Saturn's Plasma Density Depletions Along Magnetic Field Lines Connected to the Main Rings. Issue 16 (22nd August 2018)

• Record Type: Journal Article
• Authors: Farrell, W. M.; Hadid, L. Z.; Morooka, M. W.; Kurth, W. S.; Wahlund, J.‐E.; MacDowall, R. J.; Sulaiman, A. H.; Persoon, A. M.; Gurnett, D. A.
• Is Part Of: Geophysical research letters. Volume 45:Issue 16(2018)
• Page Start: 8104
• Page End: 8110
• Publication Date: 2018-08-22
• Subjects: Saturn -- Cassini -- rings -- plasma; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078137
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: We report on a set of clear and abrupt decreases in the high‐frequency boundary of whistler mode emissions detected by Cassini at high latitudes (about ±40°) during the low‐altitude proximal flybys of Saturn. These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the edges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high‐frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low‐density plasma in the region. The observation of the electron density dropout along ring‐connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring‐connected electron density dropouts observed between 1.1 and 1.3 R s are connected to the low‐density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass. Plain Language Summary: We present Cassini observations during the close passes by the planet Saturn indicating that plasma on magnetic field lines that pass through the A and B rings is of anomalously low density. These observations are consistent with the Saturn orbit insertion observations of a plasma cavity located at equatorial regions overtop the dense B ring. Using a terrestrial analogy, we suggest that the low‐density conditions overtop the rings create an electrical force, called an ambipolar electric field that draws plasma out of the ionosphere in an attempt to replenish the plasma void found at equatorial regions. Key Points: The high‐frequency edge of a continuous whistler mode emission displayed a clear decrease in frequency near plus‐minus 40 degrees latitude at altitudes of 1.1 to 1.3 R s Closer examination reveals that these dropouts are associated with abrupt electron density decreases occurring on field lines connected to the A and B rings We conclude that magnetic field lines connected to the rings are evacuated of plasma relative to field lines just outside the A ring