HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede

Abstract

We report new observations of the spectrum of Ganymede in the spectral range 1160È1720 made A with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space T elescope (HST ) on 1998 October 30. The observations were undertaken to locate the regions of the atomic oxygen emissions at 1304 and 1356 previously observed with the Goddard High Resolution Spectrograph on HST , that A , Hall et al. claimed indicated the presence of polar aurorae on Ganymede. The use of the 2A wide STIS slit, slightly wider than the disk diameter of Ganymede, produced objective spectra with images of the two oxygen emissions clearly separated. The O I emissions appear in both hemispheres, at latitudes above o 40 o ¡, in accordance with recent Galileo magnetometer data that indicate the presence of an intrinsic magnetic Ðeld such that Jovian magnetic Ðeld lines are linked to the surface of Ganymede only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the four contiguous orbits (5.5 hr) of observation, presumably because of the changing Jovian plasma environment at Ganymede. However, the observed longitudinal nonuniformity in the emission brightness at high latitudes, particularly in the southern hemisphere, and the lack of pronounced limb brightening near the poles are difficult to understand with current models. In addition to observed solar H I Lya reÑected from the disk, extended Lya emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb, and its brightness is consistent with the Galileo UVS measurements of Barth et al.

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

“…As a result the brighter Ly-α albedos of the contaminated (visible-darkened) regions could be explained by the surrounding region of purer water ice being relatively darker in the UV. The correlation of Europa's darker visible areas with UV bright regions is also consistent with the finding that Ganymede's darker visible surface has a slightly higher FUV albedo (J2%) than Europa (27).…”

“…This mode has been previously used many times to observe the Galilean satellites (e.g., refs. [26][27][28][29] and more technical details are explained elsewhere (30). During each of the five orbits per visit two exposures with exposure times of 860 s (orbit 1) and 1,160 s (orbits 2-5) were obtained.…”

“…Global Ly-α emissions can potentially originate from sunlight scattered off atomic hydrogen in the atmosphere, which is likely also the source of the extended Ly-α emissions detected at Ganymede (27,45). Ganymede's global Ly-α emissions have peak brightnesses above 500 R and are thus considerably brighter than the global Ly-α brightnesses we find for Europa.…”

Excitation of the Ganymede Ultraviolet Aurora

“…Observations of emission in the ultraviolet lines of atomic oxygen were reported by Hall et al (1998) Feldman et al (2000) and are reproduced in Figure 1, which is taken from the above paper. A summary of the observations and exposure parameters may be found in Feldman et al (2000).…”

“…The ultraviolet aurora was discovered by Hall et al (1998) and observed in greater detail by Feldman et al (2000). Energetic electron data from the magnetosphere of Ganymede were obtained from the energetic particle detector (EPD) on board Galileo (Williams, Mauk, & McEntire 1997a ;Williams et al 1997b ;Williams & Mauk 1997 ;Paranicas et al 1999 ;Eviatar et al 2000).…”