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.