Photometric evidence of electron precipitation induced by first hop whistlers

Abstract

Electron precipitation events induced by discrete VLF whistler mode waves have previously been detected by photometers at Siple Station, Antarctica. This paper presents the first observations of ionospheric optical emissions correlated with VLF waves at the conjugate location, near Roberval, Quebec. Since most whistlers recorded at Siple or Roberval originate in the north, Roberval affords a clear perspective on the direct precipitation induced during the first pass of the wave as it propagates southward. For such a wave the direct precipitation and that induced in the "mirrored mode" by the returning two‐hop wave should differ in arrival time by roughly twice the wave propagation time between hemispheres, while at Siple the effects of the direct and mirrored modes may overlap in time. A well defined series of observations of structured λ4278 optical emissions was observed on August 30, 1979 in the aftermath of an intense magnetic storm. The optical emissions were found to lead the arrival time of the two‐hop waves by about 0.7 s instead of lagging the local waves by about 1‐2 s as had been previously observed for whistler driven events at Siple. The observed arrival time relationships are consistent with the predictions of a cyclotron resonance interaction model, and thus support previous observations of x‐rays at Roberval. The importance of the first pass of the wave is further emphasized by an approximate proportionality between the amplitude of the VLF waves recorded at Siple and the intensity of the optical emission bursts at Roberval. Although structured optical emissions correlated with wave bursts can clearly be detected at Roberval, relatively large magnetospheric particle fluxes may be required to produce such events.