Figure 1. A pattern of the progress of the storm on Aragats on 23 May. Disturbances of the NS electric field (blue), count rate of ASNT spectrometer (black), and distance to lightning (red).
During the second storm that occurred the next day on May 24 and lasted for 3 hours (see Fig. 2), the electrification of the atmosphere above the station was much smaller. Multiple nearby lightning flashes occurred only during the first hour of the storm from 13:30 to 14:30. After the active phase of the storm, we detect the end-of-storm-oscillation (EOSO). The NS electric field at the ground beneath decaying thunderstorm makes several polarity changes over a period of ≈60 minutes, beginning with a positive field dominated by positive charge overhead, in 2 minutes electric field reversal occurred and the NS field was for 8 minutes in the deep negative domain. We use the atmospheric electricity sign convention, according to which the downward directed electric field or field change vector is positive. This period of negative field was followed by a slower return to a weaker than the initial positive field, which returns in an hour after to additional oscillation to fair weather value. During mostly negative NS electric field (dominated by the negative charge overhead) the TGE started and smoothly finished without being disturbed by a lightning flash. The total duration of TGE was (≈12 minutes) and the intensity of gamma ray and electron fluxes were sufficient to allow the differential energy spectra recovery, see Table 1.
Thus, owing to the possibility of observing EOSO simultaneously with registered electron and gamma ray fluxes, we can classify TGEs according to the pattern of the NS electric field disturbances. In the classification of NS electric field disturbances, we outline 4 categories (Chilingarian & Mkrtchyan, 2012), and the fourth most complicated type, namely, “Multiple disturbances of a near-surface electrical field accompanied by numerous flashes of lightning”, we do not analyze, leaving it for the later publications.