University of Kansas
X-Ray Emission in the Solar System
(DRAFT)
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University of KansasX-Ray Emission in the Solar System |
(DRAFT)
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Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
In March 1996, X-ray (0.1 - 2 keV) and extreme ultraviolet (.09-.2 keV) emission was observed from comet Hyakutake by the HRI and WFC instruments, respectively, on the ROSAT satellite (Lisse et al., 1996) and by the EUVE satellite (Mumma et al., 1997). C/Hyakutake 1996 B2 had a total X-ray luminosity of about 109 W. The X-ray emission varied by a factor of about 2 over a few hour period and by a factor of 4 over about a one day time interval. The spectral resolution was quite low, but the emission was predominantly in the soft X-ray part of the spectrum. X-ray emission has been observed (Dennerl et al., 1997; Krasnopolsky et al., 1997) at several other comets. Extreme ultraviolet emission (.07 - .18 keV) was observed from comet Hale-Bopp by the EUVE satellite (Mumma et al., 1997). Dennerl et al. found that the X-ray luminosity decreased with heliocentric distance and also varied with cometary gas production rate.
Images of cometary X-ray and EUV emission (e.g., Lisse et al., 1996) indicate that the X-rays are produced over a very large volume, extending well beyond cometocentric distances of 105 km. The emission also appears to be centered sunward of the nucleus by about 20,000 km as well as being elongated in the direction perpendicular to the sun-comet line (see Figure 4).
Any mechanism proposed to explain cometary X-rays must be able to explain the following observational facts: (1) the total luminosity for Hyakutake and other comets, as well as the variation of this luminosity with heliocentric distance and comet gas production rate, (2) the time-dependence of the Hyakutake emission, (3) the spatial distribution, and (4) the spectrum.
Several mechanisms have been suggested to explain the cometary X-ray emission, including scattering or fluorescence of solar X-rays by cometary gas or dust, electron bremsstrahlung from gas or dust, and charge transfer of solar wind ions. Krasnopolsky (1997) reviewed and evaluated these various mechanisms and concluded that the most promising mechanism is the charge transfer mechanism first suggested by Cravens (1997). The electron bremsstrahlung mechanism (Bingham et al., 1997; Northrop et al., 1997) easily explains the broad spectral distribution in the soft X-ray region but is orders of magnitude too low in total luminosity if reasonable assumptions are made about the electron energy distribution. A major problem is that X-ray emission is seen at great distances from the cometary nucleus where the electrons are essentially unperturbed solar wind electrons which have a temperature of about 10 eV, not the several hundred eV needed for bremsstrahlung to explain the observations. Krasnopolsky (1997) suggested that solar X-ray scattering by small "nanogram" dust grains might also make some contribution to cometary X-ray emission, but this is a hard mechanism to quantify.
The essence of the charge transfer mechanism (Cravens, 1997) is that highly stripped heavy (Z > 2) solar wind ions charge transfer with cometary neutrals leaving the product ions with one more electron but still highly stripped and usually in an excited state. For example, for oxygen ions, reaction (2) would apply although the cometary neutral is more likely to be an oxygen atom or a water molecule than an H2. However, whereas the incident Jovian oxygen and sulfur ions are energetic, solar wind ions have energies only about 1 keV/amu. Many different heavy ion species are present in the solar wind, reflecting the composition and the charge state distribution of the solar corona (see Figure 1). The charge state distribution of the solar corona is frozen into the solar wind and is characteristic of the very high coronal temperatures (cf. Bame, 1972; Hundhausen et al., 1968).
Cravens (1997) estimated an x-ray production rate using the known solar wind composition, the average solar wind speed, reasonable charge transfer cross sections, and a conservative estimate of the excitation efficiency. He found the following expression for the X-ray power produced per unit volume for a neutral density nnJand a solar wind density nsw:
This power density integrated over the volume of comet Hyakutake (Cravens, 1997) yielded a total X-ray luminosity for comet Hyakutake of about 5 x 1015 ergs/s (or approx. 109 W) which is in agreement with the measured luminosity. Now let us consider how well the heavy ion charge transfer mechanism works for the spectrum and spatial distribution of the cometary X-rays.
The soft X-rays are produced from dozens of lines and from a large number of ion species. Figure 5 shows the results of a detailed theoretical calculation of the cometary X-ray spectrum (Wegmann et al., 1998). It is quite evident that the emission is distributed over a large range of photon energies. When such a "multi-line" spectrum is convolved with an instrumental spectral bandwidth with poor resolution then the spectrum will have the appearance of a continuum.
Next consider the spatial distribution of the cometary X-rays predicted from the charge transfer mechanism. Haberli et al. (1997) used a global MHD model of the solar wind interaction with comet Hyakutake to assess the spatial distribution of X-ray emission associated with the charge transfer (see Figure 4). The observed and modeled spatial distributions agree very well. Haberli et al. (1997) also produced simulated images of the X-ray emission that agree very well with the observed images (Lisse et al., 1996).
Overall, the charge transfer mechanism appears to work very well, although some issues still need to be resolved. In particular, the X-ray time variability observed for comet Hyakutake can probably be explained by the known temporal variability of the solar wind flux and the solar wind composition (this variability is considerable), but this has yet to be demonstrated.
Next: Other Possible Solar System X-Ray Sources
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Tizby Hunt-Ward tizby@ku.edu |