Recent results from EUVE observations of the Io plasma torus and Jupiter

Abstract

Between 1993 and 1996 the Extreme Ultraviolet Explorer (EUVE) satellite has observed Jupiter and the Io plasma torus (IPT) for a total of over 380 hours. The data consist of time‐tagged photons at wavelengths in the 7–76 nm range. By using appropriate ephemerides for Jupiter and EUVE, these data can be made into spectral images, or “overlappograms,” of the IPT. Most of the emissions from the IPT appear in the bandpass of the long‐wavelength (LW) spectrograph (28–76 nm), which has 0.2 nm spectral resolution and spatial resolutions of 60 arc sec full width at half maximum (FWHM) in the dispersion direction and 24 arc sec FWHM in the direction perpendicular to dispersion. Presented here is a subset of the EUVE data, comprising 83 consecutive orbits during June 19–24, 1996. The dusk (west on the sky) half of the IPT is, on average, ∼25–30% brighter than the dawn half of the IPT, in the most prominent EUV emission features. In addition, there is no noticeable correlation of total IPT brightness with Io orbital phase during this time period. On Jupiter, the brightness of the reflected He I 58.4 nm emission was measured in June 1996 to be 1.3±0.5 R. This level is substantially less than the 4–5 R measured during the Voyager flybys and is attributed to solar cycle variability of the Sun's He I 58.4 nm emission.