Computing the tilt angle of solar active regions

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

San Juan

Conferencia; Towards Future Research on Space Weather Drivers; 2019

The tilt angle of an active region (AR) is defined as the acute angle between the segment that joints the magnetic barycenters of the main AR polarities and E-W solar direction. Current methods to estimate the tilt angle rely on the magnetic flux distribution of the AR obtained from line-of-sight (LOS) magnetograms. However, the LOS projection of magnetic field in twisted emerging structures forming ARs produces an elongation of the magnetic 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 in LOS magnetograms and, consequently, impact on the determination of the tilt angle. As a first approach, we use a simple flux emergence model to test a method to isolate and remove the flux associated with the tongues from the tilt angle estimation. Then, with our experience gained from the model, we develop a new procedure to determine the tilt angle of solar ARs. We apply this method to 150 ARs observed by the Michelson Doppler Imager. We compute the evolution of the corrected tilt during the emergence the ARs and we quantify the effect of the magnetic tongues in the determination of the tilt. Finally, using this method we correct the mean size of the ARs and we estimate the intrinsic rotation of the emerging bipoles. Knowledge of an AR intrinsic rotation can be a good proxy for the free magnetic energy stored during the AR emergence.