Latitudinal and seasonal variations of quasiperiodic and periodic VLF emissions in the outer magnetosphere

Abstract

[1] We have analyzed ELF-VLF receiver and search coil magnetometer data from five Antarctic stations from 1998 and 1999 to study quasiperiodic emissions (QPs) and periodic emissions (PEs), which occur as ULF-range modulations of ELF-VLF signals between 0.5 kHz and $4 kHz. QPs are modulated at frequencies of $20-50 mHz, and PEs are modulated at frequencies of $200-500 mHz. The stations used covered a range of magnetic latitudes from À62°(Halley) to À74°(South Pole Station); three automated geophysical observatories (AGOs) were located at intermediate latitudes.Consistent with earlier studies, most QPs were observed with magnetic pulsations of identical period in the Pc3 range (type I QPs). Of those QPs not observed with simultaneous magnetic pulsations (type II QPs), nearly all were accompanied by PEs. Type I QPs, PEs, and events during which both appeared together (QPPEs) were found to have different latitudinal, seasonal, and diurnal occurrence patterns: QPs of both types were more likely to occur between À65°and À70°magnetic latitude, while PEs occurred more often around À60°magnetic latitude. QPs were more common during the months of October though March, while PEs were more common during the months of May through September. QPs, whether with or without simultaneous PEs or magnetic pulsations, were predominantly a dayside phenomenon, with a broad maximum near local noon. The occurrence of QPs unaccompanied by PEs was restricted to the dayside, however, while a small number of QPPEs appeared even during nighttime hours. PEs, on the other hand, could be seen at all local times, but with latitudinally dependent diurnal patterns. Most higher-latitude QPs were type I events (observed with magnetic pulsations), while type II QP events (without simultaneous magnetic pulsations) occurred relatively more often at lower latitudes. A case study from 1 August 1999 using wideband data from South Pole and Halley provides evidence of a transition from echoing whistler activity to PE activity and then to QP activity and suggests a causal relationship.INDEX TERMS: 2772 Magnetospheric Physics: Plasma waves and instabilities; 2752 Magnetospheric Physics: MHD waves and instabilities; 2483 Ionosphere: Wave/particle interactions; 7867 Space Plasma Physics: Wave/particle interactions;

New results of structured VLF emissions observed simultaneously at two closely located stations near <i>L</i> ~ 5.5

“…Later it was found both in space (on board the DEME-TER spacecraft) and on the ground at KAN (Bespalov et al, 2010). Earlier, the occurrence and seasonal variations of the similar short-period VLF emissions were studied by Engebretson et al (2004) based on the observations at Antarctic stations. They showed that their repetition periods were similar to the two-hop travel time of echoing whistlers and that the signal occurrence was strong in the local winter.…”

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

“…Omura et al [2009] have simulated the nonlinear generation of rising-tone chorus waves in the equatorial plane of the magnetosphere. QP emissions have been further categorized into emissions associated with geomagnetic pulsations (type 1) or not thus associated with them (type 2) [e.g., Sato et al, 1974;Sazhin and Hayakawa, 1994;Engebretson et al, 2004]. Pasmanik et al [2004] reproduced the amplitude/frequency modulation of type 2 QP emissions without geomagnetic pulsations by using a self-consistent set of equations from quasilinear plasma theory of energetic electrons and whistler wave spectral energy density.…”

Observation of pulsing hiss at low latitudes

“…VLF emissions having periodic or quasi-periodic structures with periods ranging from less than a second up to several minutes have been reported from mid and high latitude stations (Sazhin and Hayakawa, 1994;Engebretson et al, 2004). Periodic emissions with periods less than 10 s are divided into three main groups: periodic emissions, hisslers and pulsing hiss.…”