Determining the 3D Dynamics of Solar Flare Magnetic Reconnection

TL;DR

The paper demonstrates, for the first time in a fully 3D eruptive flare model, that plasmoid dynamics in a turbulent flare current sheet leave distinct, spiraling imprints on flare ribbons when mapped to the solar surface via field-line length maps ($L$-maps). Using a high-resolution ARMS-based MHD simulation with adaptive refinement, the authors link coronal plasmoid formation, propagation, and annihilation to the fine-scale ribbon morphology, showing that the ribbons encode the life cycle of plasmoids. They introduce robust ribbon proxies based on $L$-maps and compare them to $Q$-maps and $J_r$, showing $L$-maps provide clearer, topology-driven insights with less sensitivity to cadence and resolution. The study also provides synthetic observations, predicting observable plasmoid signatures in white-light and ribbon-like spirals, offering concrete targets for Solar Orbiter, IRIS, and DKIST to test plasmoid-dominated reconnection in solar flares and advancing our understanding of energy release and particle acceleration in three dimensions.

Abstract

Solar flares are major space weather events that result from the explosive conversion of stored magnetic energy into bulk motion, plasma heating, and particle acceleration. While the standard flare model has proven highly successful in explaining key morphological features of flare observations, many aspects of the energy release are not yet understood. In particular, the turbulent three-dimensional structure of the flare current sheet is thought to play an important role in fast reconnection, particle acceleration, and bursty dynamics. Although direct diagnosis of the magnetic field dynamics in the corona remains highly challenging, rich information may be gleaned from flare ribbons, which represent the chromospheric imprints of reconnection in the corona. Intriguingly, recent solar imaging observations have revealed a diversity of fine structure in flare ribbons that hints at corresponding complexity in the reconnection region. We present high-resolution three-dimensional MHD simulations of an eruptive flare and describe our efforts to interpret fine-scale ribbon features in terms of the current sheet dynamics. In our model, the current sheet is characterized by many coherent magnetic structures known as plasmoids. We derive a model analogue for ribbons by generating a time series of field-line length maps (L-maps) and identifying abrupt shortenings as flare reconnection events. We thereby demonstrate that plasmoids imprint transient 'spirals' along the analogue of the ribbon front, with a morphology consistent with observed fine structure. We discuss the implications of these results for interpreting SolO, IRIS, and DKIST observations of explosive flare energy release.

Figures (15)

• Figure 1: Initial configuration and driver for the eruptive flare model. (a-b) Magnetic topology shown from two orthogonal perspectives: (a) east and (b) above. White (black) shading at the inner boundary corresponds to positive (negative) polarity. Four distinct flux regions are indicated in cyan, green, and red (northern and southern sets of green field lines represent distinct flux regions). (c) Illustration of initial solar surface grid (shown at half resolution for clarity) and coordinate system. Green, red, and blue curves indicate the radial, polar $(\theta)$/latitude and azimuth ($\phi$)/longitude coordinates, respectively, with arrows indicating their directions of increase. The polarity inversion lines (PILs) are indicated in yellow, with the oval PIL encircling the active region extending approximately $\pm34^\circ$ in longitude and $\pm22^\circ$ in latitude. (d) Profile of $B_\phi$ component of the tangential flux injected by the STITCH method (see text for details) at the inner boundary. Contours of the radial component of the magnetic field are overlaid in grayscale.
• Figure 2: Overview of the eruptive flare model evolution. (a-e) Erupting flux rope (orange field lines), overlying flux, and flare loops at five different times during the simulation. Magenta, blue, and cyan indicate field lines progressively further from the PIL (yellow). Grayscale contours show the magnitude of the radial component of the magnetic field. (f) Time-evolution of globally integrated quantities, including the free magnetic energy (black), kinetic energy (red), and reconnection rate (blue). Energies are normalized to the initial potential field energy. The temporal profile for the shear injection is shown in the dashed magenta line. An animation of this figure is available, showing the eruption of the flux rope and formation of the flare arcade during the interval 12,800 s $\leq$ t $\leq$ 14,000 s at a 10 s cadence (the animation duration is 5 s).
• Figure 3: Three-dimensional volumetric visualization of the total current density in the flaring region at $t=13,400s$. Magenta indicates the most intense currents. Grayscale color table indicates the radial component of the magnetic field at the inner boundary of the domain, and yellow curves indicate polarity inversion lines. Orange field lines trace the erupting magnetic flux rope.
• Figure 4: Three-dimensional plasmoid structures in the flare current sheet. (a) Context image showing contours of the vertical current density at the inner boundary, illustrating the flare ribbon locations. Yellow field lines indicate the location of the erupting CME flux rope. Five 2D slices of the flare current sheet are also shown, with a color table indicating the guide magnetic field component ($B_\phi$); concentrations of the guide field indicate plasmoids. Panels (b-f) show distinct sets of field lines traced from the slices indicated in panel (a), illustrating the twist and topology of the plasmoids. Examples of plasmoids sunward and anti-sunward of the flare reconnection X-line are shown in (g) and (h), respectively.
• Figure 5: Illustration of the link between flare reconnection and field line length length at four different times during the evolution of the eruptive flare (the times are identical to those shown in Fig. \ref{['fig:eruption_summary']}). The field-line length map (L-map) is shown at the inner boundary ($r=R_s$) where white/light gray indicate short field lines and black/dark gray indicate long field lines. Yellow curves indicate PILs. The magenta, blue, and cyan field lines are identical to those shown in Fig. \ref{['fig:eruption_summary']}. As flare reconnection proceeds, these field lines expand outward, then reconnect, and form the flare arcade. The resulting lengths of the associated field lines abruptly shorten, as reflected in the white region that expands outward from the central PIL. Simultaneously, the footpoints of the orange field lines (representing the erupting twisted flux rope) darken, corresponding to the outward expansion of the flux rope. An animation of this figure is available, showing the evolution of the $L$-map on the solar surface during the eruption of the flux rope and formation of the flare arcade during the interval 12,800 s $\leq$ t $\leq$ 14,000 s at a 10s cadence (the animation duration is 5 s).