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Ion Anisotropy in Earth's Magnetotail: Importance of High‐Energy Ions.

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Bibliographic Details
Title: Ion Anisotropy in Earth's Magnetotail: Importance of High‐Energy Ions.
Authors: Shi, Xiaofei1 (AUTHOR) sxf1698@g.ucla.edu, Angelopoulos, Vassilis1 (AUTHOR), Richard, Louis2 (AUTHOR), Artemyev, Anton1,3 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Jun2026, Vol. 131 Issue 6, p1-12. 12p.
Subject Terms: *Geomagnetism, Ions, Current sheets, Magnetotails, Fast ions, Plasma pressure, Plasma physics
Company/Entity: THEMIS (Artificial satellites)
Abstract: The reconfiguration of the magnetotail current sheet during substorms often includes the formation of a thin current sheet (TCS) with a strong magnetic field line tension force. This force cannot be balanced by isotropic plasma pressure gradients, and force balance in such a TCS requires ion anisotropy and/or agyrotropy of plasma pressure. A statistical investigation of these plasma properties in the magnetotail is challenging because a significant contribution to ion anisotropy or agyrotropy stems from the suprathermal ion population, which often has energies beyond the upper limit of electrostatic analyzer measurements. In this study, we compare ion measurements from Cluster, MMS, and THEMIS to investigate the ion anisotropy in the magnetotail current sheet. We show that the central region of the magnetotail is characterized by transverse anisotropy of suprathermal ions, whereas subthermal field‐aligned anisotropic ions populate the magnetotail current sheet boundaries. Our results indicate that different ion populations (subthermal, thermal, and suprathermal ions) exhibit distinct behaviors within the current sheet, showing that different combinations of these populations contribute differently to the stability of current sheets. Key Points: We collect ion measurements from multiple missions to statistically investigate ion pressure anisotropy in the magnetotail current sheetNear the equatorial plane, suprathermal ions contribute to the transverse anisotropyAt the current sheet boundaries, subthermal ions contribute to the field‐aligned anisotropy [ABSTRACT FROM AUTHOR]
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Description
Abstract:The reconfiguration of the magnetotail current sheet during substorms often includes the formation of a thin current sheet (TCS) with a strong magnetic field line tension force. This force cannot be balanced by isotropic plasma pressure gradients, and force balance in such a TCS requires ion anisotropy and/or agyrotropy of plasma pressure. A statistical investigation of these plasma properties in the magnetotail is challenging because a significant contribution to ion anisotropy or agyrotropy stems from the suprathermal ion population, which often has energies beyond the upper limit of electrostatic analyzer measurements. In this study, we compare ion measurements from Cluster, MMS, and THEMIS to investigate the ion anisotropy in the magnetotail current sheet. We show that the central region of the magnetotail is characterized by transverse anisotropy of suprathermal ions, whereas subthermal field‐aligned anisotropic ions populate the magnetotail current sheet boundaries. Our results indicate that different ion populations (subthermal, thermal, and suprathermal ions) exhibit distinct behaviors within the current sheet, showing that different combinations of these populations contribute differently to the stability of current sheets. Key Points: We collect ion measurements from multiple missions to statistically investigate ion pressure anisotropy in the magnetotail current sheetNear the equatorial plane, suprathermal ions contribute to the transverse anisotropyAt the current sheet boundaries, subthermal ions contribute to the field‐aligned anisotropy [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2026JA035223