Figure 3. 25 May 2020: Location of the box array used for extracting the brightness variation of the green ghost.
A representative plot of the brightness variation from each of the two events is shown in Figure 4. In the first few frames the green signal is near saturation, but after the main sprite turns off, the green emission is seen to increase towards a maximum around frame 8 in the two sequences. After that it decreases and an exponential has been fitted to the time series. We assume that the green ghost is from atomic oxygen emission at 557.7 nm, an emission that is well known in aurora and airglow. It is a forbidden emission with a radiative life-time of 0.7 s. Thus we would expect the emissions to trail the main sprite and to decay exponentially, which is indeed the case.