Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of precipitation-induced enhancement of the ionospheric Pedersen conductivity

摘要

We consider the effect of precipitation-induced enhancement of the Jovian ionosphericPedersen conductivity on the magnetosphere-ionosphere coupling current system which isassociated with the breakdown of the corotation of iogenic plasma in Jupiter's middle magnetosphere.In previous studies the Pedersen conductivity has been taken to be simply a constant, while it isexpected to be significantly enhanced in the regions of upward-directed auroral field-alignedcurrent, implying downward precipitating electrons. We develop an empirical model of themodulation of the Pedersen conductivity with field-aligned current density based on the modellingresults of Millward et al. and compute the currents flowing in the systemwith the conductivity self-consistently dependent on the auroral precipitation. In addition, weconsider two simplified models of the conductivity which provide an insight into the behaviour ofthe solutions. We compare the results to those obtained when the conductivity is taken to beconstant, and find that the empirical conductivity model helps resolve some outstandingdiscrepancies between theory and observation of the plasma angular velocity and current system.Specifically, we find that the field-aligned current is concentrated in a peak of magnitude ~0.25µAm-2in the inner region of the middle magnetosphere at ~20 R_J, rather than being more uniformlydistributed as found with constant conductivity models. This peak maps to ~17° in the ionosphere,and is consistent with the position of the main oval auroras. The energy flux associated with thefield-aligned current is ~10mWm-2 (corresponding to a UV luminosity of ~100kR), in a region~0.6° in width, and the Pedersen conductivity is elevated from a background of ~0.05mho to~0.7mho. Correspondingly, the total equatorial radial current increases greatly in the region ofpeak field-aligned current, and plateaus with increasing distance thereafter. This form is consistentwith the observed profile of the current derived from Galileo magnetic field data. In addition, wefind that the solutions using the empirical conductivity model produce an angular velocity profilewhich maintains the plasma near to rigid corotation out to much further distances than the constantconductivity model would suggest. Again, this is consistent with observations. Ourresultstherefore suggest that, while the constant conductivity solutions provide an important indicationthat the main oval is indeed a result of the breakdown of the corotation of iogenic plasma, they do notexplain the details of the observations. In order to resolve some of these discrepancies, one musttake into account the elevation of the Pedersen conductivity as a result of auroral electronprecipitation.Key words. Magnetospheric physics (current systems,magnetosphere-ionosphere interactions, planetary magnetospheres)70d