The production of X ray fluxes in thunderstorms has been attributed to bremsstrahlung. Assuming this, another question arises. How can a thunderstorm modulate the number density of electrons which are sufficiently energetic to produce X rays? As a partial answer to this question, we consider the effects of typical thunderstorm electric fields on a background population of energetic electrons, such as produced by cosmic ray secondaries and their decays or the decay of airborne radionuclides. The observed variation of X ray flux is shown to be accounted for by a simple model involving typical electric field strengths. A necessary background electron number density is found from the model and is determined to be more than 2 orders of magnitude higher than that available from radon decay and a factor of 8 higher than that available from cosmic ray secondaries. Finally, the ionization enhancement due to energetic electrons and X rays is discussed. ¿ American Geophysical Union 1992