Nightside ULF Waves Observed in the Topside Ionosphere by the DEMETER Satellite

Abstract

AbstractWe study ultralow‐frequency (ULF) waves at frequencies 17–100 mHz observed in the topside ionosphere by the DEMETER satellite in an ~5‐year period from January 2006 to November 2010. Our results show that two types of ULF oscillations occur on the nightside in the L < 2 region. These two kinds of ULF oscillations are separated based on cross‐covariance analysis between electric field in the DC/ULF range and electron density. Type I ULF oscillations, accompanied by electron density perturbations (average |δNe/Ne0| > 5%), are found to lag behind density variations, and the longitudinal distribution of type I ULF oscillations is quite similar to the longitudinal distribution of plasma irregularities at ~600 km. These signatures suggest that type I ULF oscillations are related to plasma irregularities that are common phenomena in the nightside F region ionosphere. The characteristics of type II ULF oscillations (without significant electron density perturbations) agree well with those features of midlatitude ionospheric electric field fluctuations, which are not thought to be related to magnetospheric origins.