Secular and local time dependence of Jovian X ray emissions

Abstract

Jovian soft X rays are believed to be line emissions from energetic oxygen and sulfur ions generated through charge exchange and stripping reactions when the ions enter Jupiter's upper atmosphere. The X rays are brightest in the polar regions (generally about twice as bright in the north as in the south) but also have a large component from low latitudes. Observations of Jupiter's X ray emissions at several epochs by the high‐resolution imager (HRI) instrument on the ROSAT satellite show a general decrease in overall brightness by ∼50% from 1994 to 1996 and reveal an interesting dependence of brightness on local time. The emissions are not of uniform brightness across the disk but are strongly weighted toward one limb of the planet. The X ray limb can be on either the duskside or dawnside of Jupiter, and so far it correlates with position of the subsolar point relative to the sub‐Earth point (i.e., the X ray limb is the bright visible limb). Except during the Shoemaker‐Levy 9 impacts, the total X ray emissions are not strongly correlated with System III central meridian longitude. However, a composite map of the emissions shows that they are generally located in regions of low surface magnetic field strength at low latitudes, and in regions of large horizontal surface magnetic field strength gradients at high latitudes.