Geoeffectiveness of Interplanetary Alfvén Waves. I. Magnetopause Magnetic Reconnection and Directly Driven Substorms

In particular during the descending phase of the solar cycle, Alfvén waves in the high-speed solar wind streams are a major form of interplanetary disturbances. The fluctuating southward interplanetary magnetic field (IMF) of Alfvén waves has been suggested to induce geomagnetic activities through intermittent magnetic reconnection at the magnetopause. In this study, we provide in situ observational evidence for dayside magnetopause reconnection induced by such interplanetary Alfvén waves. Using multipoint conjunction observations, we show that the IMF B _z from interplanetary Alfvén waves is transmitted through and amplified by the Earth’s bow shock. Associated with the intensified southward B _z to the magnetopause, in situ signatures of magnetic reconnection are detected. Repetitively, interplanetary Alfvén waves transmit the intensified B _z to the magnetosheath, leading to intervals of large magnetic shear angles across the magnetopause and magnetopause reconnection. Such intervals are promptly followed by hundreds of nanoTesla (nT) increases in the auroral electrojet indices (AE and AU) within 10–20 minutes. These observations are confirmed in multiple events in corotating interaction region-driven geomagnetic storms. To put the observations into context, we propose a phenomenological model of a strongly driven substorm. The substorm electrojet is linked to the enhanced magnetopause reconnection in the short timescale of re-establishing the ionosphere electric field and the two-cell convection. These results provide insights on the temporal patterns of solar wind magnetosphere–ionosphere coupling, especially during the descending phase of the solar cycle.