University of Kansas X-Ray Emission in the Solar System

Draft Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals by Cravens et al.

Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.

2. X-Ray Production by the Solar Wind Charge Exchange Mechanism

In the SWCX X-ray mechanism, highly stripped, heavy solar wind ions undergo charge exchange reactions with neutrals leaving each product ion with one additional electron but still in a high charge state and in an excited state. Approximately 0.1% of the solar wind consists of species more massive than helium, and these species are found in high charge states [Bame, 1972; Schwadron and Cravens, 2000] including species such as O⁷⁺, O⁶⁺, O⁵⁺, Fe¹³⁺, Fe¹²⁺. For example, the charge exchange reaction of O⁷⁺ with a neutral atom or molecule can be represented by

The neutral M could be H₂O, OH, O, or H in comets, H or He for the heliosphere, or H in the geocorona. The resulting ion (e.g., the O⁶⁺ in (1)) is almost always highly excited [cf. Haberli et al., 1997a; Wegmann et al., 1998; Greenwood et al., 2000; Kharchenko and Dalgarno, 2000] and hence emits an X-ray or EUV photon due to the high charge state. The relevant cross sections are very large (in excess of 10^-15 cm²) for most neutral targets [Greenwood et al., 2000; Phaneuf et al., 1982]. In this study, we are interested in the total soft X-ray or EUV intensity (i.e., photon energies roughly 100 eV to 1 keV; most of the observed emission is at roughly 500 eV and below) and not with the details of the spectrum or the atomic transitions. The following expression for the overall soft X-ray and EUV power density is adopted, as discussed by Cravens [1997]:

where the neutral density is denoted n_n and the solar wind density and speed are denoted n_sw and u_sw, respectively. All the detailed atomic cross sections, transition information, and solar wind heavy ion composition, etc., are lumped into the coefficient a. The coefficient is linearly proportional to the heavy ion flux relative to the solar wind proton flux and has somewhat different values for low or high speed solar wind [Schwadron and Cravens, 2000]. For the slow solar wind a reasonable value of this coefficient is a ~= 6 x 10^-16 eV cm² [Schwadron and Cravens, 2000; Wegmann et al., 1998; Kharchenko and Dalgarno, 2000]. As more information on the relevant atomic transitions and cross sections becomes available [cf. Kharchenko and Dalgarno, 2000; Greenwood et al., 2000], the optimum value(s) of a will evolve, but the value used here should be approximately correct. In any case, this paper is mainly concerned with relative variations of the X-ray intensity rather than with absolute intensities. A more significant issue here is that expression (2) only allows for X-ray variations due to changes in the solar wind proton flux and not for variations in the relative abundance of heavy species (lumped into a).

Next: 3. Heliospheric X-Ray Production by the Solar Wind Charge Exchange Mechanism

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Last modified January 8, 2004 Tizby Hunt-Ward tizby@ku.edu