Saturn's Auroral Field‐Aligned Currents: Observations From the Northern Hemisphere Dawn Sector During Cassini's Proximal Orbits

Abstract

We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field-aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F-ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LTs). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept-forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO-independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass-by-pass, we find that the main upward current at dawn is stronger compared to near noon. This results in the current density being~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO-independent upward current electron power is~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.

Seasonal Dependence of the Magnetospheric Drag Torque on Saturn's Northern and Southern Polar Thermospheres and its Relation to the Periods of Planetary Period Oscillations

“…An additional important result of the above Cassini magnetic data studies has been the finding that the primary rotating oscillatory field‐aligned currents of the northern and southern PPO systems are collocated with and superposed on the primary subcorotation field‐aligned currents in the auroral region (Bradley et al, 2018; Hunt et al, 2014, 2015, 2018, 2020; Provan, Lamy, et al, 2019). They are also of comparable magnitude.…”

“…with and superposed on the primary subcorotation field-aligned currents in the auroral region (Bradley et al, 2018;Hunt et al, 2014Hunt et al, , 2015Hunt et al, , 2018Hunt et al, , 2020Provan, Lamy, et al, 2019). They are also of comparable magnitude.…”

“…Data from sufficient such passes must also be obtained that the mean current profile associated with plasma subcorotation, of main interest here, can be separated from the comparably large rotating oscillatory currents of the PPO systems. In addition to the optimal orbit intervals outlined above, current profiles have also been derived spanning the dawn, dusk, and dayside sectors from further sequences of highly inclined orbits during the final phase of the Cassini mission spanning northern summer solstice in May 2017, specifically by Hunt et al (2018) for the F ring orbits (Revs 251-270) between October 2016 and April 2017, and by Provan, Lamy, et al (2019) and Hunt et al (2020) for the proximal orbits (Revs 271-293) between April 2017 and end of mission in September 2017. In regions of latitudinal overlap these profiles generally show similar properties to those derived by Hunt et al (2014Hunt et al ( , 2015 and Bradley et al (2018), apart from small latitude shifts associated with the solar wind-related daynight asymmetry about the pole.…”

The Response of Saturn's Dawn Field‐Aligned Currents to Magnetospheric and Ring Current Conditions During Cassini's Proximal Orbits: Evidence for a Region 2 Response at Saturn

“…These first two vertical dashed lines mark the boundaries of the upward auroral current which was determined by Hunt et al. (2020), where the direction of the field‐aligned current is given by the red arrow. Current continuity then requires that the total field‐aligned current per radian of azimuth flowing between two colatitude points, θ iN 1 and θ iN 2 , where θ iN 2 > θ iN 1 , is given by $$${I}_{\vert \vert }=-\left({I}_{m}\left({\theta }_{iN2}\right)-{I}_{m}\left({\theta }_{iN1}\right)\right).$$$ …”

“…To explore this further we combine all the field‐aligned current observations ( I || ) from Hunt et al. (2020) with the magnetospheric state determined by Bradley et al. (2020), and the total ring current values ( I T ) from Provan et al.…”