Modeling the magnetic field in active regions: LFFF versus NLFFF

NUEVO, F.A.; VALORI, G. ; MANDRINI, C.H.; LÓPEZ FUENTES, M.C.; VÁSQUEZ, A.M.

San Juan

Workshop; Towards Future Research on Space Weather Drivers; 2019

The knowledge of the three-dimensional coronal magnetic eld (B) at high-resolutionis key to better understand the physical mechanisms that trigger eruptive phenomena in theinner corona, the ultimate drivers of space weather conditions. High-resolution, direct coronalmeasurements of B are not available, so we must rely on suitable coronal eld models based onthe available photospheric eld measurements (magnetograms). For active region's (ARs) FIeld,where plasma beta << 1, the force-free regime (rotB = alpha B) is the simplest approximation. If is takento be constant across field lines, then the model field is called a linear force-free field (LFFF),while if alpha depends on position, then the model field is a non-linear force free field (NLFFF).While LFFF models are fully determined by the line-of-sight magnetograms as boundary condition,NLFFF models require information from the full vector magnetogram. In ARs with ahigh degree of non-potentiality, currents are observed to be concentrated in limited region ofcoronal space. The observational determination of using vector magnetograms confirms thatthis quantity is not constant in complex ARs. In such cases, large variations of are expectedthat cannot be accomodated for by LFFF models. In this work we test and compare specificnumerical implementations of both LFFF and NLFFF models applied to two ARs exhibitingdifferent levels of non-potentiality. We first compare the small-scale and the large-scale structureof the field, using observed coronal loops as proxies for the field-line tracing. Then, wecompare the amount of magnetic free energy and helicity obtained with both models for thesame integration box, and discuss the implications of our results for the determination of thecharacteristics of active phenomena with impact on Space Weather conditions. We also analyzethe convenience of using one method over the other for dierent applications.