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

Abstract

We 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.

Features of Nightside ULF Wave Activity in the Ionosphere

“…As reported by Ouyang et al (), ULF oscillations of electric field observed by the DEMETER satellite mostly occur on the nightside. According to latitudes where these nightside ULF oscillations occur, we may categorize them into three main types.…”

“…Types I and II wave events have a clear seasonal dependence with increased occurrence rates in the local summer in the Northern Hemisphere (in the following referred to as NH) and in both local winter and summer in the Southern Hemisphere (SH). This characteristic also suggests that these waves are not of magnetospheric origin, and a detailed discussion about the nonmagnetospheric origin of Types I and II is given in Ouyang et al (). Figure c shows that the occurrence of Type III events maximizes close to equinoxes (March and April, and September and October), this being particularly visible at high latitudes in the SH and low latitudes in the NH.…”

“…According to latitudes where these nightside ULF oscillations occur, we may categorize them into three main types. The first two types mainly occur at middle and low latitudes (L < 2) and are defined in Ouyang et al (2018). Types I and II are respectively defined as electric field oscillations with and without simultaneously significant electron density perturbations, both types being classified as nonmagnetospheric ULF waves according to Ouyang et al (2018).…”

“…Similar to ultralow frequency (ULF) waves in the magnetosphere, ULF waves observed in the ionosphere can also be generated by external or internal sources. The internal sources are related to processes in the disturbed ionosphere, such as plasma irregularities (Lühr et al, 2002;Ouyang et al, 2018;Stolle et al, 2006). External sources arise from active processes in regions above and below the ionosphere.…”

ULF Wave Activity Observed in the Nighttime Ionosphere Above and Some Hours Before Strong Earthquakes

“…Therefore, the lower and upper cutoff frequencies are set to 0.0509 and 1 Hz, and the minimum width of the spectral peak is 0.118 Hz (2 f s ) in our detection algorithm. Since ULF oscillations of electric field recorded by the DEMETER satellite are mostly observed on the nightside (Ouyang et al, 2018), the automatic detection algorithm is only applied to the nightside observations. A total of 64,403 ULF wave events in the nightside ionosphere are obtained during our ~5.5 year period.…”