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.
Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness.
more » « less- Award ID(s):
- 1744269
- PAR ID:
- 10441555
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 15
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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