Active-region Tilt Angles from White-light Images and Magnetograms: The Role of Magnetic Tongues

POISSON, MARIANO; DÉMOULIN, PASCAL; MANDRINI, CRISTINA H.; FUENTES, MARCELO C. LÓPEZ

The Astrophysical Journal

IOPscience

Año: 2020 vol. 894

The presence ofelongations in active-region (AR) polarities, called magnetic tongues, is mostlyvisible during their emergence phase. AR tilts have beenmeasured thoroughly using long-termwhite-light (WL) databases,sometimes combined with magnetic-field information. Since theinfluence of magnetic tongues on WL tilt measurements has not beentaken into account before, we aim to investigate their role intilt-angle values and to compare them with those derived from LOSmagnetograms. We apply fourmethods to compute the tilt angleof generally bipolar ARs: one applies the k-means algorithm toWL data, a second one includes the magnetic-field sign of thepolarities to WL data, and a third one uses the magneticflux-weighted center of each polarity. The tilt valuescomputed in any of these ways are affected by the presence ofmagnetic tongues. Therefore, we apply the newly developed CoreField Fit Estimator (CoFFE)method to separate the magneticflux in the tongues from that inthe AR core. We compare the fourcomputed tilt-angle values, as well as these with the onesreported in long-term WL databases. For ARs with low-magnetic-fluxtongues, the different methodsreport consistent tilt-anglevalues. But for ARs with high-flux tongues, there are noticeablediscrepancies between all methods, indicating that magnetic tonguesdifferently affect WL andmagnetic data. However, ingeneral, CoFFE achieves a better estimation of the main bipoletilt because it removes both theeffect of tongues as well as the emergence ofsecondary bipoles when it occurs in between themain bipole magnetic polarities.