Plasmaspheric Mass Density Variation During a Moderate Magnetic Storm

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 allows the radial density distribution, and its temporal variation, to be remotely monitored. We have ground magnetometer arrays spanning low and middle latitudes to examine changes in magnetospheric mass density during a moderate geomagnetic storm. After several very quiet days, the storm was initiated on 9 March 2004 by a recurrent high-speed solar wind stream with maximum velocity of 800 km/s. On 10 March, at the beginning of the storm recovery phase (Kp = 6+), the field line eigenfrequency (obtained by cross-phase analysis) for the UK sector at L = 2.63 reached 44 mHz in the morning sector, dropping to 32 mHz in the afternoon sector. This corresponds to very low mass densities, around 500 amu/cc and 650 amu/cc respectively. Over 10-13 March the eigenfrequency at this L value progressively decreased to 17 mHz, indicating refilling of the flux tube to pre-storm levels, superimposed upon diurnal variations. We compare similar density measurements during this storm from a range of L shells, and discuss these results in the context of (i) the location of plasma boundaries such as the plasmapause, (ii) the refilling rate as a function of L and time, (iii) the movement of heavy ions, and (iv) consequences for model representations of the field-aligned density distribution.