A 2.2-Hz modulation of auroral electrons imposed at the geomagnetic equator

Abstract

The direct observation of high frequency ∼3 Hz modulation of electron intensities during pulsating aurora is comparatively recent although, from TV and photometric studies, modulation of the visual aurora at these frequencies has been recognized for over a decade. We report here 2.2 ± 0.5 Hz oscillations in the intensities of 4–25 keV electrons producing a pulsating aurora. The electrons were measured from a Petrel sounding rocket launched from Kiruna, Sweden on 25 January 1979. The oscillations like the slower 1–20 s pulsations, exhibit a marked velocity dispersion implying an equatorial origin for both forms of modulation. VLF—hiss observed at about the same time at the Satellite GEOS 2, operating in the same equatorial region, shows remarkable similarities to the modulation in the particle fluxes. A connection between VLF emissions and auroral pulsations has been suggested by Coriniti and Kennel who argue that low frequency, 5–300 s period, micropulsations modulate whistler-mode wave amplitudes leading to variations in the rate of pitch-angle scattering of electrons from the magnetosphere. The isotropic angular distributions reported here suggest that this mechanism acting alone cannot explain the modulation in the present case.