  In the spring of 2017 an ER-2 aircraft campaign was undertaken over
continental United States to observe energetic radiation from thunderstorms and
lightning. The payload consisted of a suite of instruments designed to detect
optical signals, electric fields, and gamma rays from lightning. Starting from
Georgia, USA, 16 flights were performed, for a total of about 70 flight hours
at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm
regions. An analysis of two gamma ray glow events that were observed over
Colorado at 21:47 UT on 8May 2017 is presented.We explore the charge structure
of the cloud system, as well as possible mechanisms that can produce the gamma
ray glows. The thundercloud system we passed during the gamma ray glow
observation had strong convection in the core of the cloud system. Electric
field measurements combined with radar and radio measurements suggest an
inverted charge structure, with an upper negative charge layer and a lower
positive charge layer. Based on modeling results, we were not able to
unambiguously determine the production mechanism. Possible mechanisms are
either an enhancement of cosmic background locally (above or below 20 km) by an
electric field below the local threshold or an enhancement of the cosmic
background inside the cloud but then with normal polarity and an electric field
well above the Relativistic Runaway Electron Avalanche threshold
