Sodium in Io's Extended Atmosphere

Abstract

This dissertation combines several new observations of the Io sodium cloud to create a consistent picture of the extended Io atmosphere and its interaction with the Jovian plasma torus. I used the LPL echelle spectrograph to obtain three types of high-resolution spectra of the extended sodium cloud at the sodium D lines (5890 Â, 5896 Â). The first class of observations made use of the mutual satellite eclipses of 1985 to probe the density profile of the atmosphere in the range 1.4 to 10 Io radii, a previously unstudied region. The second type of observation examined the sodium emission in the immediate vicinity of Io, allowing an accurate measurement of the velocity structure around Io. The final method employed a high-sensitivity detector to study faint jets of high-speed sodium farther out in the extended cloud. The synthesis of these three data sets results in a better understanding of how sodium is distributed about Io as a function of position and velocity.The extended atmosphere of Io is composed of many kinematically distinct components. The distribution in space is linked to their characteristic velocities, with low-energy sodium confined near Io and faster atoms (10 to 100 km sec -1 ) prevalent beyond ~ 25 Io radii. The sodium density falls rapidly near Io and more slowly outside 5.6 Io radii, the effective limit of the gravity of Io.The data indicate that the atmosphere is collisionally thick near the surface but becomes thin by an altitude of ~ 700 km. The upper limit of the exobase location is derived from reliable sodium density measurements made during the satellite eclipses. The lower limit is indirectly inferred from the velocity distribution of sodium near Io and the nature of high-speed jets far from Io.The high-speed jets reveal a new type of close interaction between the corotating plasma and the atmosphere of Io. The morphology and brightness of the jets require a two-reaction process in which atmospheric sodium is ionized, accelerated to high speeds, and then charge-exchanges with other sodium atoms. These processes must occur near the atmospheric exobase, indicating that the atmosphere of Io is not completely protected from the plasma flow.

The Dual Sources of Io's Sodium Clouds

“…In situ measurements by the Galileo spacecraft have revealed significantly slowed plasma near Io (Frank et al 1996) and a dense ionosphere around it (Hinson et al 1998), suggesting that Io's collisionally thick neutral atmosphere interacts with a significant flux of plasma. Added to these points are the observed lack of the thermal exospheric charge exchange fan in this work and the observed presence of the jet found here and by many other observers (Goldberg et al 1984, Pilcher et al 1984, Schneider 1988, Cremonese et al 1992, Mendillo et al 1997, Burger et al 1999, Takahashi et al 2000. The lack of the exospheric charge exchange fan does not rule out charge exchange between torus ions and atmospheric neutrals.…”

“…This adaptation of the stream model to the jet by Wilson (1996) and Wilson and Schneider (1999) was originally inspired by images and spectra of Pilcher et al (1984), Goldberg et al (1984), and Schneider (1988), as well as the groundbreaking realization by Schneider (1988) that charge exchange of pickup ions at Io could lead to narrowly directed ejection of fast neutrals. The ability of our model to reproduce the long (∼10 R J ) jet seen in the 1995 medium FOV of Fig.…”