Bifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause

A Radioti,W Pryor,S E Milan,J.-C Gérard,D Grodent,J Gustin,B Bonfond

doi:10.1029/2011ja016661

Abstract

[1] This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously.

Highlights

[2] Several studies suggest that the main auroral ring at Saturn is produced by field aligned currents and plasma precipitation associated with the solar wind magnetosphere interaction [e.g., Cowley et al, 2004], to the terrestrial aurora at Earth [e.g., Paschmann et al, 2002]
[13] the consecutive bifurcations of the main ring between 11 and 18 magnetic local time (MLT) appear within ∼60 min each, on the same order of magnitude with the 20–30 min reconnection interpulse period estimated for Saturn [Bunce et al, 2005], in analogy with the flux transfer events (FTEs) seen at Earth
The consecutive bifurcations of the main ring appear between 11 and 18 MLT, within ∼60 min each, a timescale of the same order of magnitude with the 20– 30 minutes reconnection interpulse period estimated for Saturn [Bunce et al, 2005], in analogy with the FTEs seen at Earth

Summary

Introduction

[2] Several studies suggest that the main auroral ring at Saturn is produced by field aligned currents and plasma precipitation associated with the solar wind magnetosphere interaction [e.g., Cowley et al, 2004], to the terrestrial aurora at Earth [e.g., Paschmann et al, 2002]. [8] If the auroral features under study are the ionospheric signatures of consecutive reconnection events at the magnetopause of Saturn, they should represent the total magnetic flux opened during dayside reconnection processes.

Results

Conclusion