Dynamics of the Spectra of Multiband Pc1 Pulsations in the Presence of Multiple Regions of Ion–Cyclotron Instability in the Magnetosphere

Abstract

We study the dynamics of the spectra of multiband sporadic magnetic pulsations in the Pc1 range (0.2–5.0 Hz) during the event of 5–6 March 2011 by using ground-based magnetic measurements at stations largely spaced from each other in latitude and longitude. The event is characterized by a long duration (about 16 h), the presence of several bands with varying frequencies, splitting of these bands into narrower subbands, significant variations in the amplitude and polarization of the signals on the ground, and their observation in a wide range of latitudes and longitudes. On the basis of a joint analysis of the Pc1 pulsation properties and the data of low-orbiting spacecraft detecting localized precipitations of energetic protons into the ionosphere, we infer the possible generation regions of these waves in the magnetosphere and conclude that they are multiple. The results of analysis allowed us to determine the mechanisms of broadening and splitting of Pc1 frequency bands even in the absence of direct wave observations in the magnetosphere. We also propose an explanation of the atypical (for ground-based detection) character of two-band Pc1 spectra when the signal at frequencies above the helium ion gyrofrequency has a higher amplitude than at lower frequencies. We also explain the inhomogeneous frequency profile of polarization in different frequency bands. Possible variations in the magnetospheric plasma parameters that resulted in the observed dynamics of amplitude and polarization spectra of Pc1 pulsations are revealed by using calculations of the wave cyclotron amplification by energetic protons in the magnetosphere.