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The middle corona is a region of significant transitions in the corona, including changes in magnetic topology and plasma properties, between 1.5 and 6 R⨀ (West et al. 2022). The 1.5–3 R⨀ region, in particular, is dominated by complex dynamics where the magnetic field transitions from mostly closed loops to mostly open, radial structures, in the vicinity of the streamer cusps, strongly modulating outflow into the solar wind (Seaton et al. 2021). This region sets the connectivity between surface magnetic structures and the solar wind, but the transition between these two magnetic morphology regimes is not well understood. The middle corona is difficult to observe because it is too faint for ground-based coronagraphs, and the required resolution is too fine for existing space-based instruments to trace detailed connectivity through the cusp region. The high-resolution polarized optics planned for CATE 2024 permit 3D rendering and disambiguation of the fine structure observed by CATE 2017 and other projects (e.g., DeForest et al. 2013, 2017, 2022), along with direct measurements of electron density (Sivaraman et al. 1984; Habbal et al. 2011). Achieving this objective requires an equatorial field of view (FOV) extent of at least ±2.5 R⨀, at a spatial resolution of 5″ to separate threadlike structures, and 2–4% level polarimetry of coronal features.

 

Fig. 3. CATE-2024 will see significant evolution in the middle corona, illustrated by these processed frames of the outer corona from STEREO/COR2 (DeForest et al. 2018). The inner black annulus highlights the gap in prior and current observations that CATE-2024 will fill. Coronal flows are traced by a “riotous torrent” of inhomogeneities, which CATE tracks through the middle corona during a single 1-hr extended-eclipse observation. Compared to COR2, CATE has finer resolution and a more inhomogeneous target, indicating the method is feasible.
Fig. 3. CATE-2024 will see significant evolution in the middle corona, illustrated by these processed frames of the
outer corona from STEREO/COR2 (DeForest et al. 2018). The inner black annulus highlights the gap in prior and
current observations that CATE-2024 will fill. Coronal flows are traced by a “riotous torrent” of inhomogeneities, which CATE tracks through the middle corona during a
single 1-hr extended-eclipse observation. Compared to COR2, CATE has finer resolution and a more inhomogeneous
target, indicating the method is feasible.

The association of slow solar wind flow with particular features in the low corona is a long outstanding open question and distinguishes between major classes of currently-open solar wind models. The solar wind is highly inhomogeneous and can be tracked visually throughout the corona over time (e.g., Fig. 3; DeForest et al. 2018). Typical eclipse measurements at one fixed location last only 2–4 minutes, making them essentially still frames with such a short interval compared to typical coronal evolution time scales. CATE 2024 offers over 60 consecutive minutes of observations, a sufficient duration to distinguish where the solar wind is flowing in the lower and middle corona. Solar wind speeds at 2 R⨀ are typically ~50–150 km/s (e.g., DeForest et al. 1997; Kohl et al. 1997). During the ~1-hour observation, density features in the middle corona will move 4′–18′, easily observable at the CATE resolution, yielding a unique measurement of solar wind flow. CATE 2024 will thereby distinguish between models that include release or diffusion through the closed-field region, from those requiring release at coronal hole edges. This objective requires an equatorial FOV extent of ±3 R⨀, spatial resolution of 30″, and duration of at least 0.5 hours. Polarimetry at the 5% level is required to separate the 3-D location of outbound features, particularly at altitudes above 1.5 R⨀.

 

Fig. 4. Morphological changes associated with non-flaring loop reconnection in the corona are accessible to CATE-2024 on few-minute timescales. This X-point and current sheet observed with SDO/AIA (Srivastava et al. 2019) developed within just 20 minutes. CATE-2024 will search for hypothesized smaller-scale events throughout the structure of the low corona.
Fig. 4. Morphological changes associated with non-flaring loop reconnection in the corona are accessible to CATE-2024 on few-minute timescales. This X-point and current sheet observed with SDO/AIA (Srivastava et al. 2019) developed within just 20 minutes. CATE-2024 will search for hypothesized smaller-scale events throughout the structure of the low corona.
Magnetic reconnection in the low to middle corona is essential to maintaining the smooth “combed” appearance of the corona as a whole and to nearly all coronal heating and wind origination models. This inferred non-flare reconnection in the “bulk quiet corona” has historically been invisible to most instruments. However, it has been detected with EUV imaging of bright, well-presented loops (e.g., Li et al. 2018; Srivastava et al. 2019) and is associated with changing morphology over minutes to hours (Fig. 4). Observations of in-progress reconnection in EUV are rare partly because of the low signal-to-noise ratios of existing EUV images and partly because of the narrow temperature sensitivity of collisionally excited (or resonantly scattered) emission lines. Thomson-scattered light offers a broad thermal response and direct sensitivity to line-of-sight density; this is reflected in the rich loop detail available in even moderately enhanced images (e.g., Fig. 1). Conventional
eclipse observations do not span sufficient time to capture changing coronal  topology, but the extended observation with CATE 2024 does. Achieving this objective requires an equatorial FOV extent of ±2 R⨀, a spatial resolution of 10″ to distinguish changing topology, sensitivity of 1% of coronal brightness, and duration of one hour.