Pc1 pearl waves with magnetosonic dispersion

Results of the analysis of 15 unusual Pc1 pearl wave events with inverse dispersion in comparison with the dispersion of well-known electromagnetic ion-cyclotron (EMIC) waves in the form of classic pearl pulsations are presented. Pulsations with the dynamical spectrum consisting of both falling tones only (first type) and events with structures, which start with the falling tones and then develop into rising tones (second type), have been discovered. The first type corresponds to the frequency dispersion of magnetosonic waves (R-waves), and the second type corresponds to the mixed frequency dispersion of R-waves and EMIC waves (L-waves). All events were observed during quiet geomagnetic periods. The duration of the events is about 20–30 min. For the interpretation of these phenomena, the cyclotron instability driven by energetic proton beams with relative mean velocity v0 directed along the background magnetic field and corresponding to an energy ∼10–100 keV is considered. The interaction of such proton beams with waves having frequencies ω<ωi (ωi is the ion gyrofrequency) leads to the instability, which allows the fastest growth of electromagnetic oscillations with the dispersion of R-wave type. When the velocity of the proton beam decreases (v0≈0), R-waves attenuate and L-waves (for the proton temperature T⊥>T∥) will be amplified. This instability is the reason for the generation of classic Pc1 pearl pulsations with the usual dispersion and allows explaining the transition of the dispersion from R- to L-waves.