Energy spectrum for auroral-zone X rays: 1. Diurnal and type effects

Abstract

Balloon observations of X rays produced by energetic (∼50–250 kev) electron precipitation into the auroral zone suggest a rather consistent diurnal pattern of occurrence for certain types of bremsstrahlung activity. Distinguishing spectral characteristics and variability, represented by approximate e-folding energies in the range 10–50 kev, are emphasized. Whereas the harder spectrums (E0 ≈ 25–50 kev) may be precipitated at any local hour, the softer spectrums (E0 ≈ 10–25 kev) appear primarily between the midnight and noon meridians. The softest spectrums (E0 ≈ 10–15 kev) observed in this energy range occur preferentially around midnight and into the early morning in association with pronounced negative magnetic bays and post-breakup auroral activity. The results suggest the existence of an intermediate spectrum (E0 ≈ 10–20 kev), in addition to the very soft (E0 ≲ 5 kev) precipitation characteristic of the luminous aurora and the rather harder precipitation (E0 ≳ 25 kev) characteristic of the ‘average’ auroral-zone X-ray event.