Mid-Latitude Ground Magnetic and Electric Field Responses Associated with SCW Development During Substorms

This study elucidates the development of the global three-dimensional current system from polar to equatorial regions during substorms. We extracted 50 cases from 1934 isolated substorms between 2006 and 2013. These 50 cases met two criteria: the electric field was calm for 1 h before the substorm, and clear electric field fluctuations were observed during substorm onset. We compared the 210 MM and mid-latitude magnetic field data to capture each event's distribution and temporal evolution of the substorm current wedge. Additionally, we captured the electric field response using the mid-latitude Paratunka station. The results revealed that the electric field amplitude was proportional to the SuperMAG aurora electrojet index magnitude (correlation coefficient = 0.67). In the central region of the substorm current wedge (SCW), predominantly westward electric fields were observed between the upward and downward field-aligned currents (FACs) (i.e., among 12 cases, wherein the observation points were at the center of the SCW, 10 showed westward electric fields). Moreover, a tendency for larger amplitudes of these westward electric fields was observed at the SCW center. This may reflect the projection of the primary potential formed by the charges injected into the upward and downward FAC regions on the ionosphere. Contrarily, a tendency for eastward electric fields was observed near the upward FAC of the SCW (i.e., among 24 cases, wherein the observation points were near or slightly inside the upward FAC, 14 showed eastward electric fields), which might be due to the twisting of the potential distribution associated with Hall polarization.