Pivot of the Emerging Bipolar Magnetic Region in the Birth of Sigmoidal Solar Active Regions

We present an augmentation to long-standing evidence from observations and MHD modeling that (1) every solar emerging bipolar magnetic region (BMR) is made by an emerging Ω-loop flux rope, and (2) a twist in the flux-rope field makes the emerged field sigmoidal. Using cotemporal full-disk coronal EUV images, magnetograms, and continuum images from the Solar Dynamics Observatory (SDO), we found and tracked the emergence of 42 emerging single-BMR sigmoidal active regions (ARs) that have sunspots in both polarity domains. Throughout each AR’s emergence, we quantified the emerging BMR’s tilt angle to the east–west direction (the x -direction in SDO images) by measuring in the continuum images the tilt angle of the line through the (visually located) two centroids of the BMR’s opposite-polarity sunspot clusters. As each AR emerges, it becomes either S-shaped (shows net right-handed magnetic twist) or Z-shaped (shows net left-handed magnetic twist) in the coronal EUV images. Nineteen of the ARs become S-shaped, and 23 become Z-shaped. For all 42 ARs, in agreement with published MHD simulations of the emergence of a single-BMR sigmoidal AR from a subsurface twisted flux rope, if the AR becomes S-shaped, the emerging BMR pivots counterclockwise, and if the AR becomes Z-shaped, the emerging BMR pivots clockwise. For our 42 ARs, the pivot amount roughly ranges from 10° to 90° and averages about 35°. Thus, at the onset of the emergence of our average emerging Ω-loop flux rope, the magnetic field’s twist pitch angle at the flux rope’s top edge is plausibly about 35°.