Network-based quantification of the Substorm Current Wedge

Abstract:

Magnetospheric and space weather observations are increasingly becoming a data analytical challenge as we move from a data-poor to a data-rich era. This requires novel approaches to the data analysis and our research presents an example of this. We have for the first time obtained the time-varying directed network that represents the observations from 100+ ground-based magnetometers (collated by SuperMAG) and this quantitatively captures the characteristics of the ground magnetic field associated with the substorm. If the canonical cross-correlation between vector magnetic field perturbations, observed at two magnetometer stations, exceeds an event and station specific threshold, they form a network connection. The time lag at which cross-correlation is maximal determines the direction of propagation or expansion of the structure captured by the network connection. Network parameters, such as the modularity, quantify spatially coherent behaviour and allow us to make a quantitative statistical study across many events rather than looking at individual events on a case by case basis. If spatial correlation reflects ionospheric current patterns, network properties can test different models for the evolving substorm current system. This offers an entirely new way to synthesize multi-point magnetospheric observations into a form that readily tests physical model hypothesis. It is essentially information extraction. We report that the SCW displays large-scale coherent behaviour which puts significant doubt on the recent hypothesis that this current system consists of a series of meso-scale wedgelets. For the first time we perform community detection on the data-derived network, where communities identify locally dense but globally sparse groups of connections in the network. We consistently find robust structural change from many small, uncorrelated current systems before substorm onset, to one large spatially-extended correlated system during the expansion phase. All 40+ substorms analysed ultimately form a single large-scale structure, approximately 10 minutes after onset. This establishes that a single large-scale SCW is central to substorm physics and that substorms do not proceed solely by meso-scale wedgelets.