Observations of horizontal transport effects on high latitude metastable O( 1D), N( 2D) auroral emissions

Abstract

An analysis is presented of photometric measurements of the NI ( λ = 520nm),OI( λ = 630nm)and other emissions made at Nord, where the invariant latitude is Λ = 80°4. The time variations of the intensities are interpreted in the following way by comparison with simultaneous ground based or satellite measurements. The N( 2D) atoms formed in the dayside cleft are carried by the neutral wind in a plume across the polar cap, so that the ratio of λ(630 nm) to λ(520 nm) intensities decreases along the plume with increasing distance from the source region. In the polar cap, but outside the plume region, 630 nm emission is produced by electron impact of polar rain and by substorms that reach high latitudes. Ionization produced at the same time, especially by the substorms, will produce further 630 nm emission through dissociative recombination. In any case, the region outside the plume may be regarded as a source region, with a high value of the ratio I(630) I(520) . This explains in part the diurnal variations, since this ratio is depressed as Nord crosses the dayside plume. The electron energy along the oval increases progressively from the dayside to the nightside. The intensity ratio increases with increasing electron energy because N( 2D) is quenched more rapidly than O( 1D). Thus the ratio rises progressively from noon to midnight. An effect of the interplanetary magnetic field is superimposed on this pattern : as its North-South component B z increases, the oval contracts so that Nord becomes nearer from the cleft source and the intensity ratio increases on the dayside. The inverse effect is also observed. On the nightside, negative B z is associated with substorms that produce poleward expansions of the poleward oval boundary, that brings more energetic precipitation to Nord. This causes the intensity ratio to increase with decreasing B z in a way that is opposite to that for the dayside.