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1. Improving Coronal Hole Detections and Open Flux Estimates

   https://doi.org/10.3847/1538-4357/ad01b6  

   Caplan, Ronald M; Mason, Emily I; Downs, Cooper; Linker, Jon A (November 2023, The Astrophysical Journal)

   Abstract One systematic limitation of solar coronal hole (CH) detection at extreme ultraviolet (EUV) wavelengths is the obscuration of dark regions of the corona by brighter structures along the line of sight. Another problem arises when using CHs to compute the Sun’s open magnetic flux, where surface measurements of the radial magnetic field, $B_r^{\odot }$, are situated slightly below the effective height of coronal EUV emission. In this paper, we explore these two limitations utilizing a thermodynamic magnetohydrodynamic (MHD) model of the corona for Carrington rotation (CR) 2101, where we generate CH detections from EUV 193 Å images of the corona forward-modeled from the MHD solution, and where the modeled open field is known. We demonstrate a method to combine EUV images into a full Sun map that helps alleviate CH obscuration called theminimum intensity diskmerge(MIDM). We also show the variation in measured open flux and CH area that is due to the effective height differences between EUV and $B_r^{\odot }$measurements. We then apply the MIDM method to SDO/AIA 193 Å observations from CR 2101, and conduct an analogous analysis. In this case, the MIDM method uses time-varying images, the effects of which are discussed. We show that overall, the MIDM method and an appreciation of the effective height mismatch provide a useful new way to extract a broader view of CHs, especially near the poles. In turn, they enable improved estimates of the open magnetic flux, and help facilitate comparisons between models and observations. 

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2. High-resolution Observation and Magnetic Modeling of a Solar Minifilament: The Formation, Eruption, and Failing Mechanisms

   https://doi.org/10.3847/1538-4357/ad50d0  

   Teng, Weilin; Su, Yingna; Liu, Rui; Chen, Jialin; Liu, Yanjie; Dai, Jun; Cao, Wenda; Shen, Jinhua; Ji, Haisheng (July 2024, The Astrophysical Journal)

   Abstract Minifilaments are widespread small-scale structures in the solar atmosphere. To better understand their formation and eruption mechanisms, we investigate the entire life of a sigmoidal minifilament located below a large quiescent filament observed by Big Bear Solar Observatory/Goode Solar Telescope on 2015 August 3. The Hαstructure initially appears as a group of arched threads, then transforms into two J-shaped arcades, and finally forms a sigmoidal shape. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly observations in 171 Å show that two coronal jets occur around the southern footpoint of the minifilament before the minifilament eruption. The minifilament eruption starts from the southern footpoint, then interacts with the overlying filament and fails. The aforementioned observational changes correspond to three episodes of flux cancellations observed by SDO/Helioseismic and Magnetic Imager. Unlike previous studies, the flux cancellation occurs between the polarity where the southern footpoint of the minifilament is rooted and an external polarity. We construct two magnetic field models before the eruption using the flux rope insertion method and find a hyperbolic flux tube above the flux cancellation site. The observation and modeling results suggest that the eruption is triggered by the external magnetic reconnection between the core field of the minifilament and the external fields due to flux cancellations. This study reveals a new triggering mechanism for minifilament eruptions and a new relationship between minifilament eruptions and coronal jets. 

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3. Coronal Hole Detection and Open Magnetic Flux

   https://doi.org/10.3847/1538-4357/ac090a  

   Linker, Jon A.; Heinemann, Stephan G.; Temmer, Manuela; Owens, Mathew J.; Caplan, Ronald M.; Arge, Charles N.; Asvestari, Eleanna; Delouille, Veronique; Downs, Cooper; Hofmeister, Stefan J.; et al (August 2021, The Astrophysical Journal)

   Abstract Many scientists use coronal hole (CH) detections to infer open magnetic flux. Detection techniques differ in the areas that they assign as open, and may obtain different values for the open magnetic flux. We characterize the uncertainties of these methods, by applying six different detection methods to deduce the area and open flux of a near-disk center CH observed on 2010 September 19, and applying a single method to five different EUV filtergrams for this CH. Open flux was calculated using five different magnetic maps. The standard deviation (interpreted as the uncertainty) in the open flux estimate for this CH ≈ 26%. However, including the variability of different magnetic data sources, this uncertainty almost doubles to 45%. We use two of the methods to characterize the area and open flux for all CHs in this time period. We find that the open flux is greatly underestimated compared to values inferred from in situ measurements (by 2.2–4 times). We also test our detection techniques on simulated emission images from a thermodynamic MHD model of the solar corona. We find that the methods overestimate the area and open flux in the simulated CH, but the average error in the flux is only about 7%. The full-Sun detections on the simulated corona underestimate the model open flux, but by factors well below what is needed to account for the missing flux in the observations. Under-detection of open flux in coronal holes likely contributes to the recognized deficit in solar open flux, but is unlikely to resolve it. 

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4. Fine Structures of Tiny Quiet Sun Jets Observed by Solar Orbiter and Big Bear Solar Observatory

   https://doi.org/10.3847/2041-8213/ae0df2  

   Lee, Jeongwoo; Longcope, Dana; Youn, Junmu; Panesar, Navdeep K; Huang, Nengyi; Wang, Haimin (October 2025, The Astrophysical Journal Letters)

   Abstract We present the first joint high-resolution observations of small-scale EUV jets using Solar Orbiter (SolO)’s Extreme Ultraviolet Imager and High Resolution Imager (EUI/HRI_EUV) and Hαimaging from the Visible Imaging Spectrometer installed on the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. These jets occurred on 2022 October 29 around 19:10 UT in a quiet Sun region, and their main axis aligns with the overarching magnetic structure traced by a cluster of spicules. However, they develop a helical morphology, while the Hαspicules maintain straight, linear trajectories elsewhere. Alongside the spicules, thin, elongated red- and blueshifted Hαfeatures appear to envelope the EUV jets, which we tentatively call sheath flows. The EUI jet moving upward at a speed of ∼110 km s⁻¹is joined by a strong Hαredshift at ∼20 km s⁻¹to form bidirectional outflows lasting ∼2 minutes. Using AI-assisted differential emission measure analysis of SolO’s Full Sun Imager, we derived total energy of the EUV jet as ∼1.9 × 10²⁶erg with 87% in thermal energy and 13% in kinetic energy. The parameters and morphology of this small-scale EUV jet are interpreted based on a thin flux tube model that predicts Alfvénic waves driven by impulsive interchange reconnection localized as narrowly as ∼1.6 Mm with a magnetic flux of ∼5.4 × 10¹⁷Mx, belonging to the smallest magnetic features in the quiet Sun. This detection of intricate corona–chromospheric coupling highlights the power of high-resolution imaging in unraveling the mechanisms behind small-scale solar ejections across atmospheric layers. 

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5. Solar Eruptions in Nested Magnetic Flux Systems

   https://doi.org/10.3847/1538-4357/ad2eaa  

   Karpen, Judith T; Kumar, Pankaj; Wyper, Peter F; DeVore, C Richard; Antiochos, Spiro K (April 2024, The Astrophysical Journal)

   Abstract The magnetic topology of erupting regions on the Sun is a key factor in the energy buildup and release, and the subsequent evolution of flares and coronal mass ejections (CMEs). The presence/absence of null points and separatrices dictates whether and where current sheets form and magnetic reconnection occurs. Numerical simulations show that energy buildup and release via reconnection in the simplest configuration with a null, the embedded bipole, is a universal mechanism for solar eruptions. Here we demonstrate that a magnetic topology with nested bipoles and two nulls can account for more complex dynamics, such as failed eruptions and CME–jet interactions. We investigate the stalled eruption of a nested configuration on 2013 July 13 in NOAA Active Region 11791, in which a small bipole is embedded within a large transequatorial pseudo-streamer containing a null. In the studied event, the inner active region erupted, ejecting a small flux rope behind a shock accompanied by a flare; the flux rope then reconnected with pseudo-streamer flux and, rather than escaping intact, mainly distorted the pseudo-streamer null into a current sheet. EUV and coronagraph images revealed a weak shock and a faint collimated outflow from the pseudo-streamer. We analyzed Solar Dynamics Observatory and Solar TErrestrial RElations Observatory observations and compared the inferred magnetic evolution and dynamics with three-dimensional magnetohydrodynamics simulations of a simplified representation of this nested fan-spine system. The results suggest that the difference between breakout reconnection at the inner null and at the outer null naturally accounts for the observed weak jet and stalled ejection. We discuss the general implications of our results for failed eruptions. 

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