The 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, ∼10
The magnetotail current sheet carries the current responsible for the largest fraction of the energy storage in the magnetotail, the magnetic energy in the lobes. It is thus inextricably linked with the dynamics and evolution of many magnetospheric phenomena, such as substorms. The magnetotail current sheet structure and stability depend mostly on the kinetic properties of the plasma populating the magnetotail. One of the most underinvestigated properties of this plasma is electron temperature anisotropy, which may contribute a large fraction of the total current. Using observations from five missions in the magnetotail, we examine the electron temperature anisotropy,
- Award ID(s):
- 1902699
- NSF-PAR ID:
- 10375026
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 125
- Issue:
- 1
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract R E , to the lunar orbit ∼60R E ) 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. -
more » « less
Abstract Statistical and case studies, as well as data‐mining reconstructions suggest that the magnetotail current in the substorm growth phase has a multiscale structure with a thin ion‐scale current sheet embedded into a much thicker sheet. This multiscale structure may be critically important for the tail stability and onset conditions for magnetospheric substorms. The observed thin current sheets are found to be too long to be explained by the models with isotropic plasmas. At the same time, plasma observations reveal only weak field‐aligned anisotropy of the ion species, whereas the anisotropic electron contribution is insufficient to explain the force balance discrepancy. Here we elaborate a self‐consistent equilibrium theory of multiscale current sheets, which differs from conventional isotropic models by weak ion anisotropy outside the sheet and agyrotropy caused by quasi‐adiabatic ion orbits inside the sheet. It is shown that, in spite of weak anisotropy, the current density perturbation may be quite strong and localized on the scale of the figure‐of‐eight ion orbits. The magnetic field, current and plasma density in the limit of weak field‐aligned ion anisotropy and strong current sheet embedding, when the ion scale thin current sheet is nested in a much thicker Harris‐like current sheet, are investigated and presented in an analytical form making it possible to describe the multiscale equilibrium in sharply stretched 2D magnetic field configurations and to use it in kinetic simulations and stability analysis.
-
more » « less
Abstract We report an event of two‐satellite measurements of simultaneous earthward and tailward fast flows of ~500 km/s in the midtail at
X ~ −63 REand evaluate magnetic reconnection as a responsible mechanism by comparing the observations with a particle‐in‐cell (PIC) simulation. The two satellites were near midnight separated mainly along theX direction by ~5 RE. As they moved across the current sheet from the northern to southern lobes, the one closer to the Earth crossed the x line with fast flows changing from tailward to earthward, while the other one simultaneously observed tailward flows. The observed plasma and fields showed several key reconnection signatures, including the Walén relation, the fast reconnection rate of ~0.1, the Hall magnetic and electric fields, and counterstreaming electrons in the separatrix, indicating the fast flow was the reconnection exhaust. The observed temporal variations of flow speeds and magnetic fields suggested that the x line was moving tailward to a location between the two satellites and the exhaust was moving up and down. Within the exhaust, plasma pressure was highly anisotropic, and the current sheet can be unstable to the mirror, ion cyclotron, and firehose instabilities. Current sheet flapping and enhanced compressional waves near proton's local gyro frequencies were observed around the current sheet. Comparing with the PIC simulation suggests that the waves were mainly a result of oblique firehose instability. -
more » « less
Abstract Reconnection in the magnetotail occurs along so‐called X‐lines, where magnetic field lines tear and detach from plasma on microscopic spatial scales (comparable to particle gyroradii). In 2017–2020, the Magnetospheric MultiScale (MMS) mission detected X‐lines in the magnetotail enabling their investigation on local scales. However, the global structure and evolution of these X‐lines, critical for understanding their formation and total energy conversion mechanisms, remained virtually unknown because of the intrinsically local nature of observations and the extreme sparsity of concurrent data. Here, we show that mining a multi‐mission archive of space magnetometer data collected over the last 26 yr and then fitting a magnetic field representation modeled using flexible basis‐functions faithfully reconstructs the global pattern of X‐lines; 24 of the 26 modeled X‐lines match (
B z = 0 isocontours are within ∼2 Earth radii orR E ) or nearly match (B z = 2 nT isocontours are within ∼2R E ) the locations of the MMS encountered reconnection sites. The obtained global reconnection picture is considered in the context of substorm activity, including conventional substorms and more complex events. -
more » « less
Abstract The spatial scale and intensity of Earth’s magnetotail current sheet determine the magnetotail configuration, which is critical to one of the most energetically powerful phenomena in the Earth’s magnetosphere, substorms. In the absence of statistical information about plasma currents, theories of the magnetotail current sheets were mostly based on the isotropic stress balance. Such models suggest that thin current sheets cannot be long and should have strong plasma pressure gradients along the magnetotail. Using Magnetospheric Multiscale and THEMIS observations and global simulations, we explore realistic configuration of the magnetotail current sheet. We find that the magnetotail current sheet is thinner than expected from theories that assume isotropic stress balance. Observed plasma pressure gradients in thin current sheets are insufficiently strong (i.e., current sheets are too long) to balance the magnetic field line tension force. Therefore, pressure anisotropy is essential in the configuration of thin current sheets where instability precedes substorm onset.
