Optical emissions from the mid-day aurora

Abstract

Airborne measurements of optical emissions from mid-day aurora on 13 December 1969 UT yielded an average intensity of 520 R for OI 6300 Å, 183 R for OI 5577 Å, 11 R for NI 5200 Å, 7 R for Hβ 4861 A, 26 R for N 2 + I NG (0, 1) band at 4278 Å, and 32 R for N 2 2 P (0, 0) band at 3371 Å. Simultaneous satellite particle measurements showed that protons and electrons precipitating along the magnetospheric cleft have much lower characteristic energies than particles which excite normal nighttime auroras. Combining the available particle and photometric data as well as excitation cross-sections for atmospheric constitutents, it is shown that most of the OI red and green line emissions in the mid-day aurora are excited by the low energy electrons precipitating in the cusp region but only part of the N 2 and N 2 + emissions can be so accounted; precipitating protons excite the rest of N 2 and N 2 + emission. Additionally, the ratios of 4278, 3371 and 5577 to Hβ during precipitation of 1.3 kev protons are derived from the optical and particle data. It is also shown that neither resonant scattering of sunlight by N 2 + ions nor anomalous vibrational distribution of N 2 + ING bands, resulting from impact of low energy hydrogen atoms and electrons on N 2, will account for the total emission around 3886 Å, which in mid-day aurora has an intensity comparable to that of N 2 + ING (0, 0) band. Resonant scattering of sunlight by He (2 3 S) at 3888 Å will only account for this spectroscopic feature if the helium abundance in the northern polar region is three to four times higher than at midlatitudes during winter.