Figure 4. Plot of green emissions for box 4 (24 May 2020) and 5 (25 May 2020). The ordinate is in arbitrary units, and the abscissa in sequence frame number. An exponential has been fitted to the decay phase of the events. The decay time constant is given on the plots.
In the two events the main sprite lasts 2 and 3 frames respectively (60-100 ms) with little change. Based on our experience observing sprites with high-speed cameras most large sprites are rarely active for more than 30 ms, and we think there may be some instrumental issues related to the internal camera image processing and settings. We are looking into those issues.
The green atomic oxygen emission is affected by atmospheric quenching. Thus we would expect the decay time constant to be less that the 0.7 s radiative decay life time, and further, that the observed decay time constant should be smaller at lower altitudes because of the increasing atmospheric density. While the observed decay time constants are all less than 0.7 s, the time constant only decreases with lower altitude for the 24 May event. For the 25 May 2020 event we observe a time constant of 0.4 s for all altitudes. While the decay appears to be exponential, as expected, the observed time constants are larger than the expected 0.1 to 0.2 s based on equations and rate constants given by Vallance Jones (1974) in chapter 4.2. The observed maximum after the sprite has turned off and the decay time constant being larger than expected indicate that there are processes present following the main sprite leading to the green ghost.