HR: 17:15h
AN: P54A-05    [Abstracts]
TI: Low-Altitude ENA Emission from Energetic Ions Trapped in Saturn's Exosphere
AU: * Roelof, E C
EM: edmond.roelof@jhuapl.edu
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Brandt, P C
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Mitchell, D G
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Krimigis, S M
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Mauk, B H
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Paranicas, C P
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Saur, J
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AU: Demajistre, R
AF: Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099 United States
AB: The Ion and Neutral Camera (INCA), one three components of the MIMI experiment on the Cassini orbiter, viewed the low-latitude northern hemisphere of Saturn in a sequence of 16-minute images from an altitude of only 0.4-1.4 R$_{S}$ during two hours (0108-0316 UT on 1 July 2004) following the Saturn Orbit Insertion (SOI) engine burn. A low-altitude band of energetic neutral atom (ENA) emission appeared to be centered $\sim$N10$\deg$ and extended from post-noon to at least midnight in the TOF energy range $\sim$10-100 keV/nucleon. The feature was observed in both hydrogen and oxygen ENAs, and appeared to be eclipsed by the inner edge of the D-ring (r=1.11 R$_{S}$). It was $\sim$1/10 the brightness of the ENA emission from the ring current region (3-8 R$_{S}$), also visible in the same images. We interpret this ENA emission to be produced by double charge exchange. ENAs are generated by singly-charged energetic ions from the ring current region. Those that enter Saturn's molecular hydrogen (H$_{2}$) exosphere are stripped and thus become ions temporarily trapped on magnetic field lines several thousand kilometers above the 1-bar level. Subsequently, these ions undergo a second charge exchange collision and are emitted from the exosphere as ENAs once again. The brightness of the exospheric emission, relative to the ring current source region, implies that the ENA emission is optically thick. The double charge-exchange mechanism was identified at Earth as the source of a low-altitude (L=1.1) radiation belt in the early 1970s. The main difference is that the Earth's (atomic) hydrogen geocorona is optically thin to ENA, while Saturn's (molecular) hydrogen exosphere is optically thick.
DE: 5739 Meteorology (3346)
DE: 6275 Saturn
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting