Flux rope formation through flux cancellation of sheared coronal arcades in a 3D convectively-driven MHD simulation

TL;DR

This work demonstrates that coronal flux ropes can form above the solar photosphere through convectively driven flux cancellation, using a self-consistent 3D radiative MHD simulation with an inserted linear force-free arcade. By ramping in a controlled LFFF and letting stochastic convection drive footpoint motions, the study observes a flux rope assembling over 2.5 hours via multiple events, including slipping reconnection, U-loop emergence, Omega-loop submergence, and a novel thick-photosphere tether-cutting reconnection. The results show that while not all cancellations contribute to rope formation, the end state—a twisted flux rope with footpoints separated by up to ~12 Mm and extending ~2 Mm into the corona—emerges without prescribed boundary driving, reinforcing flux cancellation as a viable formation mechanism in a realistic solar environment. The findings bridge idealized boundary-driven models and reality by revealing how fragmentation of cancellations and reconnection within a thick photosphere shape pre-eruptive magnetic structures with potential CME relevance.

Abstract

Context. Space weather and its potential negative consequences for life on Earth has received increasing attention in recent decades. Particularly predicting CME onset has become important from a security perspective. To predict CMEs, one must first understand the dynamics leading to pre-eruptive magnetic field configurations such as flux ropes. Aims. In this study, we investigate the realistic formation of coronal flux ropes above the solar photosphere. The aim is to find if and how flux ropes can form there, and how the formation is related to flux cancellation at the photosphere. Methods. We run a convective non-symmetric 3D radiative MHD simulation with the code Bifrost. A linear force-free field with sheared coronal arcades is slowly inserted in the 24Mmx24Mmx30Mm simulation box. After this, the self-consistent stochastic plasma flows of the convection zone drive several small-scale flux cancellations and magnetic reconnection, without external influence. Lagrangian markers called corks are used to track the dynamic evolution of the magnetic field. Results. Over a period of 2.5 h, a flux rope is generated with photospheric footpoints separated by up to 12Mm. The flux rope forms gradually through several individual events, such as slipping reconnection, U-loop emergence, and thick-photosphere tether-cutting reconnection. Conclusions. Flux ropes can be formed in the solar atmosphere solely driven by convection and flux cancellations at the photosphere. However, not all flux cancellations contribute to the build-up of the flux rope, and some coronal reconnection events that do are not clearly related to flux cancellation. The formation process of flux ropes from coronal sheared arcades driven by convection is therefore more complex than in the original smooth flux cancellation model. But the end result is qualitatively the same. Flux cancellation works. A flux rope is formed.

Figures (8)

• Figure 1: Horizontal averages of the Bifrost simulation. The magnetic pressures ($P_B$) are given in $\dyne\per cm^2$ for the QS simulation before the ramp at snapshot 1000 (green dashed), for the time-independent LFFF (red dashed), and for the simulation after the ramp at snapshot 1120 (purple dashed). The pressure (orange), the density (light blue), and the temperature (dark blue) of the atmosphere are plotted at snapshot 1000. The density is given in $g\per cm^3$, multiplied with a factor $10^{12}$ to fit on the same axis as the magnetic pressures, while the temperature is given on the right axis. The gray shaded area (${z\in[0,-1]~Mm}$) represents the "thick" photosphere, up to the altitude of the temperature minimum, i.e. the height where the density stratification is the strongest (see Sect. \ref{['sec_exp_thick']} for more details).
• Figure 2: 2D cross sections of the magnetic field at ${y=10Mm}$. The streamlines show field lines, but their density does not signify the field strength. The field strength is shown by the color. (a) is the QS field right after the relaxation, (b) is the LFFF, (c) is the sum of the two proceeding, while (d) shows the EN right after the ${20min}$ long ramp of the LFFF into the QS simulation.
• Figure 3: Magnetograms at the photosphere (${z=0}$) in the Bifrost simulation. (a) is just after the relaxation of the QS. (b) is just after the ramp of the LFFF. The white polarity (north pole) signifies flux coming out of the solar surface, being negative here as $z$ is positive along the LOS. The black polarity (south pole) signifies flux entering the solar surface.
• Figure 4: Gradual formation and evolution of a flux rope in the Bifrost simulation. Each subfigure contains two panels, an angled view above a vertical top view of the same set of field lines. The angled views are identical in all subfigures, oriented such that the $x$-axis increases down to the right and the $y$-axis increases down to the left. The numbers on the axes are in units of $Mm$. (a) is right after the end of ramp of the LFFF, and differs from the rest by only showing field lines seeded at ${x=12Mm}$, ${y\in\{5,10,15\}~Mm}$, and ${z\in (0,-5)~Mm}$ in gradually brighter colors. (b)-(f) show field lines representative for the flux rope in yellow-red colors, darker colors are shown for larger values of ${\abs{J}/\abs{B}}$, and overlying arcades in cyan. (d) also shows the four poles W1, W2, B1, and B2 referred to in the text.
• Figure 5: Slipping reconnection in the Bifrost simulation. Combined, the subfigures labeled (a)-(f) show the evolution until the time ${t_a=\text{245m40s}}$ when this slipping process is complete. Each subfigure contains a top panel with an angled view of three families of 3D field lines (cyan, green, magenta), tracked by corks, above a 2D magnetogram. The angled view is angled from the bottom right corner of the 2D magnetogram toward the top left corner. On the bottom of each subfigure is a panel with only the 2D magnetogram , seen directly from above. The red circles in the bottom panels of each subfigure all highlight the same area of interest.