Detection of a strongly negative surface potential at Saturn's moon Hyperion

Abstract

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (∼ −200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

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“…De and Criswell (1977) and Lee (1996) used the value of 3.0×10 5 V/m as the electric field strength across lunar terminator. As on the Moon, it is observed that Hyperion has intense electric fields at its terminator (Nordheim et al 2014). An experimental study, setting a plane with a potential of 10 V adjacent to a plane with a potential of 0 V owing to differential UV charging, showed that strong electric fields, ~1 kV/m, can be generated across the boundary (Wang et al 2007).…”

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“…One finding has been that of long-term reconnection in the tail (Arridge et al 2016), confirmation of the Vasyliunas cycle, plasmoids in the tail taking products from Enceladus away from the Saturn system (Hill et al 2008, Jackman et al 2014, observation of a "polar wind" like Earth's (Felici et al 2016), new radiation belts unknown before Cassini arrived (Krimigis et al 2005), and charging of the surface at Hyperion (Nordheim et al 2014) and other moons.…”