On the origin of frequency modulation of serpentine emission

Abstract

We consider a problem of the interpretation of the so-called "serpentine emission" (SE), which is ULF electromagnetic emission in the range of Pc1–2 (0.1–5 Hz) found in Antarctica. A distinctive feature of SE complicating its understanding within the standard theory of the magnetospheric oscillations is a deep modulation of the SE carrier-frequency. The characteristic time of the frequency modulation varies from case to case from 1 minute to 1 hour. It is assumed that the frequency-modulated oscillations penetrate into the polar caps from the interplanetary medium, where they are excited in the form of ion-cyclotron waves as a result of the instability of plasma with anisotropic ion velocity distribution. To verify the hypothesis of the SE extra-magnetospheric origin a test is proposed. A previously unknown property of SE is discovered, which supports the hypothesis. The property is that at times the emission carrier frequency undergoes a pronounced 5-minute modulation. Apparently, a 5-minute modulation SE is genetically related to the 5-minute oscillations of the solar photosphere.