Long-duration X-ray emissions observed in thunderstorms

In 1995, a series of four balloon flights with an X‐ray spectrometer and an electric field meter were conducted to examine if strong electric fields could accelerate, and perhaps multiply, cosmic ray secondary electrons and produce bremsstrahlung X‐rays. X‐ray intensities between 10 and 1000 times that of normal background were observed in conjunction with strong electric fields. Both negative and positive polarity electric fields (as referenced to the vertical field) produced X‐rays, which lasted for time scales on the order of tens of seconds. It was also observed that the increased X‐ray intensity would return to near background levels after lightning reduced the local electric field. The observations indicate that X‐rays observed above background are most likely produced by a runaway electron process occurring in the strong static electric field present in thunderstorms. The production of runaway electrons can occur over long periods of time without causing an electrical breakdown. This may provide a leakage current that limits the large scale electric field to values near the runaway threshold, especially in regions where the thunderstorm charging rate is low. Key Points Unique data sets of X‐ray and electric fields in thunderstorms Evidence of energetic electron production by electric fields Possible mechanism for limitation of electric fields in thunderstorms