On the morphology of auroral absorption during substorms

Abstract

Multistation riometer measurements of ionospheric radio-wave absorption have been used to infer the characteristics of the higher-energy component of electron precipitation during substorms. Two analyses are presented which deal separately with longitudinal and latitudinal variations. The first is based on data from four riometer stations located at L ≈ 5.2 and at about 90° longitude intervals, and the second on a chain of five riometer stations spaced at about 3° intervals across the auroral zone. Previous work has shown that substorms detected at night are accompanied by daytime precipitation. In the present study, the average substorm absorption is found to have two broad maxima, one at night within 15 min of the absorption onset, and another during the morning about an hour after this onset. There is very little effect in the evening.From the latitude study, a transition is observed during the nighttime phase of substorms between the abrupt, irregular, intense absorption at higher latitudes and the smoother, less intense absorption at lower latitudes. This transition, designated as Λa, is observed at a median latitude of Λa = 65 ± 1.5°. For the majority of a sample of 181 events selected from 1964 and 1967 records, the absorption was observed to start in the vicinity of Λa. With increased magnetic disturbance (measured by Kp), Λa is observed to shift to lower latitudes. The Kp variation is compared with that of energetic-particle boundaries.