Plasma refilling rates for L = 2.3-3.8 flux tubes

Measurements of the eigenfrequency of geomagnetic field lines can provide information on the plasma mass density near the equatorial plane of the magnetosphere. Data from an extended meridional array of ground magnetometers therefore allow the radial density distribution, and its temporal variation, to be remotely monitored. Using cross-phase analysis of magnetometer array data, we determined the equatorial mass density during three moderate geomagnetic storms in March 2004 and June and April 2001. In each case the field line eigenfrequency increased markedly, corresponding to reductions in mass density and indicating that the plasmapause moved earthward and these flux tubes were depleted. We then measured the rate at which these flux tubes were refilled to prestorm levels. This took 2–3 days for L = 2.3 flux tubes, 3 days at L = 2.6, and over 4 days for L > 3.3. Plasmaspheric refilling progressed with a clear diurnal variation associated with linearly increasing plasma density in the daytime and decreasing plasma density at nighttime. The daytime increases in plasma mass density related to refilling rates ranging from ~250 to ~13 amu cm⁻³ h⁻¹ over L = 2.3–3.8. The resultant upward plasma flux at the 1000 km level was in the range 0.9–5.2 × 10⁸ amu cm⁻² s⁻¹. We also determined the daily averaged refilling rate to be ~420 amu cm⁻³ d⁻¹ at L = 2.9–3.1, including the nighttime downward flux. By comparison with Imager for Magnetopause-to-Aurora Global Exploration–EUV and VLF whistler data we were able to estimate the plasma composition and found the O⁺ proportion was of order 3%–7% at L = 2.3 and 6%–13% at L = 3.0