University of Kansas X-Ray Emission in the Solar System

Draft X-Ray Emission from the Terrestrial Magnetosheath by Robertson and Cravens

Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.

Introduction

X-ray emission from comet Hyakutake was discovered in 1996 [Lisse et al., 1996]. Subsequently, X-ray emission from a number of other comets, planets, interstellar gas throughout the heliosphere and even the moon has been observed [Dennerl et al., 1997; Lisse et al., 1999a,b; Cravens, 2002a,b; Krasnopolsky and Mumma, 2001]. Cravens [1997] proposed that this X-ray emission could be produced by charge exchange between heavy solar wind ions and cometary neutrals. In these charge exchange collisions, an electron is transferred from a neutral to a high charge state heavy solar wind ion. The heavy ion is left in an excited state and consequently emits a photon in the extreme ultraviolet or soft X-ray region of the spectrum. Recent higher resolution spectra of the cometary X-rays by Chandra [Lisse et al., 2001] and of the extreme ultraviolet emission (EUV) by the EUVE satellite [Krasnopolsky and Mumma, 2001] show individual spectral lines, which has confirmed that the solar wind charge exchange (SWCX) mechanism was the main source of these emissions.

Cox [1998] suggested that the SWCX mechanism applied to interstellar neutrals and neutrals in the Earth's geocorona could account for part of the observed soft X-ray background. He also suggested [1998] that the same mechanism could explain some of the temporal variations in the soft X-ray background, and in particular the Long Term Enhancements (LTE) as seen by ROSAT [Snowden et al., 1994]. Freyberg [1998] also attributed the LTE to variations in the solar wind and speculated that the SWCX mechanism applied to the vicinity of Earth might be responsible.

Cravens [2000] constructed a simple model of heliospheric X-ray emission from charge exchange between the solar wind and interstellar helium and hydrogen. A comparison of the LTE part of the ROSAT X-ray background for the 1/4 keV channel with the solar wind proton flux proved promising [Cravens et al., 2001; Robertson et al., 2001]. Daily averages of measured solar wind proton fluxes were compared with ROSAT LTE data for all days for which both data types were available, and a correlation coefficient of R = 0.71 was found. A model of the X-ray emission was used to show that the time variations came from the SWCX mechanism for both the interstellar helium and geocoronal hydrogen. The geocoronal hydrogen density outside the magnetopause was assumed to vary as n_H = n_Ho(10 R_E/r)³, and the solar wind flow outside the magnetopause was assumed to be uniform. We now report on a more elaborate model of the geocoronal SWCX X-ray contribution. In particular, simulated X-ray images of the magnetosheath are generated. In a similar study, Holmstrom et al. [2001] predicted X-ray intensities and simulated images associated with the solar wind interacting with exospheric neutrals at Mars. Remote observations of the magnetosheath have also been made by the LENA instrument on the IMAGE spacecraft using low energy neutral hydrogen atoms produced by the charge transfer of solar wind protons with geocoronal hydrogen [Collier et al., 2001].

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Last modified January 21, 2004 Tizby Hunt-Ward tizby@ku.edu