A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere

Abstract

We propose a simple illustrative axisymmetric model of the plasma flows and currents that occur in Saturn's polar ionosphere which are due to both internal magnetospheric plasma processes and the solar wind interaction. The features of the model are based on previous physical discussion, guided quantitatively by both Voyager plasma observations on closed field lines and remote‐sensing IR Doppler observations on open field lines. With increasing latitude the flow features represented include a region poleward of ∼25° colatitude where the angular velocities decrease continuously from rigid corotation to ∼60% of rigid corotation due to plasma production from internal sources in the central magnetosphere, a narrow band of higher but still subcorotating angular velocities mapping to Dungey cycle return flow and Vasyliunas cycle flow regions in the outer closed field magnetosphere, and, finally, a region of low angular velocities, ∼30% of rigid corotation, on open field lines in the polar cap. We show that these flows require a four‐region pattern of field‐aligned currents. With increasing latitude, these consist of regions of upward and downward current on closed field lines peaking at a few tens of nanoamperes per square meter (for an effective ionospheric Pedersen conductivity of 1 mho), a narrow ring of upward field‐aligned current across the open‐closed field line boundary of order 100 nA m−2, and distributed downward currents on open field lines of order 10 nA m−2. Of the upward currents, only that at the open‐closed field line boundary is of sufficient intensity to require significant acceleration of magnetospheric electrons, resulting in total precipitating electron powers of ∼0.03–0.06 TW, together with auroral UV emissions of a few tens of kilorayleighs. The latter emissions occur in a ring of a few hundred kilometers latitudinal width at ∼13° colatitude, which we thus associate with Saturn's main auroral oval. We also estimate similar powers in unaccelerated electron precipitation on closed field lines equatorward of the boundary, leading to distributed “diffuse” UV emissions of a fraction of a kilorayleigh. However, by far the most important energy input to the polar upper atmosphere is due to Joule heating by the ionospheric Pedersen currents, which we estimate as typically several milliwatts per square meter poleward of ∼20° colatitude. The overall Joule powers are estimated to be ∼2.5 TW on both open and closed field lines in each hemisphere (for a conductivity of ∼1 mho and no slippage of the neutral atmosphere), thus representing a very significant energy input to Saturn's thermosphere, more than an order of magnitude larger than the globally averaged solar input. Joule heating is thus likely to make a significant contribution to an explanation of why Saturn's thermosphere is observed to be hot, ∼400–600 K, compared with less than ∼200 K expected on the basis of solar heating alone.