While comets provide the cleanest case for studying charge exchange interactions, this is a general process that occurs whenever highly charged ions interact with gas. One such place is the geocorona, an extended, tenuous cloud of hydrogen around the Earth, which is well visible in the EUV because of fluorescent La scattering of solar EUV radiation (Fig. 9.3, right).
Although the existence of the geocorona was known for a long time, its relevance to X-ray astronomy was realized only after the discovery of cometary X-ray emission, which demonstrated the presence of an efficient process capable of converting a tenuous cold cloud into an X-ray emitter. Once it was realized that many X-ray observations are made with satellites from inside an X-ray emitting geo-corona, a straightforward explanation was offered for peculiarities in the soft X-ray
Fig. 9.3 Left: First X-ray image of Venus, obtained with Chandra . The X-rays result mainly from fluorescent scattering of solar X-rays on C and O in the upper Venus atmosphere, at heights of 120-140 km. In contrast to the Moon, the X-ray image of Venus shows evidence for brightening on the sunward limb. This is caused by the fact that the scattering takes place on an atmosphere and not on a solid surface. Right: EUV images of the Earth, taken with DE-1 SAI . They show the three basic components, which constitute the X-ray properties of Earth: the auroral oval, the sunlit crescent, and the geocorona. The image at right was taken when the Sun was behind the Earth; the two bands straddling the magnetic equator in the premidnight section are caused by airglow background, which were not understood before. Occasional brightenings of the X-ray sky observed during the ROSAT all-sky survey could be explained by variations of the geocoronal X-ray brightness in response to a temporally variable solar wind, and the presence of X-ray emission, which appeared at the dark side of the Moon, a long-standing puzzle, finally found a straightforward answer (Fig. 9.1, left). Although compelling evidence had accumulated over the last years that the geo-corona does influence soft X-ray observations, the direct proof was obtained only recently, when high-resolution spectra of the dark Moon, taken with Chandra , revealed the presence of charge-exchange signatures. The same lines were also found in spectra of the diffuse X-ray sky . X-ray observations of the geocorona are complicated by the fact that they cannot be performed from a sufficiently large distance. The narrow field of view of the satellites that are capable of sensitive imaging at low X-ray energies introduces a mixture of temporal and spatial effects, and does not provide full coverage of the geocorona. This complication, however, does not apply to other planets.
In the first X-ray observation of Mars, with Chandra (Fig. 9.4, left), evidence for a faint, extended X-ray halo produced by charge exchange was detected in addition to the bright X-ray fluorescence . A subsequent observation with XMM-Newton confirmed the presence of the halo and made it possible to study its unique charge exchange signatures with an unprecedented spectral resolution of ^4 eV. The O6+ multiplet was found to be dominated by the spin-forbidden 23S1 ^ 11S0 transition, proving that charge exchange is the origin of the emission. Several X-ray flares
Fig. 9.4 Left: First X-ray image of Mars, obtained with Chandra . The X-rays result mainly from fluorescent scattering of solar X-rays on C and O in the upper Mars atmosphere, at heights of 110-130 km, similar to Venus. The X-ray glow of the Martian exosphere is too faint to be directly visible in this image. Right: Chandra X-ray image of Jupiter . It shows pronounced auroral emission at the poles superimposed on a uniform brightness distribution. According to the current understanding, the auroral emission is caused by the precipitation of highly ionized oxygen and sulfur into the polar regions, while the nonauroral emission is the combined result of predominantly elastic scattering of solar X-rays and energetic heavy ion precipitation of the halo were observed, obviously caused by the solar wind, because they were not related to the solar X-ray flux. X-ray emission from the exosphere could be traced in individual emission lines out to at least 8 Mars radii, revealing a highly structured emission with morphological differences between individual ions and ionization states . This first X-ray detection of an exosphere around another planet should also lead to a better understanding of the X-ray properties of our geocorona.
On a larger scale, also the interstellar gas, which is streaming through the solar system, provides electrons for charge exchange with the solar wind ions. Model calculations show that the X-ray surface brightness of the heliosphere due to this process should be roughly comparable with that of the geocorona . However, in contrast to the geocorona, which responds almost instantaneously to variations in the solar wind, the heliosphere is expected to react more slowly and smoothly, because of its large extent.
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