2. Event Statistics

[5]  The LF monitoring of some radio stations was carried out at the Polar Geophysical Institute of the Russian Academy of Sciences (Apatity, 67.5^oN, 33.3^oE) by M. I. Beloglazov during 1974-1992. The registration was continuous except for summer vacations (1-1.5 months) and some induced technical interruptions. So the summary continuous registration time was equal to the 85-90% of a year. The data of registration for the second half of 1980 and for 1981 are absent. The quality of registration before and after these years was different. During the first interval of registration, the 10.2 and 13.6 kHz signals of OMEGA station in Aldra (66.5^oN, 13.2^oE) and the 16.4 kHz signal of JXZ station (66.4^oN, 13.2^oE) were received by the apparatus with the amplitude channels of 200 Hz range and with a quartz base generator in the phase channels. During the second interval of registration all three OMEGA signals (including one with 12.1 kHz) and GBR signal (16 kHz, 52.3^oN, 1.15^oW, 1982-1989) were monitored by the 20 Hz amplitude channels and by the 0.01 Hz phase channels with a highly stable base signal [Beloglazov and Remenets, 1982].

[6]  The first period refers to the 20th-21st solar activity cycle, and the second period refers to the 21st-22nd cycle. It is important to make a note of the fact that before 1981 the LF disturbances which may be caused by REPs do not exceed 30-50 events and only five of them may be strong disturbances (StD), weaker than PwDs. There was one definitely StD on 29 October 1977, 1620-1635-1700 UT, and there was no powerful disturbance (PwD) for a period of 5.5 years.

[7]  For the second period of registration (after 1981), there were more than 270 disturbances (connected with REPs), all quite reliable. The uncertainty of the statements for the first period is caused by the relatively broad intervals between the frequencies used (near 3 kHz) for the middle source zone of radio registration, by the relatively low quality of the registration apparatus indicated above and by the discrete character of operating of the JXZ station.

Figure 1

[8]  Figure 1 is a histogram of the amount of the anomalous LF disturbances (MEREPs, HEREPs, and UrEPs) per year and the curve of monthly mean sunspot numbers. The white part of the histogram is the estimated result, and the black part is the real result. The estimation was derived as the amount of events for which all (3 amplitudes and 3 phases), or because of all kind of technical causes mentioned above, only a part of the magnitudes was registered. So we consider that the real amount of the events for 1974-1979 is some times less than according to the estimation data of Figure 1. The statement of Beloglazova and Beloglazov [1982] on 700 events of REPs in 1974-1977, as we now understand on the basis of the quantitative analysis of LF disturbances, is on erroneous one. These hundreds of events are the fluctuations of the phase of the second ionospheric ray relative to the sum of the ground and the first ionospheric ray due to the weak (the riometers did not "feel" them) casual fluctuations of the effective height of the Earth-ionosphere waveguide. Also, we state that during the minimum of solar activity in 1974-1976, there was a relative minimum of REPs but for the next minimum of solar activity (1984-1987) the REPs activity was not suppressed just as in the outer radiation belt [Callis et al., 1991; Remenets and Beloglazov, 1999].

[9]  An additional comment on Figure 1 is that for 1989 the amount of events is equal to 23 and significantly larger than one (six) which Remenets and Beloglazov [1999] reported. The cause of this difference is technical and is caused by the fact that during half of the year the phases of OMEGA radio signals with frequencies 10.1 and 13.6 kHz were registered instead of the Aldra station from a distant OMEGA station. So the disturbances for the period were not taken into consideration in the earlier paper.

[10]  The main conclusion due to the represented statistical result for the period 1974-1992 is a potential 22-year periodicity in REPs activity which, as one may expect, is controlled by the 22-year periodicity of the Sun's magnetic field. The direct measurements of the HEREs [Baker, 2000] for 1995-1997 (solar activity minimum) may be adjust this supposition.

[11]  For the classification due to the intensity of the LF anomalous disturbances measured by the ground monitoring (GM) and caused by EPs of all relativistic energy we used four gradations [Remenets and Beloglazov, 1999].