Optical and radio emissions from different high-energy electron acceleration mechanisms

Electric fields in thunderclouds can accelerate electrons to relativistic energies, which leads to bremsstrahlung production of gamma radiation. This radiation was recently recorded by the ALOFT experimental aircraft campaign [1], and may be classified into various types according to their lightcurve shapes, for example, flickering gamma flashes (FGF), single and multiple terrestrial gamma flashes (TGF), and extended gamma-ray glows (GRG). Electromagnetic field in radio and optical range was also recorded, and has different features for the enumerated gamma radiation types.

The relativistic runaway electrons may be produced in various ways. We consider two different mechanisms: (1) electrons are accelerated from low energies in high fields at the tips of long streamers, and (2) runaway electrons grow in large-scale (km-size) avalanches sustained by relativistic feedback mechanism [2].

The first mechanism (long streamers) is analyzed using the novel Streamer Parameter Model (SPM) [3]. This model had been shown to agree with experiments for laboratory-size streamers, and here it is applied to streamers exceeding several meters in length. Such long streamers may describe the fast positive and negative breakdown (FPB/FNB), experimentally observed in thunderstorms. The long streamers, compared to regular laboratory-observed streamer, are predicted to have higher (subluminal) velocities, higher electric fields at the tip, and wider tips. These factors all facilitate production of large quantities of relativistic runaway electrons and, therefore, efficient radiation of x-rays in the form of short pulses, which may be observed as TGF. The currents radiate a short electromagnetic pulse similar to the observed narrow bipolar events (NBE).

The second mechanism (large-scale feedback) is analyzed using the recently developed 0.5D FGF model [4] which is a dynamic model of electric field and cloud conductivity connected through production of relativistic runaway electrons, secondary electrons and ions. This model describes a system in which oscillations may be excited by changing external field [2]. For various set of parameters (such as the system size, and time scale and strength of the external field change), as analyzed by [4], one may obtain gamma radiation lightcurves similar to all the observed types listed above. Charge redistributions and electric currents, for certain sets of parameters, may produce detectable electromagnetic fields.

For both mechanisms, we also calculate optical radiation excited by secondary electrons and estimate its detectability.

[1] N. Østgaard et al, Flickering gamma-ray flashes, the missing link between gamma glows and TGFs. Nature, 634, p. 53-56, 2024. doi:10.1038/s41586-024-07893-0.

[2] N. Liu and J. R. Dwyer. Modeling terrestrial gamma ray flashes produced by relativistic feedback discharges. J. Geophys. Res.–Space, 118 (5), p. 2359-2376, 2013. doi:10.1002/jgra.50232.

[3] N. G. Lehtinen (2021). Physics and Mathematics of Electric Streamers, Radiophys Quantum El, 64, p. 11-25, doi:10.1007/s11141-021-10108-5.

[4] Færder et al, this session.