University of Kansas
X-Ray Emission in the Solar System
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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.
X-ray emission from Jupiter's auroral region was first observed by the Einstein X-ray observatory (Metzger et al., 1983). Additional auroral X-ray observations were subsequently made by the ROSAT observatory (Waite et al., 1994). Recently, X-ray emission from Jupiter's equatorial region has also been observed (Waite et al., 1997a; Gladstone et al., 1998). Only a brief review is given here; additional review material can be found in Waite et al. (1997b). The estimated total auroral X-ray power is of the order of 109 W as is the total low-latitude (roughly equatorial ) X-ray power. The total power emitted from the Jovian aurora is much greater than this, about 1013 W, and primarily comes from the H2 Lyman and Werner bands in the ultraviolet part of the spectrum (cf. Atreya, 1986). The X-rays emitted from Jupiter are primarily "soft" X-rays with energies of a couple hundred eV (see Figure 2), although the spectral resolution of the observations made to date is quite poor.
What is responsible for the Jovian X-ray emission? Two basic mechanisms have been proposed for the auroral emission, neither of which can currently be entirely excluded: (1) electron bremsstrahlung associated with electron precipitation (Barbosa, 1990, 1992) and (2) heavy (mainly sulfur and oxygen) ion precipitation followed by line emission from highly excited, high charge state ions (Waite et al., 1994; Cravens et al., 1995). A variety of evidence points to the existence of both electron and ion precipitation into the Jovian atmosphere (e.g., Waite et al., 1988; Gehrels and Stone, 1983: Waite et al., 1997a) with the current opinion being that electron precipitation is the dominant source of the UV aurora and ion precipitation the dominant source of the X-ray aurora.
The heavy ion emission mechanism can be summarized as follows (Cravens et al., 1995; Kharchenko et al., 1998). Energetic magnetospheric S or O ions precipitate into the Jovian atmosphere and lose energy in collisions with H2. Several collisional processes are possible:
(1) Charge transfer. The ion (e.g., O+q, where charge state q = 0, 1, ... 8 is possible) captures an electron from a target neutral. Note that molecular hydrogen is the main atmospheric constituent at Jupiter (Atreya, 1986).
(2) Stripping, or electron removal, in which the ion loses an electron.
(3) Ionization and excitation of the target molecule (H2) by impacts of the fast ion.
At higher ion energies, the stripping process dominates over the charge transfer process and the precipitating ions become highly stripped, whereas at lower energies the reverse is true and lower charge state ions predominate. For example, for oxygen at 1 MeV/amu, O+6 and O+7 ion species are the most abundant. X-ray emission results because the oxygen ion resulting from the charge transfer reaction (2) is usually left in a highly excited state and quickly emits an energetic photon, as discussed in the introduction. Figures 2 and 3 show some X-ray emission spectra from theoretical calculations of the Jovian ion aurora.
X-ray emission was also observed from low latitudes at Jupiter (Waite et al., 1997a; Gladstone et al., 1998) and hence cannot be associated with high latitude auroral processes. Waite et al. suggested that the mechanism is the same as at high latitudes -- heavy ion precipitation. In this case, however, the ions originate in the inner magnetosphere of Jupiter rather than in the middle magnetosphere, which has significant implications for the magnetospheric dynamics as well as for the energetics of the equatorial thermosphere. This low latitude ion precipitation represents a significant heat source for the upper atmosphere (Waite et al., 1997a).
Two other related mechanisms for the low-latitude X-ray emission were recently suggested by the author and A. Maurellis. Only a preliminary analysis has been carried out so far (Maurellis, 1998) but the work is being completed now. First, X-ray emission can come from scattering of solar X-ray photons by H2 molecules, and, second, emission can result from fluorescence associated with the carbon K-shell. Carbon is contained in the methane located below the homopause (cf. Atreya, 1986). Preliminary estimates indicate intensities maximizing near the subsolar region, but quantitatively the estimates seem to be somewhat less than the observed intensities.
Next: Cometary X-Ray Emission
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Tizby Hunt-Ward tizby@ku.edu |