Dawn-Dusk Asymmetries in Rapidly Rotating Magnetospheres

Abstract:

Spacecraft measurements reveal perplexing dawn-dusk asymmetries of field and plasma properties in the magnetospheres of Saturn and Jupiter. Here we describe a previously unrecognized source of dawn-dusk asymmetry in a rapidly rotating magnetosphere. As plasma rotates from dawn to noon on a dipolarizing flux tube, it flows away from the equator at close to the sound speed. As plasma rotates from noon to dusk on a stretching flux tube, it is accelerated back to the equator by centrifugal acceleration at flow speeds typically smaller than the sound speed. Correspondingly, the plasma sheet remains far thicker in the afternoon than in the morning. Using two magnetohydrodynamic simulations, we analyze the forces that account for flows along and across the field in Saturn's magnetosphere and point out analogous effects at Jupiter. Different radial force balance in the morning and afternoon sectors produces net dusk to dawn flow, or equivalently, a large-scale electric field oriented from post-noon to pre-midnight.