Electromagnetic emissions in the MHz and GHz frequency ranges driven by the streamer formation processes.

We provide comprehensive data on the spectral and temporal characteristics of low-frequency (LF) (MHz) and high-frequency (HF) (GHz) radio emissions and investigate their correlation with the streamer formation. We show that the propagation of streamers from the cathode is accompanied only by the LF radio emission (10-150 MHz). In contrast, the HF radio emission (1-4 GHz) arises during the travel of counterstreamers from the anode, which is also indicated by radio interferometric measurements. The power of the LF radio emission sharply increases almost synchronously with that of the HF radio emission. We find that the HF radio emission has a complex spectral and temporal structure and appears as multiple short (less than 1 ns) bursts characterized by various frequency components, existing in subnanosecond time intervals.