Researchers led by ZHANG Yan and YAN Xiaoli from the Yunnan Observatories (YNAO) of the Chinese Academy of Sciences have revealed the formation mechanism of solar double-decker filament.

The study was published in The Astrophysical Journal on July 14.

Filaments, embedded in the solar corona, are 100 times cooler and denser than the coronal material. The filaments that appear at the limb as bright features are called prominences, and those that appear on the disk as a feature darker than their background are called filaments.

A double-decker filament refers to two filaments vertically distributed above the same polarity inversion line. There are two possibilities for magnetic configuration of a double-decker filament: One is that both the upper and lower branches are flux ropes, and the other is that the upper branch is the flux rope and the lower one is the sheared arcade. Up to now, the formation process of double-layer filaments is controversial.

In this study, the researchers used the multi-band data of the Solar Dynamics Observatory and the Hα image data of the Global Oscillation Network Group to investigate the filament in the active region NOAA 12665 from July 8 to 14, 2017.

They found that due to the photospheric magnetic motion and the magnetic reconnection, two small filaments joined together to form a longer magnetic structure of the filament. Then, the newly formed filament splitted into two branches and finally formed a double-decker filament.

Sunspot rotation, internal reconnections and the movement of the negative magnetic field were found to drive the filament split into two branches and finally form the double-decker filament. The internal reconnection can accelerate the separation of filaments.

The researchers used non-linear force-free field (NLFFF) extrapolation to obtain the magnetic configuration of the filaments. The results showed that the upper and the lower filaments were both magnetic flux ropes.

This work was supported by the National Science Foundation of China, Yunnan Science Foundation for Distinguished Young Scholars and Yunnan Key Laboratory of Solar Physics and Space Science.

 The magnetic structures of the double-decker filament from non-linear force-free field (NLFFF) extrapolation (Image by YNAO)