Avalanches of runaway relativistic electrons in electric and geomagnetic fields in the atmosphere: Numerical experiment

The development of upward propagating avalanches of runaway electrons in el ectric and geomagnetic fields above stratiform thunderclouds has been simul ated. It is assumed that the quasistationary electric ge to the cloud top. The field above the cloud was generated by a lightning, which transferred t he negative charge avalanche of background electrons has been calculated fo r an altitude of 30 km. and higher, where the electric field was stronger t han the threshold (for runaway electrons) electric field. It has been shown that, in the considered cases, the avalanche either faded or slightly deve loped: the amount of runaway electrons increased by a factor of 3-4, which is insufficient for an initiation of the recently revealed high-altitude (3 0-80 km) optical flashes called red sprites. The strong dependence of the o btained results on the values and mutual direction of the electric and geom agnetic fields has been corroborated. For example, the horizontal magnetic field (50 muT) perpendicular to the electric field excludes the development of an avalanche and even the removal of a primary electron with an energy of 2 MeV from a starting point. If the geomagnetic field (50 muT) is strict ly vertical and 4D parallel to the electric field, the avalanche develops s lowly (as in the case with no magnetic field) and has a considerably smalle r cross section. If the geomagnetic field has a strength (B) of (0, 30, 30) muT, the avalanche does C, not develop, but a small portion of primary ele ctrons reached the ionosphere with an energy of up to 5-6 MeV The calculati ons have indicated that an electron start altitude of about 38 km is optima l for avalanche enhancement under the accepted external conditions.