Strange VLF bursts in northern Scandinavia: case study of the afternoon "mushroom-like" hiss on 8 December 2013

Abstract

Abstract. We investigate a non-typical very low frequency (VLF) 1-4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency-time domain, each one lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, ϕ = 67.74 • N, λ = 26.27 • E; L ∼ 5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2-3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8-1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth-ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron-cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1-10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF-VLF (ultra low frequency-very low frequency) wave interaction and the ionosphere waveguide propagation.

New Type of Short High‐Frequency VLF Patches (“VLF Birds”) Above 4–5 kHz

“…Despite the importance of direct VLF measurements in the space with satellite instruments, the continuous ground-based observations can provide the unique possibility to study the temporal dynamics of the waves. In particular, still now many findings of properties and dynamics of different types of natural VLF emissions such as chorus, hiss, long series of QP emissions, have been found basing on the ground-based observations at Kannuslehto station (KAN) in Northern Finland (e.g., Bezděková et al, 2020;Demekhov et al, 2020;Kleimenova et al, 2015Kleimenova et al, , 2019Manninen et al, , 2014Manninen et al, , 2015Manninen et al, , 2016Manninen et al, , 2017Manninen et al, , 2020Manninen et al, , 2012Martinez-Calderon et al, 2019;Shklyar et al, 2020;Titova et al, 2017Titova et al, , 2015.…”