Magnetotail Current Sheet Research Articles

Nightside Electron Precipitation Patterns as Observed by ELFIN and CIRBE CubeSats

AbstractThe rapidly expanding fleet of low‐altitude CubeSats equipped with energetic particle detectors brings new opportunities for monitoring the dynamics of the radiation belt and near‐Earth plasma sheet. Despite their small sizes, CubeSats can carry state‐of‐the‐art instruments that provide electron flux measurements with finer energy resolution and broader energy coverage, compared to conventional missions such as POES satellites. The recently launched CIRBE CubeSat measures 250–6,000 keV electrons with extremely high energy resolution, however, CIRBE typically only measures locally‐trapped electrons and cannot directly measure the precipitating electrons. This work aims to develop a technique for identifying indications of nightside precipitation using the locally‐trapped electron measurements by the CIRBE CubeSat. This study focuses on two main types of drivers for nightside precipitation: electron scattering by the curvature of magnetic field lines in the magnetotail current sheet and electron scattering by resonance with electromagnetic ion cyclotron (EMIC) waves. Using energy and pitch‐angle resolved electron fluxes from the low‐altitude ELFIN CubeSat, we reveal the features that distinguish between these two precipitation mechanisms based solely on locally‐trapped flux measurements. Then we present measurements from four CIRBE orbits and demonstrate the applicability of the proposed technique to the investigation of nightside precipitation using CIRBE observations, enabling separation between precipitation induced by curvature scattering and EMIC waves in nearby regions. Our study underscores the feasibility of employing high‐energy‐resolution CIRBE measurements for detecting nightside precipitation of relativistic electrons. Additionally, we briefly discuss outstanding scientific questions about these precipitation patterns that could be addressed with CIRBE measurements.

Characteristics of Thin Magnetotail Current Sheet Plasmas at Lunar Distances

AbstractThe magnetotail current sheet plays a key role in the dynamics of Earth's magnetosphere. Specifically, the formation and subsequent reconnection of thin (ion‐gyroscale) current sheets are critical components of magnetospheric substorms. However, the precise mechanisms governing the configuration and distribution of current density in these thin current sheets remain elusive. By analyzing a data set consisting of 453 thin current sheet crossings observed by the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission, we explore the statistical properties of the ion and electron pressures and current densities, Ji and Je, in the spacecraft rest frame. Using magnetotail flapping and magnetic field measurements to estimate the total current density, J0, we find that it agrees well with the sum of those from direct ion and electron measurements, Ji + Je, respectively. In 65% of thin current sheets, electrons were found to dominate the contribution to the total current density in the spacecraft frame, with a typical dawnward drift velocity of ≳100 km/s. Diamagnetic drifts of electrons and ions estimated from their respective vertical pressure profiles (along the current sheet normal) reveal that the gradient of electron pressure alone cannot fully account for the observed high values of Je/Ji. Counter‐intuitively, for most (52% of) thin current sheets the electron vertical pressure profile is wider than the ion pressure profile, again suggesting that electron diamagnetism is an insufficient contributor to the current density at such sheets. These findings suggest the presence of a significant E × B dawnward drift that the electrons can fully acquire but ions cannot, being partially unmagnetized. We compare our results with those previously reported for the near‐Earth magnetotail and discuss them in the context of magnetotail current sheet modeling.

The Key Role of Magnetic Curvature Scattering in Energetic Electron Precipitation During Substorms

AbstractEnergetic electron precipitation (EEP) during substorms significantly affects ionospheric chemistry and lower‐ionosphere (<100 km) conductance. Two mechanisms have been proposed to explain what causes EEP: whistler‐mode wave scattering, which dominates at low latitudes (mapping to the inner magnetosphere), and magnetic field‐line curvature scattering, which dominates poleward. In this case study, we analyzed a substorm event demonstrating the dominance of curvature scattering. Using ELFIN, POES, and THEMIS observations, we show that 50–1,000 keV EEP was driven by curvature scattering, initiated by an intensification and subsequent earthward motion of the magnetotail current sheet. Using a combination of Swarm, total electron content, and ELFIN measurements, we directly show the location of EEP with energies up to ∼1 MeV, which extended from the plasmapause to the near‐Earth plasma sheet (PS). The impact of this strong substorm EEP on ionospheric ionization is also estimated and compared with precipitation of PS (<30 keV) electrons.

Magnetic reconnection in the magnetodisk of centrifugally dominated giant planets

AbstractMagnetic reconnection is crucial in understanding magnetospheric dynamics and aurorae processes at planets. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisk, where thin-current-sheet conditions are conducive to reconnection. At the Earth, the magnetopause and magnetotail current sheets are primarily controlled by the upstream solar wind. At Jupiter and Saturn, their fast rotation and internal mass sources lead to an additional current sheet that encircles the planet, forming a magnetodisk inside the magnetosphere. The reconnection processes in the magnetodisk current sheet are associated with centrifugal force-driven dynamics. The magnetodisk reconnection is not limited to the nightside but is discretely distributed at all local times inside the magnetosphere. The reconnection sites also rotate with the magnetosphere. These widely distributed small-scale reconnection sites can result in the global release of energy and mass from the magnetosphere.

The Energetic Oxygen Ion Beams in the Martian Magnetotail Current Sheets: Hints From the Comparisons Between Two Types of Current Sheets

AbstractUsing data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we explore the plasma properties of Martian magnetotail current sheets (CS), to further understand the solar wind interaction with Mars and ion escape. There are some CS exhibit energetic oxygen ions that show narrow beam structures in the energy spectrum, which primarily occurs in the hemisphere where the solar wind electric field (Esw) is directed away from the planet. On average, these CS have a higher escaping flux than that of the CS without energetic oxygen ion beams, suggesting different roles in ion escape. The CS with energetic oxygen ion beams exhibits different proton and electron properties to the CS without energetic oxygen ion beams, indicating their different origins. Our analysis suggests that the CS with energetic oxygen ion beams may result from the interaction between the penetrated solar wind and localized oxygen ion plumes.

Thin Current Sheets in the Magnetotail at Lunar Distances: Statistics of ARTEMIS Observations

AbstractThe magnetotail current sheet's (CSs) spatial configuration and stability control the onset of magnetic reconnection ‐ the driving process for magnetospheric substorms. The near‐Earth CS has been thoroughly investigated by numerous missions, whereas the midtail CS has not been adequately explored. This is especially the case for the long‐term variation of its configuration in response to the solar wind. We present a statistical analysis of 1261 magnetotail CS crossings by the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission orbiting the moon (X ∼ −60 RE), collected during the entirety of Solar Cycle 24. We demonstrate that the magnetotail CS typically remains extremely thin, with a characteristic thickness comparable to the thermal ion gyroradius, even at large distances from Earth's dipole. We also find that a substantial fraction (∼one quarter) of the observed CSs have a partially force‐free magnetic field configuration, with a significant contribution of the magnetic field shear component to the pressure balance. Further, we quantify the impact of the changing solar wind driving conditions on the properties of the midtail around the lunar orbit. During active solar wind driving conditions, we observe an increase in the occurrence rate of thin CSs, whereas quiet solar wind driving conditions seem to favor the formation of partially force‐free CSs.

Reconnection Inside a Dipolarization Front of a Diverging Earthward Fast Flow

AbstractWe 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 ∼ 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.

Open Magnetic Fields in the Martian Magnetosphere Revealing Dipole-like Intrinsic Magnetic Fields at Mars

Mars’s magnetosphere is hybrid, having contributions from both an induced magnetosphere like Venus and the localized crustal magnetic fields. However, the planetary fields also include large-scale, more global components. In this study, we investigate their role in Mars’s magnetospheric topological responses to the interplanetary magnetic field (IMF) clock angle using observations from the Mars Atmospheric Volatile and EvolutioN mission. We show that the large-scale planetary field has a “dipole-like” influence on the Mars global magnetosphere by examining the open field topology. We find that the “dipole-like” planetary field, as at Earth, results in a more open magnetosphere during southward IMF. The clock angle effects on the twisted magnetotail current sheet are similarly consistent with this analogy. It reinforces the idea that Mars’s magnetosphere and solar wind interaction are more Earth-like than previously thought.

Observations of Locally Generated Whistler-mode Waves in the Martian Magnetotail Current Sheet

The whistler-mode wave is an electromagnetic wave that commonly occurs in space plasma and has been extensively studied, especially within the Earth's magnetosphere. They have also been reported in the near-Mars space, such as Martian upstream solar wind, crustal magnetic field, ionopause, and the magnetic reconnection ion diffusion region. However, the generation of whistler-mode waves in the Martian magnetotail current sheet is still unclear. Based on observations made by Mars Atmosphere and Volatile Evolution spacecraft, we report whistler-mode waves observed within a train of proton-scale magnetic dips during a Martian magnetotail current sheet crossing. The linear growth rate analyses demonstrate that the whistler-mode waves are locally generated within the magnetic dips. Unlike in Earth's plasma environment, the train of magnetic dips in the Martian plasma sheet is attributed to electron mirror-mode instability. Our finding suggests that the mirror-mode structure in the Martian magnetotail can be an important source region for generating whistler-mode waves. This provides a new insight into how whistler-mode waves are generated in unmagnetized planets.

Impact of Changing Stellar and Planetary Magnetic Fields on (Exo)planetary Environments and Atmospheric Mass Loss

The magnetic activity of a star—which modulates the stellar wind outflow—shapes the immediate environments of orbiting (exo)planets and induces atmospheric loss, thereby impacting their habitability. We perform a detailed parameter space study using three-dimensional magnetohydrodynamic simulations to understand the effect of changing stellar wind magnetic field and planetary magnetic field strengths on planetary magnetospheric topology and atmospheric losses. It is observed that the relative strengths of stellar and planetary magnetic fields play a significant role in determining the steady-state magnetospheric configuration and atmospheric erosion. When the stellar field is strengthened or the planetary field is weakened, stellar magnetic field accumulation occurs at the dayside of the planet, forcing the magnetopause to shift closer to its surface. The magnetotail opens up, leading to the formation of Alfvén wings in the nightside wake region. We demonstrate how reconnection processes and wind conditions lead to the bifurcation of the magnetotail current sheet. With increasing stellar wind magnetic field strength, the dayside reconnection point approaches the planet, thereby enhancing mass loss. We establish an analytic equation relating the atmospheric mass-loss rates to stellar and planetary magnetic field strengths, which successfully explains the modeled behavior. Our results are relevant for understanding how the interplay of stellar and planetary magnetism influence (exo)planetary environments and their habitability in star–planet systems with differing relative magnetic field strengths or in a single star–planet system over the course of their evolution with age.

The Influence of Oxygen Ions on the Formation of a Thin Current Sheet in the Magnetotail

A thin current sheet in the Earth’s magnetotail with characteristic thickness of one to several proton gyroradii is often observed during magnetospheric disturbances named substorms, when a relatively thick current configuration in the magnetotail is narrowed to an extremely small thickness and then can spontaneously be destroyed. The current-sheet destruction is usually accompanied by such active processes as plasma acceleration and heating, as well as generation of an induced electric field and magnetohydrodynamic waves. In this paper, we developed and investigated a model of formation of a thin current sheet in which, along with protons, we have taken into account single-charged oxygen ions coming from the ionosphere into the magnetotail current sheet during magnetically active periods. The aim of this simulation is to study the peculiarities of thin current-sheet formation in two-component plasma and investigate its structure. It is shown that equilibrium configuration can have some characteristic properties. In particular, if the system consists only of protons or heavy ions, single-scale current equilibrium supported by quasi-adiabatic particles is formed. When a current sheet is formed in plasma that consists of a mixture of protons and oxygen ions in comparable concentrations, a current sheet can be formed with heavy ions as current carriers and chaotic proton trajectories that make a negative contribution to the current, due to which the current-density profile becomes bifurcated with the minimum at the center and maxima at the periphery of the sheet. The results may be useful for the interpretation of observational data in the Earth’s magnetotail.

Kelvin-Helmholtz instability-driven magnetopause dynamics as turbulent pathway for the solar wind-magnetosphere coupling and the flank-central plasma sheet communication

About sixty years ago it was proposed that the solar wind entry and changes in magnetospheric magnetic topology via dayside magnetic reconnection initiate the magnetospheric convection over the poles. On the other hand, the quasi-viscous interaction via Kelvin-Helmholtz waves/vortices was proposed to lead to the solar wind entry and magnetospheric convection. Since then, the two processes have been thought to regulate the solar wind and earth’s magnetosphere coupling. However, their relative efficiency and importance leave a lot of room for enhanced and quantitative understanding. Kelvin-Helmholtz instability operating on the entire surface of the magnetopause also provide a place for not only solar wind transport but also energetic particle transport or escape, thus, being an efficient channel for two-way transport. Recent observations and simulations indicate that the flanks of the earth’s magnetosphere can act as a pathway to/from the central magnetotail current sheet. Possible causality between the flank-side dynamics and magnetotail current sheet stability has never been explored. In this paper we discuss our perspective on these unsolved areas of Heliophysics research with brief suggestions of observational and numerical approaches.

MESSENGER Observations of Reconnection in Mercury's Magnetotail Under Strong IMF Forcing

AbstractThe MESSENGER spacecraft typically crossed Mercury's magnetotail current sheet relatively close to the planet, that is, less than 2.5 RM (planet radius; 2,440 km). Magnetometer measurements are used to detect active reconnection events by identifying the quadrupole Hall magnetic field signatures that form about X‐lines. Statistical analyses of the 51 active reconnection events detected in this manner indicate that they occur most frequently on the duskside and typically at a mean altitude greater than 1.5 RM. In contrast, the dawnside events occur at altitudes of ∼1 RM. In addition, a higher recurrence rate of flux ropes formed in the Hall region was observed on the dawnside. Applying the Kan‐Lee solar wind‐magnetosphere coupling function confirmed that these near‐tail reconnection events at Mercury are observed under strong forcing by the interplanetary magnetic field. We further propose that the reconnection‐driven nightside magnetosphere‐planet interaction is characterized by a pronounced dawn‐dusk asymmetry and may significantly influence the near‐Mercury space environment.

Two Types of Martian Magnetotail Current Sheets: MAVEN Observations of Ion Composition

AbstractUsing measurements from the Mars Atmosphere and Volatile EvolutioN mission, we investigate the densities of H+ (), O+ (), and O2+ (), respectively, in the Martian magnetotail current sheet. We find that the current sheet when it is closer to the terminator than 0.75 Mars radii is mostly dominated by heavy ions (()>2 ), regardless of the variation of the upstream solar wind, but that it is sometimes dominated by H+ ( >2()) at downstream distances exceeding 0.75 Mars radii. The occurrence rate of the dominant H+ weakly increases (and that of the heavy ions decreases) with solar wind density and dynamic pressure. Our results suggest that solar wind protons could enter the Martian tail and may become the dominant ion species in the current sheet, particularly when the solar wind density or dynamic pressure is high.

Division of Magnetic Flux Rope via Magnetic Reconnection Observed in the Magnetotail

AbstractUsing high‐resolution data from Magnetospheric Multiscale (MMS) mission, we report an intense current layer at the center of a flux rope (FR) in the magnetotail. The intense current layer is caused by the compression of the ion bulk flows at the center of a FR rather than two interlaced flux tubes reported at the magnetopause previously. The intense current layer has been identified as an electron diffusion region of the magnetic reconnection, and the hall magnetic field generated by magnetic reconnection makes the FR show crater‐shaped. The reconnecting current layer is supported by the poloidal magnetic field of the FR, and it is dividing the FR into two secondary FRs. The observations suggest that magnetic FR can be compressed easily to excite instability inside it as it is propagating in the magnetotail current sheet, thus changing its magnetic topology.

The Search for Magnetotail Twisting at Mercury: Comparing MESSENGER Observations With the Terrestrial Case

AbstractPrevious studies reported that the terrestrial and Martian magnetotails can become twisted due to the solar wind‐planetary interaction; however, the associated physical processes proper of intrinsic and induced magnetospheres are still under debate. In particular, there is evidence that the Interplanetary Magnetic Field (IMF) dawn‐dusk component (By) plays a major role in both environments, affecting the sense of twist. Here, we analyze all MErcury, Surface, Space ENvironment, GEochemistry and Ranging Magnetometer observations to investigate the IMF By influence on Mercury's magnetotail. We find that Mercury's tail twist is very small (≲3°), for a median downtail distance of ∼2 Mercury radii. We also identify a correlation between the IMF By and the local By component around the magnetotail current sheet. These results suggest the small (or lack of) twist may be explained by the dipolar field strength in the near‐magnetotail. We examine this hypothesis by putting these observations into context with studies on the terrestrial magnetotail.

The Role of Magnetic Flux Rope in Ion Acceleration: MHD Simulations and Test-particle Tracing

Magnetic flux ropes (MFRs), playing a crucial role in particle energization and energy transport in the solar–terrestrial space, are helical structures produced by magnetic reconnection. It has been both theoretically predicted and observationally confirmed that MFRs and associated processes are inherently three-dimensional in space. Although such structures have been widely suggested as a favorable place for electron acceleration, whether large-scale MFRs can lead to ion acceleration has been rarely investigated. In this study, an MHD model is used to examine the evolution of large-scale MFRs in the magnetotail, and a test-particle simulation is further employed to study the associated ion energization. Results show that magnetic reconnections take place at multiple X-lines in the magnetotail current sheet, generating a twisted MFR with a scale of about 10 R e in azimuth. It propagates earthward following the tail reconnection but its east and west wings are significantly distorted azimuthally. Test-particle tracing reveals that ions (0.1–100 keV) can be trapped within the rope while being effectively accelerated. The rope therefore brings in energetic plasma sources into the inner magnetosphere as it transports earthward. These results demonstrate that the MFR is an important source carrier for the ring-current formation in the inner magnetosphere.

Statistical Study of the Relationship Between Pi1/2‐Band Wave Powers and Firehose Instability Criterion During Fast Flows in the Magnetotail Plasma Sheet

AbstractFast flows play a significant role in the transport of magnetic flux, mass, and energy in the terrestrial plasma sheet. Abundant wave activities occur during fast flows, however, what determines their wave powers is not fully understood. Here, we statistically investigate the relation of Pi1‐band (10–40 s) and Pi2‐band (40–150 s) mean wave amplitudes (MWAs) to the threshold of firehose instabilities during fast flows based on observations of the Magnetospheric Multiscale mission. To reduce the effect of dipolarization processes on the MWAs, we only analyze the fast flows with an average BZ < 3 nT, where BZ is the Z component of the magnetic field in the geocentric solar magnetospheric coordinate system. The compressional and transverse components of the magnetic field fluctuations during fast flows are mostly comparable for each frequency range. The probability of reaching firehose unstable condition tends to be larger for the faster fast flow. The Pi1/2‐band MWAs tend to be larger as the probability of the firehose unstable condition near the neutral sheet is larger for the fast flows with a certain average velocity near the neutral sheet, although they have a weak relationship. Our results suggest that the firehose instability is a possible factor to affect the Pi1/2‐band MWAs during the fast flow in the magnetotail current sheet.

Thinning of the Magnetotail Current Sheet Inferred From Low‐Altitude Observations of Energetic Electrons

AbstractThe magnetospheric substorm is a key mode of flux and energy transport throughout the magnetosphere associated with distinct and repeatable magnetotail dynamical processes and plasma injections. The substorm growth phase is characterized by current sheet thinning and magnetic field reconfiguration around the equatorial plane. The global characteristics of current sheet thinning are important for understanding of magnetotail state right before the onset of magnetic reconnection and of the key substorm expansion phase. In this paper, we investigate this thinning at different radial distances using plasma sheet (PS) energetic (>50 keV) electrons that reach from the equator to low altitudes during their fast (∼1 s) travel along magnetic field lines. We perform a multi‐case study and a statistical analysis of 34 events with near‐equatorial observations of the current sheet thinning by equatorial missions and concurrent, latitudinal crossings of the ionospheric projection of the magnetotail by the low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats at approximately the same local time sector. Energetic electron fluxes thus collected by ELFIN provide near‐instantaneous (<5 min duration) radial snapshots of magnetotail fluxes. Main findings of this study confirm the previously proposed concepts with low‐altitude energetic electron measurements: (a) Energy distributions of low‐altitude fluxes are quantitatively close to the near‐equatorial distributions, which justifies the investigation of the magnetotail current sheet reconfiguration using low‐altitude measurements. (b) The magnetic field reconfiguration during the current sheet thinning (which lasts ≥ an hour) results in a rapid shrinking of the low‐altitude projection of the entire PS (from near‐Earth, ∼10RE, to the lunar orbit ∼60RE) to 1–2° of magnetic latitude in the ionosphere. (c) The current sheet dipolarization, common during the substorm onset, is associated with a very quick (∼10 min) change of the tail magnetic field configuration to its dipolar state, as implied by a poleward expansion of the PSPS at low altitudes.

Response timescales of the magnetotail current sheet during a geomagnetic storm: Global MHD simulations

The response of the Earth’s magnetotail current sheet to the external solar wind driver is highly time-dependent and asymmetric. For example, the current sheet twists in response to variations in the By component of the interplanetary magnetic field (IMF), and is hinged by the dipole tilt. Understanding the timescales over which these asymmetries manifest is of particular importance during geomagnetic storms when the dynamics of the tail control substorm activity. To investigate this, we use the Gorgon MHD model to simulate a geomagnetic storm which commenced on 3 May 2014, and was host to multiple By and Bz reversals and a prolonged period of southward IMF driving. We find that the twisting of the current sheet is well-correlated to IMF By throughout the event, with the angle of rotation increasing linearly with downtail distance and being more pronounced when the tail contains less open flux. During periods of southward IMF the twisting of the central current sheet responds most strongly at a timelag of ∼ 100 min for distances beyond 20 RE, consistent with the 1–2 h convection timescale identified in the open flux content. Under predominantly northward IMF the response of the twisting is bimodal, with the strongest correlations between 15 and 40 RE downtail being at a shorter timescale of ∼ 30 min consistent with that estimated for induced By due to wave propagation, compared to a longer timescale of ∼ 3 h further downtail again attributed to convection. This indicates that asymmetries in the magnetotail communicated by IMF By are influenced mostly by global convection during strong solar wind driving, but that more prompt induced By effects can dominate in the near-Earth tail and during periods of weaker driving. These results provide new insight into the characteristic timescales of solar wind-magnetosphere-ionosphere coupling.