Dynamics of Ionospheric Alfvén Resonances from the End of Cycle 21 through Cycle 24 of Solar Activity

Abstract

The results of the study of the dynamics of ionospheric Alfven resonances (IARs) in a range of 0–10 Hz based on data from magnetic field observations at the midlatitude Borok observatory (L = 2.8) from the end of solar cycle 21 through solar cycle 24 are presented. It is shown that IARs in 30% of events are accompanied by the simultaneous observation of structured Pc1 geomagnetic pulsations (“pearls”) and IARs are recorded in 70% of events without Pc1 excitation. A specific feature of pearls is that they are observed predominantly at a frequency of the IAR first resonance band. The qualitative coincidence of the dynamics of frequencies of IAR and Pc1 wave packets is detected in 80% of events. The probability maximum of IAR observation falls in the hours before midnight (2000–2200 MLT). IAR seasonal variation is characterized by the presence of two equinoctial maxima. It is shown that the 11-year variation in the IAR emission is controlled by the dynamics of some parameters of the solar wind and IMF. The probability of IAR observation is maximal (in years of solar activity minima) when the ratio of the proton density to the helium ion (α particle) density Np/Na (we note that it is customary to use the inverse, i.e., Na/Np, for the solar wind) and parameter β (which characterizes the ratio of thermal pressure to magnetic pressure) reach the maximum values, while the dynamic pressure of solar wind Pdyn (which controls the magnetosphere compression) is decreased. The coincidence of the dynamics of the frequencies of the IAR first resonance band and pearls, as well as of their seasonal and cyclic variations, may be evidence of the interrelation of these oscillatory processes and the possible common mechanism of their generation.