Constraints to do realistic modeling of the electric field ahead of the tip of a lightning leader

TL;DR

This paper establishes new realistic constraints on electric fields ahead of lightning leaders, improving models of electron acceleration and assessing their potential to produce terrestrial gamma-ray flashes, though some observed energies remain unexplained.

Contribution

It introduces a set of constraints based on in situ measurements to improve the realism of electric field modeling ahead of lightning leaders.

Findings

Maximum electric field strength of 50 kV/cm at sea level.

Potential difference in acceleration region is approximately 52 MV.

Leaders with about 600 MV potential difference are candidates for TGF production.

Abstract

Several computer models exist to explain the observation of terrestrial gamma-ray flashes (TGFs). Some of these models estimate the electric field ahead of lightning leaders and its effects on electron acceleration and multiplication. In this paper, we derive a new set of constraints to do more realistic modeling. We determine initial conditions based on in situ measurements of electric field and vertical separation between the main charge layers of thunderclouds. A maximum electric field strength of 50 kV/cm at sea level is introduced as the upper constraint for the leader electric field. The threshold for electron avalanches to develop of 2.86 kV/cm at sea level is introduced as the lower value. With these constraints, we determine a region where acceleration and multiplication of electrons occur. The maximum potential difference in this region is found to be $\sim$52 MV, and the corresponding number of avalanche multiplication lengths is $\sim$3.5. We then quantify the effect of the ambient electric field compared to the leader field at the upper altitude of the negative tip. Finally, we argue that only leaders with the highest potential difference between its tips ($\sim$600 MV) can be candidates for the production of TGFs. However, with the assumptions we have used, these cannot explain the observed maximum energies of at least 40 MeV. Open questions with regard to the temporal development of the streamer zone and its effect on the shape of the electric field remain.