Horizontal photospheric flows trigger a filament eruption^⋆

¹ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, UPS, CNES 14 Avenue Édouard Belin, 31400 Toulouse, France
e-mail: thierry.roudier@irap.omp.eu
² Observatoire de Paris, LESIA, 5 Place Jules Janssen, 92195 Meudon, France
³ PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, France
⁴ Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN), Troitsk, Moscow, 108840 Russia
⁵ Department of Physics, DSB Campus, Kumaun University, Nainital, 263 001 India

Received: 2 March 2018
Accepted: 9 July 2018

Abstract

Context. A large filament composed principally of two sections erupted sequentially in the southern hemisphere on January 26, 2016. The central, thick part of the northern section was first lifted up and lead to the eruption of the full filament. This event was observed in Hα with the Global Oscillation Network Group (GONG) and Christian Latouche IMageur Solaire (CLIMSO), and in ultraviolet (UV) with the Atmospheric Imaging Assembly (AIA) imager on board the Solar Dynamic Observatory (SDO).

Aims. The aim of the paper is to relate the photospheric motions below the filament and its environment to the eruption of the filament.

Methods. An analysis of the photospheric motions using Solar Dynamic Observatory Helioseismic and Magnetic Imager (SDO/HMI) continuum images with the new version of the coherent structure tracking (CST) algorithm developed to track granules, as well as large-scale photospheric flows, has been performed. Following velocity vectors, corks migrate towards converging areas.

Results. The supergranule pattern is clearly visible outside the filament channel but difficult to detect inside because the modulus of the vector velocity is reduced in the filament channel, mainly in the magnetized areas. The horizontal photospheric flows are strong on the west side of the filament channel and oriented towards the filament. The ends of the filament sections are found in areas of concentration of corks. Whirled flows are found locally around the feet.

Conclusions. The strong horizontal flows with an opposite direction to the differential rotation create strong shear and convergence along the magnetic polarity inversion line (PIL) in the filament channel. The filament has been destabilized by the converging flows, which initiate an ascent of the middle section of the filament until the filament reaches the critical height of the torus instability inducing, consequently, the eruption. The n decay index indicated an altitude of 60 Mm for the critical height. It is conjectured that the convergence along the PIL is due to the large-scale size cells of convection that transport the magnetic field to their borders.