|
|
 |
|
|
Ann. Geophys., 23, 1405-1431, 2005
www.ann-geophys.net/23/1405/2005/
© European Geosciences Union 2005
Interplanetary magnetic field control of Saturn's polar cusp aurora
E. J. Bunce, S. W. H. Cowley, and S. E. Milan
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
Abstract. Dayside UV emissions in Saturn's polar ionosphere have been suggested to be
the first observational evidence of the kronian "cusp" (Gérard et al.,
2004). The emission has two distinct states. The first is a bright arc-like
feature located in the pre-noon sector, and the second is a more diffuse
"spot" of aurora which lies poleward of the general location of the main
auroral oval, which may be related to different upstream interplanetary
magnetic field (IMF) orientations. Here we take up the suggestion that these
emissions correspond to the cusp. However, direct precipitation of electrons
in the cusp regions is not capable of producing significant UV aurora. We
have therefore investigated the possibility that the observed UV emissions
are associated with reconnection occurring at the dayside magnetopause,
possibly pulsed, akin to flux transfer events seen at the Earth. We devise a
conceptual model of pulsed reconnection at the low-latitude dayside
magnetopause for the case of northwards IMF which will give rise to pulsed
twin-vortical flows in the magnetosphere and ionosphere in the vicinity of
the open-closed field-line boundary, and hence to bi-polar field-aligned
currents centred in the vortical flows. During intervals of high-latitude
lobe reconnection for southward IMF, we also expect to have pulsed
twin-vortical flows and corresponding bi-polar field-aligned currents. The
vortical flows in this case, however, are displaced poleward of the
open-closed field line boundary, and are reversed in sense, such that the
field-aligned currents are also reversed. For both cases of northward and
southward IMF we have also for the first time included the effects associated
with the IMF By effect. We also include the modulation introduced by the
structured nature of the solar wind and IMF at Saturn's orbit by developing
"slow" and "fast" flow models corresponding to intermediate and high
strength IMF respectively. We then consider the conditions under which the
plasma populations appropriate to either sub-solar reconnection or
high-latitude lobe reconnection can carry the currents indicated. We have
estimated the field-aligned voltages required, the resulting precipitating
particle energy fluxes, and the consequent auroral output. Overall our model
of pulsed reconnection under conditions of northwards and southwards IMF, and
for varying orientations of IMF By, is found to produce a range of UV
emission intensities and geometries which is in good agreement with the data
presented by Gérard et al. (2004). The recent HST-Cassini solar wind
campaign provides a unique opportunity to test the theoretical ideas
presented here.
Full Article in PDF (2271 KB) |
|
|
