Spectral enhancements associated with Pi1B events observed at high latitude

摘要

Spectral enhancements observed in conjunction with Pi1B by a ground-based search-coil magnetometer located at Longyearbyen in Svalbard, Norway, are examined as evidence of resonant structures in the ionosphere excited by broadband ultra low frequency geomagnetic micropulsations. We examine the morphology of spectral enhancements in dynamic Fourier spectra using a series of 102 Pi1B events recorded from February through October, 2008, followed by a quantitative analysis of a 49-event subset that allows us to derive an estimate of the range of quality factors that can be expected for Pi1B-associated spectral enhancements. Finally, we use European Incoherent Scatter (EISCAT) radar electron density data recorded during two Pi1B events from the original data set-one during a sunlit ionosphere and one during a dark ionosphere-in conjunction with Mass-Spectrometer Incoherent Scatter (MSIS) and International Model Ionosphere (IRI) model data as input to a model of eigenmodes of the ionospheric Alfvén resonator. We compare the results of this model to the observed spectra and show that the observations can be partially explained as (instantaneous) resonant frequencies of the ionospheric Alfvén resonator.