Dynamical consequences of two modes of centrifugal instability in Jupiter's outer magnetosphere

Abstract

Important global‐scale properties of the middle and outer portions of the Jovian magnetosphere can be interpreted in terms of plasma experiencing either marginal or explosive centrifugal instability as a response to rotational stresses. The process accounts for characteristics of the plasmasheet, particularly a strong dawn‐dusk asymmetry, inferred from magnetic field data. Confining forces externally imposed at the magnetopause are critical to this analysis. Because of the vast length of magnetospheric flux tubes, most plasma‐disk ions do not bounce between mirror points as a flux tube rotates through local time sectors. Rotational stresses can lead to irreversible heating and increase plasma anisotropy, creating the conditions for firehose instability. The outer edge of the plasma disk appears marginally stable prenoon, but there is weak loss of plasma from its outer edge. The plasma disk thickens further postnoon as it assimilates the empty flux tubes of the outer magnetosphere where small perpendicular scale instability enables Bohm diffusion of plasma to refill depleted flux tubes. On the nightside, the plasmasheet becomes strongly unstable, material flows out down tail, the plasma disk thins, and its outer portions can break off leaving depleted closed flux tubes behind. This outflow must dominate solar wind‐controlled reconnection in the magnetotail. The depleted flux tubes at large radial distance on the morningside form a distinct low‐density plasma/magnetic regime that overlays a thin plasma disk. The dawn‐dusk asymmetry in the UV aurora is likely associated with the larger tapping of the ionospheric flywheel in the afternoon sector to drive the heating and instability that the model proposes.