Statistical study on recently discovered high frequency emissions at Kannuslehto, Finland.

Abstract:

Applying a sferics filtering above f = 4 kHz, previously unknown differently structured high-frequency waves has been observed on the ground station of Kannuslehto in northern Finland (KAN, MLAT=64.4°N, L=5.46). These high frequency emissions are referred to as KHF-VLF, standing for Kannuslehto High-Frequency Very Low Frequency emissions. First reported by Manninen et al., 2016, they are observed above the local electron gyrofrequency for the L-shell of KAN (f_ce ~ 5 - 6 kHz). While these waves show different spectral features, they seemed to be mostly composed of hiss bursts with a duration of a few seconds. Bbetween 2006 and 2015, KHF-VLFs have been detected in at least 60% of the observation campaign days at KAN, and appear to be related to high solar activity.

Here we present the results of a statistical analysis of the 2017-2020 campaigns to determine the specific properties of KHF-VLF waves. We will show the different types of waves observed, their occurrence rate depending on MLT, wave type, periodicity, and number of bursts detected. Using observed frequencies combined with ray tracing, we will discuss the likely location of the source of these emissions, and the mechanisms allowing for propagation to the L-shells of KAN. In addition, we also investigate the relationship to other more commonly known natural VLF waves, such as chorus or hiss, and the influence of solar activity and magnetic field. Preliminary results show that while occurrence of KHF-VLF waves are positively correlated with AE index, their detection on the ground is coupled with a decrease of the magnetic field.

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