Determination of the wave mode contribution into the ULF pulsations from combined radar and magnetometer data: Method of apparent impedance

To identify the physical nature of Pc5 pulsations and to determine relative contributions of different MHD modes into their structure, we introduce the method of apparent impedance. An approximate analytical relationship from the theory of ULF wave transmission through a thin ionosphere can be compared with the measured ratio between simultaneous ionospheric electric and ground magnetic fields. The analytical expressions show that the impedances of Alfven and compressional modes should be essentially different. To validate this technique, we have analyzed global Pc5 pulsations during the recovery phase of the strong magnetic storm on October 31, 2003 using ground magnetometricdata and tri-static EISCAT radar. This radar facility gives a possibility to calculate vectors of the F-region ionospheric plasma flow and electric field. From these observations we conclude that the global Pc5 pulsations during storm conditions above the ionosphere are predominantly composed from Alfven waves with a small contribution of fast compressional mode.

► New method identifies nature of ULF ionospheric disturbances detected by radar and magnetometer. ► Apparent impedances of Alfven, fast compressional and acoustic modes are to be different. ► We reveal relative contribution of Alfven and compressional modes into Pc5 pulsation structure.