Scale size-dependent characteristics of the nightside aurora

TL;DR

This study analyzes the spatiotemporal variability of nightside aurora using innovative Fourier analysis, revealing scale-dependent stability and linking auroral features to magnetosphere-ionosphere coupling during substorm events.

Contribution

It introduces a novel Fourier-based method to analyze auroral imaging data, uncovering scale size-dependent variability and linking auroral dynamics to magnetosphere-ionosphere coupling.

Findings

Large scale auroral features are stable over minutes.

Small scale features show rapid variability.

Auroral characteristics align with nightside field-aligned currents.

Abstract

We have determined the spatiotemporal characteristics of the magnetosphere-ionosphere (M-I) coupling using auroral imaging. Observations at fixed positions for an extended period of time are provided by a ground-based all-sky imager measuring the 557.7 nm auroral emissions. We report on a single event of nightside aurora ($\sim$22 magnetic local time) preceding a substorm onset. To determine the spatiotemporal characteristics, we perform an innovative analysis of an all-sky imager movie (19 min duration, images at 3.31 Hz) that combines a two-dimensional spatial fast Fourier transform with a temporal correlation. We find a scale size-dependent variability where the largest scale sizes are stable on timescales of minutes while the small scale sizes are more variable. When comparing two smaller time intervals of different types of auroral displays, we find a variation in their characteristics. The characteristics averaged over the event are in remarkable agreement with the spatiotemporal characteristics of the nightside field-aligned currents during moderately disturbed times. Thus, two different electrodynamical parameters of the M-I coupling show similar behavior. This gives independent support to the claim of a system behavior that uses repeatable solutions to transfer energy and momentum from the magnetosphere to the ionosphere.