Ion velocity filter effect observed in dayside hydrogen aurora

Abstract

Observations of dayside auroral hydrogen emissions of Hα (λ656.3 nm) were carried out using spectrometers on Svalbard at Ny‐Ålesund (NYA: 76.3°N, 111.0°E CGM) and Longyearbyen (LYR: 75.3°N, 111.9°E CGM). Using a Monte Carlo model, simulated Doppler profiles were fitted to the spectra to estimate precipitating proton energy. A difference in energy was found between the sites for approximately 45 minutes. When combined with measurements of antisunward convection, the energies are consistent with the inverse variation of ion energy with latitude observed by satellites in the cusp region, known as the ion velocity filter. This is the first measurement of the ion velocity filter effect in the dayside aurora using ground‐based optical instrumentation. The increasing difference in energy observed is interpreted as a measure of the decreasing merging rate at the magnetopause.