Field‐Aligned Currents in Saturn's Nightside Magnetosphere: Subcorotation and Planetary Period Oscillation Components During Northern Spring

Abstract

We newly analyze Cassini magnetic field data from the 2012/2013 Saturn northern spring interval of highly inclined orbits and compare them with similar data from late southern summer in 2008, thus providing unique information on the seasonality of the currents that couple momentum between Saturn's ionosphere and magnetosphere. Inferred meridional ionospheric currents in both cases consist of a steady component related to plasma subcorotation, together with the rotating current systems of the northern and southern planetary period oscillations (PPOs). Subcorotation currents during the two intervals show opposite north-south polar region asymmetries, with strong equatorward currents flowing in the summer hemispheres but only weak currents flowing to within a few degrees of the open-closed boundary (OCB) in the winter hemispheres, inferred due to weak polar ionospheric conductivities. Currents peak at 1 MA rad À1 in both hemispheres just equatorward of the open-closed boundary, associated with total downward polar currents~6 MA, then fall across the narrow auroral upward current region to small values at subauroral latitudes. PPO-related currents have a similar form in both summer and winter with principal upward and downward field-aligned currents peaking at~1.25 MA rad À1 being essentially collocated with the auroral upward current and approximately equal in strength. Though northern and southern PPO currents were approximately equal during both intervals, the currents in both hemispheres were dual modulated by both systems during 2012/2013, with approximately half the main current closing in the opposite ionosphere and half cross field in the magnetosphere, while only the northern hemisphere currents were similarly dual modulated in 2008.

Saturn's Planetary Period Oscillations During the Closest Approach of Cassini's Ring‐Grazing Orbits

“…Therefore, we suggest that the difference shown in Figure 3c between the 2008 and F-ring amplitudes could be due to a combination of the local time asymmetry observed by , the weaker PPO-related field-aligned currents, and secular change of the PPOs. Similar behavior for the PPO currents was shown by Bradley et al (2018) during the nightside high-latitude 2013 interval. This value of k is consistent within errors with the k value determined by Provan et al (2018) from analysis of the beat modulations of data at larger distances in the magnetosphere but over the same time interval as studied here.…”

Driving Planetary Period Oscillations From the Hall Conducting Layer of Saturn's Upper Atmosphere

“…Figure 7a summarizes the total interhemispheric currents due to L1* and U1*. It is this mixed regime that fits with the Bradley et al (2018) data, which showed that about half of the current closes in the magnetosphere. The triple dash-dotted line is the difference, that is, the current that must close in the magnetosphere.…”

“…Bradley et al (2018) noted that in their 2012-2013 data (in early northern summer), both hemispheres were modulated by both PPO systems, whereas in the 2008 data (in late northern winter) analyzed by Hunt et al (2014Hunt et al ( , 2015, only the northern hemisphere was dual modulated, indicating that northern PPO currents were not penetrating to the southern hemisphere. Bradley et al (2018) noted that in their 2012-2013 data (in early northern summer), both hemispheres were modulated by both PPO systems, whereas in the 2008 data (in late northern winter) analyzed by Hunt et al (2014Hunt et al ( , 2015, only the northern hemisphere was dual modulated, indicating that northern PPO currents were not penetrating to the southern hemisphere.…”

“…Both Hunt et al (2015) and Bradley et al (2018) did find phase differences between the hemispheres; however, the uncertainties on the phases are large, and so it is difficult to draw a clear comparison with the data. In particular, the peak of the field-aligned currents in the opposite hemisphere should lie further west than the peak in the driving hemisphere.…”

“…In addition, recent studies have continued to characterize the magnetic perturbations associated with the PPO disturbances in greater detail (e.g., Hunt et al, 2018;Provan et al, 2018;Bradley et al, 2018): of particular interest here is the study by Bradley et al (2018), in that it characterizes the magnitude and distribution of the field-aligned currents associated with the PPO disturbances. The observed peak magnitude of the PPO currents-approximately 1 MA/rad-is in line with those observed previously.…”