Proposal

Understanding the Onset of Solar Eruptions

Team lead by Georgios Chintzoglou (Lockheed Martin Advanced Technology Center) and Tibor Török (Predictive Science Inc.)

Abstract: Despite intense research efforts over the past decades, the physical mechanisms that initiate large-scale solar eruptions such as flares and coronal mass ejections (CMEs) are still not well understood, which strongly hampers our ability to forecast their occurrence and, ultimately, their impact on Earth and other planets. Uncovering these mechanisms, therefore, remains one of the most significant and challenging tasks in Solar and Heliospheric Physics. To achieve progress, we need to (1) better understand the roles of key processes such as ideal magnetohydrodynamic (MHD) instabilities and magnetic reconnection and (2) unravel the contribution of the preceding slow rise phase, which entails a potential paradigm shift in our conception of the onset of solar eruptions. Upcoming missions will, for the first time, enable us to directly observe reconnection in the corona and study it in the whole chromosphere-to-corona region (rapid Doppler imaging in EUV with the MUlti-slit Solar Explorer [MUSE]; wide temperature range [full VUV] plasma diagnostics with the Extreme UltraViolet high-throughput Spectroscopic Telescope [EUVST] onboard Solar-C). Paired with the continuous advances in the numerical modeling of solar eruptions, this creates a timely opportunity for a concentrated effort to tackle these scientific tasks. We propose to assemble an international team of world-leading observers and modelers to systematically address the most important open questions related to the onset of solar eruptions and to formulate strategies for making best use of the observational capabilities of upcoming missions such as MUSE and Solar-C.

1. Scientific Rationale

Solar eruptions are the largest energy release processes in the solar system. It has long been recognized that these events, which are mainly observed as flares (a sudden, localized increase of radiation in the low solar atmosphere) and CMEs (a massive expulsion of magnetized plasma from the low corona into inter- planetary space), are manifestations of a single underlying pro- cess: a sudden and violent reconfiguration of the coronal mag- netic field (e.g., Forbes, 2000). CMEs, in particular, have been recognized to be the most severe driver of space-weather distur- bances at Earth, and are the primary cause of major geomag- netic storms (e.g., Baker and Lanzerotti, 2016). They are also associated with major Solar Energetic Particle (SEP) events, which can arrive at Earth orbit within tens of minutes after the commencement of a large eruption (e.g., Reames, 1999). The rapid transit of SEPs makes the prediction of eruptions crucial, if mitigating actions are to be employed.

Figure 1: Schematic plot of eruption phases, showing the CME velocity in blue and the flare emission in red (from Zhang and Dere, 2006).

It is well-established that the evolution of solar eruptions takes place in several phases. In the build-up phase, the required magnetic energy is accumulated over days to weeks in the corona, driven by slow processes (velocities v . 1km/s) such as flux emergence, shear flows, and flux cancellation, and stored in current-carrying, sheared/twisted magnetic fields (e.g., Patsourakos et al., 2020), which always form along and above polarity inversion lines of the surface magnetic field. These core fields (which may harbor a filament/prominence) are stabilized by ambient coronal “strapping” fields. During this phase, the pre- eruptive configuration is thought to evolve quasi-statically along a sequence of approximately force-free equilibria (e.g., Klimchuk and Sturrock, 1992). If, by some mechanism, this force-balance is destroyed, a stable equilibrium can no longer be maintained and an eruption occurs.

The quasi-static energization during the build-up phase is followed by three consecutive phases, which are schematically illustrated in Figure1. Just prior to an eruption, the sheared/twisted magnetic flux often undergoes an “initiation phase” or slow rise phase that lasts several minutes to hours, with velocities ranging from a few up to ⇠ 102 km/s, and is typically accompanied by a gradual increase in EUV and soft X-ray emission and/or a series of small, confined flares. This phase is followed by the “acceleration phase” or main phase, during which a CME is ejected from the low corona at speeds ⇠ 102 103 km/s and the associated flare grows impulsively. In the final propagation phase, the CME travels in the outer corona and interplanetary space, and flaring activity decays (e.g., Manchester et al., 2017).

Despite intense research efforts and constant improvement of observational and modeling capabilities, the detailed mechanisms that govern the onset of solar eruptions (particularly, the specific roles of key physical processes such as ideal MHD instabilities and magnetic reconnection) are still not well understood (e.g., Aulanier, 2014; Green et al., 2018; see also Questions (Q1)–(Q2) below). In addition to the scientific importance of this topic and its relevance for space weather research/prediction, upcoming missions will provide excellent opportunities for scrutinizing proposed mechanisms. In particular, a main goal of the NASA MUlti-slit Solar Explorer (MUSE) and the NASA/JAXA Solar-C missions will be to address the initiation of solar eruptions (Cheung et al., 2022; Shimizu et al., 2020). Specifically, the high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an AR-scale from MUSE will enable us, for the first time, to directly observe reconnection at the relevant scales of eruptions, and the scanning spectroscopy in the full vacuum-UV (VUV, i.e., EUV and FUV) band and “slit-jaw” imaging (in NUV) from Solar-C will probe reconnection in the full chromosphere-to-corona temperature range. It is, therefore, vitally important and timely to (1) systematically tackle this long-standing open problem and, at the same time, (2) prepare ourselves for these new observational capabilities. To this end, we propose to assemble a team of world-leading experts in solar physics. Combining state-of-the-art observations and MHD simulations in a concentrated, synergistic effort, our Team will focus on (i) the Slow Rise Phase (SRP) and (ii) the early Main Phase (MP) of solar eruptions, as those are most relevant for understanding their onset. Two outstanding open scientific questions will guide our research:

(Q1) How is the MP of solar eruptions initiated? Several mechanisms for initiating the MP, associated primarily with ideal MHD flux-rope instabilities (helical kink instability [KI; Fan and Gibson, 2004]; torus instability [TI; Kliem and Török, 2006]) or reconnection (magnetic breakout [Antiochos et al., 1999; Lynch et al., 2008]; tether cutting [Moore et al., 2001; Jiang et al., 2021]; flux emergence [Kusano et al., 2012]) have been suggested and partly evaluated in MHD simulations (for an extensive compilation, see Green et al., 2018). It is, however, still very challenging to determine the occurrence and specific role of these not mutually exclusive mechanisms in observed events, since they are (i) often difficult to identify unambiguously (especially due to the lack of direct observations of reconnection in the corona), and (ii) quantitative onset thresholds are insufficiently known. Our Team will systematically address these two obstacles, as described in our Work Plan below. After the initiation, the acceleration of the erupting flux is believed to be driven predominantly by the combined action of the TI and fast flare reconnection underneath the rising flux rope (e.g., Karpen et al., 2012; Welsch, 2018), even though their respective contributions are a matter of ongoing debate (e.g., Zhong et al., 2023; Liu et al., 2024). These two processes are closely coupled, supporting each other in a mutual feedback (e.g., Vršnak, 2008), which makes their contributions very challenging to disentangle. An investigation of this question, therefore, warrants a separate, dedicated research effort and will not be a part of this proposal.

(Q2) What is causing the SRP and how frequent is it? Recent high-cadence observations have shown that the SRP is distinct from both the build-up phase and the MP, as clear break points between these three phases have been found (e.g., Cheng et al. 2020). This challenges the ideas that the SRP marks a beginning instability (Zhang and Dere, 2006) or continuous photospheric driving (Vršnak, 2019). Rather, it strongly suggests that a different, presently unidentified physical mechanism acts in this phase. This entails a potential paradigm shift in the modeling of the onset of solar eruptions, namely from a single onset to a two-step onset. Another important question concerns the frequency of occurrence of the SRP. If it should turn out that the vast majority of eruptions is preceded by an SRP, then the detection of gradual rise of the pre-eruptive flux (and/or associated pre-flare activity) could be employed as a warning sign that an eruption is imminent. McCauley et al. (2015) found a large fraction, but their data set was strongly biased towards quiescent prominence eruptions. The occurrence rate of the SRP in AR eruptions, which are typically the strongest and most relevant for space weather, is presently not known.

2. Work Plan

We propose a two-year work plan, with two team meetings separated by one year. In Year 1, focusing on (Q1), we will (1) critically review the current eruption models and limitations of the observational and modeling capabilities; (2) use the synergy between MHD simulations and observations to test and discriminate between models, and (3) develop suggestions for how MUSE and Solar-C observations can improve on the discrimination between models. Specifically, we will:

Figure 2: High-resolution, spatiotemporal rasters as provided by MUSE will allow us to directly capture the initiation of eruptions. Shown here are the line intensity and Doppler velocity of the Fe XV 284 Å line synthesized from a radiative MHD simulation of a C-class flare and flux-rope eruption. Five minutes before eruption, the Doppler map shows bidirectional reconnection outflows (black circle). This coronal reconnection event is the mechanism that destabilized the magnetic flux rope (from Cheung et al., 2022).

• Build a largely uniform set of representative MHD simulations of the main eruption mod- els by updating models already available in the team, carefully perform new data-constrained real-event simulations (as, e.g., Török et al., 2018; Fan, 2022) of a moderate set (⇠3–6) of well observed events in bipolar, quadrupolar, and null-point source regions, and compute synthetic observables (rise profiles, EUV images, Doppler shifts, emission measures, etc.) for comparison with observations. Data from SDO, Solar Orbiter, ASO-S, IRIS provide a rich source for such com- parisons, which have the goal to reveal the initiation mechanism(s) of the selected events.

• In preparation for the new capabilities expected from MUSE (launch in 2027) and Solar-C (launch in 2028), we will synthesize expected observables from idealized simulations of the main eruption model categories, as well as from the real-event simulations. We will focus on spectroscopic quantities such as line intensity, Doppler shift, and line width for lines at various temperatures observed in different parts of the erupting source region, and thus map out expected signatures of reconnection (e.g., Fig. 2). Key properties of the reconnection such as location (high/low in the source volume), timing (before/after the threshold of ideal instability is reached), and requirement on emerging flux (location, orientation, and magnitude) differ between the model categories. In this way, we will significantly extend the proposed “use cases” of how MUSE and Solar-C are expected to reveal the key mechanisms of solar eruptions (Cheung et al. 2022) and tailor them to the different models, facilitating their confirmation or rejection.

• Systematically evaluate the validity of proposed eruption thresholds, especially for the TI and for the current sheet properties (width, height, associated free energy) required for the onset and self-sustained amplification of reconnection, as well as for the recently suggested κ-scheme (Kusano et al., 2020), which considers the ratio of core-field twist and stabilizing ambient flux. These quantitative criteria facilitate the discrimination between the different eruption models in the above simulation-observation comparisons.

In Year 2, focusing on (Q2), we will (1) evaluate candidate mechanisms for the SRP and its transition to the MP, and (2) determine how often it occurs prior to AR-eruptions. Specifically, we will:

• Systematically test several hypotheses for the physical mechanism underlying the SRP, as for example breakout reconnection (Lynch et al., 2008), localized flux emergence in the vicinity of filament channels (Török et al., 2024), relaxation of a non-equilibrium flux rope produced by confined flares (Kliem et al., 2021), flux-rope formation via collisional shearing (Chintzoglou et al., 2019; Rempel et al., 2023), HFT reconnection (Xing et al., 2024), and prominence draining for quiescent prominence-cavity systems (Jenkins et al. 2019; Fan, 2020). This will be done by building and analyzing a series of simulations carefully designed by the Team members, to determine for each of the hypotheses the conditions responsible for the onset of the SRP and for the transition from the SRP to the MP, i.e., how a specific SRP mechanism can “activate” another mechanism that is capable of initiating a full eruption (e.g., HFT reconnection slowly lifting a flux rope to the critical height for TI onset; Xing et al., 2024). We will employ several evaluation criteria, e.g., whether a given mechanism can reproduce the linear rise behavior typically observed during the SRP, and produce synthetic observables from the simulations for comparison with existing observations and as preparation for the MUSE and Solar-C missions (see Year 1 work plan).

• Determine the occurrence rate of the SRP in AR eruptions, by investigating, for the first time, a large data set of ≤100 observed events. To this end, we will select a time period of pronounced solar activity (for example 2011-2013, when 171 M-class and 14 X-class flares occurred on the Sun), and pre-flare events by inspecting GOES soft-X-ray data and EUV images from SDO/AIA. For each event, we will integrate the EUV emission in the corresponding source region only (to avoid potential contamination by other events), and use its time profile as a proxy for the SRP. To check the reliability of this approach, we will select a subset (≤20 cases), for which good observations of rising filaments or hot plasma channels are available, and determine the SRP for these cases by means of detailed height-time measurements. By carefully fitting these data, we will infer the respective onset points of the SRP and the MP, as well as determine the rise velocities. These will be confronted with our numerical results, which will allow us to further constrain the potential mechanisms at work in the SRP (see previous bullet).

During the second meeting, the Team will perform a final assessment of the overall maturity of the project, identify and assign remaining work tasks, as well as writing duties for producing journal articles and a review paper that will summarize our scientific results (see next section).

3. Expected Outputs

The science tasks outlined in our work plan are expected to yield:

• A significant number (>10) of peer-reviewed publications on the onset of solar eruptions, focusing on the mechanisms and onset criteria of the slow-rise and main phases, and systematically addressing the SRP for the first time. The specific subjects of these publications will largely correspond to the goals highlighted in bold in the above work plan.
• A review paper for publication in Space Science Reviews, summarizing our current understanding of the conditions and mechanisms that govern the onset of solar eruptions. This will include hypothesis tests specific for each eruption model by synthesizing diagnostics (from MHD simulations) that can be realized by MUSE and Solar-C (see URLs at the end of this document). Such a review paper is expected to become an important community resource, helping solar researchers to fully exploit these new data/diagnostics and to prepare for subsequent instrument projects.
• Different data products (e.g., processed satellite image sequences, height-time measurements, syn- thetic observables obtained from simulations), which will be made available to the community via web sites or online data repositories (e.g., zenodo.org). The original simulation data are typically too large to be hosted on public servers. They will be provided to other researchers upon request.

– MUlti-slit Solar Explorer (MUSE) MIDEX mission (selected by NASA in 2022; launch in 2027):https://www.nasa.gov/press-release/new-sun-missions-to-help-nasa-better-understand-ear th-sun-environment
– Extreme UltraViolet high-throughput Spectroscopic Telescope (EUVST) of the NASA/JAXA Solar-C mission (selected by NASA in 2021; launch in 2028): https://solar-c.nao.ac.jp/en/