Mesoscale Processes In The Coupled Magnetosphere - Ionosphere System: What We Know, What We Do Not Know, And Why We Need To Close the Knowledge Gap.

In the near-Earth space, in-situ observations have revealed ubiquitous fast, earthward moving, mesoscale plasma and magnetic flux transport in the plasma sheet, and hot plasma injections, well within geosynchronous orbit, during geomagnetic storms, where the global energy density builds up to form the storm-time ring current. Yet, it still remains a mystery whether these mesoscale structures constitute a fundamental mode of plasma transport and energization, with major impacts on the global development of the geomagnetic storm, or are merely ripples on top of the global plasma convection flow. At the same time, auroral phenomena, ranging from mesoscale auroral streamers to the global brightening of an auroral substorm, are a manifestation of both the energy deposition within the strongly coupled ionosphere-magnetosphere system and the ionospheric conditioning itself. Multiple missions over the years have targeted either the local or global nature of geospace with in-situ probes or global imaging, respectively. However, understanding geomagnetic disturbances to the level of predictability remains elusive, because we still do not understand the bridge between the local and global nature of geospace, that is, the mesoscale (1000 km to few RE in the magnetotail, ~10s-100s km in the ionosphere) processes and their global implications. In this presentation we summarize the current knowledge regarding mesoscale processes in the coupled Magnetosphere-Ionosphere (M-I) system, and we discuss the lack of crucial measurements, which are needed in order to understand under what conditions mesoscale processes in the coupled M-I system become geoeffective.