Observations of electromagnetic ion cyclotron waves during geomagnetic storms: Wave occurrence and pitch angle scattering

Abstract

This paper addresses the occurrence of low‐L (3.5 < L < 5) electromagnetic ion cyclotron (EMIC) waves during magnetic storms. Using ∼10 years of data, a statistical analysis indicates that EMIC waves in the equatorial magnetosphere occur 5 times more often during magnetic storms than during quiet times. EMIC wave occurrence during magnetic storms is important because EMIC waves pitch angle scatter energetic protons and thus represent a ring current loss mechanism. Observations in the equatorial magnetosphere, acquired by the Dynamic Explorer 1 (DE 1) satellite, are used to show the simultaneous occurrence of EMIC waves and enhanced proton fluxes in the loss cone. The study involves three cases selected only on the basis of observed EMIC wave activity and the availability of energetic ion data. The observations show that EMIC waves indeed scatter protons into the loss cone. Model calculations of the loss cone heat‐flux density compare favorably to the observed proton flux in the loss cone. These calculations are then used to estimate the effect of EMIC waves on ring current energy loss, assuming an effective area of wave activity and the duration of the wave activity. It is found that large‐amplitude, long‐duration EMIC waves can produce the observed recovery in the Dst index.