Science

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.

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⨀.