The spatial variation of auroral luminosity—II determination of volume emission rate profiles

Abstract

The variation in the intensity of a stable isolated auroral arc has been measured as a function of zenith angle from two stations located on the same geomagnetic meridian and separated by 226 km. The measurements were made with meridian scanning photometers which recorded simultaneously the emissions λ3914 (N 2 +, λ5577 [OI] and λ6300 [OI] at both stations. A comparison of the λ5577 and λ6300 data shows that excitation of the O [ 1D ] state by secondary electrons due to primary ionization processes cannot be the dominant process for the emission of λ6300 at least for altitudes above 250 km. Collisional deactivation of the excited O [ 1D ] atoms is essentially complete below 150 km and is important at least up to 240 km. A detailed analysis of the λ3914 (N 2 +) and λ5577 [OI] data in terms of volume emission rate profiles confirms the constancy of the ratio of λ5577 to λ3914 photon emission rates with height which has previously been inferred from single station measurements. The unexpected result of the present observations, however, is that the horizontal volume emission rate profiles are different for the two emissions, the λ5577 emission being concentrated toward the center of the λ3914 emitting region. It is concluded from these results that excitation of oxygen atoms by secondary electrons due to the primary ionization process cannot be the dominant process for the production of either the λ5577 or the λ6300 emission in the aurora.