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.