Magnetic flux cancellation in the solar atmosphere through 3D realistic numerical modeling

TL;DR

The paper tackles magnetic flux cancellation by performing a 3D RMHD simulation of an emerging bipole interacting with a preexisting field, using the Bifrost code to cover from the upper convection zone to the corona. It couples RMHD dynamics with NLTE radiative transfer via RH1.5D and coronal line synthesis to compare with observations, and uses a DBSCAN-based patch/PIL analysis to characterize flux evolution. The results show reconnection at chromospheric heights within a QSL without null points, formation of a twisted flux rope under the post-reconnection loops, and chromospheric signatures such as intensity enhancements and bidirectional jets, while the coronal response is modest in this event. The work highlights the importance of including coronal heights and realistic radiation to model flux cancellation and interpret chromospheric observables, setting the stage for future study of coronal heating and jetting in similar events.

Abstract

We present a radiation-magnetohydrodynamics (RMHD) simulation of a magnetic cancellation event. The model is calculated with the Bifrost code and spans from the uppermost convection zone to the corona. The cancellation occurs between the positive polarity of an emerged magnetic bipole and a preexisting negative polarity. We try both to understand the RMHD aspects as well as to carry out comparison to observations, in part via spectral synthesis of optically thick photospheric and chromospheric lines using the RH1.5D code, and optically thin coronal ones. The reconnection between the opposite flux systems takes place at chromospheric heights through a quasi-separatrix layer without null points. Sharp V-shaped upward-moving field lines and highly warped downward-moving post-reconnection loops are created. The chromospheric reconnection is in full swing when the colliding magnetic patches are still separated by a granular cell at the photosphere. In a later phase, photospheric cancellation takes place with submergence of the closed magnetic loops linking the opposite polarities. We carry out comparisons with the observations of the photospheric magnetic flux loss rates, as well as of the horizontal magnetic field and vertical velocity at the polarity inversion line. The reconnection outflows cause intensity brightenings, jets and different spectral features in the synthesized chromospheric spectral lines, strongly reminiscent of those found in recent observations. Coherent, twisted magnetic flux ropes are created by the flows associated with the process. Including coronal levels is crucial for proper modeling, even if no major ejection or brightening is produced in the corona in this event.

Figures (13)

• Figure 1: Magnetograms taken at different instants along the cancellation process. Top row: synthetic magnetograms obtained through the V-Stokes profile of the Fe1 $617.34$ nm line. Lower rows: horizontal maps of the vertical magnetic field component at heights $z=0.15$ Mm (second row), $0.5$ Mm (third row), $0.8$ Mm (fourth row), and $1.5$ Mm (bottom row). The accompanying animation shows the time evolution for rows 2 and 4 of this figure between $t=101.7$ min and $t=130.0$ min. (An associated animation is available for this figure.)
• Figure 2: Canceling flux patch detection on a horizontal cut at $z=0.15$ Mm with $B_{\mathrm{thr}}=300$ G. Left: Vertical field strength map (grey scale) with, superimposed, the identified canceling patches (red: negative polarity, blue: positive polarity). Right: Time evolution of the integrated unsigned magnetic flux in the positive patch (blue), negative patch (red) and their sum (black). The time evolution of the flux patches between $t=106.7$ min to $t=126.0$ min can be seen in the accompanying animation. (An associated animation is available for this figure.)
• Figure 3: Top row: the PIL between the canceling patches on a horizontal cut at $z=0.15$ Mm at $t=116.0$ min. The pixels on the PIL are colored based on the horizontal field strength ${B_{\mathrm{hor}}}$ (left) and downflow (right), with colorscale given at the top of the panels. The background map shows the vertical magnetic field, $B_z$ (colorbar on the right). The associated animation shows the evolution of those two panels in the time range $t=116.0 - 122.0$ min including average values for ${B_{\mathrm{hor}}}$, downflow speed, and the inclination angle to the horizontal $\alpha$ for the PIL in each snapshot. Middle row: cumulative diagrams for the pixels in the PIL shown in the animation: left, scatter plot of downflow vs ${B_{\mathrm{hor}}}$; right: histogram for the inclination angles; the range $\pm\langle|\alpha|\rangle$ is indicated with dashed vertical lines. Lower row: same as the top row, but now for $z=1.0$ Mm. (An associated animation is available for this figure.)
• Figure 4: Result of the synthesis of the Ca2 $854.2$ nm, Ca2 K, Mg2 k, Mg2 279.9 nm lines in the vicinity of the cancellation site. The top row shows, from left to right: the density at 1 Mm; filtergrams of the Ca2 $854.2$ nm line at $0.04$ nm from line center near the $k_{2r}$ peak; the formation height at this wavelength; and the line spectra along the cut $y=10.8$ Mm (shown by the green dashed line). The red dashed line is placed at line center, while the red dotted line shows the wavelength plotted in the second column. The second and third rows show the same quantities for the Ca2 K and Mg2 k lines, but at $0.02$ nm from line center, with the downflow speed and temperature at 1 Mm replacing the density in the left column. Finally, the fourth row shows these quantities for the Mg2 279.9 nm line, with the vertical magnetic field $B_z$ at a height of 1.0 Mm being shown in the left column. The black arrow in the vertical velocity figure shows the location of the "ridge" discussed in Sect. \ref{['sec:chromospheric_brightenings_and_jets']}.
• Figure 5: Chromospheric line profiles during the cancellation event. Vertical ($B_z$) magnetic field at $z = 150$ km (left panel), average profiles of the Ca2 $854.2$ nm (middle panel) and Mg2 k (right panel) lines for the entire computational domain (red curve) and in the small patch centered on the cancellation site (blue curve) indicated by the blue rectangle in the left panel. The line centers of Ca2 $854.2$ nm and Mg2 k are marked with green vertical dashed lines.