Modulation of Jupiter's main auroral oval emissions by solar wind induced expansions and compressions of the magnetosphere

It has recently been suggested that the jovian 'main auroral oval' is associated with the region of upward-directed field-aligned currents in the circuit formed through angular momentum exchange between the atmosphere-ionosphere and the equatorial iogenic magnetospheric plasma. It has also been suggested that the luminosity of these emissions should be modulated primarily by the dynamic pressure of the solar wind, which causes expansions and compressions of the magnetosphere and thereby changes the angular velocity of the magnetospheric plasma and the strength of the coupling currents. Here we present a quantitative demonstration of these effects. We consider an initial empirically based middle magnetosphere field and plasma configuration extending to 50R(J), which expands outwards to 70R(J), or is compressed inwards to 30R(J), due e.g. to sudden changes in the dynamic pressure of the solar wind. Changes in the angular velocity of the plasma are computed from the flux-preserving motions of the field lines using conservation of angular momentum. The initial configuration produces a band of aurora in which the luminosity falls over similar to1degrees latitude from similar to200 kR at the poleward boundary to similar to20 kR. Expansion of the current sheet to 70R(J) produces a significant increase in luminosity over this range, with the largest effect occurring at the poleward boundary where a thin (similar to50-100 km) arc-like structure of MR intensity is formed. The initial emission is reduced by about an order of magnitude for a compression to 40R(J), and essentially to zero for a compression to 35R(J), when the middle magnetosphere plasma is brought to near rigid corotation over the whole radial range such that the coupling current system is essentially switched off. Further compression to 30R(J) then induces significant super-corotation, with a reversal in sense of the magnetosphere-ionosphere coupling current system. In this case the middle magnetosphere field-aligned currents reverse to downward throughout, such that we would not expect them to be associated with bright electron-induced auroras. However, upward-directed 'return' currents then flow in the poleward region outside of the middle magnetosphere, which may be associated with an auroral oval lying poleward of the usual 'main oval' location. A 'Hill-type' equilibrium will re-form on time scales of a few tens of hours, and with it, the main oval auroras will re-emerge, corresponding to the changed size of the magnetospheric cavity. (C) 2002 Elsevier Science Ltd. All rights reserved.