The Sun’s dynamic extended corona observed in extreme ultraviolet

Abstract

The "middle corona" is a critical transition between the highly disparate physical regimes of the lower and outer solar corona. Nonetheless, it remains poorly understood due to the difficulty of observing this faint region (1.5-3 R☉). New observations from the GOES Solar Ultraviolet Imager in August and September 2018 provide the first comprehensive look at this region's characteristics and long-term evolution in extreme ultraviolet (EUV). Our analysis shows that the dominant emission mechanism here is resonant scattering rather than collisional excitation, consistent with recent model predictions. Our observations highlight that solar wind structures in the heliosphere originate from complex dynamics manifesting in the middle corona that do not occur at lower heights. These data emphasize that low-coronal phenomena can be strongly influenced by inflows from above, not only by photospheric motion, a factor largely overlooked in current models of coronal evolution. This study reveals the full kinematic profile of the initiation of several coronal mass ejections, filling a crucial observational gap that has hindered understanding of the origins of solar eruptions. These new data uniquely demonstrate how EUV observations of the middle corona provide strong new constraints on models seeking to unify the corona and heliosphere. Our EUV Observations and the Middle CoronaThe solar corona is the primary driver of almost all plasma dynamics throughout the solar system 1 . However, the precise nature of the connection between the corona and the heliosphere remains surprisingly poorly understood 2 . Recent solar and heliospheric observations taken by Parker Solar Probe, well within Mercury's orbit, revealed a highly structured environment shaped by flows and ejecta interacting with the corona's complex magnetic field 3,4,5,6 . The influence of these flows on the heliosphere and structural evolution

A Review of the Extended EUV Corona Observed by the Sun Watcher with Active Pixels and Image Processing (SWAP) Instrument

“…The misapprehension that EUV emission in the middle corona is too weak to observe with current instrumentation is discussed by Seaton et al (2021), but it stems from a lack of direct measurements of the region. It is discussed by Del Zanna et al ( 2018) that the EUV intensity of the solar disk is comparable with that of the inner corona observed off-limb, but the behavior of the coronal lines up to about 3 R was not known until their study.…”

“…The median-stacked image is composed of 34 individual exposures obtained over the 100-minute image aggregation window. Both images have been processed with an azimuthally varying radial-normalizing filter (see the methods section in Seaton et al, 2021), developed specifically for these SWAP data products, which helps compensate for the large radial gradient from the solar limb to the edge of the FOV to reveal coherent structures across the entire image. When stacking (summing) images to enhance signal, fast-moving structures become smeared as they are recorded at different positions in successive images.…”

Direct Measurement of AIA 171 Coronal Loop Transparency

“…Generally, this is a reasonable assumption. However, some studies have pointed out that the situation would be different for strong coronal lines (Schrijver et al 1994;Wood & Raymond 2000;Seaton et al 2021). In contrast, those studies suggest that coronal loops tend to have significant scattering for some typical strong coronal lines.…”

“…This is because the accurate transparency or opacity of the corona is not readily available. There are only a few indirect methods (Schrijver & McMullen 2000; Seaton et al 2021). How transparent the coronal loop is has not been directly answered yet.…”

The GOES‐R Solar UltraViolet Imager

“…A 2018 SUVI Extended Coronal Imaging (ECI) test campaign (Tadikonda et al., 2019) demonstrated a proof‐of‐concept observation strategy with SUVI, and subsequent campaigns in 2018 and 2019 (Seaton et al., 2021) fully characterized the EUV corona to heights above 4 R ⊙ over timescales of one to several solar rotations. These campaigns explored different strategies to optimize cadence, resolution, passband, and signal‐to‐noise in these observations, and ultimately selected a three‐panel mosaic approach using SUVI's 171 and 195 Å passbands.…”