Simultaneous observations of the plasma density on the same field line by the CPMN ground magnetometers and the Cluster satellites

Abstract

By applying the cross-phase method and the amplitude-ratio method to magnetic field data obtained from two ground stations located close to each other, we can determine the frequency of the field line resonance (FLR), or the field line eigenfrequency, for the field line running through the midpoint of the two stations. From thus identified FLR frequency we can estimate the equatorial plasma mass density ( ρ ) by using the T05s magnetospheric field model [Tsyganenko, N.A., Sitnov, M.I. Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms, J. Geophys. Res. 110, A03208, 2005] and the equation of Singer et al. [Singer, H.J., Southwood, D.J., Walker, R.J., Kivelson, M.G. Alfven wave resonances in a realistic magnetospheric magnetic field geometry, J. Geophys. Res. 86 (A6) 4589–4596, 1981]. In this study we compare ρ estimated from magnetometer data at two stations in the CPMN (Circum-pan Pacific Magnetometer Network) chain, Tixie (TIK, geographic coordinates: 71.59°N, 128.78°E, L = 6.05 ) and Chokurdakh (CHD, geographic coordinates: 70.62°N, 147.89°E, L = 5.61 ), with the plasma electron number density ( Ne) observed by the WHISPER (Waves of HIgh frequency Sounder for Probing the Electron density by Relaxation) instrument onboard the Cluster satellites. For the interval of January 1, 2001–December 31, 2005, we have identified 19 events in which the Cluster spacecraft were located on the field line running through the midpoint of TIK and CHD when they observed FLR, and statistically compared the simultaneously observed ρ and Ne, although the number of events are limited (19). In 15 out of the 19 events the ratio of ρ to Ne falls into a realistic range. It is also suggested that the contribution of heavy ions tends to increase when the magnetosphere is disturbed.