Saturn’s field aligned currents and their modulation by the Planetary Period Oscillations Greg Hunt 1 *, S. W. H. Cowley 1, G. Provan 1, E. J. Bunce 1, I. I. Alexeev 2, E. S. Belenkaya 2, V.V. Kalegaev 2, M. K. Dougherty 3 and A. J. Coates 4 1 Department of Physics and Astronomy, University of Leicester, 2 Skobeltsyn Institute of Nuclear Physics, Moscow State University, 3 Blackett Laboratory, Imperial College London, 4 Mullard Space Science Laboratory, University College London

Aims and motivation Determine the dependence of the field-aligned currents on the phase of the planetary period oscillations (PPOs) Separate out the PPO-independent currents from the PPO-related currents Determine each of the current systems’ structure, position and strength By better understanding these current systems we can improve upon theoretical discussions on the source the Saturn’s periodic behaviour Understanding these current systems will also play a key part in Cassini’s Grand Finale The results presented here are from two papers, both of between can be found online in JGR: Space physics, Hunt et al [2014, 2015] and the current work is in progress

Current systems [Hill [1979], Cowley and Bunce [2003], Southwood and Kivelson [2007], Andrews et al [2010], Jia et al [2012], Jia and Kivelson [2012], Southwood and Cowley [2014], Hunt et al [2014], Hunt et al [2015]]

Data set To study the field-aligned current signatures we used Cassini magnetometer data from when Cassini was highly inclined from the equatorial plane Such a set of Revs were performed in 2008 at ~fixed local time on the nightside Previously studied by Talboys et al [2009, 2011] The bottom two panels show Cassini magnetic footprint in both northern and southern hemispheres Auroral boundaries from Carbary [2012]

Analysis Procedure

Phase coordinate

Southern hemisphere data

Southern hemisphere currents We combine 31 Revs’ I m profiles into eight 45° (non-overlapping) sectors of southern PPO phase and calculate the weighted mean profiles By exploiting the symmetry of the subcorotation and expected antisymmetry of the PPO current, it is possible to separate them, by summing and differencing opposite phases We only found organisation by the southern PPO

Southern hemisphere currents

Southern hemisphere boundary motion Shown here are the positions and current magnitudes at the five boundaries to the four sheets of current We have determined the colatitude of the current layers and found how they vary as a function of southern PPO phase The current layers are displaced maximally towards the pole at 90° and a maximally towards the equator at 270°, with a point to point amplitude of ~2° The fitted sin functions for the positions in the top figure are shown below on a polar plot By considering the plasma and atmospheric flows physical implication the boundary motions with southern PPO phase can be determined

Southern hemisphere boundary motion

Northern hemisphere Now moving on to the northern hemisphere sections of the 2008 Revs Here are six NH ionospheric colatitude profiles chosen to exemplify differing conditions of PPO- related phases The top row the main PPO currents are expected to have the same sense The middle row are where the PPO currents are opposite in sense The bottom row where the PPO currents are minimal and an unusual profile

Northern hemisphere

Northern hemisphere profiles

The remaining four sectors are when the PPO systems are in phase and in antiphase, these are generated from the profile on the last slide and then compared with the data Below is the comparison between the profiles derived the northern hemisphere data and the mapped southern hemisphere profile similarly derived

Northern hemisphere

Equatorial oscillations NH Equator SH These results are from Hunt et al., 2015, JGR, submitted

Oscillations in polar regions Northern polar region Southern polar region These results are from Hunt et al., 2015, JGR, submitted Applying the same fitting method, but now to colatitude bins in the polar regions, we see as with previous studies in the polar regions the oscillations are hemispherically pure to within ~10%

Work in progress SKR power with LT [Lamy et al 2009, Andrews et al 2011] Currently, we are now comparing the field-aligned currents from different local times to the results To do this we are looking at a set of Revs from 2006/2007 which have a different local time to the 2008 Revs 2008 Revs mostly ~midnight 2006/07 Revs span pre-dawn to post noon We can then compare these two datasets to see if there are any similarities or differences which could correspond to the differences in the SKR powers So far we have done this for the southern hemisphere

Work in progress Here we compare the 2006/2007 coloured symbols with the 2008 boundary data (grey) The colatutide position has been transformed to account for the offset of the centre of the auroral oval compared to the spin/magnetic axis [Nichols et al 2008] The I m values we split into 180° sectors centred on 90° and 270° and take the mean Here we attempt to remove the sinusoidal motion of boundaries from the 2008 data and then take the mean position

Work in progress Comparison between the 2008 (dashed lines) and 2006/2007 (solid lines) calculated from the mean boundary positions and mean I m values of each boundary split into 180° of phase Then we perform the sum and differencing to get the next two plots for the PPO-independent current and PPO-related currents The key point is that the profiles are remarkably similar in strength and form, in particular the main upward current of the PPO-independent, this raises interesting questions for the current paradigm for the powerful dawn SKR and requires further work

Conclusions Saturn’s field-aligned currents are modulated in form, amplitude and position by the planetary period oscillations For the first time the subcorotation current system and PPO current systems have been separated and their individual signatures identified and shown to be co-located The PPO related currents are shown to be fully on closed lines In the southern hemisphere the boundary motion is shown to be evidence for the driving of the PPO field-aligned currents to be driven from the planetary atmosphere/ionosphere The first direct evidence of the inter-hemispheric currents associated with the southern PPO current system in the northern hemisphere’s ionosphere Both northern and southern oscillations are present on closed field lines interior to the field-aligned current region In common with previous studies in the polar regions the oscillations are hemispherically pure to within ~10% Hunt, G. J., S. W. H. Cowley, G. Provan, E. J. Bunce, I. I. Alexeev, E. S. Belenkaya, V. V. Kalegaev, M. K. Dougherty, and A. J. Coates (2014), Field-aligned currents in Saturn's southern nightside magnetosphere: Subcorotation and planetary period oscillation components, J. Geophys. Res. Space Physics, 119, , doi: /2014JA /2014JA Hunt, G. J., S. W. H. Cowley, G. Provan, E. J. Bunce, I. I. Alexeev, E. S. Belenkaya, V. V. Kalegaev, M. K. Dougherty, and A. J. Coates (2015), Field-aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations, J. Geophys. Res. Space Physics, 120, doi: /2015JA (in press) /2015JA021454

ρB ϕ ~ constant Apply Ampère’s law to the circular path I || constant along a field line due to current continuity therefore I m =2πI ||

ρB ϕ ~ constant (PPO case) Apply Ampère’s law to a short circular path either side of a thin current layer

Mapping effect

Effective Conductivities From the PPO-Independent current profile and angular velocity profile for the magnetospheric plasma. The effective Pedersen conductivity can be estimated [Cowley et al., 2008] This shows that the latitudinal variations in the effective Pedersen conductivity of the ionosphere are at least as important as the variations in plasma angular velocity in determining the structure of the field-aligned current system These results are from Hunt et al., 2015, JGR, submitted

Latitudinal variations of oscillations Select the mapped data in the grey shaded region in overlapping latitude intervals of 15° For a pair of phase offsets search for values of the three parameters in a 3D parameter cube that gives the minimum RMS deviation Vary phase offsets to find the smallest total RMS and repeat with those phase offsets fixed, to get “best” 3 parameters Constant term Northern amplitude Southern amplitude Phase offsets