Dynamic and topological features of emerging magnetic fields


We performed a pair of three-dimensional MHD simulations focused on dynamic and topological features of magnetic structures produced by emerging flux tubes with different degrees of field-line twist, to investigate how these features are related to photospheric and coronal activities. The flux tube was composed of strongly (weakly) twisted field lines for one (the other) simulation. As for the photospheric activity, in both simulations the emerging flux tube produced a bipolar region associated with a specific photospheric flow, the latter of which was decomposed into several basic flow components: rotation, expansion/contraction, and distortion. This quantitatively explained how the photospheric flow developed as a leg of the emerging flux tube became vertical; that is, the main polarity region produced by the leg apparently rotated in the direction opposite to a rotational component of the photospheric flow driven in that region. As for the coronal activity, the strongly twisted field lines with high current density at their chromospheric footpoints displayed a distinct S-shape in the corona (sigmoidal structure), which is different from a potential field-like arcade composed of the weakly twisted field lines. We discussed the following topological features of the sigmoidal structure: bald patch (BP), BP field line, BP curve, BP footpoint curve. We also discussed the formation of filament channel, flux rope, and sigmoid.


Reference

Magara, T. 2006 ApJ, 653, 1499




Figure 11: Topological features of magnetic structures produced by emerging flux tubes with different degrees of field-line twist.
(a) Snapshot of emerging field lines in the strongly twisted case. The color map shows the distribution of current density in a chromospheric plane, while contours on this plane represent vertical magnetic flux. Colors of field lines represent the strength of current density at their footpoints in this plane (bright color indicates that high current density is distributed).
(b) Same as (a), but for the weakly twisted case.
(c) Soft X-ray image of sigmoid obtained by Yohkoh.
(d) EUV image of potential field-like arcade obtained by TRACE.



Figure 12a: Distributions of current density (red contours) and vertical magnetic flux (gray contours; positive flux is shown by solid lines and negative flux dotted lines) are presented for the strongly twisted case. These contours are plotted in the same chromospheric plane as that presented in Figure 11a. The thick green curve was obtained by connecting bald patches (BPs) (BP curve), while the thin violet line shows the field line passing through one of the BPs (BP field line). The diamond and asterisks indicate the locations of the BP and footpoints of this BP field line, respectively. The thick blue curves were obtained by connecting footpoints of BP field lines (BP footpoint curves). High current density tends to be distributed along the BP curve and BP footpoint curves.

This shows multiple BP field lines.


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