Coherence as a Global Measure of Geomagnetic Activity: Phase Boundaries and a Leading-edge Disturbance Signature

Geomagnetic pulsations are carriers of energy within the magnetosphere. Pulsations are often triggered by solar wind perturbations or by internal processes in the magnetosphere such as, for example, plasmoids, dipolarizations or bursty bulk flows. Pulsations may arise through a magnetospheric response function associated with field line resonances. In this work we consider the global coherence of geomagnetic activity as a way to evaluate the magnetospheric response, and develop a quantitative "coherence index" that demonstrates a distinctive response to magnetotail disturbances. The spatial structure and extent of the magnetospheric response to a magnetotail disturbance occurring February 14-15, 1998, is considered through a time/frequency analysis of global coherence and correlation. Two global measures of the data are constructed: (1) a measure of the power in magnetic fluctuations, and (2) a measure of the coherence of the pulsations across latitude and local time, irrespective of power. We find that some pulsations exhibit a high degree of coherence, suggesting a mode that is quite global. We observe phase reversals suggestive of field line resonances, except that they seem to occur across field lines that thread the plasma sheet, irrespective of frequency. Coherence and phase reversal are observed at and below the Pc5 period range, down to about 30 minutes. Coherence exhibits a sharp peak at the onset of the disturbance, which leads a more extended peak in the power. We interpret this as an indication of a global magnetotail instability. As the magnetospheric response evolves, power and coherence get distributed to discrete frequencies. We argue that coherence, as a measure of geomagnetic activity, quantifies the extent to which the magnetosphere exhibits a global response, and suggest a coherence index that characterizes the degree of globalization.