Scientific Rationale: The region of the middle corona (MC), roughly between 0.5 and 2 solar radii (Rs) above the photosphere, is a virtually unexplored region, despite its fundamental role in shaping the inner corona and the solar wind. As reviewed recently by West et al. (2023), the quiet sun MC is where the low-β inner corona, dominated by closed magnetic structures, transitions into the high-β outer corona, dominated by radial magnetic structures. It is the crucial bound- ary where the solar wind is formed, i.e. where plasma is accelerated and escapes the corona into interplanetary space, and where eruptive space weather events, such as coronal mass ejections (CMEs), undergo the majority of their acceleration (cf. Zhang & Dere, 2006).

The fundamental questions of where the slow solar wind originates and what is the physics of the acceleration region are still unanswered (cf. Abbo et al., 2016). Major interplanetary missions, such as Parker Solar Probe and Solar Orbiter have been launched to address these questions. There are obvious temporal and spatial limitations in the solar wind information that can be gathered in-situ, hence it is fundamental to try and link those measurements with remote-sensing ones. One possibility is to use charge states as tracers, as they are frozen in the outer corona. The freezing-in distance for a charge state is the location where the timescales for ionization and recombination are longer than the expansion timescale for the ion outflow to cover a density scale height. Rough estimates of the freezing-in distance place it in the middle corona, another important reason to explore this region.

The interplay between open and closed structures is thought to occur in the MC and have a fundamental role in shaping both the inner and outer corona. For example, interchange reconnection between closed hot active region (AR) loops and the ambient field, predicted to occur in the MC (Del Zanna et al., 2011), would provide a way for AR plasma to be released into the heliosphere, but also to affect the plasma circulation flows observed in the low corona.

For over 60 years we have studied the inner corona with instruments in the X-ray and EUV (XUV), or the more distant corona in visible light with coronagraphs. We had occasional glimpses with sounding rockets, or beautiful imaging in visible light during a few total solar eclipses (cf. Habbal et al., 2011). Some information was obtained by the Mauna Loa K-coronameter (KCor), and briefly by SOHO LASCO/C1. The only instrument that surveyed the MC for an extended period of time was the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO, but with limited cadence, spatial and spectral resolution. Despite this, a host of ground-breaking results were obtained, as summarised e.g. by Kohl et al. (2006).

The presence of large-scale structures in the low MC and their dynamical behaviour was caught by PROBA2/SWAP images in the EUV (Fe X/Fe IX, formed around 1 MK), see e.g. O’Hara et al. (2019); West et al. (2022). More recent observations of such structures are clear in composite EUV images obtained by GOES/SUVI off-point campaigns, as shown e.g. by Seaton et al. (2021), indicating structures are observed over a broad range of temperatures.

An obvious question is why the MC has not been regularly observed before? The main reason was that it was difficult to build large coronagraphs in the visible light to observe close to the Sun, and previous EUV instruments had small fields of view (FOV), as it was thought that there would be little signal in the MC.

The quiescent corona emits strongly in spectral lines of highly ionized ions in the EUV. In the inner corona, these lines are mainly formed by collisional excitation by the free electrons. Some strong lines in the EUV/UV are known (with e.g. UCVS) to be visible to great distances out to several Rs as they are mainly excited by the disk radiation. The modelling of such lines is non-trivial as it depends on the distribution of disk radiation, the type of transition, the local state of the plasma, and line of sight (LOS) effects. A combination of modelling and observations provides, however, powerful diagnostics as the possibility to measure outflow velocities via the Doppler dimming (see, e.g. Noci et al., 1987), as well as other diagnostics such as measurements of densities (cf. Dudík et al., 2021).

Forbidden lines in the visible and near infrared (NIR) are also strongly radiatively excited and visible out to large distances of several Rs. Via spectropolarimetry, they offer the possibility to measure the coronal magnetic field, which so far has only been estimated, mainly by extrapolating the photospheric measurements. As for the EUV/UV lines, the modelling of the spectropolarimetric signal is non-trivial and is still being developed. The visible and especially the NIR is virtually unexplored, but offer many diagnostic possibilities to measure the plasma state (densities, temperatures chemical abundances) plus the magnetic field, see e.g. Judge (1998); Del Zanna & DeLuca (2018).

A new era to unravel the mysteries of the MC is fast approaching. UVCS observations of collisional lines out to 3 Rs have indicated that simple EUV imagers will have enough signal out to those distances (Del Zanna et al., 2018). Indeed recent Solar Orbiter EUI 17.4 nm images with an occulter have detected signal out to even larger distances. On this basis, the Sun Coronal Ejection Tracker (SunCET) wide field of view EUV imager has been selected as a new CubeSat mission for NASA’s Heliophysics Flight Opportunities in Research and Technology program (Mason et al., 2022), and the EUV CME and Coronal Connectivity Observatory (ECCCO) has been proposed as a Small Explorer mission to NASA.

Within the visible and NIR, two breakthrough space-based coronagraphs, the Visible Emission Line Coronagraph (VELC), and the ESA Proba-3 Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun (ASPIICS), will soon become operational and probe the MC. VELC is part of the first large solar Indian mission Aditya, with expected launch in June 2023. VELC will image from 1.05 to 3 Rs and a slit spectrometer in forbidden lines and white light continuum, in addition to spectropolarimetry in Fe XIII to measure the magnetic field. ASPIICS is the first ever formation-flying coronagraph, to be launched in early 2024. It will provide routine imaging at 15 s cadence in forbidden lines of Fe XIV 530.5 nm, D3 of He I 587.7 nm and continuum near 545 nm with a square FOV covering 1.1 to 3 Rs at 2.81” resolution (Shestov et al., 2021).

Within the NIR, the Airborne Infrared Spectrometer (AIR-Spec), a next-generation airborne spectrometer, performed novel measurements during two eclipses, in 2017 and 2019 (cf. Samra et al., 2022). A follow-up instrument, the Airborne Coronal Emission Surveyor (ACES: Samra et al., 2021), will survey for the first time the entire NIR spectral region in early 2024 during the total eclipse. The Coronal Spectropolarimeter for Airborne Infrared Research (CORSAIR), funded by the NSF, is being built and will be tested on a balloon flight in 2024. It will then perform ground breaking measurements of the MC with its 64 long slits and large FOV with a one-month Antarctic circumpolar flight. In addition, the inner corona is also being routinely observed in the visible and NIR with DKIST (in particular with Cryo-NIRSP) and the Upgraded Coronal Multi-channel Polarimeter (UCoMP) instrument.

To make progress in our understanding of the MC, a careful combination of key observations and modelling needs to be coordinated. As this requires specific expertise found in several different countries, an ISSI team seems the obvious and timely starting point. We are aware of several ISSI teams for studies of the inner solar corona and the solar wind, but none that have addressed the issues discussed here.

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