The aries auroral modelling campaign: characterization and modelling of an evening auroral arc observed from a rocket and a ground-based line of meridian scanners

An auroral arc system excited by soft electrons was studied with a combination of in situ rocket measurements and optical tomographic techniques, using data from a photometer on a horizontal, spinning rocket and a line of three meridian scanning photometers. The ground-based scanner data at 4709, 5577, 8446 and 6300 Å were successfully inverted to provide a set of volume emission rate distributions in the plane of the rocket trajectory, with a basic time resolution of 24 s. Volume emission rate profiles, derived from these distributions peaked at about 150 km for 5577 and 4709 Å, while the 8446 Å emission peaked at about 170 km with a more extended height distribution. The rocket photometer gave comparable volume emission rate distributions for the 3914 Å emission as reported in a separate paper by McDade et al. (1991, Planet. Space Sci. 39, 895). Instruments on the rocket measured the primary electron flux during the flight and, in particular, the flux precipitating into the auroral arc overflown at apogee (McEwen et al., 1991; in preparation). The local electron density and temperature were measured by probes on the rocket (Margot and McNamara (1991; Can. J. Phys. 69, 950). The electron density measurements on the downleg were modelled using ion production rate data derived from the optical results. Model calculations of the emission height profile based on the measured electron flux agree with the observed profiles. The height distribution of the N₂⁺ emission in the equatorward band, through which the rocket passed during the descent, was measured by both the rocket and the ground-based tomographic techniques and the results are in good agreement. Comparison of these profiles with model profiles indicates that the exciting primary spectrum may be represented by an accelerated Maxwellian or a Gaussian distribution centered at about 3 keV. This distribution is close to what would be obtained if the electron flux exciting the poleward form were accelerated by a 1–2 kV upward potential drop. The relative height profiles for the volume emission rate of the 5577 Å OI emission and the 4709 Å N₂⁺ emission were almost indistinguishable from each other for both the forms measured, with ratios in the range 38–50; this is equivalent to I(5577)/I(4278) ratios of 8–10. The auroral intensities and intensity ratios measured in the magnetic zenith from the ground during the period before and during the rocket flight are consistent with the primary electron fluxes and height distributions measured from the rocket. Values of I(5577)/I(4278) in the range 8–10 were also measured directly by the zenith ground photometers over which the arc system passed. These values are slightly higher than those reported by Gattinger and Vallance-Jones (1972) and this may possibly indicate an enhancement of the atomic oxygen concentration at the time of the flight. Such an enhancement would be consistent with our result, that the observed values of I(5577) and I(8446) are also significantly higher than those modelled on the basis of the electron flux spectrum measured at apogee.