ULF wave measurements aboard the Magion-4 subsatellite: narrow-band wave events observed in the magnetopause regions

Using ULF electromagnetic measurements obtained by the Magion-4 subsatellite of the Interball-1 spacecraft, we study the wave activity in the high-latitude magnetopause and magnetosheath. The observed ULF wave activity is characterized by two types of spectra: (a) narrow-band waves of frequency ∼0.33 Hz observed near the magnetopause and (b) broadband spectrum of turbulent nature having discrete bursts of ∼ minute duration. We focus on the case (a) in this article. These narrow-band waves occur rarely, mostly under northward IMF conditions. Their frequency seems stable while their intensity is modulated. The characteristics of the narrow-band ULF events are reminiscent of 3 s-period ULF waves observed by ISEE spacecraft in the foreshock environment, except that the waves discussed in this paper are observed near the magnetopause, inside the magnetosphere. A possible generation mechanism for the observed ULF waves is briefly proposed. We suggest that the emergence of narrow-band ULF events in the magnetopause region at a frequency below the ion cyclotron frequency may be associated with anisotropic ion fluxes flowing just inside the magnetopause.