In situ and ground-based intercalibration measurements of plasma density at L = 2.5
 Two independent ground-based experiments and two satellite-borne experiments are used to interpret the changes in plasmaspheric composition at the same point in space during moderate geomagnetic activity on 22 January and 14 February 2001. Mass density at L = 2.5 was determined from an array of magnetometers on the Antarctic Peninsula, while the electron number density along the same flux tube was determined from analysis of the group delay of man-made VLF transmissions from north-east America. The IMAGE satellite RPI experiment provided in situ measurements of the electron number density in passing the equatorial region of the same field line, while the EUV Imager experiment was able to resolve the He+ abundance by looking back toward the same place a few hours later. On 22 January 2001 all measurements were consistent with a moderately disturbed plasmasphere. On 14 February 2001 there appeared to be a significant response of the plasmasphere to the moderate (Kp = 5) activity levels. Both the electron number density and the mass density determined from the ground-based experiments were markedly higher than on 22 January 2001. Also, the IMAGE RPI gave a markedly lower electron number density than did the ground-based data; this is explained by differences in the longitude at which the measurements were made and the presence of localized plasmaspheric structures. At Antarctic Peninsula longitudes a He+ column abundance value of 6 × 1010 cm-2 is found to be equivalent to plasmaspheric electron density levels of 3000 cm-3 at L = 2.5. For these conditions the He+ mass abundance was about 12–16% compared with H+. Both decreases and increases in the He+ column abundance measured by the EUV Imager appear to be linearly correlated to changes in the percentage occurrence of He+ as determined from a combination of ground-based VLF and ULF observations.