Solar Wind 931 Comets

The Sun is emitting a wind that contains mainly H and He ions and electrons, plus a minor fraction (~0.1%) of heavier, highly charged elements. This wind is ejected at some hundred kilometers per second and expands so quickly that the ions and electrons usually have no chance to recombine. The ions remain in their highly charged state until they encounter the gas in a cometary coma, where electrons are available in large numbers. When the ions capture such electrons, they attain highly excited states and radiate a large fraction of the excitation energy in the extreme ultraviolet and X-ray range. As the process of charge exchange is characterized by a large cross section (~3 x 10~15 cm2), it works very efficiently in the tenuous cometary coma. Although this process and all its constituents were well known for a long time, the importance of the minor fraction of heavy, highly charged ions for the generation of X-rays was overlooked. So the discovery of comets as a new class of X-ray sources came as a big surprise to many scientists.

Prior to the discovery, there were two ideas about a possible generation of X-rays in comets, but charge exchange was not considered. One idea was that the interaction

1 An exception might be very small dust particles having a size similar to the X-ray wavelengths, as this dust would scatter X-rays very efficiently However, it is a controversial question whether X-ray scattering on such particles has been observed.

of the magnetized solar wind with the cometary ionosphere could create an extended magnetosphere, which might be capable of accelerating electrons under favorable conditions. These energetic electrons might then produce bremsstrahlung X-ray emission when they decelerated in the cometary coma. There were, however, great uncertainties in this model and its parameters, and the X-ray emission would have occurred only during episodes of cometary auroral substorms. The other idea considered collisions of cometary dust particles with solar system dust. If such particles collided with high velocity, they might evaporate, briefly forming a high-temperature plasma, which might emit an X-ray flash. These ideas were the motivations for searching for X-ray emission from comets, but so far no firm observational evidence has been found that X-rays are produced by these effects.

At the end of 2005, at least 20 comets were detected in X-rays, with the satellites ROSAT, EUVE, BeppoSAX, Chandra, XMM-Newton, FUSE, and Swift [26]. The first record of cometary X-ray emission dates back to July 1990, when Comet 45P/Honda-Mrkos-Pajdusakova happened to cross the field of view of ROSAT during the all-sky survey [16]. The most famous comet is C/1996 B2 (Hyakutake), where X-ray and extreme UV emission was discovered for the first time, with ROSAT [27] and EUVE [30], during its close encounter with the Earth in March 1996. The first high-resolution X-ray spectrum was obtained on Comet C/1999 S4 (LINEAR) with Chandra. It exhibited pronounced emission lines, which could be attributed to the charge exchange process [25].

Probably the best cometary X-ray data obtained so far are from comet C/2000 WM1 (LINEAR), observed in December 2001 with XMM-Newton [14]. Figure 9.2 shows the X-ray contours superimposed on an optical image. It is apparent that

Fig. 9.2 X-ray properties of Comet C/2000 WM1, observed with XMM-Newton [14]. Left: X-ray image with contours superimposed on an optical image, illustrating the different morphology and the large extent of the X-ray emission. The Sun is to the right. Right: X-ray spectrum, showing pronounced line emission due to solar wind charge exchange. The most likely transitions are labeled. Note that there is no X-ray emission above 1 keV

Fig. 9.2 X-ray properties of Comet C/2000 WM1, observed with XMM-Newton [14]. Left: X-ray image with contours superimposed on an optical image, illustrating the different morphology and the large extent of the X-ray emission. The Sun is to the right. Right: X-ray spectrum, showing pronounced line emission due to solar wind charge exchange. The most likely transitions are labeled. Note that there is no X-ray emission above 1 keV

the X-ray morphology is different from the optical appearance: the maximum of the X-ray emission is shifted toward the Sun and the X-ray coma is elongated and considerably extended perpendicularly to this direction, while the cometary tail is completely invisible. This morphology, which is characteristic for all sufficiently active comets in the undisturbed solar wind, can be well understood as a direct consequence of charge exchange [10,40]: when the highly charged ions (coming from the right in Fig. 9.2) first encounter the tenuous outer coma, the probability for charge exchange processes is low, as is the X-ray emission. Further inward, the density of available electrons is increasing. It eventually becomes so high that the ionization state of the ions is reduced so much that they are loosing their ability to emit X-rays. This explains why there is no X-ray emission from the cometary tail. The X-ray flux, which we see in the "shadow region" (left of the nucleus in Fig. 9.2), comes mainly from the outer coma and appears there due to projection along the line of sight.

Details of the charge exchange processes are revealed in the X-ray spectrum (Fig. 9.2, right). The most prominent emission line, at 0.561 eV, results when a H-like O7+ solar wind ion captures an electron from the cometary gas (usually an outer electron from a H2O or OH molecule) to become a He-like O6+ ion in an excited state, from which it deexcites to the ground state. Thus, the X-ray spectra of comets contain direct information about the heavy ion content of the solar wind, which would otherwise be only accessible by in-situ measurements.

A recent comparative analysis of the spectra of eight comets observed with Chandra showed that spectral differences can indeed be ascribed to different solar wind states, like (i) the fast, cold wind, (ii) the slow, warm wind, and (iii) disturbed, fast, hot winds associated with interplanetary coronal mass ejections [6]. Comets can thus be utilized as natural spaceprobes, which sample the heavy ion content of the solar wind at various states, various heliographic latitudes, and at various phases in the solar cycle. Because of its high cross section, the charge exchange process is so efficient in comets that it may power X-ray luminosities of several 109 W.

0 0

Post a comment