Space Climate 7 Meeting Abstract
Evidence of connections between Schumann resonances and auroral crackling sounds
Unto K. Laine (Aalto University)
Sounds observed under active aurorae have been in the focus of many tales, argumentations, and hypotheses during centuries. In Finland, auroral sounds have been measured and studied since 2000 in a project called Auroral Acoustics. A hypothesis that the sounds are created in a temperature inversion layer at about 70–90 m altitude was presented in 2016 (BNAM2016). According to the hypothesis, an inversion layer is formed during calm and cold evenings when the warm air from the ground rises up carrying negative ions. Positive ions are precipitated from the higher layers of the atmosphere. The inversion layer is a kind of lid hindering the vertical movements of the different charges and therefore they are accumulated under and above the layer as in a parallel plate capacitor. When such a layer is present, a geomagnetic storm can induce time-varying potentials that can cause sudden discharge of the voltage difference, resulting in sound, magnetic pulse, UV, and ozone. In a congress paper presented at ICSV26 conference (Montreal, July 2019), new evidence of these sounds will be published. It is shown that the temporal envelopes of the auroral crackling sounds have a clear connection to the Schumann resonances and their intermodulation distortion components. These components reveal a highly nonlinear nature of the discharging process and its sensitivity to the magnetic field fluctuations. The present poster presents new additional evidence of the connection between the Schumann resonances and the auroral crackling sounds by extending the earlier analysis to VLF signals recorded simultaneously with the sounds. A clear activation of the Schumann resonances (due to an auroral substorm) is present in the VLF signal just before the crackling is recorded on the ground. In addition, all-sky camera data shows a clear activation in the auroral light simultaneously with the measured event.
Mode of presentation: poster