Correlation as a global measure of geomagnetic activity: Phase boundaries and a precedent line of nodes

Abstract

This work considers the global correlation of geomagnetic activity as a way to evaluate magnetotail disturbances, such as the substorm. Impulsive magnetotail disturbances are generally associated with geomagnetic pulsations, which can be coherent over wide ranges of latitude and longitude, and which display distinctive phase reversals collocated with power maxima. Analyzing a disturbance period chosen for its breadth in local time, we find that pulsations can be detected from the coherence that they generate within a magnetometer array, and identify an extended line of nodes across which the phase reversals occur. Phase reversals consistent with the same line of nodes persist for five hours, beginning clearly in a 0.7 mHz pulsation one and a half hours before the disturbance, and persisting in a 5.8 mHz pulsation three and one half hours after the initial disturbance. Under the hypothesis that the line of nodes maps to a source in the central plasma sheet (CPS), we note that the persistence of this extended source of disturbances suggests memory in the CPS. We define a quantitative "coherence index" that characterizes geomagnetic activity according to the degree of global coherence that it generates, and observe that a narrowly peaked coherence signal leads a much broader peak in power. We relate these results to models of the magnetosphere based in critical phenomena.