Non-equilibrium formation and relaxation of magnetic flux ropes at kinetic scales

Magnetic flux ropes are pivotal structures and building blocks in astrophysical and laboratory plasmas, and various equilibrium models have thus been studied in the past. However, flux ropes in general form at non-equilibrium, and their pathway from formation to relaxation is a crucial process that determines their eventual properties. Here we show that any localized current parallel to a background magnetic field will evolve into a flux rope via non-equilibrium processes. The detailed kinetic dynamics are exhaustively explained through single-particle and Vlasov analyses and verified through particle-in-cell simulations. This process is consistent with many proposed mechanisms of flux rope generation such as magnetic reconnection. A spacecraft observation of an example flux rope is also presented; by invoking the non-equilibrium process, its structure and properties can be explicated down to all six components of the temperature tensor.

Properties and Acceleration Mechanisms of Electrons Up To 200 keV Associated With a Flux Rope Pair and Reconnection X-Lines Around It in Earth's Plasma Sheet

The properties and acceleration mechanisms of electrons (<200 keV) associated with a pair of tailward traveling flux ropes and accompanied reconnection X-lines in Earth's plasma sheet are investigated with MMS measurements. Energetic electrons are enhanced on both boundaries and core of the flux ropes. The power-law spectra of energetic electrons near the X-lines and in flux ropes are harder than those on flux rope boundaries. Theoretical calculations show that the highest energy of adiabatic electrons is a few keV around the X-lines, tens of keV immediately downstream of the X-lines, hundreds of keV on the flux rope boundaries, and a few MeV in the flux rope cores. The X-lines cause strong energy dissipation, which may generate the energetic electron beams around them. The enhanced electron parallel temperature can be caused by the curvature-driven Fermi acceleration and the parallel electric potential. Betatron acceleration due to the magnetic field compression is strong on flux rope boundaries, which enhances energetic electrons in the perpendicular direction. Electrons can be trapped between the flux rope pair due to mirror force and parallel electric potential. Electrostatic structures in the flux rope cores correspond to potential drops up to half of the electron temperature. The energetic electrons and the electron distribution functions in the flux rope cores are suggested to be transported from other dawn-dusk directions, which is a 3-dimensional effect. The acceleration and deceleration of the Betatron and Fermi processes appear alternately indicating that the magnetic field and plasma are turbulent around the flux ropes.

Properties of Ion-Inertial Scale Plasmoids Observed by the Juno Spacecraft in the Jovian Magnetotail

We expand on previous observations of magnetic reconnection in Jupiter's magnetosphere by constructing a survey of ion-inertial scale plasmoids in the Jovian magnetotail. We developed an automated detection algorithm to identify reversals in the B θ component and performed the minimum variance analysis for each identified plasmoid to characterize its helical structure. The magnetic field observations were complemented by data collected using the Juno Waves instrument, which is used to estimate the total electron density, and the JEDI energetic particle detectors. We identified 87 plasmoids with "peak-to-peak" durations between 10 and 300 s. Thirty-one plasmoids possessed a core field and were classified as flux-ropes. The other 56 plasmoids had minimum field strength at their centers and were termed O-lines. Out of the 87 plasmoids, 58 had in situ signatures shorter than 60 s, despite the algorithm's upper limit being 300 s, suggesting that smaller plasmoids with shorter durations were more likely to be detected by Juno. We estimate the diameter of these plasmoids assuming a circular cross section and a travel speed equal to the Alfven speed in the surrounding lobes. Using the electron density inferred by Waves, we contend that these plasmoid diameters were within an order of the local ion-inertial length. Our results demonstrate that magnetic reconnection in the Jovian magnetotail occurs at ion scales like in other space environments. We show that ion-scale plasmoids would need to be released every 0.1 s or less to match the canonical 1 ton/s rate of plasma production due to Io.

Direct evidence of secondary reconnection inside filamentary currents of magnetic flux ropes during magnetic reconnection

Magnetic reconnection is a fundamental plasma process, by which magnetic energy is explosively released in the current sheet to energize charged particles and to create bi-directional Alfvénic plasma jets. Numerical simulations predicted that evolution of the reconnecting current sheet is dominated by formation and interaction of magnetic flux ropes, which finally leads to turbulence. Accordingly, most volume of the reconnecting current sheet is occupied by the ropes, and energy dissipation occurs via multiple relevant mechanisms, e.g., the parallel electric field, the rope coalescence and the rope contraction. As an essential element of the reconnecting current sheet, however, how these ropes evolve has been elusive. Here, we present direct evidence of secondary reconnection in the filamentary currents within the ropes. The observations indicate that secondary reconnection can make a significant contribution to energy conversion in the kinetic scale during turbulent reconnection.

Magnetic reconnection is a fundamental plasma process of magnetic energy conversion to kinetic energy. Here, the authors show direct evidence of secondary reconnection in the filamentary currents within the flux ropes indicating a significant contribution to energy conversion in the kinetic scale during turbulent reconnection.

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 RE 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 ~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 ~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.

Explosive Magnetotail Activity

Modes and manifestations of the explosive activity in the Earth's magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field B z ; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased B z , prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.

Evaluating Single-Spacecraft Observations of Planetary Magnetotails With Simple Monte Carlo Simulations: 1. Spatial Distributions of the Neutral Line

A simple Monte Carlo model is presented that considers the effects of spacecraft orbital sampling on the inferred distribution of magnetic flux ropes, generated through magnetic reconnection in the magnetotail current sheet. When generalized, the model allows the determination of the number of orbits required to constrain the underlying population of structures: It is able to quantify this as a function of the physical parameters of the structures (e.g., azimuthal extent and probability of generation). The model is shown adapted to the Hermean magnetotail, where the outputs are compared to the results of a recent survey. This comparison suggests that the center of Mercury's neutral line is located dawnward of midnight by 0 . 3 7 - 1 . 02 + 1 . 21 R M and that the flux ropes are most likely to be wide azimuthally (∼50% of the width of the Hermean tail). The downtail location of the neutral line is not self-consistent or in agreement with previous (independent) studies unless dissipation terms are included planetward of the reconnection site; potential physical explanations are discussed. In the future the model could be adapted to other environments, for example, the dayside magnetopause or other planetary magnetotails.

Evaluating Single Spacecraft Observations of Planetary Magnetotails With Simple Monte Carlo Simulations: 2. Magnetic Flux Rope Signature Selection Effects

A Monte Carlo method of investigating the effects of placing selection criteria on the magnetic signature of in situ encounters with flux ropes is presented. The technique is applied to two recent flux rope surveys of MESSENGER data within the Hermean magnetotail. It is found that the different criteria placed upon the signatures will preferentially identify slightly different subsets of the underlying population. Quantifying the selection biases first allows the distributions of flux rope parameters to be corrected, allowing a more accurate estimation of the intrinsic distributions. This is shown with regard to the distribution of flux rope radii observed. When accounting for the selection criteria, the mean radius of Hermean magnetotail quasi-force-free flux ropes is found to be 58 9 - 269 + 273  km. Second, it is possible to weight the known identifications in order to determine a rate of recurrence that accounts for the presence of the structures that will not be identified. In the case of the Hermean magnetotail, the average rate of quasi-force-free flux ropes is found to 0.12 min-1 when selection effects are accounted for (up from 0.05 min-1 previously inferred from observations).

Magnetotail Fast Flow Occurrence Rate and Dawn-Dusk Asymmetry at XGSM ∼ -60 RE

As a direct result of magnetic reconnection, plasma sheet fast flows act as primary transporter of mass, flux, and energy in the Earth's magnetotail. During the last decades, these flows were mainly studied within X_GSM>-60R_E , as observations near or beyond lunar orbit were limited. By using 5 years (2011-2015) of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun) data, we statistically investigate earthward and tailward flows at around 60 R_E downtail. A significant fraction of fast flows is directed earthward, comprising 43% (v_x >400 km/s) to 56% (v_x >100 km/s) of all observed flows. This suggests that near-Earth and midtail reconnection are equally probable of occurring on either side of the ARTEMIS downtail distance. For fast convective flows (v_⊥x >400 km/s), this fraction of earthward flows is reduced to about 29%, which is in line with reconnection as source of these flows and a downtail decreasing Alfvén velocity. More than 60% of tailward convective flows occur in the dusk sector (as opposed to the dawn sector), while earthward convective flows are nearly symmetrically distributed between the two sectors for low AL (>-400 nT) and asymmetrically distributed toward the dusk sector for high AL (<-400 nT). This indicates that the dawn-dusk asymmetry is more pronounced closer to Earth and moves farther downtail during high geomagnetic activity. This is consistent with similar observations pointing to the asymmetric nature of tail reconnection as the origin of the dawn-dusk asymmetry of flows and other related observables. We infer that near-Earth reconnection is preferentially located at dusk, whereas midtail reconnection (X >- 60R_E ) is likely symmetric across the tail during weak substorms and asymmetric toward the dusk sector for strong substorms, as the dawn-dusk asymmetric nature of reconnection onset in the near-Earth region progresses downtail.

Coalescence of Macroscopic Flux Ropes at the Subsolar Magnetopause: Magnetospheric Multiscale Observations

We report unambiguous in situ observation of the coalescence of macroscopic flux ropes by the magnetospheric multiscale (MMS) mission. Two coalescing flux ropes with sizes of ∼1  R_{E} were identified at the subsolar magnetopause by the occurrence of an asymmetric quadrupolar signature in the normal component of the magnetic field measured by the MMS spacecraft. An electron diffusion region (EDR) with a width of four local electron inertial lengths was embedded within the merging current sheet. The EDR was characterized by an intense parallel electric field, significant energy dissipation, and suprathermal electrons. Although the electrons were organized by a large guide field, the small observed electron pressure nongyrotropy may be sufficient to support a significant fraction of the parallel electric field within the EDR. Since the flux ropes are observed in the exhaust region, we suggest that secondary EDRs are formed further downstream of the primary reconnection line between the magnetosheath and magnetospheric fields.

Magnetic reconnection in Saturn's magnetotail: A comprehensive magnetic field survey

Reconnection within planetary magnetotails is responsible for locally energizing particles and changing the magnetic topology. Its role in terms of global magnetospheric dynamics can involve changing the mass and flux content of the magnetosphere. We have identified reconnection related events in spacecraft magnetometer data recorded during Cassini's exploration of Saturn's magnetotail. The events are identified from deflections in the north-south component of the magnetic field, significant above a background level. Data were selected to provide full tail coverage, encompassing the dawn and dusk flanks as well as the deepest midnight orbits. Overall 2094 reconnection related events were identified, with an average rate of 5.0 events per day. The majority of events occur in clusters (within 3 h of other events). We examine changes in this rate in terms of local time and latitude coverage, taking seasonal effects into account. The observed reconnection rate peaks postmidnight with more infrequent but steady loss seen on the dusk flank. We estimate the mass loss from the event catalog and find it to be insufficient to balance the input from the moon Enceladus. Several reasons for this discrepancy are discussed. The reconnection X line location appears to be highly variable, though a statistical separation between events tailward and planetward of the X line is observed at a radial distance of between 20 and 30R_S downtail. The small sample size at dawn prevents comprehensive statistical comparison with the dusk flank observations in terms of flux closure.

Secondary magnetic islands generated by the Kelvin-Helmholtz instability in a reconnecting current sheet

Magnetic islands or flux ropes produced by magnetic reconnection have been observed on the magnetopause, in the magnetotail, and in coronal current sheets. Particle-in-cell simulations of magnetic reconnection with a guide field produce elongated electron current layers that spontaneously produce secondary islands. Here, we explore the seed mechanism that gives birth to these islands. The most commonly suggested theory for island formation is the tearing instability. We demonstrate that in our simulations these structures typically start out, not as magnetic islands, but as electron flow vortices within the electron current sheet. When some of these vortices first form, they do not coincide with closed magnetic field lines, as would be the case if they were islands. Only after they have grown larger than the electron skin depth do they couple to the magnetic field and seed the growth of finite-sized islands. The streaming of electrons along the magnetic separatrix produces the flow shear necessary to drive an electron Kelvin-Helmholtz instability and produce the initial vortices. The conditions under which this instability is the dominant mechanism for seeding magnetic islands are explored.

A Dynamic Twist in the Tail

Observations by Venus Express suggest that the magnetic tails of Venus and, by implication, comets may be more dynamic than originally thought.

Direct evidence for a three-dimensional magnetic flux rope flanked by two active magnetic reconnection X lines at Earth's magnetopause

We report the direct detection by three THEMIS spacecraft of a magnetic flux rope flanked by two active X lines producing colliding plasma jets near the center of the flux rope. The observed density depletion and open magnetic field topology inside the flux rope reveal important three-dimensional effects. There was also evidence for nonthermal electron energization within the flux rope core where the fluxes of 1-4 keV superthermal electrons were higher than those in the converging reconnection jets. The observed ion and electron energizations differ from current theoretical predictions.

MESSENGER observations of extreme loading and unloading of Mercury's magnetic tail

During MESSENGER's third flyby of Mercury, the magnetic field in the planet's magnetic tail increased by factors of 2 to 3.5 over intervals of 2 to 3 minutes. Magnetospheric substorms at Earth are powered by similar tail loading, but the amplitude is lower by a factor of approximately 10 and typical durations are approximately 1 hour. The extreme tail loading observed at Mercury implies that the relative intensity of substorms must be much larger than at Earth. The correspondence between the duration of tail field enhancements and the characteristic time for the Dungey cycle, which describes plasma circulation through Mercury's magnetosphere, suggests that such circulation determines the substorm time scale. A key aspect of tail unloading during terrestrial substorms is the acceleration of energetic charged particles, but no acceleration signatures were seen during the MESSENGER flyby.

Asymmetry of the ion diffusion region Hall electric and magnetic fields during guide field reconnection: observations and comparison with simulations

In situ measurements of magnetic reconnection in the Earth's magnetotail are presented showing that even a moderate guide field (20% of the reconnecting field) considerably distorts ion diffusion region structure. The Hall magnetic and electric fields are asymmetric and shunted away from the current sheet; an appropriately scaled particle-in-cell simulation is found to be in excellent agreement with the data. The results show the importance of correctly accounting for the effects of the magnetic shear when attempting to identify and study magnetic reconnection diffusion regions in nature.

In situ observations of a secondary magnetic island in an ion diffusion region and associated energetic electrons

Numerical simulations have predicted that an extended current sheet may be unstable to secondary magnetic islands in the vicinity of the X line, and these islands can dramatically influence the reconnection rate. In this Letter, we present the first evidence of such a secondary island near the center of an ion diffusion region, which is consistent with the action of the secondary island instability occurring in the vicinity of the X line. The island is squashed in the z direction with a strong core magnetic field. Energetic electrons with anisotropic or field-aligned bidirectional fluxes are found in the ion diffusion region, and the enhancement of energetic electron fluxes is more obvious inside the secondary island.

MESSENGER observations of magnetic reconnection in Mercury's magnetosphere

Solar wind energy transfer to planetary magnetospheres and ionospheres is controlled by magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field (IMF) and a planet's magnetic field. During MESSENGER's second flyby of Mercury, a steady southward IMF was observed and the magnetopause was threaded by a strong magnetic field, indicating a reconnection rate ~10 times that typical at Earth. Moreover, a large flux transfer event was observed in the magnetosheath, and a plasmoid and multiple traveling compression regions were observed in Mercury's magnetotail, all products of reconnection. These observations indicate that Mercury's magnetosphere is much more responsive to IMF direction and dominated by the effects of reconnection than that of Earth or the other magnetized planets.