Electron-driven instabilities in Jupiter's magnetodisc

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Abstract
Thermal electrons in Jupiter's magnetodisc have been observed to be anisotropic with higher parallel temperature compared to perpendicular. In this work, we used previously-derived electron properties (Sarkango et al. 2025) to calculate electron β ∥ and compared the observed anisotropy with electron temperature-anisotropy-driven instability thresholds. For cold electrons, the ratio of perpendicular to parallel temperatures (T ⊥ /T ∥ ) decreased with M-shell, reached a minimum near M=20-25, beyond which it increased until the distribution was quasi-isotropic. In T ⊥ /T ∥ -β ∥ space, this turnover was observed close to the oblique electron firehose instability threshold. Linear dispersion analysis using plasma parameters representative of different M-shells predicts the onset of the oblique firehose instability arising primarily due to cold electron anisotropy, though growth rates differ based on the cold and hot electron abundances, anisotropies, and β ∥ . On the basis of these observations, we argue that the cold electron anisotropy in Jupiter's magnetodisc is regulated by the oblique electron firehose instability, which may be a source of plasma waves and could contribute to cyclotron-resonant heating of the Jovian plasma.
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