The width of the frequency spectrum of Pc1 geomagnetic pulsations in quiet and disturbed conditions

The relation of the frequency spectrum width of Pc1 geomagnetic pulsations with magnetospheric parameters was theoretically studied. An analytical expression was obtained and numerical calculations were conducted. The formula for the increment (γ) was shown to include an important magnetospheric parameter, the ratio (VA/U‖) of the Alfven velocity to the average velocity of energetic protons along the field line, which significantly affects the frequency spectrum width. The calculations show that the normalized width of the Pc1 frequency spectrum (x0 = ωout/Ωi = VA/U‖) decreases with decreasing (VA/U‖). With increasing anisotropy of energetic protons A = T⊥/T‖ − 1, the normalized width of Pc1 frequency spectrum is also decreased for a fixed value of the parameter (VA/U‖). These conclusions have been confirmed by analysis of spectrograms conducted in 2005–2010 of ground-based observations of Pc1 on the Scandinavian network of induction magnetometers. Analysis of these data showed that Pc1 pulsations were observed in a narrow frequency band of around 0.2–0.4 Hz, with a central frequency of oscillations in the series f ∼ 0.5–0.7 Hz, in more than 90% of cases in magnetically quiet conditions (Kp < 2). This corresponds to their generation beyond the plasmasphere. In more disturbed conditions (Kp ∼ 2–3), the central frequency of Pc1 oscillations was almost twice greater (∼ 1.0–1.2 Hz) and the spectral width was ∼ 0.5–0.7 Hz, which makes it possible to suggest that they can be generated within the plasmasphere.