Dynamics of the auroral bifurcations at Saturn and
their role in magnetopause reconnection
A. Radioti, J.-C. Gérard, D. Grodent, J. Gusti, B. Bonfond, W. Pryor
LPAP - Université de Liège
Main auroral ring of emissions
Boundary of openclosed field lines :
layer of upwarddirected FAC flows
density requires
downward acceleration of
magnetospheric electrons
The precipitating fluxes
are sufficient to produce
auroras of several kR
Bunce et al., 2008
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
0800
1000
1200
“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
Polar arcs at Earth – Comparison with Saturn
Earth
Saturn
Each polar arc type occurs for a characteristic combination of
solar wind parameters (northward IMF, strong IMF magnitude,
high solar wind speed)
N
18 LT
During steady IMF
12 LT
S
After an IMF By
sign change
After an IMF Bz
sign change
18 LT
12 LT
S
e) Multiple arcs (all above types together)
18 LT
No characteristic IMF
click angle dependence
12 LT
(Kullen et al., 2002)
MAGNETIC RECONNECTION
Auroral signatures of dayside reconnection
HST UV images
Subsolar reconnection
• 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 highlatitude lobe reconnection
pulsed twin- vortical flows ->
bi-polar FAC and auroral
intensification poleward of the
OCFLB
High latitude reconnection
Bunce et al., 2005
Gérard et al., 2004
Bifurcations of the main auroral emission
UVIS – UV aurora
The bifurcations are associated with
reconnection events in the flank of the
magnetopause (Radioti et al., 2011)
VIMS – IR aurora
Badman et al., 2012
Bifurcations of the main auroral emission
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
July 13, 2008 (DOY 195)
Radioti et al., 2011
• 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)
The bifurcations are associated with
reconnection events in the flank of the
magnetopause
Multiple brightenings on the bifurcations
2009 021
18:00
18:15
18:30
18:45
19:00
19:25
19:40
19:55
06 LT
12 LT
20:10
20:25
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
Multiple reconnection along the same flux tube
19:00
19:25
1
19:40
19:55
2
2
1
Fasel et al., 1993
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’: 2nd brightening of the arc (EL’ and CL’ the
same ionospheric foot)
• EC’: not connected to the ionosphere -> no
additional auroral signature
Radioti et al, submitted
•Not the same ionospheric signature in the south !
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.