Event Details

Directed networks applied to ground-based magnetometer observation of substorms

Abstract: Geomagnetic substorms are a fundamental, global reconfiguration of the magnetosphere during which energy is abruptly transported to the ionosphere where it is dissipated. This fully non-linear and multi-scale process has been observed in-situ for decades, but how it evolves and is spatially structured is a fundamental problem since the various proposed models imply different and conflicting magnetospheric reconfiguration scenarios. Our studies show that the substorm current wedge (SCW) displays large-scale coherent behaviour which puts significant doubt on the recent hypothesis that this current system consists of a series of mesoscale wedgelets. We used techniques from network science to analyse data from >100 ground-based magnetometers collated by the SuperMAG collaboration. We translated this data into a time-varying directed network. 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. If spatial correlation reflects ionospheric current patterns, network properties can test different models for the evolving substorm current system. We obtained the timings for, a consistent picture in which the classic SCW forms. A current system is seen pre-midnight following the SCW westward expansion. Later, there is a weaker signal of eastward expansion. Further, we perform community detection on the network which identifies locally dense but globally sparse groups of connections. 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 substorms analysed ultimately form a 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 small-scale wedgelets.