4.5 Article

Direct Observations of Electron Firehose Fluctuations in the Magnetic Reconnection Outflow

Journal

Publisher

AMER GEOPHYSICAL UNION
DOI: 10.1029/2022JA031128

Keywords

plasma instabilities; electron firehose instability; magnetic reconnection; collisionless plasmas; spacecraft observations

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Electron temperature anisotropy-driven instabilities like the electron firehose instability (EFI) are important in collisionless plasmas, especially in space. This study uses in situ measurements to observe EFI fluctuations in the magnetic reconnection exhaust in Earth's magnetotail. The observed fluctuations agree with theoretical predictions and provide the first direct in situ observations of EFI-generated fluctuations.
Electron temperature anisotropy-driven instabilities such as the electron firehose instability (EFI) are especially significant in space collisionless plasmas, where collisions are so scarce that wave-particle interactions are the leading mechanisms in the isotropization of the distribution function and energy transfer. Observational statistical studies provided convincing evidence in favor of the EFI constraining the electron distribution function and limiting the electron temperature anisotropy. Magnetic reconnection is characterized by regions of enhanced temperature anisotropy that could drive instabilities-including the electron firehose instability-affecting the particle dynamics and the energy conversion. However, in situ observations of the fluctuations generated by the EFI are still lacking and the interplay between magnetic reconnection and EFI is still largely unknown. In this study, we use high-resolution in situ measurements by the Magnetospheric Multiscale spacecraft to identify and investigate EFI fluctuations in the magnetic reconnection exhaust in the Earth's magnetotail. We find that the wave properties of the observed fluctuations largely agree with theoretical predictions of the non-propagating EF mode. These findings are further supported by comparison with the linear kinetic dispersion relation. Our results demonstrate that the magnetic reconnection outflow can be the seedbed of EFI and provide the first direct in situ observations of EFI-generated fluctuations.

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