Energy estimation of electrons producing sprite halos using array photometer data

Abstract

Sprite halos are brief diffuse glows with an average duration of ∼1 ms occurring at altitudes from 70 to 85 km preceding the development of sprite streamers. Their horizontal scale is ∼50– 100 km and they move downward as if focusing into the center. To investigate the energies of electrons producing sprite halos and their spatial and temporal variations, we analyzed photometric and CCD imaging data recorded at Yucca Ridge Field Station, Colorado, USA, during the Sprites’99 campaign. We estimated the electron energies from the intensity ratios of blue (350– 500 nm ) to red (560– 800 nm ) emissions captured separately by two array photometers on the assumption that the electron energy distribution is given by the Maxwell–Boltzmann or Druyvesteyn distribution. It is found that the electron energies reach their peaks preceding the luminosity peaks of the red emission (mostly due to the N 2 first positive and the N 2 + Meinel band systems), exhibiting energy increases in the lower part of the sprite halo. The estimated peak electron energies are in the range 6–23 and 7– 16 eV for the assumed Maxwell–Boltzmann and Druyvesteyn distribution, respectively. These results suggest that more energetic processes occur at the initiation of the sprite halo, particularly in its lower part. Nevertheless, the estimated values of electron energies should be interpreted as a measure of the energetic processes producing sprite halos, represented by the equilibrium temperatures, since the real electron energy distributions would be undoubtedly deviated from the equilibrium states due to additional high-energy tails produced by the thermal runaway electron mechanism.