The Breathing Plasmasphere

Abstract:

Global observations of He⁺ ions from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Mission Extreme Ultraviolet Imager (EUV) are used to examine plasmasphere content using integrated mass through times of erosion and refilling. The objective for this initial study is to take a large-scale view of erosion and refilling that avoids the need to identify and follow individual magnetic flux tubes. Storm-time loss of plasmasphere content is a kind of exhalation where plasma is drawn outward through the plume toward the dayside magnetopause and inward into the ionosphere. Refilling or inhalation into the body of the plasmasphere initially comes from recapturing plasma incompletely lost to outward erosion or from the warm plasma cloak and continues over time from ionospheric outflow or energetic ion charge exchange with the geocorona. Plasmaspheric content is expressed as integrated mass in units of metric tons (), using the Craven el al. 1997 He+ to H+ ion density ratio and 1% oxygen. Three event periods of moderate storm-time erosion and recovery and four periods of recovery during quiet conditions are examined. The total loss of plasmasphere content was found to range from 30% to 62% during storm-time erosion. Plasma lost inside the eroded plasmapause boundary ranged from 22% to 42%, consistent with previous observations, but with a striking lack of discontinuity across that boundary. Refilling rates range from 0.5 Mt/d to 2.5 Mt/d and are found to be consistent with previous results out to L=4.5. Lower refilling rates were observed beyond that L-shell for these few events. The content of the plasmasphere between L=1.5 and 3.0 was also followed for 54-days during which it was observed to exhibit eight “breaths”. Only the first is “deep”. The remaining carry forward the weight of past and continued activity that prevents recovery. There appears to be a month-long declining trend in the mass of the inner plasmasphere during the last 2/3^rd of this time. The suggestion is of longer geomagnetic activity-driven persistent trends in plasmasphere content than previously demonstrated. Subsequent study will integrate over smaller volumes for the purpose of more closely examining the movement of plasmaspheric content across L-shells and magnetic local time and between the plasmasphere and ionosphere.