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.