Depletion of oxygen in aurora: Evidence for a local mechanism

Abstract

Abstract. A ground-based and rocket investigation of the response of the neutral atmosphere to E region auroral heating has been carried out at Poker Flat, Alaska. The temporal evolution of the atomic oxygen to molecular nitrogen ratio (O/N2) in the lower thermosphere has been monitored using the optical emissions from the aurora as a diagnostic. Comparisons between the changes in the O/N 2 ratio and the auroral Joule and particle heating have shown several examples of close similarity between the durations of the heating events and the depletions. Using the thermospheric winds measured during the rocket flights and the temporal structure of the depletions, the upper limit on the horizontal scale size of the depletions has been estimated at 200-400 km. Moreover, in situ rocket measurements of atomic oxygen showed significant differences at points separated horizontally by approximately 220 km. It is also concluded from the near coincidence between the depletion events and the Joule heating events that the dynamical mechanism(s) that drive the depletions were not far distant from the observing site, that is, local processes are sometimes dominant • during periods of moderate auroral activity.We suggest that the observation of a strong wind shear in the 100-to 120-km altitude region [Larsen et al., 1997] could be responsible for turbulence that contributes to the changes in minor constituent composition.

A comparison between auroral particle characteristics and atmospheric composition inferred from analyzing optical emission measurements alone and in combination with incoherent scatter radar measurements

“…This is qualitatively consistent with models that have been developed for many years. The magnitude of the observed decrease is also consistent with previous observations recently reported by Christensen et al [1997], where it was concluded that local dynamical mechanisn•s a.re causing the composition change. However, in that paper it was asserted that recent three-dimensional numerical models [Sun et al, 1995] significantly •lnderpredicted the magnitude of the observed decrease in O/N2.…”

“…However, in that paper it was asserted that recent three-dimensional numerical models [Sun et al, 1995] significantly •lnderpredicted the magnitude of the observed decrease in O/N2. In contrast to the events considered by Christensen et al [1997] where the dominant heat source was Joule dissipation, in this work the majority of the total hea,ting was due to particle precipitation. Whether or not this is an important difference will require additional model analysis.…”

Particle precipitations during NEIAL events: simultaneous ground based observations at Svalbard

“…Energetic particle precipitating and Joule heating due to dissipation of electric currents will heat the lower thermosphere during aurora and drive vertical and horizontal transport of the atmospheric gases (Fuller-Rowell, 1985). In general, heating and O/N 2 ratio are anti-correlated; the ratio O/N 2 in the lower thermosphere (E-region) is at a minimum during maximum particle and Joule heating (Hecht et al, 1995;Christensen et al, 1997). Thus, both heating and atmospheric composition will influence the optical brightness ratio.…”

Simultaneous observations of lower thermospheric composition change during moderate auroral activity from Kangerlussuaq and Narsarsuaq, Greenland

“…Any local signatures of composition change are masked in such cases. Christensen et al [1997] presented data that indicated that during some substorms, decreases in the O/N 2 density ratio of the lower E region of up to 50% occurred in a period of a few tens of minutes after the peak of local auroral energy deposition. These timescales suggested that the mechanisms that drive these changes were located not too far distant from the observation site.…”