  The source fluence distribution of terrestrial gamma ray flashes (TGFs) has
been extensively discussed in recent years, but few have considered how the TGF
fluence distribution at the source, as estimated from satellite measurements,
depends on the distance from satellite foot point and assumed production
altitude. As the absorption of the TGF photons increases significantly with
lower source altitude and larger distance between the source and the observing
satellite, these might be important factors. We have addressed the issue by
using the tropopause pressure distribution as an approximation of the TGF
production altitude distribution and World Wide Lightning Location Network
spheric measurements to determine the distance. The study is made possible by
the increased number of Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
TGFs found in the second catalog of the RHESSI data. One find is that the
TGF/lightning ratio for the tropics probably has an annual variability due to
an annual variability in the Dobson-Brewer circulation. The main result is an
indication that the altitude distribution and distance should be considered
when investigating the source fluence distribution of TGFs, as this leads to a
softening of the inferred distribution of source brightness.
