Investigating Geomagnetic Disturbances in the Expansion Phase of Strong Substorms

This study advances space weather forecasting by investigating geomagnetic disturbances (GMDs) triggered during rapid decreases in the SuperMAG Auroral Electrojet Lower (SML) index during strong substorms (SML $<$ -500 nT) at high latitudes (64$^\circ$-75$^\circ$ MLAT). Through integrated analysis of two selected strong substorm events from December 2015, combining THEMIS and GOES-13 spacecraft observations with ground magnetometer data from Arctic Canada and Spherical Elementary Current System (SECS) modeling, we reveal key relationships: (1) the SML drops during substorm expansion phase coincide with GMD onset and subsequent poleward propagation (64$^\circ$-75$^\circ$ MLAT); (2) SECS-derived current patterns show intensified horizontal ionospheric currents ($>$200 mA/m) and localized vertical current density ($\sim$8-10 $\mu$A/m$^2$) that align spatially and temporally with the largest GMDs (reaching $\sim \pm$25 nT/s) and (3) rapid SML decreases during substorm expansion phases correlate with dipolarization and earthward fast flows as observed by THEMIS-A, E in the Earth’s magnetotail and near-Earth magnetic field reconfigurations with enhanced particle fluxes (GOES-13). The horizontal current enhancements propagate coherently with the GMDs, while the SECS analysis reveals a structured vertical current density that connects magnetospheric drivers to ground-level disturbances. These observations demonstrate a complete chain of processes from magnetotail reconfiguration to ionospheric current intensification and subsequent GMD development - a progression not typically observed in weaker substorms (SML $>$ -500 nT). Our multi-instrument approach advances substorm-GMD coupling understanding, offering a robust framework for improved GMD forecasting.