Stephanie L. Yardley

The Sun continuously releases a stream of charged particles known as the solar wind, threaded by magnetic field lines that occasionally fold back on themselves in sharp reversals called magnetic switchbacks. These structures are frequently observed by spacecraft near the Sun, but their formation mechanism remains an open question. Using measurements from Solar Orbiter at 0.55 AU, we report heavy ion observations within a magnetic switchback that provide direct evidence of its solar source. Elevated charge-state ratios of oxygen and carbon, combined with a depletion of helium relative to protons, indicate that the switchback plasma originated from hot, closed coronal loop structures. A semi-empirical coronal model, constrained by electron temperature and density measurements from the Solar Dynamics Observatory, traces the ion signatures to either a coronal hole boundary or loops within the coronal hole, implicating the source mechanism as interchange reconnection, where closed loops reconnect with open magnetic field and eject plasma. The proton velocity and magnetic field fluctuations of the switchback are in accordance with an outward-propagating Alfvén wave. These findings suggest that interchange reconnection and wave or turbulence mechanisms jointly govern switchback origin and propagation, respectively. Most importantly, we show that switchbacks preserve measurable signatures, carrying information about the physical processes that heat the solar atmosphere and accelerate the solar wind.

Context. Fast and slow solar wind have distinct properties linked to their solar sources. The Alfvénic slow wind complicates the usual classification by solar-wind speed, especially at intermediate speeds. The Solar Wind Analyzer (SWA) on Solar Orbiter offers unique capabilities to investigate how Alfvénic slow wind differs from classical fast wind, and relates these differences to their solar origins. Aims. In September 2022, Solar Orbiter moved inbound toward the Sun from 0.59 to 0.38 AU, and observed several Alfvénic streams: one fast wind (F), three Alfvénic slow wind intervals (AS1, AS2, AS3), and a moderate fast wind (MF); a non-Alfvénic slow wind (S) interval was included for comparison. We analyzed these streams to highlight their similarities and differences. Methods. We combined plasma parameters from all SWA sensors with magnetic field measurements from the magnetometer (MAG). A spectral analysis of magnetic and velocity fluctuations was used to characterize the Alfvénicity. The magnetic connectivity of each stream to its solar source was examined using potential field source surface extrapolation combined with ballistic backmapping from the spacecraft. Results. Proton velocity distribution functions exhibit anisotropies and field-aligned beams typical of Alfvénic streams, while electron strahl populations become progressively broader and weaker from fast to non-Alfvénic slow wind. Heavy-ion composition shows a similarly ordered transition, where the Alfvénic slow wind displays intermediate charge-state ratios between fast and non-Alfvénic slow wind. Magnetic connectivity analysis associates the fast wind with a large coronal hole, whereas the Alfvénic slow wind intervals connect to open-field regions near pseudostreamers and overexpanded coronal-hole boundaries. Spectral analysis indicates that most Alfvénic streams remain close to energy equipartition despite their different heliocentric distances and bulk speeds. Altogether, these results support the interpretation of Alfvénic slow wind as the low-speed extension of coronal-hole-associated Alfvénic plasma undergoing a distinct acceleration and transport history. Conclusions. The combined SWA observations offer key insights into how turbulence, particle transport, and coronal source conditions jointly shape the evolution of Alfvénic solar wind streams, revealing a coherent transition between fast, Alfvénic slow, and non-Alfvénic slow wind regimes.

Upflows at coronal temperatures are typically present at the edges of active regions, where plasma escapes into the upper atmosphere. This study focuses on NOAA Active Regions 12665 and 12685 which are in different evolutionary stages, they display varying activity levels, and have a large difference in their magnetic field structure. Our targets were followed by Hinode/EIS as the ARs crossed the solar disk, and we used Fe XII maps to track their evolution. We separated time periods with nearly stationary flows from the non-stationary ones, in order to isolate the effects of solar rotation and activity on both the line-of-sight and non-thermal velocities. We confirmed the presence of a strong positive correlation between the Doppler and non-thermal velocity during both time periods and with different levels of activity. These results help us differentiate the physical origin of non-thermal velocities, such as Alfvén waves, MHD turbulence, magnetic reconnection, and multi-stream components. Our results favour the latter.

Elemental abundances provide a powerful diagnostic of the physical mechanisms and processes that heat the solar atmosphere and drive the solar wind. The first ionization potential (FIP) effect and its inverse (IFIP) are observed both on the Sun and other stars; however, the underlying fractionation mechanisms, their dependence on the magnetic field topology and the role of wave dynamics and turbulence in the chromosphere are not entirely understood. To address these challenges, a focused team, including observers, theorists, modellers and instrument scientists, spanning a range of career stages and institutions, came together for the Royal Society Theo Murphy meeting ‘Solar atmospheric abundances in space and time’. As a result of this meeting, the team worked in collaboration to produce 16 publications for this special issue. These publications are introduced here, including a discussion of the open questions and future directions in the context of advances in numerical modelling and current and upcoming solar and stellar missions. This article is part of the Theo Murphy meeting issue ‘Solar atmospheric abundances in space and time’.

Wave–plasma interactions and energy transport are fundamental processes in stellar atmospheres, shaping elemental composition through the first ionization potential (FIP) and inverse FIP (IFIP) effects. Although stellar measurements provide global evidence of abundance anomalies, the Sun offers a unique local laboratory in which to resolve how these processes operate on small spatial and temporal scales. In this overview, we summarize the current state of knowledge of the IFIP effect, its observational signatures and the theoretical framework that underpins it. We then present new insights from a detailed case study of active region (AR) NOAA AR 11967, where combined Hinode/extreme-ultraviolet imaging spectrometer (EIS), IRIS, SDO/AIA and Fermi/GBM observations and IRIS2+ inversions indicate that torsional Alfvén waves generated below the chromospheric fractionation region can account for the highly localized IFIP effect plasma observed on the Sun. This analysis highlights the possible role of sub-chromospheric processes in the formation of the IFIP effect on the Sun and informs pathways for understanding similar mechanisms in the coronae of active M dwarfs. This article is part of the Theo Murphy meeting issue ‘Solar atmospheric abundances in space and time’.

Coronal hole boundaries are the interfaces between closed and open magnetic field regions in the solar atmosphere. Many fundamental processes take place at these regions, including magnetic reconnection that is responsible for solar wind release and restructuring of the solar magnetic field. In this paper, we present a case study in which we investigate the physical properties of the boundary of a large low-latitude coronal hole. Differential Emission Measure analysis is used to derive the plasma properties of these regions. We also apply correlation dimension mapping analysis to measure the irregularities of the coronal hole boundary. We find that the leading boundary of this coronal hole has a slightly higher average plasma temperature, is associated with a stronger and more unipolar magnetic field, and has a smoother boundary line than the trailing counterpart. These differences are hypothesised to be direct consequences of the local magnetic field configurations of the coronal hole boundary: the leading boundary corresponds to large, well-organised coronal loops, and the trailing boundary corresponds to more dispersed, randomly orientated small magnetic bipoles. Hence, we suggest that the surrounding magnetic field structure and the nature of magnetic reconnection influence the properties of coronal hole boundaries.

Alfvénic fluctuations are a ubiquitous, particularly in fast streams, whereas the slow wind is typically characterized by reduced Alfvénicity and enhanced variability. However, the slow wind can display strongly Alfvénic behavior as well, with fluctuation properties comparable to those of fast streams, challenging the traditional fast–slow wind dichotomy.In this study, we perform a comparative analysis of fast solar wind and Alfvénic slow wind during the October 2022 perihelion. In particular, we investigate the solar source and the turbulent properties of the different solar wind regimes, using plasma and magnetic field measurements from the Solar Wind Analyser (SWA) and Magnetometer (MAG) instruments onboard Solar Orbiter. We further investigate possible connections between large-scale turbulence properties and small-scale dissipation by examining the relationship between inertial-range fluctuations and magnetic-field polarization at ion scales across the spectral break. By combining in situ observations with remote-sensing data and two-step ballistic backmapping, we show that Solar Orbiter was magnetically connected to the coronal hole has a bright structure within it, indicating that the observed solar wind variability is driven by spatio-temporal changes in magnetic connectivity to coronal source. Our results show that Alfvénic slow-wind interval preserve a high degree of Alfvénicity, as evidenced by large normalized cross helicity, near kinetic–magnetic energy equipartition, low magnetic compressibility, and large-amplitude magnetic and velocity fluctuations comparable to those observed in fast Alfvénic streams, despite their lower bulk speeds and higher Coulomb collisional age. These findings pose significant challenges for solar-wind models, which must account for the persistence of strong Alfvénic turbulence in slow wind originating from nearby and evolving coronal source regions while exhibiting markedly different bulk plasma properties.

Earth is surrounded by two highly dynamic concentric belts of particle radiation. The outer radiation belt exhibits coherent variability at solar-cycle, seasonal, and 27-day (Carrington) time scales. While the solar-cycle and Carrington variations have been attributed to the recurrence of coronal hole solar wind, the seasonal variation has long been explained by local geometric effects that modulate the solar wind-magnetosphere coupling. Here, we challenge this paradigm by showing that the periodic recurrence of coronal hole solar wind likewise drives the seasonal variation. We further demonstrate that the Alfvénic nature of this solar wind is responsible for the observed electron flux enhancement in the outer radiation belt. These findings provide a unifying framework linking solar magnetic topology, solar wind properties, and magnetospheric dynamics across multiple time scales at Earth and beyond.

Whether solar wind electrons expanding into the heliosphere can preserve information about their origin in the solar corona remains an open debate. The suprathermal strahl temperature has often been postulated as an indicator of source coronal electron temperature, while the core electron temperature has also been found to correlate with the solar wind velocity in the inner heliosphere. Here we investigate how well solar wind electron populations retain imprints of coronal electron temperature. Using Solar Orbiter measurements at ∼0.5 au, we fit three components (i.e., the core, halo, and strahl) of the electron velocity distribution function and compare the resulting strahl and core temperatures with heavy-ion charge-state ratios, which serve as proxies for the coronal electron temperature. We present the first clear evidence from inner heliosphere observations that, in several individual streams, a proxy for the strahl parallel temperature, T _strahl,∥ , correlates significantly and positively with the charge-state ratios O ⁷⁺ /O ⁶⁺ and C ⁶⁺ /C ⁵⁺ . However, this correlation is not universally present, implying that many electron streams are significantly affected by transport processes, such as scattering, that erase the signature. We find that, notably, the core perpendicular temperature ( T core , ⊥ ) also strongly correlates with the charge-state ratios. We interpret this result within the framework of the exospheric solar wind model. Our results suggest that both thermal and suprathermal electrons can at times retain coronal information, but that aggregating multiple streams can obscure the underlying relationships.

The unpredictability of solar filament eruptions presents major challenges for forecasting space weather, since such eruptions frequently drive coronal mass ejections that impact the heliosphere. While nearby flux emergence is often linked to their destabilization, the specific characteristics of both the emerging flux and the filament that determine whether an eruption occurs remain unclear. We report observations of a quiescent filament that did not erupt following the nearby emergence of active region NOAA 13270 and a subsequent C-class flare in 2023 April. Our analysis combines multiviewpoint extreme ultraviolet (EUV) imaging and X-ray imaging with EUV spectroscopy, radio imaging, and measurements of, and extrapolations from, the photospheric magnetic field. We identify the formation of a coronal null point and fan-spine topology at the interface between the active region and filament which exhibited persistent slow reconnection, indicated by chromospheric brightenings, persistent radio emission, and plasma upflows. Our results indicate that ongoing reconnection and jets can relieve magnetic stress and enable filament stability, even when under strong perturbation. We suggest that the orientation of emerging flux relative to the ambient field is a critical parameter in filament evolution and we provide observational constraints for models of filament stability and eruption.