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 has been observed from many objects throughout the solar system including the Sun, the Moon, the Earth, Jupiter, and comets (cf. Cravens 1999). The original discovery of x-rays from comet Hyakutake (Lisse et al. 1996) was surprising, but x-ray emission from a number of other comets has now been documented (Dennerl, Englhauser and Trumper 1997) as has extreme ultraviolet (EUV) emission (e.g., Mumma, Krasnopolsky and Abbott 1997). Cravens (1997) proposed that charge transfer of heavy solar wind ions with cometary neutrals can produce the required x-ray emission. This solar wind charge exchange (denoted SWCX) mechanism has been shown to reproduce most of the observed features of the x-ray emission from comets, including the spatial morphology (Haberli et al. 1997; Wegmann et al. 1998), the temporal variation (Neugebauer et al. 1999; Schwadron and Cravens 1999), the spectrum (Wegmann et al. 1998; Dennerl et al. 1997; Haberli et al. 1997; Kharchenko and Dalgarno 1999; Schwadron and Cravens 1999), and the total luminosity (Cravens 1997; Krasnopolsky 1997).
Cox (1998) suggested that the SWCX mechanism proposed by Cravens (1997) for comets could also apply to geocoronal atomic hydrogen and to interstellar neutrals in the solar system (or heliosphere) and might account for some of the observed soft x-ray background (SXRB). In this paper a simple model is presented for soft x-ray emission from the heliosphere due to the charge transfer of solar wind ions with interstellar neutral H and He atoms.
The heliosphere is the region surrounding the Sun that contains solar wind plasma rather than interstellar plasma. The outer boundary of the heliosphere, called the heliopause (cf. Axford 1996), is thought to be located at a heliocentric distance of roughly 100 Astronomical Units (AU). Before the supersonic solar wind reaches the heliopause region it undergoes a shock transition (the termination shock). The interstellar plasma is excluded from the heliosphere but neutral gas is able to flow into the heliosphere (with a speed of about 23 km/s) largely unimpeded by the plasma or the magnetic field.
Interstellar neutral hydrogen has been observed by means of resonantly scattered solar Lyman alpha photons (Bertaux, Lallement and Quemerais 1996; Judge, Gangopadhyay and Grzedzielski 1990) and by means of pick-up ions produced by the ionization of interstellar neutrals and detected by spacecraft such as Ulysses (cf. Gloeckler 1996). The interstellar neutral H density has been estimated to be about 0.1 cm-3, and the interstellar He density has been estimated to be about 10% of the H density. A small enhancement of the H density, called the hydrogen "wall," has also been predicted to exist between the termination shock and the heliopause due to charge transfer of incoming interstellar H atoms with the plasma in the vicinity of the heliopause (cf. Fahr 1996).
Most x-ray emission in the outer heliosphere should come from the charge transfer of the solar wind heavy ions with interstellar H whereas helium should contribute in the inner heliosphere. These x-rays observed at Earth should appear to come from the entire sky and, as suggested by Cox (1998), could be responsible for a significant part of the soft (80 - 500 eV photons) x-ray background observed by rocket experiments (e.g., the Wisconsin survey: McCammon and Sanders 1990) or by orbiting x-ray observatories such as ROSAT (Snowden et al. 1995, 1998). However, unlike the part of the x-ray background coming from hot interstellar gas, the heliospheric x-ray emission should exhibit spatial and temporal variations (Neugebauer et al. 1999) that are associated with variations in the solar wind flux and composition, which in turn reflect the changing conditions in the solar corona (Hundhausen 1968).
Next: Model of Heliospheric X-Ray Production Due to the Solar Wind Charge Transfer Mechanism
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Tizby Hunt-Ward tizby@ku.edu |