Effect of ICME-Driven Storms on Field-Aligned and Ionospheric Currents From AMPERE and SuperMAG

This study investigates the field-aligned currents (FACs) and ionospheric equivalent currents for interplanetary coronal mass ejection (ICME)-driven storms by considering 45 events with a minimum Dst <= -50 nT. The FACs and ionospheric equivalent currents are studied by applying a superposed epoch analysis to data from AMPERE and SuperMAG with the zero epoch (t(0)) centered at the onset of the storm main phase. The currents and number of substorm onsets begin to increase 3 hr before t(0) and maximizes about 1 hr after t(0). The currents and number of substorm onsets remain high throughout the entire storm main phase, until at t(0) + 14 hr they start to slowly relax back to quiet time conditions. The storms were separated into two groups based on the solar wind dynamic pressure p(dyn) around t(0). High p(dyn) storms are mostly driven by the sheath region ahead of the ejecta. These storms have short main phase durations and larger currents early in the main phase which maximize at t(0) + 50 min. The low p(dyn) group contains storms that start during the magnetic clouds (MC) and have gradually increasing currents that maximize at t(0) + 11 hr, close to the end of the storm main phase. For the first 4 hr of the storm main phase, the currents in sheath-driven storms are larger than for MC-driven storms. The Russell-McPherron effect is less important for ICME-driven storms where only 44% have a contribution, compared to 82% of high speed stream/stream interaction driven storms.