On the origin of frequency modulation of serpentine emission

We consider a problem of interpretation of 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 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 polar caps from the interplanetary medium where they are excited in the form of ion-cyclotron waves as a result of instability of plasma with anisotropic ion velocity distribution. To verify the hypothesis of the SE extra-magnetospheric origin a test was proposed. A previously unknown property of SE was found out which supports the hypothesis. The property is that at times the emission carrier frequency undergoes a pronounced 5-minute modulation. Apparently, a 5-minute SE modulation is genetically related to the 5-minute oscillations in the solar photosphere.