University of Kansas
X-Ray Emission in the Solar System
Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals
by Cravens et al.
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University of KansasX-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.
X-ray emission from comet Hyakutake was discovered in 1996 [Lisse et al., 1996]. X-ray or extreme ultraviolet (EUV) emission from a number of other comets has subsequently been observed [Dennerl et al., 1997; Mumma et al., 1997; Lisse et al., 1999a, 1999b; Dennerl, 1999; Krasnopolsky et al., 2000; Krasnopolsky and Mumma, 2001; Lisse et al., 2001]. Cravens [1997] proposed that charge exchange of heavy solar wind ions with cometary neutrals can produce the required X-ray emission. Charge transfer of a high charge state heavy ion with a neutral leaves the product ion in an excited state and results in the emission of an X-ray or extreme ultraviolet photon. Laboratory measurements demonstrate that the relevant charge exchange collision cross sections are quite large [e.g., Phaneuf et al., 1982; Greenwood et al., 2000]. The X-ray spectrum produced by the solar wind charge exchange (SWCX) mechanism contains many lines [e.g., see Kharchenko and Dalgarno, 2000, and references therein] but appears to be continuum-like when observed at a low or moderate spectral resolution [Wegmann et al., 1998; Dennerl et al., 1997; Schwadron and Cravens, 2000]. Just recently, observations of cometary soft X-rays [Lisse et al., 2001] and extreme ultraviolet (EUV) emission [Krasnopolsky and Mumma, 2001] have been made with sufficient spectral resolution to distinguish some individual lines. These important observations confirm that the SWCX mechanism provides the main source of cometary soft X-ray and EUV emission.
Most of the observed features of the X-ray emission from comets can be explained with the SWCX mechanism (see discussion and references by Lisse et al. [1999a], Neugebauer et al. [2000], Schwadron and Cravens [2000], and Cravens [2000]). X-ray and EUV radiation from comets varies with time [Lisse et al., 1997, 1999a, 1999b; Neugebauer et al., 2000], and this variability can be attributed to (1) variations of the solar wind proton flux [Neugebauer et al., 2000; Lisse et al., 1999a] and (2) variations in the solar wind composition [Neugebauer et al., 2000; Schwadron and Cravens, 2000].
Cox [1998] suggested that the SWCX mechanism applied to solar wind ions interacting both with interstellar neutrals and with atoms in Earth's geocorona outside the magnetosphere produces X-ray emission that could account for part of the observed soft X-ray background (e.g., the Wisconsin survey [McCammon and Sanders, 1990] or ROSAT [Snowden et al., 1995]). Cox [1998] went on to suggest that this mechanism might also account for some of the temporal variation of the soft X-ray background (SXRB) and, in particular, the long-term enhancements (LTE) seen by ROSAT [Snowden et al., 1994]. Dennerl et al. [1997] independently suggested that the SWCX mechanism applied to geocoronal neutrals might be able to explain the LTE part of the SXRB. Freyberg [1998] presented evidence for a correlation of observed variations in the soft X-ray background with the observed luminosity variations from comet Hyakutake by ROSAT high-resolution imager (HRI) and suggested that the SWCX mechanism operating on geocoronal neutrals could be responsible. Freyberg [1998] also noted that this X-ray source could be the explanation for the soft X-ray emission observed from the dark side of the Moon [Schmitt et al., 1991].
Cravens [2000] constructed a simple analytical, but quantitative, model of the heliospheric X-ray emission and its spectrum including both interstellar hydrogen and helium. Cravens predicted that roughly half of the SXRB (photons with energies about 100 to 500 eV) could be explained by this mechanism. The solar wind is highly variable temporally as well as spatially, and thus the heliospheric X-ray emission should also be temporally variable. Cravens [2000], following up on the suggestion of Cox [1998] that this heliospheric X-ray emission should vary with time due to solar wind variations, predicted the form that these variations should take for interstellar hydrogen and helium. The X-ray emission associated with the hydrogen is spread over several astronomical units (AU) spatially and over a couple of weeks temporally, whereas the X-ray emission associated with the helium responds to solar wind changes much faster, i.e., in a day or two.
The time response of the X-ray emission from the SWCX mechanism applied to the geocoronal hydrogen is extremely rapid as pointed out by Freyberg [1998] and other authors, as discussed above. Cox [1998] estimated that the geocoronal contribution to the SXRB was ~30 keV cm-1 s-1, whereas Cravens [2000] estimated that this contribution to the X-ray intensity was only ~4 keV cm-2 s-1. Furthermore, Cravens [2000, p. L155] stated that "only escaping neutrals make it past" the magnetopause. However, the geocoronal model of Hodges [1994], which agrees with hydrogen emission measurements [e.g., He et al., 1993], demonstrated that atoms in ballistic and satellite orbits, as well as escaping atoms, make very important contributions to the hydrogen density at the distance of the magnetopause. In this paper, we correct the geocoronal part of the Cravens [2000] X-ray intensity estimates using hydrogen densities from the Hodges [1994] study, and we now agree with Cox's earlier estimate.
In this paper, we present the results of a simple time-dependent numerical model of the X-ray emission from both the heliosphere and the geocorona. Whereas Cravens [2000] emphasized the overall emission intensity and the spectrum, in this paper we emphasize the time dependence. We use measured time histories of the solar wind proton density and speed from the OmniWeb (i.e., IMP-8 satellite measurements) for 1990-1993 and 1996-1998. IMP-8 is located in the unperturbed solar wind upstream of Earth. We use these solar wind data as input into our model, but we also directly compare the solar wind proton fluxes with the "long-term enhancement" part of the soft X-ray background measured by ROSAT. Robertson et al. [2001] explore some simple spatial variations associated with the heliospheric X-ray emission. This paper provides some initial quantitative estimates of the time-dependent heliospheric and geocoronal soft X-ray emission, but more accurate predictions will require a more elaborate model which includes detailed spatial distributions of the solar wind, the interstellar neutral densities, and the geocoronal densities.
Next: 2. X-Ray Production by the Solar Wind Charge Exchange Mechanism
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Tizby Hunt-Ward tizby@ku.edu |