Measurement of Kodaikanal White-Light Images: Relaxation of Tilts of Spot Groups as Indicator of Subsurface Dynamics of Parent Flux Loops

Abstract

We reexamine the evolution of the observed tilts θ of spot groups with life spans 2–7 days in the two latitude belts <13° and >13°. Using an iterative procedure, we refine the linear fit between θ and the daily tilt angle changes δθ and obtain reliable estimates of the fit coefficients. We interpret our results in light of the scenario implied by the theoretical model of Longcope & Choudhuri for the subsurface dynamics of parent flux loops of bipolar magnetic regions and arrive at the following conclusions: (1) the parent flux tubes of spot groups possess a nonzero tilt at the onset of rise from the depths of their origin; these “inborn tilts” are 4°–11° in latitudes <13° and 3°–15° in latitudes >13°; (2) during the rise the tilt of the omega loops of spot groups living 2–7 days get reduced to 2°–6° in both the latitude belts, and this calls for reexamination of the role of Coriolis force as understood so far; (3) after emergence of the top of the loop above the surface, magnetic tension in the legs tends to restore the tilt to the inborn tilt on timescales of 5 to 14 days; and (4) these timescales correspond to field strengths in the range 14–40 kG for the parent flux loops and are close to the limits set by Fan et al.