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Chandra Observations of the “Dark” Moon and Geocoronal Solar-Wind Charge Transfer

By SpaceRef Editor
March 12, 2004
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Chandra Observations of the “Dark” Moon and Geocoronal Solar-Wind Charge Transfer

Astrophysics, abstract

From: Maxim Markevitch [view email]
Date: Wed, 11 Feb 2004 04:34:21 GMT (366kb)

Chandra Observations of the “Dark” Moon and Geocoronal Solar-Wind Charge

B. J. Wargelin (1),
M. Markevitch (1,2),
M. Juda (1),
V. Kharchenko (1),
R. Edgar (1),
A. Dalgarno (1) ((1) CfA, (2) IKI)

Comments: 14 pages, color figures, uses emulateapj style. ApJ in press

We have analyzed data from two sets of calibration observations of the Moon
made by the Chandra X-ray Observatory. In addition to obtaining a spectrum of
the bright side that shows several distinct fluorescence lines, we also clearly
detect 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 varied
by at least an order of magnitude, up to 2×10^-6 phot/s/arcmin^2/cm^2 between
500 and 900 eV, which is comparable to the typical 3/4-keV-band background
emission measured in the ROSAT All-Sky Survey. The spectrum is also very
similar to background spectra recorded by Chandra in low or moderate-brightness
regions of the sky. Over a decade ago, ROSAT also detected soft X-rays from the
dark side of the Moon, which were tentatively ascribed to continuum emission
from energetic solar wind electrons impacting the lunar surface. The Chandra
observations, however, with their better spectral resolution, combined with
contemporaneous measurements of solar-wind parameters, strongly favor charge
transfer between highly charged solar-wind ions and neutral hydrogen in the
Earth’s geocorona as the mechanism for this emission. We present a theoretical
model of geocoronal emission and show that predicted spectra and intensities
match the Chandra observations very well. We also model the closely related
process of heliospheric charge transfer and estimate that the total charge
transfer flux observed from Earth amounts to a significant fraction of the soft
X-ray background, particularly in the ROSAT 3/4-keV band.

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