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Amplitude modulation of the VLF-emissions and cosmic noise absorption by Pc5 geomagnetic pulsations during geomagnetic storms of October 30-31, 2003
Solovyev S.I., Mullayarov V.A., Baishev D.G., Barkova E.S., Samsonov S.N.,
Strong geomagnetic disturbances on October 30-31, 2003 were accompanied by the intense wave activity: on recovery phases of Dst-variations the Pc5 geomagnetic pulsations and VLF-emissions which are modulated in amplitude by the same periods are observed practically continuously. There are also intervals of correspondence for the periods of geomagnetic pulsations and modulations of cosmic noise absorption. The greatest modulation depth of VLF- emissions (up to 100 %) at the Yakutsk station has been detected at the first one-half of all two days (UT). Thereby, the best correspondence with an envelope of geomagnetic pulsation amplitudes is observed at longitudes of their maximum intensity. In some intervals of 3-hour duration the exact coincidence of VLF-emission peaks and pulsations is noted. Numerous drops of the VLF-emission amplitude up to the background level corresponding to sharp amplifications of geomagnetic pulsations are also detected. Based on a character of VLF-emission variations associated to geomagnetic pulsations, the conclusion has been made that, in spite of a displacement of auroral activity to middle latitudes, the cyclotron instability of energetic electrons, as one of main sources of VLF-emissions, was in regime of weak pitch-angle diffusion of particles into a loss cone. The best coincidence of VLF-emission modulation with geomagnetic pulsations in other longitude sectors is explained by a global character of the pulsation excitation and by damping of their amplitude at the observation meridian of VLF-emissions which is associated with the auroral electrojet intensification. It is supposed that the damping of òÓ5 pulsations takes place at ionospheric heights due to the increase of a reflection coefficient of incident MHD-waves from the ionosphere.
The work was partially supported by RFBR grant 03-05-39011-GFEN.