Global estimates of plasmaspheric losses during moderate disturbance intervals

Abstract. For a set of five moderate disturbance events, we calculate the total number of He⁺ ions removed the plasmasphere using calibrated global EUV images. In each of the events, between ~0.6 and 2.2×10³⁰ He⁺ ions are removed from a region of the inner magnetosphere from L=1.5 to 5.5. This loss constitutes between 20% and 42% of the initial He⁺ distribution. The lost percentage is correlated with the number of hours of strongly positive solar wind electric field (E_y>2.5 mV/m). Also, the total amount of material removed from the plasmasphere is estimated by using several values of the He⁺ to H⁺ number density ratio. The total mass lost is found to be in the range of 20 to 80 metric tons although for each individual case the estimate can vary by over 50% depending on assumed density ratio. We also attempt to distinguish between losses to the ionosphere and losses to the dayside boundary layers by estimating losses interior and exterior to the newly formed plasmapause boundary. For the events studied, losses inside the new plasmapause constitute between 24% to 54% of the total number of He⁺ ions lost.