The energy spectrum of primary auroral electrons determined from auroral luminosity profiles

Abstract

The vertical luminosity profiles of sixteen auroral arcs have been measured in the light of λ3914 (N 2 +) by a technique of photometric triangulation. A comparison between these observed profiles and predicted profiles calculated using two model atmospheres and several electron spectral shapes yields information on the flux and energy distribution of the primary auroral electrons. In the energy range of about 0.1–30 keV the primary electron stream may be characterized by a differential number flux of the form N( E) ∼ E γ exp (− E/ α) where γ is usually 1 and α varies between 1 and 4 keV. The function has a maximum at energy E = γα which may be interpreted as the characteristic energy of the stream. Thus, the electron stream which pro- duces the luminous aurora has a characteristic energy between 1 and 4 keV, although extreme values of 0.7–6.0 were found in the sixteen cases studied. The energy distribution above 30 keV is not well determined from luminosity data and plays a small part in the auroral luminescence. The total electron flux associated with the sixteen arcs ranged between 2.9 × 10 9 and 1.1 × 10 11 electrons/cm 2 sec. The total energy flux carried by the incident electrons ranged between 28 and 590 ergs/cm 2 sec.