Latitudinal range of geospace currents inferred from correlation matrix analysis of ground magnetic variations during magnetic storms

Abstract

In recent years, remote sensing of magnetospheric and ionospheric currents using magnetometer arrays has improved considerably in geographic coverage, precision, and time resolution. Magnetic measurements have been used to detect and interpret a wide range of processes such as convection, solar wind (SW) compressions, substorms, magnetic storms (MSs), and more localized effects such as traveling ionospheric vortices, waves, etc. It is important to develop time- and frequency-domain methods that take advantage of these new observational capabilities. Here we apply a basic, but powerful correlation-based method for measuring the location and width of geospace current over a wide geomagnetic-latitude range. We compute the correlation matrix of the horizontal component of the magnetic field and use the structure of the matrix to measure the spatial extent of three current types. The method is applied to two recent, well-known magnetic-storm intervals (March 2015 and August 2018; 27 days each) as well as pre-storm activity intervals; but can be adapted to shorter time intervals by using higher time-resolution field data. We find that the correlation matrix is divided into three blocks, or geomagnetic latitudinal ranges, which are readily understood in terms of the footprint of the ring current, auroral electrojets, and polar convection intensifications. The matrix structure changes depending on intensity and pattern of geomagnetic activity (before, during and after the storm), and magnetic local time (MLT) range, with direct physical interpretation. The high correlations in each region are a quantitative measure of magnetospheric coherence. The results show how the correlation-matrix analysis can be used in quantitative remote sensing of spatial and temporal features of geospace activity.