Ionospheric Outflow in the Magnetosphere of Saturn

It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity, but less attention has been dedicated to studying the ionospheric mass source at Saturn. Felici et al., 2016 described an ionospheric outflow event from Saturn's magnetotail, when Cassini was located at ≃ 2200 h Saturn local time at 36 RS from Saturn. During several entries into the magnetotail lobe, a tailward flowing cold ion beam was observed directly adjacent to the plasma sheet and extending deeper into the lobe. These ions, which were mostly H+, appeared to be dispersed, dropping to lower energies with time. The ion composition showed mainly H+. Ultraviolet auroral observations showed a dawn brightening, and upstream heliospheric models suggested that the magnetosphere was being compressed by a region of high solar wind ram pressure.Here, we report the results of a survey of ionospheric outflow events using data from the Cassini Plasma Spectrometer Singles (CAPS) collected in Saturn's magnetosphere. By examining the spacecraft's position relative to the plasma sheet, flow direction, and ion composition using Cassini Magnetometer data and CAPS Time of Flight data, as described by Felici et al., 2016, we confirmed that these were ionospheric outflow events and mapped them within Saturn's magnetosphere.Auroral and solar wind activity during these events were analyzed using data from the Cassini Ultraviolet Imaging Spectrograph and results from the ENLIL solar wind propagation model.We obtained number flux values at 10000 km altitude ranging from 5.24 x106 to 1.63 x109 cm-2 s-1. These values are within the range reported by Glocer et al., 2007, which is 7.3 x106 to 1.7 x108 cm-2 s-1 However, our values are lower than those estimated by Felici et al., 2016, which range from 2.95 x109 to 1.43 x1010 cm-2 s-1. We attribute this discrepancy to the fact that their study assumed completely cold ions, overestimating speeds and therefore the number flux, for the differential energy flux calculations. In contrast, calibrated moments for Cassini CAPS data are available at the time of this study. From the number flux, source rate could be estimated if the knowledge of the area of the emitting region could be inferred without too many assumptions. Felici et al., 2016 postulated that the outflow could have originated from regions different from the polar cap, and estimated the mass rate provided by an active region that covered the main auroral oval - as aurora images were available in their case - which resulted to be between 49.7 and 239 kg/s (assuming all ions to be H+), revealing that the ionosphere could release a mass quantity the same order of magnitude as Enceladus (60–100 kg/s).Future work will include an analysis of ionospheric outflow in the Jovian system, and comparisons with models of ionospheric outflow at the Gas Giants, and between observed outflow at the two planets.AcknowledgmentsThis work was supported by NASA under award numbers 80NSSC23K1275 and 80NSSC19K0892. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration.References Felici et al. (2016), doi:10.1002/2015JA021648.Glocer et al., (2007), doi:10.1029/2006JA011755.

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