Evidence for subauroral proton flashes on the dayside as the result of the ion cyclotron interaction

Abstract

A series of proton aurora flashes produced by the precipitation of energetic protons equatorward of the main auroral oval was observed between 0700 and 1300 UT on 31 May 2005 on the dayside with the Imager for Magnetopause‐to‐Auroral Global Exploration (IMAGE) Far Ultraviolet (FUV) imager. At the same time multiple bursts of geomagnetic pulsations within the Pc1 range known as “hydromagnetic emission bursts” were observed on several ground stations located in the conjugate MLT sector. The pulsation bursts are well correlated with the appearance of the proton aurora conjugated with the ground stations. The frequency width of the pulsation burst correlates and the upper frequency anticorrelates, respectively, with the latitudinal width and lowest latitude of the proton aurora at the meridian of the ground station. Observations at the meridional network of pulsation magnetometers show that the maximum pulsation spectral density is detected at stations conjugated with proton flashes. The close relationship between pulsations and proton precipitation (aurora) indicates their common source. We conclude that the source is the cyclotron instability of the ring current ions that is stimulated by impulsive magnetospheric compressions. The magnetosphere compressions are, indeed, confirmed by plasma data from the Geotail spacecraft in the duskside magnetosheath. They show a series of plasma pressure pulses during the time interval of interest. Also standard midlatitude magnetograms indicate large‐scale magnetospheric compressions.