Diurnal variation in plasmaspheric He+ inferred from extreme ultraviolet images

Abstract

A diurnal variation in He+ density is observed using successive snapshots of the plasmasphere taken by the extreme ultraviolet (EUV) imager on the IMAGE spacecraft. Analysis of average EUV photon counts from a specific patch of plasma, which may move at a rate other than corotation, centered at L = 2.5 and L = 3.5 over successive images during an IMAGE pass, allows us to follow a flux tube and track the brightness variation across ∼6–10 h of local time. These EUV brightness observations are converted to He+ column‐integrated densities by taking into account solar EUV flux as measured by the SOHO spacecraft. Variations in He+ column density with local time provide insight into the process of plasmaspheric refilling by ionospheric plasma outflow on diurnal time scales. An analysis of 128 orbits of IMAGE EUV data in 2001 reveals that EUV photon count, and hence column‐integrated He+ density, tends to increase soon after dawn, decrease or stagnate around noon, and strongly increase in late afternoon through dusk. These observations show no significant dependence on geomagnetic activity and support the idea that diurnal refilling of the plasmasphere is largely controlled by increased photoionization at dawn by exposure of the ionosphere to sunlight. Azimuthal profiles of individual EUV images provide additional evidence for the same trend observed in the tracking technique and show that He+ column abundance values are typically 50–100% higher at dusk than dawn.