Dynamic and topological features of emerging magnetic fields


To investigate what dynamic and topological features of emerging magnetic fields are related to photospheric and coronal activities, we performed a pair of three-dimensional MHD simulations, in which an emerging flux tube was composed of strongly/weakly twisted field lines (ST/WT case). As for the photospheric activity, a bipolar region associated with a specific photospheric flow was produced in both cases. By decomposing this flow into several basic flow components (rotation, expansion/contraction, distortion), we derived evolutionary characteristics of the photospheric flow driven by an emerging flux tube; that is, as one of the legs of the emerging flux tube became vertical, the corresponding main polarity region formed by that leg apparently rotated in the opposite direction to the rotational flow. As for the coronal activity, S-shaped structure was formed in the ST case (sigmoidal structure composed of those field lines with high current density at their chromospheric footpoints), which is different from a potential field-like arcade formed in the WT case. We further investigated the following topological features of emerging magnetic fields relevant to the ST case: bald patch (BP), BP field line, BP curve, BP footpoint curve.


Reference

Magara, T. 2006 ApJ, 653, 1499



PFL represents "peak flux location" (not principal field line).



Figure 11: (a) Sigmoidal structure formed in ST case, which is composed of those field lines with high current density at their chromospheric footpoints. The color map shows the distribution of current density strength in a chromospheric layer, while contours on this map represent vertical magnetic flux density. Colors of each field line represents the strength of current density at the footpoint in this layer (bright color indicates high current density). (b) Same as (a), but for potential field-like arcade formed in WT case. (c) Soft X-ray image of a sigmoid observed by Yohkoh. (d) EUV image of a potential field-like arcade observed by TRACE.



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


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