Inertial limit on corotation

Abstract

Corotation of a planetary magnetosphere with the rotation frequency of the planet is maintained by the viscous torque exerted by ion‐neutral collisions in the planetary atmosphere, this torque being transmitted to the magnetosphere by Birkeland currents. In a steady state this torque balances the inertial drag associated with the production and/or outward transport of magnetospheric plasma. The viscous torque in the atmosphere requires some departure from rigid corotation, i.e., some difference between the average rotation velocities of the ionospheric plasma and of the un‐ionized atmosphere. In this paper we calculate the inertial corotation lag as a function of radial distance in the magnetosphere, the solution being parameterized in terms of the Pedersen conductivity of the atmosphere and the rate at which plasma mass is produced and transported outward in the magnetosphere. Although insignificant in the case of earth's magnetosphere, the calculated inertial corotation lag is significant in the case of Jupiter’s magnetosphere, where the rotation frequency may decrease by a factor of the order of 2 between the planetary surface and the magnetopause. One interesting consequence is that the active sector of Jupiter's magnetosphere (which is associated with a longitudinally restricted sector of enhanced ionospheric conductivity) should rotate faster, at a given distance, than adjacent longitude sectors and should therefore sweep up plasma from adjacent longitudes, thus amplifying the preexisting enhancement of plasma concentration in the active longitude sector.

Jupiter’s magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits

“…In contrast, IR emissions are thermally exited and provide a measurement of upper atmospheric temperature and thereby a measure of energy deposition, whether by particle precipitation or Joule heating. The IR aurora responds more slowly to energy input [28,38] and is diagnostic of the transfer of angular momentum from Jupiter to (ionized) mass flowing outward in its magnetosphere [39].Juno has provided observations of fields and particles in the polar magnetosphere of Jupiter as well as high-resolution images of the auroras at UV and IR wavelengths. While many of the observations have terrestrial analogs it appears that different processes are at work in exciting the aurora and in communicating the ionosphere-magnetosphere interaction.…”

A study of the Jovian “energetic magnetospheric events” observed by Galileo: role in the radial plasma transport

“…Its origin is still not clearly understood. It could be related to departures from the ideal MHD (existence of a finite parallel potential drop) in the ionosphere/magnetosphere current system that enforces the plasma corotation (see, for example, Hill, [1979]; Hill et al, [1983] for studies related with this current system) and/or, farther from the planet, to a regular loss of the centrifugal energy of the magnetodisc due to a still not identified mechanism. Whatever the details, it is now widely admitted that the planetary rotation is the dominant source of energy for this permanent magnetospheric activity (unlike the case of the Earth where the activity is driven by the solar wind).…”

The Jovian auroral oval

“…Although none of these assumptions is likely to be valid in detail, the resulting model does a fair job of predicting the observed departure from rigid corotation as a function of distance out to r ~ 30 Rj [Hill, 1980]. In the absence of a better alternative, this model is adopted here to illustrate the plausibility of the proposed mechanism for producing Jupiter's persistent auroral oval.…”

“…The only such model that exists for Jupiter [Hill, 1979] is based on several simplifying assumptions, including a spinaligned dipole magnetic field; a uniform height-integrated Pedersen conductivity of Jupiter's ionosphere; and a magnetospheric plasma mass outflow rate that is independent of time, radial distance, and azimuth, on a global scale. Although none of these assumptions is likely to be valid in detail, the resulting model does a fair job of predicting the observed departure from rigid corotation as a function of distance out to r ~ 30 Rj [Hill, 1980].…”