Determination of Sub-Photosphere Solar Active Region 3D Magnetic Field Structure from Emergence Observations




Briggs, Brian S.

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In this thesis, we study the emergence process of a bipolar solar active region NOAA 11416, a Hale class βγ/β active region, and reconstruct its global 3D magnetic structure through a novel image-stacking technique. The emergence began on 8 February 2012, and data was taken through 11 February 2012. Magnetograms from the Solar Dynamics Obersvatory’s (SDO) Helioseismic and Magnetic Imager (HMI) are used in this study, and we take advantage of the unprecedented high imagery cadence offered by the observatory. We describe the selection process of observed candidate active regions covering the full data set since SDO first light. Using this magnetogram imagery, we visualize the detailed 3D magnetic structure by stacking the high-cadence SDO imagery and producing 3D isosurface plots, which yield extraordinary detail of the fine magnetic structure of the AR. The 3D structure shows a distinct “asymmetric Λ” shape to the emerging flux tube with tilt characteristic of Joy’s Law. This “asymmetric Λ” shape exhibited a differing slope of the leading and trailing legs, the leading leg being 61° and the trailing leg 52° for an assumed rise velocity of 100 m/s. Close examination of the 3D structure indicates a highly fragmented flux rope whose organization appears to increase as the eruption progresses. We also find that the leading polarity is more fragmented in magnetogram imagery, which appears to support thin flux tube approximations and the results of analastic magneto-hydrodynamic simulations. Continuum imagery, however, shows the opposite situation, with the leading polarity being the more cohesive of the two. Additionally, the extracted AR parameters for center motion, tilt, polarity centroid separation, and flux per polarity are fit to functions, producing mathematical formulations for the 3D shape and magnetic flux of the flux tube. These functions not only accurately describe the shape and strength of the flux tube, but their exponential nature allows predictions of end points in terms of flux (~8x10^21 Mx), centroid separation (59.0 Mm), and centroid tilt angle (18.5°). We find that both tilt angle evolution and movement of the center fit well to underdamped harmonic oscillator equations. The former indicates interplay between the Coriolis force and shear forces resulting from differing Lorentz forces beneath and above the photosphere, producing an initially anti-Joy’s-law tilt which rapidly changes to tilt following Joy’s law, overshoots, then settles back towards the final value. The center motion’s underdamped “overshoot, then settle” behavior, on the other hand, suggests interplay between magnetic tension of the rising flux tube and convective turbulence.



Active region, Magnetic field, Solar physics, Sunspots