Production altitude effects on the X-ray spectrum from TGFs

Based on the RHESSI results it has been suggested that most of the X-rays are produced at very low altitudes. On the other hand the BATSE measurements show unabsorbed fluxes of X-rays in the 20-50 keV energy range, indicating that the production altitude of the X-rays is still an open question. As the soft X-rays are highly modified by photoelectric absorption on their way out of the atmosphere, we suggest that this is a key feature that can be utilized to determine at which altitude most of the X-rays are produced, although TGF measurements in the low energy range are still not available. Model examples taking into account both the photoelectric absorption and the Compton scattering and energy degrading will be presented to show how the detected X-ray spectrum may give information about the production altitude of the TGFs. These will be compared with the spectra observed by RHESSI and BATSE.