Temporal and spatial variations in the intensity ratio of N2 1st and 2nd positive bands in sprites

Abstract

Abstract In order to investigate the spatial and temporal variations of sprites and elves and their spectral structures, we have carried out photometric observations during the SPRITES' 97 campaign using two multi-anode array photometers (MAPs). Each MAP has 5 fields of view arrayed in vertical and a time resolution of 52 μs, which enables us to detect the rapid vertical motion of sprites/elves. Since the emissions of sprites and elves mainly consist of the 1st and 2nd positive bands of N 2 , the intensity ratio of these bands gives us information on the energy distribution of electrons which excite N 2 molecules via collision processes. Thus, one of the MAP instrument with an optical sharp cut filter was used to measure only the N 2 1st positive band emissions in the wavelength range of 560 – 800 nm, while the otehr MAP without a filter was used to measure both the N 2 1st and 2nd positive band emissions in the range 350 – 800 nm. Comparing the data from these two MAPs, we estimated the relative ratio of the 1st / 2nd positive bands of N 2 . During this campaign, we obtained 66 events data of sprites or elves with the MAPs. It is found that over the entire region of the head of column-shaped sprites the relative ratio of the 1st / 2nd positive bands of N 2 is small at the initial phase of the luminosity enhancements lasting only about 1 ms. In the second luminosity enhancement occurring 1 – 2 ms after the initial phase, the relative ratio of the 1st / 2nd positive bands decreases in the lower part of the head. These facts imply that electrons which excite N 2 molecules have higher energy at the initial phase over the entire head, while the energy of electrons is high only at lower altitude in the second enhancement.