Emily I. Mason

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.

We apply the slip-back mapping method of V. S. Titov et al. and R. Lionello et al. to a thermodynamic MHD simulation to track topological changes in the magnetic field at a range of temporal cadences. The method constitutes the logical successor to a simple open-field map for a steady-state model, as it tracks changes in the open and closed fields for a time-dependent model by tracking individual magnetic elements as they advect across the map, rather than simply tracing field line connectivity from each cell. Through careful categorization of the slip-back mapping values and analysis of the flux changes, we not only effectively track the open flux but can recover the flux processed through interchange reconnection as well. The field lines involved in these processes are shown to follow lines of high squashing factor, as proposed by interchange-reconnection-driven slow solar wind theory. The time-dependent model, which is scaled to solar-minimum-like activity, projects that a median value of 3.5% of the total open flux in any given 24 hr interval has been processed through interchange reconnection. This corresponds to a relatively high proportion of the total open flux changes over time in the heliosphere. Our results show that not only is this method a useful tool for accurately tracking topological changes in time-dependent simulations, but that its inherent complexity can be visually reduced into an intuitive 2D plot that simply and effectively communicates temporal changes.

Magnetic flux fills the heliosphere, expands outward from the solar corona, and is fundamentally related to the structure and dynamics of the solar corona and solar wind. Open magnetic flux and the fast wind are thought to originate from open magnetic field lines in coronal holes. Less understood processes in the streamer belt and the boundaries of coronal holes, associated with the more variable slow wind, may be formed by interchange reconnection between open and closed magnetic flux. Interchange reconnection is thought to give rise to field lines that are “folded,” i.e., that turn back on themselves. The properties of strahl electrons measured in the solar wind give clues to the heliospheric magnetic connectivity. Unidirectionally outward strahl indicates open field lines, while bidirectional strahl is associated with closed magnetic flux and coronal mass ejections (CMEs). Inward-directed, unidirectional strahl is believed to indicate folded flux. We use two time-dependent, flux-evolutionary magnetohydrodynamic (MHD) models of the combined corona and heliosphere, one for a solar-minimum configuration and one for the 2024 total solar eclipse, to investigate the magnetic connectivity of the corona/heliosphere system. We examine how magnetic connectivity varies with distance from the Sun in the two configurations. We evaluate the evolutionary effects by contrasting time-dependent results with the corresponding steady-state calculations and compare the model connectivities with statistical studies of strahl. The connectivities in the time-evolving simulations are roughly consistent with observed strahl occurrence rates, while those from the steady-state models are not. Our results suggest that complex magnetic connectivities are ubiquitous in the heliosphere.

Recent three-dimensional flare models suggest that flare ribbon substructure is linked to the fragmentation of the reconnecting current sheet in the corona. Flare ribbon substructure can therefore potentially serve as a unique diagnostic tool for physical processes in the flare current sheet. In this paper, we describe a new method to quantify the evolution of ribbon substructure that first extracts the ribbon’s bright leading edge and then quantifies its morphology using the box-counting dimension and correlation dimension mapping (CDM). We first test our method using synthetic observations. We then apply it to an M6.5-class solar flare on 2015 June 22 observed by the Interface Region Imaging Spectrograph (IRIS) 1330 Å slit-jaw imager. We find that when the flare ribbon boundary has more multiple-spatial-scale features (a higher box-counting dimension), hard X-ray emission and magnetic reconnection rates are the strongest. We also find that the flare ribbon complexity characterized by CDM has a moderate correlation with the IRIS Si IV 1402.77 Å nonthermal velocity (in the negative polarity ribbon) and reconnection flux rates (in ribbons of both magnetic polarities). We conclude that the buildup of the spatial complexity of the ribbons at multiple spatial scales can serve as an observational proxy for current-sheet fragmentation in the corona.

Large-scale solar eruptions, which are observed as flares, erupting prominences/filaments, and coronal mass ejections (CMEs), are powered by the free (excess) magnetic energy that is stored prior to eruption in current-carrying (sheared/twisted) magnetic fields. During an eruption, some of this magnetic energy is released and converted into kinetic energy in the form of thermal/non-thermal particle energy and bulk flow energy. It is well established that magnetic reconnection is the key driver of this energy release and conversion. However, the detailed physical conditions that determine the partitioning and distribution of the released energy are not yet well understood. Following the seminal work by Birn et al. (2009), we employ magnetohydrodynamic (MHD) simulations to study the energy conversion and transport due to reconnection in a flare current sheet, using an adiabatic energy equation. We extend the work by Birn et al. in three different ways. First, we consider a model in which the flare current layer is self-consistently formed by the eruption of a magnetic flux rope that evolves into a CME. Second, we incorporate the effect of reconnection between the legs of the flux rope. Third, we extend the analysis of the energy transport (and plasma heating) to the CME. In this presentation we summarize our main results and briefly discuss the next step, which will be the extension of our model to thermodynamic MHD.

Recent three-dimensional flare models suggest that flare-ribbon substructure is linked to the fragmentation of the reconnecting current sheet in the corona. Flare-ribbon substructure can therefore potentially serve as a unique diagnostic tool for physical processes in the flare current sheet. In this paper, we describe a new method to quantify the evolution of ribbon substructure, which first extract the ribbon's leading bright front and the quantifies its morphology using the box-counting dimension and Correlation Dimension Mapping (CDM). We first test our method using synthetic observations. We then find that when the flare ribbon boundary has more multi-spatial-scale features (higher box-counting dimension), hard X-ray (HXR) emission and magnetic reconnection rates are the strongest. We also find that the flare-ribbon complexity characterized by CDM has moderate correlation with the IRIS Si IV 1402.77 Å non-thermal velocity (in the negative-polarity ribbon) and reconnection flux rates (in ribbons of both magnetic polarities). We conclude that the build-up of the spatial complexity of the ribbons at multiple spatial scales can serve as an observational proxy for current-sheet fragmentation in the corona.

The study of solar active regions (ARs) is of central importance to a range of fundamental science, as well as the practical applications of space weather. AR emergence and life cycles are two areas of particular interest, yet the lack of consistent full-Sun observations has made long-term studies of ARs difficult. Here, we present results from a study to identify and characterize long-lived active regions (LLARs), defined as those which were observed during at least two consecutive Carrington rotations and that did not undergo significant successive flux emergence once the decay phase began. Such ARs accounted for 13% of all NOAA-identified ARs between 2011 and 2019, and their distribution closely follows the annual sunspot number. This implies that LLARs are produced by the same basic driving processes as regular ARs. LLAR areas tend to be significantly larger and contain more magnetic flux compared to other ARs, but the two categories have similar magnetic complexity distributions. The most striking result, however, is that LLARs are 3–6 times more likely than other ARs to be the source of a solar flare of GOES class C or greater. This highlights the importance of studying what makes a LLAR and how to identify them at emergence with a view toward improved space weather forecasting. The further implications of these findings for AR heating spatial and temporal patterns will be explored in an upcoming study.

The study of solar active regions (ARs) is of central importance to a range of fundamental science, as well as the practical applications of space weather. Active region emergence and life cycles are two areas of particular interest, yet the lack of consistent full-Sun observations has made long-term studies of active regions difficult. Here, we present results from a study to identify and characterize long-lived active regions (LLARs), defined as those which were observed during at least two consecutive Carrington rotations and which did not undergo significant successive flux emergence once the decay phase began. Such active regions accounted for 13% of all NOAA-identified ARs between 2011 and 2019, and their distribution closely follows the annual sunspot number. This implies that LLARs are produced by the same basic driving processes as regular ARs. LLAR areas tend to be significantly larger and contain more magnetic flux compared to other ARs, but the two categories have similar magnetic complexity distributions. The most striking result, however, is that LLARs are 3-6 times more likely than other ARs to be the source of a solar flare of GOES class C or greater. This highlights the importance of studying what makes a LLAR and how to identify them at emergence with a view towards improved space weather forecasting. The further implications of these findings for AR heating spatial and temporal patterns will be explored in an upcoming study.

Solar flares are intense bursts of electromagnetic radiation that occur due to a rapid destabilization and reconnection of the magnetic field. While preflare signatures and trends have been investigated from magnetic observations prior to flares for decades, analysis that characterizes the variability of the magnetic field in the hours prior to flare onset has not been included in the literature. Here, the 3D magnetic field is modeled using a nonlinear force-free field extrapolation for 6 hr before and 1 hr after 18 on-disk solar flares and flare quiet windows for each active region. Parameters are calculated directly from the magnetic field from two field isolation methods: the “active region field,” which isolates field lines where the photospheric field magnitude is ≥200 Gauss, and the “high current region,” which isolates field lines in the 3D field where the current, nonpotential field, twist, and shear exceed predefined thresholds. For this small pool of clean events, there is a significant increase in variation starting 2–4 hr before flare onset for the current, twist, shear, and free energy, and the variation continues to increase through the flare start time. The current, twist, shear, and free energy are also significantly stronger through the lower corona and their separation from flare quiet height curves scales with flare strength. Methods are proposed to combine variation of the magnetic fields with variation of other data products prior to flare onset, suggesting a new potential flare prediction capability.

The Sun’s corona is its tenuous outer atmosphere of hot plasma, which is difficult to observe. Most models of the corona extrapolate its magnetic field from that measured on the photosphere (the Sun’s optical surface) over a full 27-day solar rotational period, providing a time-stationary approximation. We present a model of the corona that evolves continuously in time, by assimilating photospheric magnetic field observations as they become available. This approach reproduces dynamical features that do not appear in time-stationary models. We used the model to predict coronal structure during the total solar eclipse of 8 April 2024 near the maximum of the solar activity cycle. There is better agreement between the model predictions and eclipse observations in coronal regions located above recently assimilated photospheric data.