Morphology of aurorae in λ 557.7-nm optical temperatures

An all-sky scanning Fabry-Perot spectrometer (FPS) was used to observe variations in the temperature of auroral λ 557.7-nm [O I] emissions over Poker Flat, Alaska (65.11§N, 147.12§W). The data sets were acquired using the Poker Flat Scanning Doppler Imager (SDI) during the 2002-2003 winter observing season. Temperatures derived from Doppler-broadened spectra were used to approximate the emission altitude, which in turn can be used to infer the characteristic energy of precipitating electron population. Several cases with differing magnetic local time (MLT) and auroral activity were analyzed, and the results are as follows: (1) A dramatic temperature drop (~200K) over the entire sky occurred immediately after a major auroral brightening; the temperature remained lower than the pre-brightening value for many minutes after the auroral intensity had resumed its quiescent levels. (2) A local increase of temperature, coinciding with a weak auroral arc, was observed approximately 25 minutes before a substorm onset event propagated westward across the instrument's field of view. This was followed by a sudden increase in auroral luminosity and a sharp decrease in the temperature in the vicinity of the arcs. (3) A westward-traveling surge was observed to contain a region of elevated temperature relative to that of the band itself. For the cases considered, the "heating" rates required to produce such temperature changes at a single height could only result from physically unreasonable currents for the modest geophysical activity observed here. The sudden temporal and spatial changes in Doppler temperatures observed are likely to be owing to the λ 557.7-nm emission height changing as a response to variations in the characteristic energy of the precipitating electron population. The Scanning Doppler Imager at Poker Flat Research Range is currently the only instrument capable of measuring temperatures of auroral emissions in a true all-sky imaging sense, and thus is a valuable tool for estimating geometric and energetic auroral parameters.