Dynamics of the auroral bifurcations at Saturn and their role in magnetopause reconnection LPAP - Université de Liège A. Radioti, J.-C. Gérard, D. Grodent, J. Gusti, B. Bonfond, W. Pryor

Bunce et al., 2008 Boundary of open- closed field lines : layer of upward- directed FAC flows Main auroral ring of emissions density requires downward acceleration of magnetospheric electrons The precipitating fluxes are sufficient to produce auroras of several kR These observations indicate that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the open closed-field line boundary.

Small scale structures DOY 239 of 2008, North 07:36 to 08:53UT ~77 min (3*26min) Spatial resolution ~200 km “Main Ring (Oval)”

Origin of these small scale auroral features? Complex Field-Aligned Current (FAC) structures Bunce et al. [2010], Talboys et al. [2009,2011] 1-2° (lat) wide FAC structures

During steady IMF Each polar arc type occurs for a characteristic combination of solar wind parameters (northward IMF, strong IMF magnitude, high solar wind speed) After an IMF By sign change After an IMF Bz sign change No characteristic IMF click angle dependence e) Multiple arcs (all above types together) (Kullen et al., 2002) Polar arcs at Earth – Comparison with Saturn SaturnEarth S N S 18 LT 12 LT 18 LT

MAGNETIC RECONNECTION

Bunce et al., 2005 Gérard et al., 2004 for northward IMF and pulsed reconnection at the low-latitude magnetopause -> pulsed twin vortical flows in the magnetosphere and ionosphere in the vicinity of the OCFLB for southward IMF and high- latitude lobe reconnection pulsed twin- vortical flows -> bi-polar FAC and auroral intensification poleward of the OCFLB Auroral signatures of dayside reconnection Subsolar reconnection High latitude reconnection HST UV images

Bifurcations of the main auroral emission The bifurcations are associated with reconnection events in the flank of the magnetopause (Radioti et al., 2011) UVIS – UV auroraVIMS – IR aurora Badman et al., 2012

Adapted from Lockwood and Wild (1993) Closed field lines are converted to open. A pair of bubbles of mixed magnetospheric and magnetosheath plasma is produced and moved away from the X point. Cassini observations revealed signatures of reconnection at Saturn’s magnetopause (McAndrews et al., 2008) Radioti et al., 2011 Bifurcations of the main auroral emission

Feature ‘a’: reconnection signature at low latitude dayside magnetopause for north IMF (Gérard et al., 2005 and Bunce et al., 2005) The main auroral emission expands with time to lower latitudes and the expansion is equal to the area occupied by the bifurcations The open flux is increasing and the bifurcations represent the amount of newly open flux (each reconnection event opens 10% of the flux contained in the polar cap) July 13, 2008 (DOY 195) Radioti et al., 2011 The bifurcations are associated with reconnection events in the flank of the magnetopause Bifurcations of the main auroral emission

18:0018:15 18:3018:45 19:0019:2519:40 20:10 19:55 20:25 12 LT 06 LT Multiple brightenings on the bifurcations The bifurcations: - fade and brighten up twice - move away from the main emission (Radioti et al., submitted)

Multiple brightenings Both arcs brighten quasi-simultaneously The distance of the bifurcation from the m.e. increases with time

19:0019:2519:4019:55 Multiple reconnection along the same flux tube IMFT flux tubes : CC’, DD’, EE’ Magnetospheric flux tubes: KK’,…, MM ’ t=t1 magnetic reconnection occurs between EE’ and LL’ EL’ : 1st brightening EL’ flux tube is under the influence of both magnetic tension and magnetosheath flow and begins to move antisunward -> the ionospheric arc separates from the m.e. t = t2 magnetic flux tube EL’ reconnects with CC’ (two new flux tubes : CL’ and EC’) CL’: 2 nd brightening of the arc (EL’ and CL’ the same ionospheric foot) EC’: not connected to the ionosphere -> no additional auroral signature Not the same ionospheric signature in the south ! Fasel et al., Radioti et al, submitted A. Radioti, AGU Chapman, Iceland, 2013

Summary The bifurcations are associated with reconnection in the flank of the magnetopause and they represent the amount of newly open flux We predict north-south asymmetries in the auroral emissions The conditions for multiple magnetopause reconnection at Saturn could be satisifed like at Earth. Even though the significance of magnetopause reconnection at Saturn is under debate the auroral observations demonstrate that magnetopause reconnection at Saturn is of certain significance Request for future UVIS observations: Long (several hours) sequences of images where the whole auroral region is scanned. Thus we would be able to estimate the polar cap changes and open flux and establish the significance fo magnetopause reconnection at Saturn.