University of Kansas

X-Ray Emission in the Solar System

 Draft
X-Ray Emission from the Terrestrial Magnetosheath
by Robertson and Cravens

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

Discussion

Is it possible to actually observe the X-rays emitted from the corona, and consequently to remotely image the location of the magnetopause and bow shock in the soft X-rays? The geocoronal intensities are small (approximately 25%) compared to the X-ray emission from the heliosphere or interstellar medium, but they exhibit dramatic time variability compared to the other sources, and perhaps this could be used to "filter out" the steady part of the background.

The observation of the time-variable part of the soft X-ray background from ROSAT (i.e., the LTEs) [Snowden et al., 1994] is a clear indication that a time variable signal can be detected with suitable techniques, although the LTEs are the (mainly) geocoronal emission seen from inside the magnetosphere and the images are this emission seen from the outside. Consider the following possible observing strategy for an X-ray telescope located at a distance of approximately 20 RE and with about 100 pixels, each subtending a solid angle of 3 deg. x 3 deg. The minimum total counts (photons detected) in a suitable time period would need to exceed approximately 100 counts in order to extract 20-50 counts of magnetosheath signal from the total signal (carried out by subtraction from a pointing direction well away from the target or from another time period with very different solar wind flux). Using a typical soft X-ray background intensity and a time/integration interval of approximately 2 hours, the effective detector area (for all pixels) would need to be approximately 10 cm2. For comparison, the effective area for the Roentgen satellite (ROSAT) PSPC instrument 1/4 keV channel was approximately 50 cm2 but with a much smaller solid angle per pixel.

Efficiencies for the SWCX mechanism (e.g., the a in equation (1)) are currently being re-examined (cf. Cravens, 2002a, b). Recently measured charge transfer cross sections for high charge state oxygen ions are about a factor of 2 to 3 less for helium targets than for other neutral targets [Greenwood et al., 2002]. Consequently, we now adopt an alpha value for helium that is a factor of 2 less than the value for H, although this will require further study.

Charge transfer of high charge state heavy solar wind ions produces soft X-rays, but charge transfer of solar wind alpha particles produces He+ 30.4 nm emission in the EUV part of the spectrum: He++ + H --> He+* + H+, where the excited He+* produces 30.4 nm photons [Gruntman, 2001]. The equivalent of equation (1) for this process can be determined using the fractional abundance of He++ in the solar wind (f ~= .05) and the cross section for charge exchange leading to 30.4 nm emission (2.5 x 10-16 cm2 at 1 keV/amu (usw ~= 400 km/hr) and 4.5 x 10-16 cm2 at 2 keV/amu (usw ~= 600 km/s [cf. Gruntman, 2001]). Our estimate of the a for the 30.4 nm emission is a ~= 1.3 x 10-18 cm2. By scaling this a value with the earlier one for soft X-ray emission, one can immediately convert soft X-ray intensities into He+ 30.4 nm intensities (photons/cm3/s) for all the images and figures shown.

As discussed by Gruntman [2001], a number of sources of other "background" 30.4 nm (or very nearby) emission exist, including SWCX of He++ with interstellar H, solar wind pickup ion glow, and emission from interstellar plasma. Our results indicate that the geocoronal contribution will be comparable to this other emission (few milli-Rayleigh intensities) during enhanced solar wind conditions. However, the other sources will be rather steady and the geocoronal 30.4 nm emission highly variable. Another difference is that the geocoronal 30.4 nm emission should be highly Doppler-shifted (D l ~= +/- 0.03 nm) due to the high He++ thermal speed in the magnetosheath.

Next: Conclusions

Last modified January 21, 2004
Tizby Hunt-Ward
tizby@ku.edu