Supergranular-scale magnetic flux emergence beneath an unstable filament^⋆

Space Research Group – Space Weather, Departamento de Física y MatemáticasUniversidad de Alcalá, 28801 Alcalá de Henares, Madrid, Spain
e-mail: judith.palacios@uah.es

Received: 19 December 2013
Accepted: 2 August 2015

Abstract

Aims. Here we report evidence of a large solar filament eruption on 2013, September 29. This smooth eruption, which passed without any previous flare, formed after a two-ribbon flare and a coronal mass ejection towards Earth. The coronal mass ejection generated a moderate geomagnetic storm on 2013, October 2 with very serious localized effects. The whole event passed unnoticed to flare-warning systems.

Methods. We have conducted multi-wavelength analyses of the Solar Dynamics Observatory through Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) data. The AIA data on 304, 193, 211, and 94 Å sample the transition region and the corona, respectively, while HMI provides photospheric magnetograms, continuum, and linear polarization data, in addition to the fully inverted data provided by HMI.

Results. This flux emergence happened very close to a filament barb that was very active in mass motion, as seen in 304 Å images. The observed flux emergence exhibited hectogauss values. The flux emergence extent appeared just beneath the filament, and the filament rose during the following hours. The emergence acquired a size of 33′′ in ~12 h, about ~0.16 km s^-1. The rate of signed magnetic flux is around 2 × 10¹⁷ Mx min^-1 for each polarity. We have also studied the eruption speed, size, and dynamics. The mean velocity of the rising filament during the ~40 min previous to the flare is 115 ± 5 km s^-1, and the subsequent acceleration in this period is 0.049 ± 0.001 km s^-2.

Conclusions. We have observed a supergranular-sized emergence close to a large filament in the boundary of the active region NOAA11850. Filament dynamics and magnetogram results suggest that the magnetic flux emergence takes place in the photospheric level below the filament. Reconnection occurs underneath the filament between the dipped lines that support the filament and the supergranular emergence. The very smooth ascent is probably caused by this emergence and torus instability may play a fundamental role, which is helped by the emergence.