A study of concurrent riometer and magnetometer variations in the PC 4–5 pulsation band

Abstract

We report the results of a study of concurrent, periodic variations in riometer and magnetometer signals in the ULF frequency band. We have found that the riometer signals show azimuthal phase variations consistent with the phase variations observed in the magnetometer signals. Prenoon pulsations show apparent phase motion toward the dawn terminator, while post‐noon pulsations show apparent phase motion toward the dusk terminator. The apparent phase motion of the riometer pulses toward dawn in the morning sector is in opposition to the direction of electron drift, indicating modulation of electron precipitation by the ULF magnetic perturbations. In addition, we find the magnitude of the riometer absorption pulsations are linearly correlated with the magnitude of the background riometer absorption level with δA = (0.22±0.04)A when measured in decibels from the quiet day level. This fact indicates modulation of the loss rate of an electron plasma which is at or near the stable trapping limit. These two observations, when combined with other information on variations in electron precipitation, imply that the variations in electron precipitation are controlled by the ULF magnetic perturbations through a ULF‐VLF‐electron interaction.