Observation of the Plasmaspheric Depletion and Refilling: Comparison Between ULF Waves and IMAGE-EUV Measurements

In order to study depletion and refilling of the plasmasphere, we studied magnetic storm events using two different measurements. The first way is cross-phase analysis of Ultra-Low Frequency (ULF) waves. Measurements of the eigenfrequency of geomagnetic field lines can provide information on the plasma mass density near the equatorial plane of the magnetosphere. Data of ULF waves from an extended meridional array of ground magnetometers therefore allows the radial density distribution, and its temporal variation, to be remotely monitored. Using cross-phase analysis of ground magnetometer array data, we studied temporal variation of the plasma mass density in a range of L shells and determined the depletion and subsequent refilling during the magnetic storms. The second way is analysis of the Extreme Ultraviolet (EUV) images from a polar orbit. He+ ions scatter light at its resonance wavelength of 30.4 nm. The EUV camera on the Imager for Magnetopause-to-Aurora Global Exploration satellite recorded its brightness and mapped the distribution of He+ ions in the plasmasphere. Using the global images of the plasmasphere, we observed inward motion of the plasmapause driven by the southward turning of the interplanetary magnetic field. We further studied temporal variation of He+ density in flux tubes which are fixed on several ground-based points. In this paper, we will compare these two measurements and discuss the results in the context of (i) the location of ions loss into the open field, (ii) supply and hold of ions into depleted flux tubes, (iii) different of movements between ion species.