Abstract

We report the results from a statistical study of a new type of naturally occurring, but differently structured very low frequency (VLF) radio waves called “VLF bursty-patches.” Their main characteristic is that they are detected at frequencies above the electron gyrofrequency (f_ce) in the equatorial plane for the L-shell of observation. Here we use the data from the station of Kannuslehto, Finland (KAN, MLAT = 64.4°N, L = 5.5) with f_ce ∼ 5–6 kHz where VLF bursty-patches are commonly detected on at least 43% of its campaign days. We analyzed all available data for 2017 (∼8 months) separating the observations by spectral type, periodicity, number of bursts, observed frequency, and accompanying usual VLF emissions at lower frequencies (f < 6 kHz). We found that the overall highest occurrence rate is in the morning sector (05–12 MLT) correlating with general VLF occurrence rate at KAN (07–13 MLT, MLT = UT + 1.5 hr). Grouping usual VLF emissions and bursty-patches showing similar spectral features, suggests that both groups are being generated by the same mechanisms (e.g., temperature anisotropy magnetospheric plasma distribution). However, unlike the situation for usual VLF emissions, the daily occurrence of bursty-patches did not show a clear relationship with AE index. Superposed epoch analysis of magnetic components from the Sodankylä magnetometer (∼45 km from KAN) shows that VLF bursty-patches are detected when the horizontal component of the magnetic field shows minimum variations. This suggests that propagation to the ground of bursty-patches is conditioned to localized low variations of magnetic activity.