Ionization emissions associated with N2+ 1N band in halos without visible sprite streamers

Abstract

We report the ionization emission associated with N2+ 1N band in a halo event without visible sprite streamers. To avoid the lightning contamination to the ionization emission, we find halos whose parent lightning light is blocked by the Earth's limb. Five halos in the 2004-2010 Imager of Sprites and Upper Atmospheric Lightning data set were identified as halos without visible sprite streamers. A halo with maximum N2 1P brightness had significant ionization emission of N2+ 1N. The time-integrated photon intensity of N2 1P, N2 2P, and N2+ 1N emission is 2.2 × 105, 2.1 × 104, and 7.4 × 102 photons cm-2, respectively at a distance of 4130 km. The total number of photons of N 2 1P, N2 2P, and N2+ 1N band emissions are 4.6 × 1023, 4.3 × 1022, and 1.6 × 1021 photons, respectively. In the halo region, the electron density increased as 1-2 orders of magnitude higher than ambient electron density. From the emission ratio of N2+ 1N to N 2 2P, the reduced electric field is estimated to be 275-325 Td that is higher than the conventional breakdown electric field. The recorded electric field related to this halo event is produced by a lightning discharge that has a total charge moment change of -1450 C km. Based on the estimated electric field from optical emissions, it is found that the lightning-induced electric field in the bright halo region is significantly relaxed with a rate faster than that estimated using ambient electron density, in agreement with previous modeling results showing that the electron density enhancement due to the ionization processes leads to a short dielectric relaxation time inside the halo region. Key Points The ionization emission N2+ 1N in a halo event was observed. The electron density in the halo increased as 1-2 orders higher than background. The reduced electric field in the halo was estimated to be 275 - 325 Td. ©2013. American Geophysical Union. All Rights Reserved.