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

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

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2020 vol. 633

0004-6361

The magnetic polarities of bipolar active regions (ARs) exhibit elongationsin line-of-sight magnetograms during their emergence. These elongations arereferred to as magnetic tongues and attributed to the presence of twist in theemerging magnetic flux-ropes (FRs) that form ARs. The presence of magnetictongues affects the measurement of any AR characteristic that depends on itsmagnetic flux distribution. The AR tilt-angle is one of them. We aim to developa method to isolate and remove the flux associated with the tongues todetermine the AR tilt-angle with as much precision as possible. As a firstapproach, we used a simple emergence model of a FR. This allowed us to developand 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 appliedour method to photospheric observations of bipolar ARs that show clear magnetictongues. Using the developed procedure on the FR model, we can reduce thedeviation in the tilt estimation by more than 60%. Next we illustrate theperformance of the method with four examples of bipolar ARs selected for theirlarge magnetic tongues. The new method efficiently removes the spuriousrotation of the bipole. This correction is mostly independent of the methodinput parameters and significant since it is larger than all the estimated tilterrors. We have developed a method to isolate the magnetic flux associated withthe FR core during the emergence of bipolar ARs. This allows us to compute theAR tilt-angle and its evolution as precisely as possible. We suggest that thehigh dispersion observed in the determination of AR tilt-angles in studies thatmassively compute them from line-of sight magnetograms can be partly due to theexistence of magnetic tongues whose presence is not sufficiently acknowledged.