Injection of Magnetic Energy and Magnetic Helicity into Solar Atmosphere by Emerging Flux Tube


We performed a three-dimensional MHD simulation of an emerging twisted flux tube to investigate flow and magnetic structure involved in flux emergence, especially focusing on how emerging field lines and associated flows contribute to injecting magnetic energy and relative magnetic helicity into the solar atmosphere. The flux emergence started with appearance of a simple bipolar region (a pair of main polarity regions) in a solar surface (photosphere), which was subsequently deformed and fragmented, leading to a quadrupolar region (the pair of main polarity regions + one satellite polarity region for each of these main polarity regions). The flux emergence generated not only a vertical flow but also horizontal flow in the photosphere, both of which contributed to injecting magnetic energy and relative magnetic helicity. The emergence term related to the vertical flow was dominant at the early phase of the flux emergence, while the shear term related to the horizontal flow (shear and/or rotational flow) became a dominant contributor at the late phase. We also investigated spatial distributions of force and flow along selected emerging field lines, which suggests that emerging field lines may be classified into two types based on the aspect ratio of field line (ratio of height to footpoint distance): expansion type and undulation type.


References

Magara, T. & Longcope, D. W. 2003 ApJ, 586, 630
Magara, T. 2004 ASP, 325, 185




Three-dimensional evolution of selected emerging field lines (purple and orange => outer (envelope) field lines, red => inner (axis) field line). Contours and colors indicate vertical magnetic flux density and vertical flow velocity in solar surface (photosphere).



Expressions for magnetic energy flux (Eq. 22) and relative magnetic helicity flux (Eq. 23) in photosphere. In both equations, the first term (shear term) represents contribution of horizontal flow (vx, vy), while the second term (emergence term) contribution of vertical flow (vz).



Flux emergence generates not only vertical flow but also horizontal flow in photosphere, both of which contribute to injecting magnetic energy and relative magnetic helicity. The emergence term related to the vertical flow is dominant at the early phase of the flux emergence, while the shear term related to the horizontal flow (shear and/or rotational flow) becomes a dominant contributor at the late phase.




Evolution of main polarity region and associated flow.




When a leg of emerging twisted flux tube is vertical to photosphere, only main polarity region is produced there (left panel). On the other hand, when the leg is inclined, satellite polarity region is produced in addition to the main polarity region (right panel; Magara 2004).


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