ChandraObservations of the “Dark” Moon and Geocoronal Solar Wind Charge Transfer

We have analyzed data from two sets of calibration observations of the Moonmade by the Chandra X-ray Observatory. In addition to obtaining a spectrum ofthe bright side that shows several distinct fluorescence lines, we also clearlydetect time-variable soft X-ray emission, primarily O VII Ka and O VIII Lya,when viewing the optically dark side. The apparent dark-side brightness variedby at least an order of magnitude, up to 2x10^-6 phot/s/arcmin^2/cm^2 between500 and 900 eV, which is comparable to the typical 3/4-keV-band backgroundemission measured in the ROSAT All-Sky Survey. The spectrum is also verysimilar to background spectra recorded by Chandra in low or moderate-brightnessregions of the sky. Over a decade ago, ROSAT also detected soft X-rays from thedark side of the Moon, which were tentatively ascribed to continuum emissionfrom energetic solar wind electrons impacting the lunar surface. The Chandraobservations, however, with their better spectral resolution, combined withcontemporaneous measurements of solar-wind parameters, strongly favor chargetransfer between highly charged solar-wind ions and neutral hydrogen in theEarth's geocorona as the mechanism for this emission. We present a theoreticalmodel of geocoronal emission and show that predicted spectra and intensitiesmatch the Chandra observations very well. We also model the closely relatedprocess of heliospheric charge transfer and estimate that the total chargetransfer flux observed from Earth amounts to a significant fraction of the softX-ray background, particularly in the ROSAT 3/4-keV band.Comment: 14 pages, color figures, uses emulateapj style. ApJ in pres