MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Open-Close Field line Boundary Characterization of Saturn‘s magnetosphere using Cassini MIMI-LEMMS data and auroral

observations from HST and Cassini-UVISNorbert Krupp, Elias Roussos

MPS, Katlenburg-Lindau, GermanyAikaterini Radioti, Denis Grodent

LPAP, Liege, BelgiumDonald G. Mitchell

JHUAPL, Laurel, MD, USAMichele K. Dougherty

Imperial College, London, UK

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Previous work: Mitchell et al, 2009

• electron beam width < 3 deg• acceleration region < 1 RS• electron beams associated with

ion conics

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Previous work: Belenkaya et al, 2011

HST UV images of Saturn’s southern auroras, projected onto a spheroidal surface 1100 km above the atmospheric1 bar level,

The locations of the polewardand equatorward boundaries of the emission in each imageare shown by red crosses,

The superposed solid orange lines show the open field regions

Mappings of field lines ito quatorial plane (blue and purple squares for the equatorward and poleward boundaries)

the two inner circles centred on the planet (black dot) show the inner and outer boundaries of the equatorial ring current,

the red line corresponds to the boundary between open and closed field lines.

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Previous work: Gurnett et al. GRL 2010

• low-energy electron density drop

• auroral hiss

• ratio of diff. intensities from field-aligned energetic electrons with the same energy but oppositie directions is a proxy of open-closed field line configuration:

ratio = 1 (CLOSED)ratio ≠ 1 (OPEN)

Indications for open-closed field line boundary:

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Cassini MIMI/LEMMS orientation and pitch angle coverage

HE LE

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Cassini MIMI/LEMMS orientation and pitch angle coverage

HE

LE

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Example 1

65 deg SR = 5 RSL = 30-40

• MIMI/LEMMS pointing along field direction during whole period

• S/C staring• large asymmetry in intensities

measured in low-energy end compared to high-energy end

• pitch angle 180 degrees in the south

• possible crossing of open-closed field line boundary

• intensity increase by 3 orders of magnitude !!

electrons move along the open field line away from the planet‘s south pole towards the S/C

closed open ???

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Example 2

40 deg NR = 18 RSL = 40

• MIMI/LEMMS pointing along field direction only shortly during S/C rotation

• S/C rotating• large asymmetry in high

energy end• pitch angle 0 degrees in the

north• beams only every second S/C

rotation (?)

electrons move along the open field line away from the north pole towards the S/C

highly collimated beams

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Example 3

53 deg NR = 23 RSL = 65

• MIMI/LEMMS pointing along field direction only shortly during S/C rotation

• lS/C rotating• large asymmetry in low-

energy end first then in high energy end

• pitch angle 0 degrees in the north

electrons move along the open field line away from the planet‘s north pole

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Time periods of asymmetries in field-aligned electron intensities in Saturn‘s magnetosphere

Cassini MIMI/LEMMS data July 2004-July 2011

50 time periods interpreted as periods when Cassini was on open field lines

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Locations of open field lines in Saturn‘s magnetosphere using offset dipole (green) or Khurana field model (red)

noon-midnight cut dawn-dusk cut

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Traces of the open field line locations in the auroral zone

Northern hemisphere

Southern hemisphere

questionable

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

2008_195 NORTH

2008_129 NORTH 2008_334 NORTH

2008_195 NORTH

Emissions that are associated with open field lines (bifurcations: signatures of reconnection at the magnetopause, Radioti et al, submitted )

The location of the main ring of emission varies, but mainly the tracing is poleward of the emission, thus on open field lines

08/07/2011

2008_197 SOUTH

RED: Current sheet modelGREEN: dipoleBLUE: Khurana

12 LT

18 LT

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Comparison with recent Cassini/CAPS results: mapping of the cold and hot beam-like populations down to the ionosphere using a dipole within 20 RS

Northern Southern

see Poster by Arridge et al.

MOP 2011, BOSTON, MA, USA July 14, 2011 Norbert Krupp

Conclusions and next steps

• 50 events of highly collimated field-aligned electrons are found in Cassini MIMI/LEMMS data interpreted as open line regions in Saturn‘s high latitude

magnetosphere• Observations of open field lines when Cassini was at 35-75

S and 20-72 N degrees latitudesopen field lines map nicely poleward of the main auroral oval

when compared with Cassini/UVIS images

• To do list:– include closed field line tracing when looking along the field for

a better determination of open-closed boundary– Determine width of electron beams– Comparison with other data sets