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Abstract In astronomical environments, the high-temperature emission of plasma mainly depends on ion charge states, requiring accurate analysis of the ionization and recombination processes. For various phenomena involving energetic particles, non-Maxwellian distributions of electrons exhibiting high-energy tails can significantly enhance the ionization process. Therefore, accurately computing ionization and recombination rates with non-Maxwellian electron distributions is essential for emission diagnostic analysis. In this work, we report two methods for fitting various non-Maxwellian distributions by using the Maxwellian decomposition strategy. For standardκ-distributions, the calculated ionization and recombination rate coefficients show comparable accuracy to other public packages. Additionally, our methods support arbitrary electron distributions and can be easily extended to updated atomic databases. We apply the above methods to two specific non-Maxwellian distribution scenarios: (i) accelerated electron distributions due to magnetic reconnection revealed in a combined MHD–particle simulation; and (ii) the high-energy truncatedκ-distribution predicted by the exospheric model of the solar wind. During the electron acceleration process, we show that the ionization rates of high-temperature iron ions increase significantly compared to their initial Maxwellian distribution, while the recombination rates may decrease due to the electron distribution changes in low-energy ranges. This can potentially lead to an overestimation of the plasma temperature when analyzing the Fe emission lines under the Maxwellian distribution assumption. For the truncatedκ-distribution in the solar wind, our results show that the ionization rates are lower than those for the standardκ-distribution, while the recombination rates remain similar. This leads to an overestimation of the plasma temperature when assuming aκ-distribution. 

Abstract Using optical and near-infrared images of the Cassiopeia A (Cas A) supernova remnant covering the time period 1951–2022, together with optical spectra of selected filaments, we present an investigation of Cas A’s reverse shock velocity and the effects it has on the remnant’s metal-rich ejecta. We find the sequence of optical ejecta brightening and the appearance of new optical ejecta indicating the advancement of the remnant’s reverse shock in the remnant’s main shell has velocities typically between 1000 and 2000 km s⁻¹, which is ∼1000 km s⁻¹less than recent measurements made in X-rays. We further find that the reverse shock appears to move much more slowly and is nearly even stationary in the sky frame along the remnant’s western limb. However, we do not find the reverse shock to move inward at velocities as large as ∼2000 km s⁻¹as has been reported. Optical ejecta in Cas A’s main emission shell have proper motions indicating outward tangential motions ≃3500–6000 km s⁻¹, with the smaller values preferentially along the remnant’s southern regions, which we speculate may be partially the cause of the remnant’s faint and more slowly evolving southern sections. Following interaction with the reverse shock, ejecta knots exhibit extended mass ablated trails $0\stackrel{″}{.}2$– $0\stackrel{″}{.}5$in length, leading to extended emission indicating reverse shock induced decelerated velocities as large as ≃1000 km s⁻¹. Such ablated material is most prominently seen in higher ionization line emissions, whereas denser parts of ejecta knots show surprisingly little deceleration. 

Abstract JWST observations of the young Galactic supernova remnant Cassiopeia A revealed an unexpected structure seen as a green emission feature in colored composite MIRI F1130W and F1280W images—hence dubbed the Green Monster—that stretches across the central parts of the remnant in projection. Combining the kinematic information from NIRSpec and the MIRI Medium Resolution Spectrograph with the multiwavelength imaging from NIRCam and MIRI, we associate the Green Monster with circumstellar material (CSM) that was lost during an asymmetric mass-loss phase. MIRI images are dominated by dust emission, but their spectra show emission lines from Ne, H, and Fe with low radial velocities indicative of a CSM nature. An X-ray analysis of this feature in a companion paper supports its CSM nature and detects significant blueshifting, thereby placing the Green Monster on the nearside, in front of the Cas A supernova remnant. The most striking features of the Green Monster are dozens of almost perfectly circular 1″–3″ sized holes, most likely created by interaction between high-velocity supernova ejecta material and the CSM. Further investigation is needed to understand whether these holes were formed by small 8000–10,500 km s⁻¹N-rich ejecta knots that penetrated and advanced out ahead of the remnant’s 5000–6000 km s⁻¹outer blast wave or by narrow ejecta fingers that protrude into the forward-shocked CSM. The detection of the Green Monster provides further evidence of the highly asymmetric mass loss that Cas A’s progenitor star underwent prior to its explosion. 

Heavy ion signatures of coronal mass ejections (CMEs) indicate that rapid and strong heating takes place during the eruption and early stages of propagation. However, the nature of the heating that produces the highly ionized charge states often observed in situ is not fully constrained. An MHD simulation of the Bastille Day CME serves as a test bed to examine the origin and conditions of the formation of heavy ions evolving within the CME in connection with those observed during its passage at L1. In particular, we investigate the bimodal nature of the Fe charge state distribution, which is a quintessential heavy ion signature of CME substructure, as well as the source of the highly ionized plasma. We find that the main heating experienced by the tracked plasma structures linked to the ion signatures examined is due to field-aligned thermal conduction via shocked plasma at the CME front. Moreover, the bimodal Fe distributions can be generated through significant heating and rapid cooling of prominence material. However, although significant heating was achieved, the highest ionization stages of Fe ions observed in situ were not reproduced. In addition, the carbon and oxygen charge state distributions were not well replicated owing to anomalous heavy ion dropouts observed throughout the ejecta. Overall, the results indicate that additional ionization is needed to match observation. An important driver of ionization could come from suprathermal electrons, such as those produced via Fermi acceleration during reconnection, suggesting that the process is critical to the development and extended heating of extreme CME eruptions, like the Bastille Day CME. 

Abstract Nonequilibrium ionization (NEI) is essentially required for astrophysical plasma diagnostics once the plasma status departs from the assumption of ionization equilibrium. In this work, we perform fast NEI calculations combined with magnetohydrodynamic (MHD) simulations and analyze the ionization properties of a Petschek-type magnetic reconnection current sheet during solar eruptions. Our simulation reveals Petschek-type slow-mode shocks in the classical Spitzer thermal conduction models and conduction flux-limitation situations. The results show that under-ionized features can be commonly found in shocked reconnection outflows and thermal halo regions outside the shocks. The departure from equilibrium ionization strongly depends on plasma density. In addition, this departure is sensitive to the observable target temperature: the high-temperature iron ions are strongly affected by the effects of NEI. The under-ionization also affects the synthetic SDO/AIA intensities, which indicates that the reconstructed hot reconnection current sheet structure may be significantly underestimated either for temperature or apparent width. We also perform an MHD-NEI analysis on the reconnection current sheet in the classical solar flare geometry. Finally, we show the potential reversal between the under-ionized and over-ionized states at the lower tip of reconnection current sheets where the downward outflow collides with closed magnetic loops, which can strongly affect multiple SDO/AIA band ratios along the reconnection current sheet. 

Community honours, such as those bestowed by professional scientific societies like the American Geophysical Union (AGU) are an important element of both individual career advancement and contributes to the historical record of scientific progress. The process by which honours are bestowed is not widely shared amongst the community. The purpose of this article is to share the recent experiences of several members of the AGU Space Physics and Aeronomy (SPA) Fellows committee. We outline the criteria for selection, the evaluation process, difficulties encountered by the committee, and steps taken to mitigate these difficulties. Of particular note is the impact of implicit bias in the award system. Steps could be taken by the awarding scientific societies to reduce the impact of these biases, but in the meantime individual award committees can employ some of the strategies we outline in this article. By sharing our experiences, we hope to improve the process of granting awards and honours for the scientists putting together award nominations, future committee members, and the scientific societies granting these awards. 

This paper outlines key scientific topics that are important for the development of solar system physics and how observations of heavy ion composition can address them. The key objectives include, 1) understanding the Sun’s chemical composition by identifying specific mechanisms driving elemental variation in the corona. 2) Disentangling the solar wind birthplace and drivers of release by determining the relative contributions of active regions (ARs), quiet Sun, and coronal hole plasma to the solar wind. 3) Determining the principal mechanisms driving solar wind evolution from the Sun by identifying the importance and interplay of reconnection, waves, and/or turbulence in driving the extended acceleration and heating of solar wind and transient plasma. The paper recommends complementary heavy ion measurements that can be traced from the Sun to the heliosphere to properly connect and study these regions to address these topics. The careful determination of heavy ion and elemental composition of several particle populations, matched at the Sun and in the heliosphere, will permit for a comprehensive examination of fractionation processes, wave-particle interactions, coronal heating, and solar wind release and energization that are key to understanding how the Sun forms and influences the heliosphere. 

The middle corona, the region roughly spanning heliocentric distances from 1.5 to 6 solar radii, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. The solar wind, eruptions, and flows pass through the region, and they are shaped by it. Importantly, the region also modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, the middle corona is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the region has been poorly studied by both major solar remote-sensing and in-situ missions and instruments, extending back to the Solar and Heliospheric Observatory/(SOHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions that it covers, and the underlying physics believed to shape the region. This article aims to define the middle corona, its physical characteristics, and give an overview of the processes that occur there.