Clear perspective on the formation mechanism of filament is revealed, according to a new research published on The Astrophysical Journal Letter. Dr. YANG Bo, from Yunnan Observatories of the Chinese Academy of Sciences (CAS), is the lead author of the research.

Filaments are filled with cool, dense plasma, manifesting as cool clouds suspended in the surrounding hot tenuous corona. They are common products of the solar atmosphere. Understanding the formation and evolution of these structures can provide valuable information on the physics of the solar atmosphere.

As a promising candidate that accounts for the filament formation, chromospheric evaporation-coronal condensation model proposed that chromospheric plasma can be heated to several million kelvin and evaporated into the corona, and then thermal instability or thermal nonequilibrium causes catastrophic cooling and coronal condensation to form the cool dense filament. To date, observational evidence of chromospheric evaporation has not been detected in filament formation events and the origin of the footpoint heating has been unclear for a long time.

Using the coordinated observations by Hinode and Solar Dynamics Observatory (SDO), researchers study in detail the complete formation process of an active region filament in NOAA AR 11967 on 2014 February 2.

They found that the filament is formed through chromospheric evaporation and subsequent coronal condensation after an M-class confined flare. The observations from the SDO show that this confined flare is triggered by a sequential tether-cutting reconnection process as strong magnetic shear occurring during flux emergence. Owing to the reconnection, an elongated magnetic structure is newly created above the flaring the polarity inversion lines (PILs), which separately bridges its footpoints at two conjugate compact brightening regions.

Furthermore, the spectroscopic data from the EUV Imaging Spectrometer (EIS）on board Hinode reveal that explosive chromospheric evaporation takes place at the two compact brightening regions in the impulsive phase of the flare. In this course, because of the explosive injection of heated chromospheric plasma from its footpoints, the newborn elongated magnetic structure soon manifests as an X-ray sigmoid.

After the flare, cool material continuously condenses in the middle section of the sigmoid and then moves in opposite directions along the magnetic field lines of the sigmoid, eventually resulting in the formation of a filament.

The researchers’ observations imply that magnetic reconnection can not only form the magnetic field structure of the filament but also heat the chromospheric footpoints during their formation and drive chromospheric evaporation. As a result, the heated chromospheric plasma may be evaporated into the magnetic field structure of the filament, where the accumulated hot plasma might suffer from thermal instability or nonequilibrium, causing catastrophic cooling and coronal condensation to form the cool, dense material of the filament.

Finding the explosive evaporation prior to thermal nonequilibrium is the highlight of the research. As the anonymous referee stressed, "This observation can explain the footpoint heating hypothesized in evaporation-condensation model. Though the footpoint heating is essential in the evaporation-condensation model, the origin has been unclear for a long time. This observation witnesses that the origin is the flare."

Contact:

YANG Bo, YNAO, CAS

boyang@ynao.ac.cn