Abstract
All lightning strokes generate electromagnetic pulses –atmospherics– which can travel over distances of thousands of kilometers. Night-side atmospherics show typical frequency dispersion signatures caused by sub-ionospheric propagation. Their analysis can be used to determine the distance to the source lightning, and therefore it represents a safe tool for investigation of distant thunderstorms, as well as for indirect observations of the lower ionosphere. However, such analysis has never been done on the dayside. Here we present the first results which show unusual daytime atmospherics with dispersion signatures originating from strong thunderstorms which occurred during winter months 2015 in the North Atlantic region. Using newly developed analysis techniques for 3-component electromagnetic measurements we are able to determine the source azimuth and to attribute these rare atmospherics to both positive and negative lightning strokes in northern Europe. We consistently find unusually large heights of the reflective ionospheric layer which are probably linked to low fluxes of solar X rays and which make the dayside subionospheric propagation possible. Although the atmospherics are linearly polarized, their dispersed parts exhibit left handed polarization, consistent with the anticipated continuous escape of the right-hand polarized power to the outer space in the form of whistlers.
Highlights
According to the waveguide theory[9] an impulsive signal excites different transmission modes
This brought us to the idea to search for possible unusual electromagnetic traces of European winter thunderstorms using newly developed analysis techniques of our 3-component VLF measurements conducted in Southern France, sufficiently far from the North Atlantic region for possible tweeks to be observed
The obtained propagation distances of these daytime tweeks are 2–3 times shorter than tweek propagation paths which are typically observed during the night[12] but their reflection heights are on average by ~5–10 km higher than heights typically obtained for standard daytime ionospheric conditions
Summary
According to the waveguide theory[9] an impulsive signal (in this case generated by a lightning discharge) excites different transmission modes. Many atmospherics exhibit a strong dispersion close to the critical frequencies of waveguide modes thanks to low attenuation and long propagation paths of electromagnetic VLF waves. These atmospherics are known as “tweeks” because of their typical sound when heard in a loudspeaker[1,10]. In Japanese winter storms[15], indicate that there might be specific and unusual state of the daytime ionosphere or specific types of thunderstorms, which might lead to the formation of daytime tweek signatures This brought us to the idea to search for possible unusual electromagnetic traces of European winter thunderstorms using newly developed analysis techniques of our 3-component VLF measurements conducted in Southern France, sufficiently far from the North Atlantic region for possible tweeks to be observed
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