Microfluctuations of the Lower Ionosphere on the Time Scale of Transient Luminous Events Inferred from High Resolution Array Analysis
Abstract:Transient luminous events above thunderclouds result from the quasi-static heating of the lower ionosphere by intense cloud to ground lightning discharges (e.g., Pasko, 2010). More recently, it was suggested that gravity wave instabilities (Liu et al., 2015) can cause small scale ionospheric irregularities (Qin et al., 2014, Fullekrug et al., 2014) which can assist the initiation of transient luminous events such that the response of the lower ionosphere to the forcing by thunderstorms is of major interest (e.g., Shao et al., 2012).
The variability of the lower ionosphere is well studied on time scales ranging from decades (solar cycle) down to tens of ms (microbursts), but on the time scale of transient luminous events ~<1 ms, plasma fluctuations in the lower ionosphere are less well known. This study uses recordings of the low frequency radio sky with a small aperture array of radio receivers operating in the frequency range from ~20-250 kHz (Fullekrug et al., 2015) to infer the existence of microfluctuations in the lower ionosphere from a high resolution array analysis. It is found that the observed microfluctuations result from very fast processes acting on time scales < 1 ms which result in a correlated multipath propagation of low frequency radio waves and a characteristic dilution of precision of radio noise source locations in the sky.
The observed dilution of precision has several physical causes. The general shape of the radio source results from the array geometry of the radio receivers, the splitting of the radio sources results from a birefringent wave propagation and the correlated multipath propagation of low frequency radio waves results from microfluctuations in the lower ionosphere. The influence of the array geometry on the dilution of precision is reduced by taking into account the impulse response of the array. This procedure reveals the splitting of one single radio source into two distinct source locations which result from a birefringent wave propagation. The remaining dilution of precision from the correlated multipath propagation of low frequency radio waves strongly suggests the existence of very fast processes acting on time scales < 1 ms in the lower ionosphere which might have an impact on the initiation of transient luminuous events above thunderstorms.