Sounding of the plasmasphere by Mid-continent MAgnetoseismic Chain (McMAC) magnetometers

We present a statistical analysis on the plasmaspheric mass density derived from the field line resonance (FLR) observations by the Mid-continent MAgnetoseismic Chain (McMAC). McMAC consists of nine stations in the United States and Mexico along the 330 degrees magnetic longitude, spanning L-values between 1.5 and 3.4. Using the gradient method and an automated procedure for FLR detection, we studied a full year of McMAC observations between July 2006 and June 2007. We find that the rate of FLR detection can reach as high as 56% around local noon at L=2.7, and the detection rates at higher and lower L-values decline due to the occasional presence of the plasmapause and weaker FLR signals, respectively. At L-values between 1.8 and 3.1, the inferred equatorial plasma mass density follows the L-dependence of L-4. By comparing the mass density with the electron density, we found that the ion mass gradually decreased from 1.7 amu at L=1.8 to 1 amu at L=3.1. The plasma mass density exhibits an annual variation that maximizes in January, and at L=2.4 the ratio between January and July densities is 1.6. Our observations also show a local time dependence of plasmaspheric mass density that stays steady in the morning and rises postnoon, a phenomenon that may be attributed to the equatorial ionization anomaly as a part of the plasma neutral coupling at low latitude.