Abstract:
The middle corona, the region roughly spanning heliocentric distances from 1.5 to 6 so lar radii, encompasses almost all of the influential physical transitions and processes that
govern the behavior of coronal outflow into the heliosphere. The solar wind, eruptions, and
flows pass through the region, and they are shaped by it. Importantly, the region also modu lates inflow from above that can drive dynamic changes at lower heights in the inner corona.
Consequently, the middle corona is essential for comprehensively connecting the corona to
the heliosphere and for developing corresponding global models. Nonetheless, because it
is challenging to observe, the region has been poorly studied by both major solar remote sensing and in-situ missions and instruments, extending back to the Solar and Heliospheric
Observatory (SOHO) era. Thanks to recent advances in instrumentation, observational pro cessing techniques, and a realization of the importance of the region, interest in the middle
corona has increased. Although the region cannot be intrinsically separated from other re gions of the solar atmosphere, there has emerged a need to define the region in terms of its
location and extension in the solar atmosphere, its composition, the physical transitions that
it covers, and the underlying physics believed to shape the region. This article aims to define
the middle corona, its physical characteristics, and give an overview of the processes that
occur there.
