The effect of magnetic tongues on the determination of the tilt angle of active regions

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

Simposio; IAU Symposium 327, Fine structure and dynamics of the solar atmosphere; 2016

We study the evolution of solar active regions (ARs) to determine how their magnetic helicity propertiesa↵ect the evolution of the photospheric flux observed in SOHO/MDI magnetograms. The amount ofmagnetic helicity accumulated in the solar atmosphere is one the main triggers for solar flares and otheractive phenomena. Several proxies to estimate the magnetic helicity of ARs have been used in recentdecades (Pevtsov et al., 2014, Space Sci. Rev., 186, 285324). Poisson et al. (2015, Solar Phys., 290,727-751) used, as a proxy of the twist in emerging ARs, the so-called magnetic tongues that are observedin line-of-sight magnetograms. These tongues are due to the line-of-sight projection of the azimuthalcomponent of the magnetic field in toroidal emerging flux-tubes that form ARs. Analysing series ofobserved magnetograms, in this work we study the e↵ect of the tongues on the evolution of the tilt angleand the polarity inversion line. In bipolar ARs the tilt is defined as the angle that the segment that joinsthe main centers (or barycenters) of positive and negative polarities forms with the solar Equator. Theretraction of the tongues during the emergence of ARs can produce a spurious apparent evolution of thetilt. Previous works have used AR tilt evolutions to infer a large torsions of the main axis of emergingmagnetic flux-tubes (the writhe helicity component). We show that the apparent rotation of the ARpolarities is not necessarily due to the torsion of the main flux-rope axis but to the e↵ect of the tongues.We test several methods to reduce this e↵ect on the tilt angle determination.