Computing the tilt angle of solar active regions

POISSON, M.; MANDRINI, C.H.; LÓPEZ FUENTES, M.C.; DÉMOULIN, P.

San Juan

Workshop; Towards Future Research on Space Weather Drivers; 2019

The tilt angle of an active region (AR) is defined as the acute angle betweenthe segment that joins the magnetic barycenters of the main AR polarities and E-W solardirection. Current methods to estimate the tilt angle rely on the magnetic flux distributionof the AR obtained from line-of-sight (LOS) magnetograms. However, the LOS projectionof magnetic field in twisted emerging structures forming ARs produces an elongation of themagnetic polarities known as magnetic tongues. In Poisson et al. (2016, Solar Phys., 291,1625-1646) we have shown that magnetic tongues affect the photospheric field distribution inLOS magnetograms and, consequently, impact on the determination of the tilt angle. As a firstapproach, we use a simple flux emergence model to test a method to isolate and remove the fluxassociated with the tongues from the tilt angle estimation. Then, with our experience gainedfrom the model, we develop a new procedure to determine the tilt angle of solar ARs. We applythis method to 150 ARs observed by the Michelson Doppler Imager. We compute the evolutionof the corrected tilt during the emergence the ARs and we quantify the effect of the magnetictongues in the determination of the tilt. Finally, using this method we correct the mean sizeof the ARs and we estimate the intrinsic rotation of the emerging bipoles. Knowledge of anAR intrinsic rotation can be a good proxy for the free magnetic energy stored during the ARemergence.