S. Kellock, M. K. Dougherty, Imperial College, M.G. Kivelson, K.K. Khurana, UCLA, A. Lecacheux, Observatoire de Paris, Meudon, V.M. Vasyliunas, MPI für Sonnensystemforschung, Lindau, Acknowledgements : D.J. Southwood 1,2,3 1 Imperial College, London, UK/ 2 ESA, Paris, France/ 3 UCLA, Los Angeles, USA.

Espinosa et al. 2001/2003 Led to “Camshaft” model vindicated by Cassini era observations

B r, B  B q, B t,

BB B r

Plots of the azimuthal magnetic field component, B , and sine wave at the SKR frequency against the model phase of the SKR signal (middle/lower panel). The middle/lower panel shows a plot of the same signal with a phase correction for rotation/without correction. There is little doubt that the SKR phase corrected for rotation orders the data better. Data comes form the July pass.

The amplitude varies little over 15 > R > 2.7

The amplitude of B  varies little with latitude

0 5  Jan Feb Mar Apr May Jun 29-Jul Jul Aug

02  4  6  8  SKR phase (radians)

-  -  /2 0  /2  SKR phase (radians)

Analysis of magnetic cam through periapsis passes using SKR phase model of Lecacheux/Kurth All data from inside 15 R s has been put into a ‘time’ coordinate System based on the SKR phase model. Moreover models have been compared: - Kurth/Lecacheux and Southwood give little difference - Gurnett phase (from paper in press with Science) seems less effective. - work has been standardised on Kurth formula.

Data from 8 consecutive Cassini periapsis passes Jan-Aug The upper panel shows a superposition of 90 min. averaged Bf plots. The abscissa is the rotationally adjusted phase of the predicted SKR period. In the bottom panel the average of the field over the eight passes is given as a function of phase. In each panel, the continuous red trace represents the cosine of the phase. The dotted curve in each panel is one where a phase shift has been introduced which is linearly dependent on radial displacement such as to produce a 10% phase shift at the centre of the plot.

Now let us move to latitudes beyond +/- 30° Major result of preceding section are: 1.The phase difference with radial distance – rotation produces a better ordering of magnetic cam signal. But rigid rotation is not perfect. There is a systematic phase delay between 15R s and C/A. (appeared as a frequency shift using my earlier phase-tracking algorithm). 2. Jitter from pass to pass is high enough (up to ¼ cycle – see slide 3) that use of SKR phase model for a longitude definition to be problematic.

This plot is taken from the periapsis pass of November 7-10 th 2006 The orientation of the orbit typifies the orbits of October – December The primary planetary field detected is in the radial direction until some time after C/A (– because of the high orbital inclination). C/A is around noon. The upper plot shows B r and B  field and the SPV model field (dashed trace) for comparison. Below (on different scale!) is the B  field. Note the sharp reversal in B  around 2100UT. This appears to be a field-aligned current sheet and is tentatively identified as the cusp. How do these sheets relate to SKR?

Elapsed time, minutes (record #, data inside R Jan 1, 2004) Sep Oct Oct Nov 7-10 Nov Dec Dec Dec BB Sep Oct Oct Nov 7-10 Nov Dec Dec Dec The plot above concatenates all B  data (black trace – scale on the left) recorded inside R = 15 R s in the period Sep 2006 – Dec Dates are indicated above the plot. The red trace (scale on the right) is the magnetic latitude of the spacecraft (on basis of offset dipole field model). From October 10 onwards, all passes reach magnetic latitudes less than - 36° (36°S) before C/A (closest approach). The shaded regions mark when the spacecraft is below 36°S. Note how except on the Oct pass, the exit is marked by a steep gradient in B  – the crossing of a sheet of field aligned current.

 20  30      SKR phase adjusted for rotation (using Lecacheux/Kurth formula) The abcissa in the plot above is the SKR phase adjusted for rotation B  data (black trace – scale on the left) recorded inside R = 15 R s in the period Oct 2006 – Dec The red trace (scale on the right) is magnetic latitude as before. The blue trace is local time, LT. The horizontal blue line marks midday (12LT). Note how midday delimits the position of the sharp gradient of Bf well. It is proposed that the field aligned current (FAC) is located near midday and not before. At the same time the vertical lines mark the SKR cycle. The largest FAC’s are seen on the vertical line (odd multiple of  This phase corresponds to a node  of B   and so an antinode of  r B , LT

The linkage between SKR and cam seems tied down by the result given previous slide. The SKR strobe is excited by the cam on each rotation. At this epoch, it seems that the actual cam – where the spark occurs - is the region where the field is tilted outward below the equator. Evidently, one expects (as Bunce et al indicate) that s.w. pressure can override the system.

solar wind Meridional Cut

This is the latest Cassini periapsis pass Day 12 through C/A is at 12.59R s at around 50°S magnetic latitude and11:00LT. Large magnetic cavity near C/A

Hi resolution view (1s data)

Interchange motion

B r maximum Field tilt maximum  r antinode B r node, and no tilt Meridian view, Br oscillation Equator: B  maximum Field tilt maximum   antinode B  node, and no tilt  r antinode B r node, and no tilt Edge on view B  oscillation

Internal source with sinusoidal variation with longitude External source with sinusoidal variation with longitude Potential theory :

At large r one has a rotating constant field ………….. Maintaining the cos (  –  t) dependence, one can combine potential and produce fields like…… At r = b, B r is zero – prevents flux crossing r = b……….. A Clue :

R = 2.25 R s R = 1.53 R s R = R s Sketch of the time varying magnetic field in plane perpendicular to rot’n axis Note - 1.The rotating off-axis dipole confined within the rings. 2.No rotating field threading the rings themselves. 3.Re-appearance of dipole near outer edge of ring – in order that [Br] = 0 there. 4.Dipole swamped beyond ring outer edge by quasi-constant “cam field”.

Dipole mt! radius where shield is 2d  4  b 3 = 4  (2.27) 3 = 47 nT-Rs 3 So using 4nT as amplitude ……. Aligned dipole  20  10 3 nT-Rs 3 Overall tilt is small  O(1/1000) rad