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

Abstract

We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17-25 February 2012 at subauroral latitudes at Athabasca (L = 4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

Polarization analysis of VLF/ELF waves observed at subauroral latitudes during the VLF-CHAIN campaign

“…During the VLF-CHAIN campaign, several interesting features of VLF/ELF emissions were observed, such as QP emissions, falling-tone and rising-tone chorus, hiss, as well as 'bursty-patch' emissions, first reported by Shiokawa et al (2014). We visually inspected the VLF spectra from ATH from 17 to 25 February, looking for clear chorus, QP, and other emissions, then applied the FFT to obtain spectral and polarization parameters.…”

“…In all of the occurrences, intervals of polarization angle sign changes lasted for several tens of minutes, within which sign changes took place every few minutes (from 2 to 10 min). Shiokawa et al (2014) reported short time-scale bursty emissions during geomagnetically disturbed periods, observed during the VLF-CHAIN campaign, giving them the name 'bursty-patches' . Figure 6 shows a 10-min interval of the polarization and spectral parameters of the very long event observed on 19 February (also shown as an example in Figure 5), accompanied by a bursty-patch.…”

“…We suggest that this could be the result of the changing wave duct structure between the source in the magnetosphere and the ionospheric exit point of the emission, and not a direct consequence of the movement of the magnetospheric source (e.g., Strangeways (1991)). It was believed that bursty-patches, first reported by Shiokawa et al (2014), might be the upper frequency band of chorus emissions. However, as shown in Figure 6b, the main chorus emission and the bursty-patch have almost opposite polarization angle values, the first one being positive (yellow) and the second one negative (dark blue).…”

“…On 17 to 25 February 2012, we installed VLF antennas in Athabasca (ATH, L = 4.3) and Fort Vermillion (FVM, L = 5.4), Canada (i.e., Shiokawa et al 2014). Our first goal is to obtain spectral and polarization parameters for different types of VLF/ELF emissions by applying the fast Fourier transform (FFT) to the data from ATH.…”

Energetic Electron Precipitation Associated With Pulsating Aurora Observed by VLF Radio Propagation During the Recovery Phase of a Substorm on 27 March 2017

“…In this case, the ambiguity of the horizontal position of PsA depends on the height distribution assumed (Brown et al, ; Hosokawa & Ogawa, ; Kaila et al, ; Kataoka et al, ) (see Text S1 and Figure S1). We used a VLF loop antenna and induction magnetometer data observed at ATHA to obtain VLF and extreme‐low‐frequency (ELF) activities (Shiokawa et al, , , ).…”