Defining the Middle Corona

TL;DR

The paper defines the middle corona as a crucial yet understudied region of the solar atmosphere, emphasizing its role in solar wind and eruption propagation, and highlights recent advances enabling its better understanding.

Contribution

It provides a comprehensive definition of the middle corona, outlining its physical characteristics, importance, and the need for focused study in this region.

Findings

Highlights the physical transitions in the middle corona

Emphasizes its role in solar wind and eruption dynamics

Notes recent observational advances enabling better study

Abstract

The middle corona, the region roughly spanning heliocentric altitudes from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. Eruptions that could disrupt the near-Earth environment propagate through it. Importantly, it modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, this region is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the middle corona has been poorly studied by major solar remote sensing missions and instruments, extending back to the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent advances in instrumentation, observational processing 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 regions 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 it covers, and the underlying physics believed to be encapsulated by the region. This paper aims to define the middle corona and give an overview of the processes that occur there.