Topside ionosphere He+ density depletions: Seasonal/longitudinal occurrence probability

The idea of an equatorial origin of the He+ density depletions (HDDs) was validated once again. For this purpose, detailed comparative analysis of their longitudinal occurrence distributions (for all seasons and different hemispheres) was made. The distributions during the equinox periods were found to be very similar to the distributions of the equatorial F-region irregularities (EFIs), equatorial spread F (EFS) and equatorial plasma bubbles (EPBs). However, the similarity was not so good for the solstice periods due to complex influence of the seasonal factor, “magnetic declination effect” and thermospheric meridional winds. The results obtained may be considered as new evidence supporting the idea of the plasma bubble origin of the HDDs.

► Seasonal/longitudinal statistics of the topside ionosphere He+ density depletions. ► Comparison with the statistics of the equatorial F region irregularities. ► Statistics are the most similar for the equinox periods. ► New validation of an equatorial ionospheric origin of the He+ density depletions is obtained.