Abstract
AbstractWe examine and compare the magnetic field perturbations associated with field‐aligned ionosphere‐magnetosphere coupling currents at Saturn, observed by the Cassini spacecraft during two sequences of highly inclined orbits in 2006/2007 and 2008 under late southern summer conditions. These sequences explore the southern currents in the dawn‐noon and midnight sectors, respectively, thus allowing investigation of possible origins of the local time (LT) asymmetry in auroral Saturn kilometric radiation (SKR) emissions, which peak in power at ~8 h LT in the dawn‐noon sector. We first show that the dawn‐noon field data generally have the same four‐sheet current structure as found previously in the midnight data and that both are similarly modulated by “planetary period oscillation” (PPO) currents. We then separate the averaged PPO‐independent (e.g., subcorotation) and PPO‐related currents for both LT sectors by using the current system symmetry properties. Surprisingly, we find that the PPO‐independent currents are essentially identical within uncertainties in the dawn‐dusk and midnight sectors, thus providing no explanation for the LT dependence of the SKR emissions. The main PPO‐related currents are, however, found to be slightly stronger and narrower in latitudinal width at dawn‐noon than at midnight, leading to estimated precipitating electron powers, and hence emissions, that are on average a factor of ~1.3 larger at dawn‐noon than at midnight, inadequate to account for the observed LT asymmetry in SKR power by a factor of ~2.7. Some other factors must also be involved, such as a LT asymmetry in the hot magnetospheric auroral source electron population.
Highlights
Electric currents flowing along magnetic field lines are of central importance in planetary magnetospheres in transferring momentum and energy between the upper atmosphere and ionosphere of the planet and the magnetospheric plasma and solar wind at large distances
The midnight profile is very similar to those determined previously from the 2008 data by Hunt et al [2014], as may be expected, once allowance is made for the colatitude transformation employed here. These results provide no support for the suggestion by Southwood and Kivelson [2009] that the higher modulated Saturn kilometric radiation (SKR) powers emitted in the dawn-to-noon sector (Figure 2) result from the superposition of the PPOrelated upward current on a quasisteady current system that has larger upward currents at dawn-noon than at dusk-midnight, whether those currents result from the solar wind interaction or otherwise
We have investigated the southern hemisphere signatures of auroral field-aligned currents observed during the 2006/2007 high-latitude interval of Cassini orbits and have compared these with the signatures observed in the 2008 high-latitude interval studied previously by Hunt et al [2014]
Summary
Electric currents flowing along magnetic field lines are of central importance in planetary magnetospheres in transferring momentum and energy between the upper atmosphere and ionosphere of the planet and the magnetospheric plasma and solar wind at large distances. During the second sequence of high-latitude orbits, the radius of periapsis was initially sufficiently low that the full auroral current system was crossed on a sequence of more than 30 orbits, corresponding to Revs 59–95 as marked by the dashed vertical lines and the gray shaded area, spanning most of Figure 1. Similar solar illumination conditions of the southern upper atmosphere prevailed during the two intervals, with a central region of the polar cap being permanently illuminated, extending to ~14° colatitude during the first interval and to ~6° during the second, while the southern auroral region centered near ~18° underwent day-night cycles, being sunlit between ~03 h and ~21 h LT via noon during the 2006/2007 interval, and between ~05 and ~19 h LT during the 2008 interval, and in darkness in the remaining region via midnight Similarity in both PPO and seasonal conditions renders plausible the search for LT effects in the subcorotation and PPO-related currents in the southern data set for the two intervals combined, spanning midnight to noon via dawn.
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