3. Intensity Types of the LF-GM-REPs

[12]  It is comfortable to introduce four types of anomalous LF variations on the auroral Aldra-Apatity radio trace according to their intensity at a disturbance maximum: (1) weak disturbance (WkD) if a variation is larger but comparable with the apparatus errors (0.5-1  m s for phases and 0.1-0.2 cm on a recording paper with 5 cm range for the amplitudes); (2) moderate disturbance (MdD) if it is more intensive than a WkD but an amplitude decrease for any of the three frequencies not larger than to 50%; (3) strong disturbance (StD) if it is more intensive than a MdD but an amplitude decrease for any of the three frequencies not larger than to 80-90%; and (4) powerful disturbance (PwD) if it is more intensive than a StD, that is, the amplitude for one or more frequencies is comparable with the atmosphere noise level.

Figure 2

[13]  The first trustworthy SrD and PwD for the second period of registration were recorded in 1982 [Remenets, 1994; Remenets and Beloglazov, 1985] and 1983. A normal LF auroral disturbance and an anomalous one (StD) are illustrated in Figure 2 for 27 January 1992 at 2100-2150 UT and 2230-2243-2320 UT, respectively (polar night conditions). The descriptive letters in Figure 2 (and the following ones) are as follows: A_i and j_i are the indicators of amplitude and phase curves where the number i=1, 2, 3 represents the frequency f=10.2, 12.1, or 13.6 kHz, respectively. Figure 2 (left) represents one of the frequent cases of an ordinary auroral disturbance generated by the precipitations of auroral electrons with an energy from some tens to some hundreds keV. A qualitative difference of the variations in Figure 2 is evident. In the case of the ordinary disturbance a receiver gets the sum of a second ionospheric ray with a first ionospheric ray and a ground one. As a result of their interference, there is a strong frequency dependence without the qualitative similarity in the time variations of the amplitudes. In the REP case (Figure 2, right) the second ray was absorbed by the ionized atmosphere and the variations of the signals for the three frequencies are qualitatively similar. This is the main symptom of the anomalous LF disturbances which we connect with the REPs only, and it makes the LF ground monitoring (GM) a unique method of ground detection of REPs [Remenets, 1997; Remenets and Beloglazov, 1999]. In all other geophysical disturbances [Remenets, 1997] a second ray is present.

[14]  A PwD for the trace with a length of 885 km (Aldra-Apatity) is possible only for conditions when the atmosphere along the radio trace is intensively ionized at the height interval 20-40 km. Because of such ionization a first ionospheric ray may compensate a ground wave which does not depend on middle atmosphere ionization. It is interesting to note that this compensation is an effect of a zero-order interference due to the curvature of a "ground-ionized atmosphere layer" waveguide. In the case of a plane model of the waveguide the thickness of it for the named zero-order compensation should be zero. The cause of anomalous ionization at such altitudes are the UrEPs. The electrons with energy more than some tens of MeV effectively generate the friction radiation of X rays and gamma rays. A list of all PwDs is shown in Table 1 (published events are indicated). The amount of the disturbances of the other three types per year are presented in Table 2.

[15]  The most prolonged of the disturbances (9 hours) is a PwD on 27 March 1988. After 0940 UT at this day all three amplitudes turned to "zeroes" during 40 min and amplitude "restoration" of the signals began after 1400 UT and continued till 1800 UT [Beloglazov et al., 2000].

[16]  The duration of the disturbances has a direct relation to their space range. Indeed, the Earth with a concrete radiotrace is rotating inside the magnetosphere. The possibility of a REP event to be "tied" only to this trace is very small. So if a LF disturbance continues some hours and the length of the west-east trace is near 1000 km, as in our case, then the longitude scale of the REP at the auroral zone is some thousand kilometers too. An experimental estimation of the latitude scale of the events has been performed by Remenets and Beloglazov [1999]. The pointed estimation due to LF-GM-UrEPs data does not contradict the direct measurements of highly relativistic electron fluxes in the outer and inner radiation belts [Baker et al., 1997; Pesnell et al., 1999].

[17]  Now let us discuss the time asymmetry in the increase and decrease phases of the most of the LF-GM-UrEPs. The necessary data are summarized in Table 3. The parameters t_incr, t_const, t_decr are the time magnitudes of increase, constant, and decrease phases of the PwDs.

[18]  As is seen in Table 3, in most of the cases the time t_decr is some times larger than the t_incr time. This is a typical feature of the UrEPs.

Figure 3

Figure 4

[19]  If the ionization of the middle atmosphere in this cases takes place without dissociation reactions (without generation of atomic oxygen), then because of the extremely small lifetime of the free electrons at 20-40 km the intensity of LF variation is determined by the top boundary of electron energy of the flux and by the flux intensity of the precipitating electrons. The most asymmetric disturbances (in time) were recorded on 3 April 1991 (1200-1430 UT) and on 22 January 1992 (a polar night) and are presented in Figures 3 and 4. The parameters in Figures 3 and 4 are the same as for Figure 2. In connection with the experimental data of Figure 3 it is necessary to point out two peculiarities: (1) "sudden" abrupt decreases (with a characteristic timescale near 100 s) of the amplitudes near 1220 UT and 1255 UT; (2) the recurrent disturbance during 1500-1900 UT. This case of recurrence is not a singular one, which was registered in Apatity for StDs and PwDs. The recurrence is seen in Figure 2 of Beloglazov et al. [1999] and is present in the above note of all PwD for 1 April 1988. We have finished the phenomenon description, and now we shall give some results of the quantitative analysis.