Modeling LOS magnetograms of emerging active regions

LOPEZ FUENTES, M.; POISSON, M.; MANDRINI, C. H.; GRINGS, F.; DÉMOULIN, P.

Simposio; IAU Symposium 365; 2023

Active regions (ARs) appear in the solar atmosphere as a consequence of the emergence of magnetic flux tubes [1]. The magnetic field of these flux-tubes acquire twist during their formation and rise through the convection zone, producing so called magnetic flux-ropes (FRs). Due to the presence of twist, during the emergence of these FRs the photospheric line-of-sight (LOS) magnetograms show an elongation of the AR polarities known as magnetic tongues. Since the magnetic tongues can affect the measurement ofAR characteristics obtained during their emergence phase (e.g. their tilt angle, magnetic flux and size, among others), direct estimations of the FR global quantities which do not consider this effect have to be revised [2]. Obtaining a good estimation of tilt angle evolutions and spatial variations plays a key role in constraining flux-transport dynamo models, as Joy’s law is fundamental for the formation and evolution of the polar field [3].In this work we use an analytical 3D model of a toroidal FR from which synthetic LOS magnetograms can be constructed by projecting the vertical component of the field over successive planes at different heights. We have seen in previous works that this simple model with 8-free parameters can reproduce most of the global features observed during the emergence of bipolar ARs (e.g. tilt angle and magnetic tongues extension) [4]. We study the emergence of 8 bipolar ARs using magnetograms from the Michelson DopplerImager (MDI) on board SOHO. Our method uses a probabilistic scheme based on the Bayes theorem to infer the most probable intrinsic parameters of the emerging FR, assuming a normal distribution for the differences between the model and the observations. We propose upgraded versions of the simple model by introducing different improvements (temporal correlations of the parameters, radial twist profile of the FR, and variable cross-section); this allows us to estimate expectation values for the tilt angle, magnetic helicity and magnetic flux, which are consistent with the observed ARs.[1] Y. Fan, Living Rev. Sol. Phys., 2009, 6, 4.[2] M. Poisson, P. D´emoulin,C.H. Mandrini, M. L´opez Fuentes, Astrophys. J., 2020, 894, 131.[3] Y. Wang, Space Sci. Rev., 2017, 210, 4.[4] M. Poisson, F. Grings, C.H. Mandrini, M. L´opez Fuentes, P. D´emoulin, Astron. and Astroph., 2022,665, A101.