Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

"Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 163 ■165, 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute SOUTHERN BOUNDARY OF THE ULTRA RELATIVISTIC ELECTRON PRECIPITATION ON MAY 13,1987 G.F. R em enets, A.M . A stafiev (Physics Department o f St.-Petersburg State University, Russia) Introduction The phenomenon of the ultra-relativistic electron precipitation (UREP) into the polar atmosphere from the near- Earth was analyzed in a cycle o f publications [1-6]. The analysis was based on the very low frequency (VLF) data in the cases o f abnormal disturbances. It was shown that the phenomenon was accompanied with the effect of the geomagnetic cutoff. It was observed that energy of the precipitating electrons was such (about 100 MeV) that they could generate bremsstrahlung X- rays, which were capable to create an electric conductivity sporadic D-layer at the altitudes 1 0 - 4 0 km. Such sporadic layer formed the reflected signals (due to the partial reflections from the altitude intervals where the conductivity gradient differed from zero) from the on ground monochromatic VLF radio sources ( 1 0 - 1 6 kHz). The effective height h of radio wave reflection is changing during a disturbance from the undisturbed value h ~ 60 km to h ~ 30 km in the maximum of powerful disturbance (PwD’s) at daytime. In addition to these results the same VLF data can be used for estimation of the latitude of southern boundary of a disturbance. This capability is realized in this paper for the UREP on May 13, 1987. Amplitude and phase experimental data on May 13,1987 [6] The amplitudes of the on ground signals as functions of time for 4 frequencies 10.2, 12.1, 13.6 and 16 kHz are represented on Fig. 1 (left). The signals with first three frequencies were gotten for a shorter radio pass Aldra (North Norway) - Apatity (Russia), its length Sj being equal to 885 km. The radio pass was completely in auroral zone and directed W - E. The 4th signal came to Apatity from Great Britain (GBR station, its length S2 = 2497 km). This radio pass was partly auroral due to its northern part. Intensity of this abnormal disturbance was comparable with the ones caused by the proton precipitations but the disturbance was qualitatively different due to the absence in the received Aldra - Apatity signals of the rays with two reflections from above. 17.00 17:30 U T time 18:00 16:30 16:30 17:30 UT lime 16:00 Fig. 1 Qualitative similarity of the amplitude (left) and phase (right) variations for shorter and long radio-passes. The amplitudes are given in relative units. The ratios of amplitudes for different frequencies are unknown. The phases o f the on ground signals as functions of time for the same 4 frequencies are represented on Fig. 1, right. These data were gotten by Beloglazov M.I., a member of the Polar Geophysical Institute in Apatity, RAS. The qualitative similarity of the amplitude and phase data represented on Fig. 1 for both radio-passes indicates on a mutual physical cause of the disturbance, which had place at the northern auroral end of the long radio- pass. The problem named in the Introduction was solved in two stages. The first stage is finding of a solution o f the VLF problem o f first kind according to the data for the auroral shorter radio-pass only. In frames of this problem we found the reflection coefficient of the first ray from the ionized middle atmosphere and the effective height of the radio-pass, the near Earth waveguide being considered homogeneous at every moment of the disturbance. The solution o f the problem was begun in [7]. Here we finished it more accurately. The latitude position of a boundary between perturbed and non-perturbed parts of the long radio- pass (16 kHz) is an object of determination by the help of the inverse problem of second kind. This problem is solved in the final part of the report. 163