MEASUREMENT OF AURORAL RAYS FROM THE SPACE-SHUTTLE

Videotapes of auroras from two space shuttle missions were analyzed to determine the locations of auroral rays. One hundred and forty aurora l ray locations were determined by triangulating from pairs of images taken several seconds (20-100 km) apart. The rays, observed over a ran ge of 18 hours in local magnetic time between 62-degrees and 85-degree s magnetic latitude, were mostly confined to the Feldstein oval in the evening and midnight sectors but were uniformly scattered well into t he polar cap in the morning sector. Plots of ray border altitude versu s magnetic time show a steady increase in lower border altitude, sugge sting a decrease in energy from near 10 keV in the early evening to ne ar 2 keV by midmorning. This variation for the energy in discrete raye d arcs is opposite to previous reports for the average energy of auror al electrons and suggests that the acceleration mechanism within discr ete arcs differs from that pertaining to the auroral oval in general. Comparison of upper and lower border altitudes suggests a nearly monoe nergetic electron flux shortly before midnight and a more extended dis tribution at both earlier and later times. The apparently independent variation in maximum energy and energy spread is difficult to reconcil e with models involving acceleration of auroral electrons by parallel electric fields. There was only a slight dependence of border altitude s on magnetic latitude within the auroral oval, while rays in the pola r cap (northward B(z)) were significantly higher.