they continue to accelerate charged particles inside larger FTEs. We conclude that magnetic reconnectionâ driven charged particle acceleration is longâ lasting and can take place far from the exhaust regions.Key PointsThe relative roles of parallel electric fields, betatron, and Fermi processes in plasma heating inside 55 subsolar FTEs are determinedParallel electric fields dominate plasma energization at FTEs’ leading edge. Betatron and Fermi processes overtake at FTEs’ trailing edgeMMS observations reveal strong plasma acceleration inside FTEs that is inversely proportional to the square root of FTE diameterPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152496/1/grl59844_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152496/2/grl59844-sup-0001-2019GL084843-SI.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152496/3/grl59844.pd

MMS Multi-Point Analysis of FTE Evolution: Physical Characteristics and Dynamics

Previous studies have indicated that flux transfer events (FTEs) grow as they convect away from the reconnection site along the magnetopause. This increase in FTE diameter may occur via adiabatic expansion in response to decreasing external pressure away from the subsolar region or due to a continuous supply of magnetic flux and plasma to the FTEs’ outer layers by magnetic reconnection. Here we investigate an ensemble of 55 FTEs at the subsolar magnetopause using Magnetospheric Multiscale (MMS) multi-point measurements. The FTEs are initially modeled as quasi-force-free flux ropes in order to infer their geometry and the spacecraft trajectory relative to their central axis. The MMS observations reveal a radially-inward net force at the outer layers of FTEs which can accelerate plasmas and fields toward the FTE’s core region. Inside the FTEs, near the central axis, plasma density is found to decrease as the axial net force increases. It is interpreted that the axial net force accelerates plasmas along the axis in the region of compressing field lines. Statistical analysis of the MMS observations of the 55 FTEs indicates that plasma pressure, Pth, decreases with increasing FTE diameter, λ, as Pth,obsv - λ-0.24. Assuming that all 55 FTEs started out with similar diameters, this rate of plasma pressure decrease with increasing FTE diameter is at least an order of magnitude slower than the theoretical rate for adiabatic expansion (i.e., Pth,adiab. - λ-3.3), suggesting the presence of efficient plasma heating mechanisms, such as magnetic reconnection, to facilitate FTE growth.Key PointsThe forces inside FTEs observed by MMS suggest plasma acceleration toward and along the FTE’s central axis causing plasma to escapeThe roles of adiabatic expansion and reconnection in FTE growth are explored using MMS observationsThe observed sub-adiabatic decrease of plasma pressure as FTE size increases requires plasma heating mechanisms such as reconnectionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151362/1/jgra55065_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151362/2/jgra55065.pd

Comparative Analysis of the Vlasiator Simulations and MMS Observations of Multiple X-Line Reconnection and Flux Transfer Events

The Vlasiator hybrid-Vlasov code was developed to investigate global magnetospheric dynamics at ion-kinetic scales. Here we focus on the role of magnetic reconnection in the formation and evolution of magnetic islands at the low-latitude magnetopause, under southward interplanetary magnetic field conditions. The simulation results indicate that (1) the magnetic reconnection ion kinetics, including the Earthward pointing Larmor electric field on the magnetospheric side of an X-point and anisotropic ion distributions, are well-captured by Vlasiator, thus enabling the study of reconnection-driven magnetic island evolution processes, (2) magnetic islands evolve due to continuous reconnection at adjacent X-points, "coalescence" which refers to the merging of neighboring islands to create a larger island, "erosion" during which an island loses magnetic flux due to reconnection, and "division" which involves the splitting of an island into smaller islands, and (3) continuous reconnection at adjacent X-points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross-sectional growth rates up to + 0.3 R-E(2)/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion-scale flux transfer events (FTEs) sandwiched between two dominant X-lines. The MMS measurements similarly reveal (1) anisotropic ion populations and (2) normalized reconnection rate similar to 0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion-scale FTEs may grow Earth-sized within similar to 10 min, which is comparable to the average transport time for FTEs formed in the subsolar region to the high-latitude magnetopause. Future simulations shall revisit reconnection-driven island evolution processes with improved spatial resolutions.Peer reviewe

MESSENGER Observations of Mercury’s Nightside Magnetosphere Under Extreme Solar Wind Conditions: Reconnectionâ Generated Structures and Steady Convection

Mercury’s nightside magnetosphere is investigated under the impact of a coronal mass ejection (CME) and a highâ speed stream (HSS) with MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) observations. The CME was shown to produce a low plasma β (ratio of thermal pressure to magnetic pressure) magnetosheath, while the HSS creates a higher β magnetosheath. Reconnection at the dayside magnetopause was found to be stronger during the CME than the HSS, but both were stronger than the average condition (Slavin et al., 2014, https://doi.org/10.1002/2014JA020319). Here we show that the CME and HSS events produced large numbers of flux ropes and dipolarization fronts in the plasma sheet. The occurrence rates for the structures were approximately 2 orders of magnitude higher than under average conditions with the rates during CME’s being twice that of HSS’s. The flux ropes appeared as quasiperiodic flux rope groups. Each group lasted approximately 1 min and had a few large flux ropes followed by several smaller flux ropes. The lobe magnetic flux accounted for around half of the Mercury’s available magnetic flux with the flux during CME’s being larger than that of HSS’s. The CME produced a more dynamic nightside magnetosphere than the HSS. Further, for the CME event, the tail magnetic reconnection produced a distorted Hall magnetic field pattern and the Xâ line had a dawnâ dusk extent of 20% of the tail width. No magnetic flux loading and unloading events were observed suggesting that, during these intense driving conditions, Mercury’s magnetosphere responded with a type of quasiâ steady convection as opposed to the tail flux loadingâ unloading events seen at Earth.Key PointsCoronal mass ejections drive more intense nightside reconnection than high speed streamsUnder extreme conditions, magnetic reconnection produces a distorted Hall magnetic field pattern in the plasma sheetContinued intense solar wind forcing does not produce substorm magnetic flux loading and unloading of tail lobe instead steady convectionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154381/1/jgra55534_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154381/2/jgra55534.pd

MMS Examination of FTEs at the Earth’s Subsolar Magnetopause

Determining the magnetic field structure, electric currents, and plasma distributions within flux transfer event (FTE)â type flux ropes is critical to the understanding of their origin, evolution, and dynamics. Here the Magnetospheric Multiscale mission’s highâ resolution magnetic field and plasma measurements are used to identify FTEs in the vicinity of the subsolar magnetopause. The constantâ α flux rope model is used to identify quasiâ force free flux ropes and to infer the size, the core magnetic field strength, the magnetic flux content, and the spacecraft trajectories through these structures. Our statistical analysis determines a mean diameter of 1,700 ± 400 km (~30 ± 9 di) and an average magnetic flux content of 100 ± 30 kWb for the quasiâ force free FTEs at the Earth’s subsolar magnetopause which are smaller than values reported by Cluster at high latitudes. These observed nonlinear size and magnetic flux content distributions of FTEs appear consistent with the plasmoid instability theory, which relies on the merging of neighboring, smallâ scale FTEs to generate larger structures. The ratio of the perpendicular to parallel components of current density, RJ, indicates that our FTEs are magnetically forceâ free, defined as RJ < 1, in their core regions (<0.6 Rflux rope). Plasma density is shown to be larger in smaller, newly formed FTEs and dropping with increasing FTE size. It is also shown that parallel ion velocity dominates inside FTEs with largest plasma density. Fieldâ aligned flow facilitates the evacuation of plasma inside newly formed FTEs, while their core magnetic field strengthens with increasing FTE size.Key PointsFlux ropes observed at subsolar magnetopause have a mean diameter of 1,700 km, which is 3 to 7 times smaller than highâ latitude flux ropesFieldâ aligned current dominates perpendicular current in the central regions of all quasiâ force free flux ropesPlasma density dropping inside flux ropes as the core magnetic field strengthens indicates temporal evolution upon flux rope formationPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142974/1/jgra54082.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142974/2/jgra54082_am.pd

High order algorithms for the management of uncertainties with applications in space situational awareness

Space situational awareness program, for both NEO and debris segments, have to face the challenging problem of accurately managing uncertainties in highly nonlinear dynamical environments. The uncertainties affect all the main phases necessary for the successful realization of the program; i.e., orbital determination, ephemeris prediction, collision probability computation, and collision avoidancemaneuver planning and execution. Since the amount of data that must be processed is huge, efficient methods for the management of uncertainties are required. Differential algebraic (DA) techniques can represent a valuable tool to address this tasks. Differential algebra supplies the tools to compute the derivatives of functions within a computer environment. This technique allows for the efficient computation of high-order expansions of the flow of ordinary differential equations (with respect to initial conditions and/or model parameters) and the approximation of the solution manifold of implicit equations in Taylor series. These two features constitute the building blocks of a set new algorithms for the nonlinear and efficient management of uncertainties. Applications to 1) angles-only preliminary orbit determination 2) propagation of orbital dynamics 3) nonlinear filtering 4) space conjunction prediction 5) robust optimal control are presented to prove the efficiency of DA based algorithms

MMS Study of the Structure of Ionâ Scale Flux Ropes in the Earth’s Crossâ Tail Current Sheet

This study analyzes 25 ionâ scale flux ropes in the Magnetospheric Multiscale (MMS) observations to determine their structures. The high temporal and spatial resolution MMS measurements enable the application of multispacecraft techniques to ionâ scale flux ropes. Flux ropes are identified as quasiâ oneâ dimensional (quasiâ 1â D) when they retain the features of reconnecting current sheets; that is, the magnetic field gradient is predominantly northward or southward, and quasiâ 2â D when they exhibit circular cross sections; that is, the magnetic field gradients in the plane transverse to the flux rope axis are comparable. The analysis shows that the quasiâ 2â D events have larger core fields and smaller pressure variations than the quasiâ 1â D events. These two types of flux ropes could be the result of different processes, including magnetic reconnection with different dawnâ dusk magnetic field components, temporal transformation of flattened structure to circular, or interactions with external environments.Plain Language SummaryMagnetic flux ropes are fundamental magnetic structures in space plasma physics and are commonly seen in the universe, such as, astrophysical jets, coronal mass ejections, and planetary magnetospheres. Flux ropes are important in mass and energy transport across plasma and magnetic boundaries, and they are found in a wide range of spatial sizes, from several tens of kilometers, that is, ionâ scale flux ropes, to tens of millions of kilometers, that is, coronal mass ejections, in the solar system. The ionâ scale flux ropes can be formed during magnetic reconnection and are hypothesized to energize electrons and influence the reconnection rate. Previous examinations of the structure of ionâ scale flux ropes were greatly limited by measurement resolution. The unprecedented Magnetospheric Multiscale (MMS) mission high temporal and spatial resolution measurements provide a unique opportunity to investigate flux rope structures. By employing multispacecraft techniques, this study has provided new insights into the magnetic field variations and dimensionality of ionâ scale flux ropes in the Earth’s magnetotail. The results are consistent with the evolution of ionâ scale flux ropes from initially flattened current sheetâ like flux ropes near the time of formation into lower energy state with circular cross section predicted by theory and termed as the â Taylorâ state.Key PointsIonâ scale flux ropes are observed to have either flattened or circular cross sections using MDD and GS reconstructionAnalysis of 25 flux ropes show that circular crossâ section flux ropes have stronger core field and smaller thermal pressures than flattened flux ropesThe two types of flux ropes may be the results of reconnection, temporal evolution, or interactions with external environmentPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150544/1/grl59049.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150544/2/grl59049_am.pd

Parker solar probe: four years of discoveries at solar cycle minimum

Launched on 12 Aug. 2018, NASA’s Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission’s primary science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-Alfvénic solar wind, which is one of the mission’s primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles

Discontinuity analysis of the leading switchback transition regions

Context. Magnetic switchbacks are magnetic structures characterized as intervals of sudden reversal in the radial component of the pristine solar wind’s magnetic field. Switchbacks comprise of magnetic spikes that are preceded and succeeded by switchback transition regions within which the radial magnetic field reverses. Determining switchback generation and evolution mechanisms will further our understanding of the global circulation and transportation of the Sun’s open magnetic flux. Aims. The present study juxtaposes near-Sun switchback transition regions’ characteristics with similar magnetic discontinuities observed at greater radial distances with the goal of determining local mechanism(s) through which switchback transition regions may evolve. Methods. Measurements from fields and plasma suites aboard the Parker Solar Probe were utilized to characterize switchback transition regions. Minimum variance analysis (MVA) was applied on the magnetic signatures of the leading switchback transition regions. The leading switchback transition regions with robust MVA solutions were identified and categorized based on their magnetic discontinuity characteristics. Results. It is found that 78% of the leading switchback transition regions are rotational discontinuities (RD). Another 21% of the leading switchback transition regions are categorized as “either” discontinuity (ED), defined as small relative changes in both magnitude and the normal component of the magnetic field. The RD-to-ED event count ratio is found to reduce with increasing distance from the Sun. The proton radial temperature sharply increases (+ 29.31%) at the leading RD-type switchback transition regions, resulting in an enhanced thermal pressure gradient. Magnetic curvature at the leading RD-type switchback transition regions is often negligible. Magnetic curvature and the thermal pressure gradient are parallel (i.e., “bad” curvature) in 74% of the leading RD-type switchback transition regions. Conclusions. The leading switchback transition regions may evolve from RD-type into ED-type magnetic discontinuities while propagating away from the Sun. Local magnetic reconnection is likely not the main driver of this evolution. Other drivers, such as plasma instabilities, need to be investigated to explain the observed significant jump in proton temperature and the prevalence of bad curvature at the leading RD-type switchback transition regions