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.
Two cases were considered: (1) a simple limited time period solar wind enhancement and (2) realistic solar wind variations.
5.1. Simple Solar Wind Enhancement
For the first case, a finite time width (105 s, a little over 1 day) enhancement was introduced into a previously steady state solar wind (density at 1 AU of 10 cm-3 and speed of 400 km s-1). The enhancement ramped up and down linearly in the initial and final thirds of the total time width and was flat for the middle third of the time period. The corresponding radial extent of the enhancement as it propagates away from the Sun is about a third of an AU. The maximum enhancement in the solar wind density is a factor of 10 and the solar wind speed is kept at 400 km s-1. The fraction of heavy ions in the solar wind is assumed to remain constant, but as the overall proton flux is enhanced, then so is the heavy ion flux. This finite length enhancement results in a time-dependent X-ray intensity at Earth as it interacts with the heliospheric neutrals. We determined this intensity by numerical integration at each time step (of 104 s).
Hydrogen and helium contributions to the X-ray emission associated with the solar wind enhancement are shown separately (Figure 1). The X-ray intensity enhancement due to hydrogen does not exceed ~20% (which is very small given the large solar wind enhancement), and overall, the X-ray enhancement is spread over a time period of 2-3 weeks. This effect is caused by the large volume over which the X-ray emission is produced for H, which is due to the large value of the effective attenuation coefficient (l = 5 AU) we adopted. However, there is a sharp, but small, response as the enhancement first passes 1 AU due to the geometrical effect of the observation ray being tangent to the spherical shell of the enhanced solar wind. Also note that the risetime of the X-ray emission is much more rapid than the "decay" time.
The maximum X-ray intensity enhancement associated with the interstellar helium is a factor of 2-3 higher than the steady background value. The X-ray enhancement extends over a period of only a day or so, which is much less than for the H contribution and comparable to the extent of the solar wind enhancement itself. The reason is that the emitting volume is smaller and closer to the Earth than for H due to the smaller attenuation coefficient for helium (l ~= 1 AU). Clearly, the interstellar helium contribution is more relevant than the hydrogen contribution for time variations of the X-ray emission.
The actual interstellar density distribution is significantly more complicated (see earlier references) than our simple distribution, but the size-scale and magnitude of the density distribution should be about right so that Figure 1 should provide a reasonable guide to the types of X-ray variations that would be produced by a localized solar wind perturbation. This argument also holds for the modeling of X-ray emission from the more realistic solar wind variations to be discussed in section 5.2.
5.2. Model With Actual Solar Wind Fluxes
For the second case, we used solar wind proton fluxes measured by the IMP-8 spacecraft for the time period 1996-1998. Again, the X-rays are produced from the heavy ion component of the solar wind, but we assume a constant heavy ion fraction so that as the proton flux varies so does the heavy ion flux. Figure 2 shows our calculated X-ray intensities for just one 27-day time period in 1998, although our calculations were carried out for the entire time interval. Hydrogen, helium, and geocoronal contributions are shown separately, as is the total intensity. In Figure 2 the timescale for the solar wind flux is shifted so that time refers to the time when the solar wind left the Sun. The relative variation of the solar wind proton flux as measured at 1 AU can be seen in the geocoronal contribution, since that is directly proportional to the proton flux. The three solar wind structures located near day 852 (this refers to days after the first day of January, 1996) clearly are associated with the three X-ray features located near day 857; the time difference is just the solar wind propagation time from the Sun to the emitting volume (located at ~1 AU for both the interstellar helium and the geocoronal sources). The X-ray variations associated with interstellar H are almost entirely smoothed out, whereas the variations associated with either the interstellar helium or the geocorona are much larger. The relative variation of the helium part of the predicted intensity is about a factor of 2, whereas the relative variation for the total X-ray intensity is ~30-50%.
These calculations are not appropriate for the downwind direction due to the very simple functional form adopted for the interstellar neutral densities. For the downwind direction (which corresponds to a December/January time period) the hydrogen contribution to the intensity would be much less than what is shown in Figure 2, but the helium contribution would either be about the same or greater, depending on whether or not Earth was in the region of gravitational focusing at the time of observation [Mobius et al., 1995]. The geocoronal component of the X-ray intensity is less than either of the heliospheric contributions overall, but because most of the heliospheric emission is rather steady and because the time variation of the geocoronal component directly mimics solar wind variations, the geocoronal component's contribution to the time-dependent part of the total intensity is very important.
Next: 6. ROSAT Long-Term Enhancement Data
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Tizby Hunt-Ward tizby@ku.edu |
