Dayside dynamics of Alfvén waves during geomagnetic storms, including the initial phase

Abstract

In this study, we investigated the dynamics of Alfvén waves as a global phenomenon in the solar wind-magnetosphere coupling as it occurs at the entry point to the dayside auroral acceleration region. It was found that total Alfvén wave power over the dayside auroral zone increases from 1.3 to 5.5 GW with increasing -Dst value, or alternatively, from minor, to moderate, and to major storm. As the storm intensity increases, the Alfvénic “band” (Alfvénic oval) expands to lower latitude with a peak at 76° invariant latitude (ILT) during the initial phase and a peak at 72° ILT during the storm phase of moderate to strong storms. In comparison with the nightside Alfvénic activity, the most striking result was that during the initial phase, enhanced Alfvén wave power mostly occurred on the dayside, which is in contrast to the storm phase which shows strong enhancement on both dayside and nightside. The different sources of initial phase and storm phase suggest different underlying Alfvén wave generation mechanisms in the dayside. Furthermore, similarity in distribution to the dayside storm aurora during the initial phase suggests contributions of Alfvénic auroral acceleration.