Ground‐based observations of F region aurora associated with the magnetospheric cusp

Abstract

A multinational program of optical observations of the dayside aurora was carried out at Longyearbyen and Ny Ålesund, Svalbard, during the winter of 1978–1979. Observations were made of the temporal and spatial variations of the particle precipitation region characterized by a predominance of [O I] 6300‐Å emission previously mapped by satellites and associated with the magnetospheric cusp. This region expands and moves equatorward at a rate proportional to the negative excursion of the H component of the disturbance geomagnetic field measured at a station in the nightside at the latitude of maximum disturbance (approximately 10° farther south than the latitude of the cusp). The intensity ratios of 5577‐Å [O I], 6300‐Å [O I], and 4278‐Å N2+, normally used to determine the energy of the incoming particles, are often not quantitatively useful for this purpose, owing to the predominating enhancement of N2+ emissions by resonantly scattered sunlight; however, comparisons of relative values show that there is a considerable variation in the energy of the incoming electrons during magnetic activity. The derivation of realistic values of energy of auroral primaries must rely on a combination of spectrometer and meridian‐scanning photometer observations. A few examples of such a derivation yield characteristic energies of auroral primaries which vary between 2 keV and <100 eV in the cusp aurora. The higher values are associated with occasional rayed bands, and the lower with the large, structureless emitting region associated with the cusp. Because of the high altitude of this region, resonance scattering is the most important source of N2+ emission, and the process O+(²D) + N2 → N2+ (A²Σ) + O is an important source of N2+ ions. Thus the observed N2+ 4278‐Å emission is greater than would be expected for impact excitation alone, and the ‘midday gap’ in 5577‐Å emission is simply a result of the low characteristic energy of the incoming particle spectrum. The rayed bands occurring occasionally throughout the cusp region are spectroscopically very similar to polar cap arcs and appear simultaneously with increasing nightside activity, an indication that they could not have drifted from the nightside. Spectrometry of auroral and atmospheric emissions reveals enhanced atomic emissions as well as redistribution of vibrational and rotational emissions which should be expected for sunlit aurora produced in the F region by relatively low energy (100 eV) particles precipitating in the cusp region. F region neutral winds display a strong, steady cross‐polar flow in the direction of the magnetic noon‐midnight meridian with vortices near 6 and 18 hours MLT. The wind strength is relatively greater on the dawnside and on the nightside, increasing with magnetic activity.