Driving mechanisms of solar active region geysers: The role of interacting magnetic flux

TL;DR

This study addresses how recurrent coronal jets in a decaying active region are triggered and sustained by complex photospheric flux dynamics. It combines multiwavelength observations (AIA/HMI), NLFFF extrapolations, and 3D radiative MHD Bifrost simulations to diagnose the magnetic drivers, focusing on moving magnetic features (MMFs) and flux cancellation. The authors identify a three-step MMF-driven mechanism—flux convergence, interaction with reconnection, and post-reconnection relaxation—that powers nine jets (J1–J9) from a single footpoint, with persistent high $B_h$ and $F_z$ near the footpoints enabling repeated energy release. The results highlight the significance of MMF-driven flux interactions in sustaining geyser-like jet activity and shed light on how energy is stored and released in the lower solar atmosphere, contributing to mass and energy transport to the corona and solar wind; they also note limitations of idealized simulations and point to future high-resolution observations to fully capture the recurrence dynamics.

Abstract

Active region recurrent jets are manifestations of episodic magnetic energy release processes driven by complex interactions in the lower solar atmosphere. While magnetic flux emergence and cancellation are widely recognized as key contributors to jet formation, the mechanisms behind repeated magnetic reconnection remain poorly understood. In this letter, we report a sequence of nine recurrent jets originating from active region AR 12715 during its decay phase, where the jet activity was associated with a complex distribution of fragmented magnetic flux. Non-linear force-free field (NLFFF) extrapolations reveal the presence of low-lying, current-carrying loops beneath overarching open magnetic fields near the jet footpoints. These magnetic structures were perturbed by (i) emerging flux elements and (ii) interactions between oppositely polarized moving magnetic features (MMFs). To interpret these observations, we compare them with 3D radiative MHD simulation from the Bifrost model, which reproduce jet formation driven by interacting bipolar MMFs, leading to subsequent flux cancellation in the photosphere. Our results emphasize the critical role of MMF-driven flux interactions in initiating and sustaining recurrent jet activity in active regions.

Figures (5)

• Figure 1: Panels show an overview of NOAA AR 12715 highlighting the recurrent jet–producing Geyser Region (GR). (a) Photospheric magnetic field distribution from the SDO/HMI LOS magnetogram, with region enclosed in black square highlighting the Geyser Region (GR) located at the south-eastern penumbral boundary of the active region. (b) Zoomed-in view of GR overlaid with AIA 171 Å emission, showing fragmented magnetic patches along with a jet observed at 16:35 UT. Cyan “+” symbols mark the likely footpoint of the recurrent jets, and the location of the artificial slit (S1) is indicated in lime. (c) Temporal evolution of the photospheric magnetic field in GR from 15:30 to 19:30 UT, with averaged magnitudes plotted alongside 4th-order polynomial fits. The time–distance evolution of jet activity along slit S1 is overlaid on the magnetic field evolution. Vertical dashed lines denote the onset times of the individual jet events (J1–J9).
• Figure 2: Temporal evolution of photospheric magnetic field and its derived parameters within the GR during 15:36–17:48 UT, encompassing the period of recurrent jet activity. Panels (a–f) show HMI continuum intensity maps overlaid with contours of positive (red) and negative (blue) magnetic polarities thresholds at $\pm50$ G. Panels (g–l) show the corresponding LOS magnetic field ($B_{z}$) from HMI magnetograms. Small boxed regions, labeled 'B$_1$' and 'B$_2$', indicate bipolar magnetic structures with interacting polarities linked with jet eruptions. Panels (m-r) show the vertical Lorentz force component ($F{z}$), and panels (s-x) display the horizontal magnetic field ($B_{h}$). Both $F_{z}$ and $B_{h}$ exhibit noticeable variations near the recurrent jet footpoint (marked by a cyan '+' symbol). run:figures/Fig2_hmi_contimuum.avi.
• Figure 3: Panels show Bifrost simulation results highlighting a photospheric bipolar magnetic feature and its associated jet. (a)-(c) Line-of-sight magnetic field ($B_{z}$), horizontal magnetic field ($B_{h}$), and vertical Lorentz force ($F_{z}$) at the photosphere ($Z = 0.1$ Mm) in the simulation domain. (d)-(f) Plasma properties of the erupted jet at $Y = 18.9$ Mm: LOS outflow velocity ($U_{z}$), temperature ($T$), and electron density ($N_{e}$) at logarithmic scale. (g) Time-distance plot along slit S2 (marked in panel a) showing the evolution of the bipolar magnetic features. The three-step process of jet triggering is evident: flux convergence ($t = 8790$ s), interaction ($t = 9050$ s), and post-interaction phase ($t = 9480$ s). run:figures/Fig3_bifrost_R1.mpeg.
• Figure 4: Comparison between numerical simulation (Bifrost, resampled to HMI resolution) and observational data of the photospheric magnetic field and derived parameters, demonstrating the three-step process leading to jet formation. Panels (a)-(f) show LOS magnetic field ($B_{z}$) maps highlight the evolution of interacting bipolar magnetic patches at the jet footpoint. Slit S2, marked in panel (a), indicates the position used on the downsampled LOS magnetic field ($B_{z}$) map for correlation with Figure \ref{['fig3']}(g). Panels (g)-(l) depict the vertical component of the Lorentz force ($F_{z}$), revealing the disappearance of its positive polarity at the jet footpoint prior to eruption. Panels (m)-(r) display the enhanced concentrations of horizontal component of the photospheric magnetic field ($B_{h}$), with a gradual decrease in magnitude near the flux cancellation site in both simulation and observations.
• Figure 5: Magnetic field topology of the Geyser Region (GR) associated with recurrent jet activity. Panels (a)-(c) show the coronal extension of magnetic fields in the Bifrost simulation at three distinct stages of jet evolution: pre-interaction (t = 8790 s), interaction (t = 9050 s), and post-interaction (t = 9480 s). Jet structures emanates from a null point formed above the interacting magnetic bipoles (indicated by an arrow in panel (a)), accompanied by enhanced current density. Panels (d)-(i) show the evolution of extrapolated magnetic field lines, revealing the formation of small-scale loop structures, associated with photospheric MMF features, beneath inclined open magnetic fields extending from the active region. The region show localized enhancements in current density (marked by an arrow) that facilitate recurrent jet formation. Both the numerical and extrapolated magnetic fields indicate the presence of magnetic dips at the sites of enhanced current density, coinciding with subsequent jet formation. run:figures/Fig5_nlfff.avi.