Variation of the serpentine emission carrier frequency under conditions of a quiet magnetosphere

The variation of the carrier frequency of serpentine emission f_SE observed in the frequency range 0.1 – 5.0 Hz under conditions of a quiet magnetosphere (Kp ~ 0 – 2) was studied. The data of magnetic field registration at the Antarctic Vostok Observatory (corrected geomagnetic coordinates F'=−85.41°, L'=69.01°) for the 1970‒1972 were used in an analysis. The 90 cases of serpentine emission observation, the central carrier frequency of which smoothly decreased (several times, sometimes to 0) and then increased almost to the initial level at time intervals significantly exceeding the maximum modulation period (1 hour) were analyzed using dynamic spectra of ULF emission. In this case, the typical modulation of the emission carrier frequency with periods of 1 – 60 min was persisted. The most likely time of observation of the detected effect was in the hours before midnight. It is shown that a decrease of the f_SE and its subsequent increase were observed against the background of weak geomagnetic activity and relative stability of the dominant number of solar wind and IMF parameters. Taking into account the discovered synchronous coincidence of the behavior of the f_SE and the dynamics of the AE index, as well as the observation of the effect of a decrease in the carrier frequency near local midnight, it is assumed that serpentine emission is most likely excited near the polar cusp and then penetrates into the region of the polar cap. The behavior of f_SE observed over long time intervals is presumably controlled by the plasma parameter β and the ratio of the proton density to the density of helium ions Np/Na, the dynamics of which are similar to the average variation of f_SE.