Solar cycle variation of geosynchronous plasma mass density derived from the frequency of standing Alfvén waves

We have studied the solar cycle variation of equatorial plasma mass density ρ eq in the plasma trough at geosynchronous altitude. The density was indirectly determined from the frequency, f T3 , of the third harmonic of toroidal standing Alfvén waves detected over a 12 year period from 1980 to 1991 with magnetometers on five Geostationary Operational Environmental Satellites (GOES). Realistic models of the ambient magnetic field and field line mass distribution were used in numerically solving the wave equation to relate f T3 to ρ eq . Scanning the magnetometer data in a 30 min time window that moved forward in 10 min steps, we obtained 228,382 f T3 samples equivalent to 1586 days of data. The detection rate of f T3 is highest (∼50%) in the prenoon sector, and f T3 and ρ eq samples from this sector were used to examine their dependence on F 10.7 , Kp, and Dst. Overall, F 10.7 exhibits the highest correlation with f T3 and ρ eq , implying that the solar UV/EUV control of ion production at the ionospheric height is strongly reflected in mass density variations at geosynchronous orbit. Using 27 day medians computed excluding periods of plasmasphere expansion to geosynchronous orbit and geomagnetic storm, we obtained the empirical formula f T3 (mHz) = 38 − 0.097 F 10.7 and log ρ eq (amu cm −3 ) = 0.42 + 0.0039 F 10.7 , where F 10.7 is given in the solar flux units 10 −22 W · m −2 · Hz −1 . This last formula means that with the 27 day F 10.7 in the range of 68–255 in the selected solar cycle, the mass density varied by a factor of ∼5 from ∼5 to ∼26 amu cm −3 . During extremely quiet times (Kp averaged using a 3 day time scale <1), for which the plasmasphere may extend out to geosynchronous orbit, and during storm periods (Dst < −50 nT), the mass density may be enhanced beyond these values.