Postmodern view of M‐regions

Abstract

The discovery of coronal holes in the early 1970s was heralded as the discovery of the mysterious M ‐regions responsible for recurrent geomagnetic activity. High‐speed flow from the holes, sweeping past Earth on successive solar rotations, was directly correlated with sustained increases in activity indices. We argue that this view of M ‐regions as coronal holes is incomplete because it ignores work dating from the 1960s demonstrating that peak recurrent activity coincides with passage of corotating interaction regions between high‐ and low‐speed flows. Thus, in an important way, M ‐regions include the boundary between coronal holes and coronal streamers, since streamers supply the slowest flows. In addition, we suggest that coronal mass ejections propagating through the streamer belt can contribute to the peak intensities of recurrent storms. Finally, we demonstrate that both peak recurrent activity and the following sustained activity that correlates with the high‐speed flows are controlled by the Russell‐McPherron effect: They are prominent only when the azimuthal component of the Parker spiral field projects a southward component in Earth's tilted dipole frame.