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

TL;DR

This study combines observations and modeling to show that the twist in emerging flux ropes determines the sigmoidal shape of active regions and their magnetic pivoting behavior during emergence.

Contribution

It provides empirical evidence linking flux rope twist and pivoting behavior to the sigmoidal shapes of emerging solar active regions, supporting MHD simulation predictions.

Findings

Emerging BMRs become S-shaped or Z-shaped in EUV images.

The pivot direction correlates with the sigmoidal shape and twist.

Average pivot angle during emergence is about 35 degrees.

Abstract

We present an augmentation to longstanding evidence from observations and MHD modeling that (1) every solar emerging bipolar magnetic region (BMR) is made by an emerging omega-loop flux rope, and (2) twist in the flux-rope field makes the emerged field sigmoidal. Using co-temporal full-disk coronal EUV images, magnetograms, and continuum images from 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{\deg} to 90{\deg} and averages about 35{\deg}. Thus, at the onset of the emergence of our average emerging omega-loop flux rope, the magnetic field's twist pitch angle at the flux rope's top edge is plausibly about 35{\deg}.