Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations

S W H Cowley,A J Coates,J.-C Gérard,M K Dougherty,C S Arridge,D Grodent,J D Nichols,E J Bunce,J T Clarke,D L Talboys

doi:10.5194/angeo-26-2613-2008

Abstract

Abstract. The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphere-ionosphere coupling at Saturn (Cowley et al., 2004), that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the height-integrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, requiring downward acceleration of outer magnetosphere electrons through a ~10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ~1° width with UV emission intensities of a few tens of kR.

Highlights

Observations of planetary auroras provide a means of remotely sensing global magnetospheric dynamics, projected along field lines into the polar upper atmosphere
Imaging of the southern polar UV emissions by the Hubble Space Telescope (HST) has shown that the main oval at noon is associated with a major layer of upward-directed field-aligned current spanning the cusp boundary between open and closed field lines (Bunce et al, 2008b)
We note that this enhanced conductivity applies to the southern ionosphere under summer conditions. These factors increase the total upward-directed field-aligned current per radian of azimuth flowing in the model open-closed field line boundary by a factor of about five compared with the CBO model, from ∼0.8 MA rad−1 to ∼4 MA rad−1, in accordance with the observations

Summary

Introduction

Observations of planetary auroras provide a means of remotely sensing global magnetospheric dynamics, projected along field lines into the polar upper atmosphere. It is generally understood that for the giant planets the principal momentum exchange is from planetary rotation to magnetospheric plasma, and that for the planetary field polarities present at both Jupiter and Saturn, upward currents flow where the plasma angular velocity decreases with increasing latitude. At such locations a bright “auroral oval” may form if the downward acceleration of magnetospheric electrons required by the density of the upwarddirected field-aligned current produces precipitating electron energy fluxes of sufficient intensity. Cowley et al.: Auroral current systems in Saturn’s magnetosphere

Objectives

Conclusion