X-ray emission from planets and comets: relationship with solar X-rays and solar wind

Abstract

Scattering of solar X-ray radiation mainly produces the non-auroral X-ray emissions from Jupiter, Saturn, and Earth, those from the disk of Mars, Venus, and Moon, and from the rings of Saturn. Recently X-ray flares are observed from the low-latitude disk of giant planets Jupiter and Saturn in the energy range of 0.2-2 keV. These flares are found to occur in tandem with the occurrence of solar X-ray flare, when light travel time delay is accounted, suggesting that X-rays from these planets can be used to study flaring on the hemisphere of the Sun invisible to near-Earth space weather satellites. Also by proper modeling of the observed planetary X-rays the solar soft X-ray flux can be derived. X-ray flares are also observed recently from Mars in direct response to solar flares. The X-ray emission from comets, the heliosphere, the geocorona, and the Martian and Venusian halo are all largely driven by charge exchange collision between highly ionized minor heavy ions in the solar wind and gaseous neutral species in the bodies' atmosphere or exosphere - a process known as solar wind charge exchange (SWCX). In particular, the cometary X-ray spectrum can be used to derive abundances of high-charge state ions of O, C, Ne as well as the speed of the solar wind. Thus cometary X-rays can provide a diagnostic of the solar wind properties even at far off distances from the Earth. This paper provides a brief overview of X-rays from some of the solar system bodies and their connection with solar X-rays and solar wind, and how planetary and cometary X-rays can be used to study the solar X-ray radiation and solar wind properties.