Variation in the Serpentine Emission Carrier Frequency under Quiet Magnetosphere Conditions

The article studies variation in the serpentine emission carrier frequency f_SE observed in the 0.1–5.0 Hz frequency range under quiet magnetosphere conditions (Kp ~ 0–2). The data of magnetic field recording at the Antarctic Vostok Observatory (corrected geomagnetic coordinates Φ′ = −85.41°, Λ′ = 69.01°) for 1970‒1972 were used in the analysis. Using the dynamic spectra of ULF emission, we analyzed 90 cases of serpentine emission observation, the center carrier frequency of which gradually decreased (several times, sometimes to 0), then increased almost to the initial level in time intervals significantly exceeding the maximum modulation period (1 h). In this case, typical modulation of the emission carrier frequency with periods of 1–60 min persisted. The most likely time of observation of the detected effect was in the hours before midnight. It is shown that a decrease and subsequent increase in f_SE were observed versus 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 f_SE and dynamics of the AE-index, as well as observation of the effect of a decrease in the carrier frequency near local midnight, it is suggested that serpentine emission is most likely excited near the polar cusp, then penetrates the polar cap region. The behavior of f_SE observed over long time intervals is presumably governed by the plasma parameter β and ratio of the proton density to the helium ion density Np/Na, the dynamics of which are similar to the average variation in f_SE.