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

• Published in: Journal of geophysical research. Space physics 2013-06, Vol.118 (6), p.3077-3086
• Main Authors: Chi, P. J., Engebretson, M. J., Moldwin, M. B., Russell, C. T., Mann, I. R., Hairston, M. R., Reno, M., Goldstein, J., Winkler, L. I., Cruz-Abeyro, J. L., Lee, D.-H., Yumoto, K., Dalrymple, R., Chen, B., Gibson, J. P.
• Format: Article
• Language: English
• Subjects: Annual variations; field line resonance; Geophysics; ground-based magnetometers; Ionization; Magnetic fields; Plasma physics; plasmaspheric density; Statistical analysis; ULF waves
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/jgra.50274

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° 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. Key Points Ground observations reveal statistical properties of plasmaspheric density. Plasmaspheric mass density exhibits an annual variation that peaks in winter. Plasmaspheric mass density stays steady in the morning and rises post‐noon.