Author: Yang Guo

School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China

One of the key unsolved puzzles in solar physics is understanding the "slow-rise phase" that often precedes solar eruptions (Zhong et al. 2021, Xing et al. 2024). This phase, during which magnetic structures ascend gradually at speeds between photospheric flows and eruption velocities, has remained enigmatic—especially in terms of whether it stems from magnetic reconnection or ideal MHD processes. In a recent study published in Research in Astronomy and Astrophysics, Liu et al. (2025) take a significant step toward resolving this question.

By performing a series of 3D MHD simulations with unprecedentedly low bottom-driving speeds—approaching realistic photospheric values (~1 km/s)—the authors demonstrate that the slow-rise phase can be entirely accounted for by ideal MHD evolution in a sheared magnetic arcade, without invoking early reconnection. The simulations are enabled by the high-accuracy models of Jiang et al. (2021). Crucially, they find that both the core and overlying magnetic fields exhibit expansion speeds far exceeding the imposed boundary motions, with the overlying loops rising over ten times faster than the driving speed.

These findings confirm that the slow-rise phase is a generic, pre-eruptive feature of the corona’s ideal MHD evolution, modulated by the speed of photospheric shearing. Moreover, by matching their simulated slow-rise velocities with those observed in a real X-class flare event, the authors provide a critical bridge between theory and observation.

This study not only strengthens the case for current-sheet formation and eruption onset as consequences of a quasi-static expansion process, but also underscores the importance of high-fidelity simulations constrained by real-world boundary conditions. It sets the stage for further investigation into flux rope configurations and reconnection-driven scenarios—where the physics may differ substantially.

References: 

Jiang, C., Feng, X., Liu R. et al. 2021, NatAs, 5, 1126

Liu, Q., Jiang, C., & Liu, Z. 2025, RAA, 25, 051002

Xing, C., Aulanier, G., Cheng, X., Xia, C., & Ding, M. 2024, ApJ, 966, 70

Zhong, Z., Guo, Y., & Ding, M. 2021, NatCo, 12, 2734