Correcting the effect of magnetic tongues on the tilt angle of bipolar active regions

Abstract

Context. The magnetic polarities of bipolar active regions (ARs) exhibit elongations in line-of-sight magnetograms during their emergence. These elongations are referred to as magnetic tongues and attributed to the presence of twist in the emerging magnetic flux-ropes (FRs) that form ARs. Aims. The presence of magnetic tongues affects the measurement of any AR characteristic that depends on its magnetic flux distribution. The AR tilt-angle is one of them. We aim to develop a method to isolate and remove the flux associated with the tongues to determine the AR tilt-angle with as much precision as possible. Methods. As a first approach, we used a simple emergence model of a FR. This allowed us to develop and test our aim based on a method to remove the effects of magnetic tongues. Then, using the experience gained from the analysis of the model, we applied our method to photospheric observations of bipolar ARs that show clear magnetic tongues. Results. Using the developed procedure on the FR model, we can reduce the deviation in the tilt estimation by more than 60%. Next we illustrate the performance of the method with four examples of bipolar ARs selected for their large magnetic tongues. The new method efficiently removes the spurious rotation of the bipole. This correction is mostly independent of the method input parameters and significant since it is larger than all the estimated tilt errors. Conclusions. We have developed a method to isolate the magnetic flux associated with the FR core during the emergence of bipolar ARs. This allows us to compute the AR tilt-angle and its evolution as precisely as possible. We suggest that the high dispersion observed in the determination of AR tilt-angles in studies that massively compute them from line-of sight magnetograms can be partly due to the existence of magnetic tongues whose presence is not sufficiently acknowledged.

Long-term studies of photospheric magnetic fields on the Sun

“…The magnetic tongues, or elongations of two main polarities of an active regions during their emergence were first reported by López Fuentes et al (2000). Poisson et al (2020) have shown that the active regions with weaker magnetic tongues show the magnetic tilts similar to those derived from white light images. However, for active regions with strong magnetic tongues, there could be significant difference in tilts derived from white light images and magnetograms.…”

“…For example, larger tilts will enhance the poleward transport of the magnetic flux of trailing polarity, and depending on the polarity orientation, could either strengthen or weaken the polar field (e.g., Baumann et al, 2004;Whitbread et al, 2017;Petrovay and Talafha, 2019). Several studies of the Hale polarity rule and the tilt angle of active regions were published in recent years (Illarionov et al, 2015;Li, 2018;Tlatova et al, 2018;Kuzanyan et al, 2019;Kleeorin et al, 2020;Poisson et al, 2020). Thus, for example, Poisson et al (2020) emphasized the importance of, so called, magnetic tongues in the determination of active region tilts using longitudinal magnetograms.…”

“…Several studies of the Hale polarity rule and the tilt angle of active regions were published in recent years (Illarionov et al, 2015;Li, 2018;Tlatova et al, 2018;Kuzanyan et al, 2019;Kleeorin et al, 2020;Poisson et al, 2020). Thus, for example, Poisson et al (2020) emphasized the importance of, so called, magnetic tongues in the determination of active region tilts using longitudinal magnetograms. The magnetic tongues, or elongations of two main polarities of an active regions during their emergence were first reported by López Fuentes et al (2000).…”