The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N 2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N 2 , and O 2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002-2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N 2 densities are lower than the NRLMSISE-00 model, but O 2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

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.

[1] Thermospheric O/N 2 column density ratios referenced at a N 2 column density of 10 17 cm À2 are obtained using the IMAGE SI-13 and TIMED/GUVI far-ultraviolet (FUV) dayglow data, AURIC simulation results, and MSIS86 model. Each of the magnetic storms occurring during a 4-day period (1-4 October 2002) caused significant O/N 2 depletion that was detected by both of the IMAGE SI-13 and GUVI instruments. The depletion extended down to latitudes of 10°and À5°in the Northern and Southern Hemispheres, respectively. Simultaneous measurements show an excellent agreement between the SI-13 and GUVI O/N 2 on both global and local scales. The IMAGE SI-13 O/N 2 data provide direct optical evidence that the O/N 2 depletion corotates with the Earth. The GUVI O/N 2 indicate the depletion in both of the hemispheres is not symmetric owing to the seasonal effect and differences in heating and convection induced winds. Both the IMAGE SI-13 and GUVI O/N 2 maps also provide a good opportunity for future modeling efforts.

No abstract

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space T elescope and compare them to similar phenomena on Earth. We Ðnd that the tenuous nature of GanymedeÏs atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots :1. Birkeland-type currents and associated magnetic ÐeldÈaligned electric Ðelds. 2. The stochastic heating of plasma electrons by the Landau damping of electron plasma oscillations generated by precipitated energetic electrons.We conclude that the electron density in the bright regions may attain local values as high as 105 cm~3.