Dr. WANG Jincheng and his collaborators from Yunnan Observatories of the Chinese Academy of Sciences, utilizing data from the one-meter New Vacuum Solar Telescope (NVST) and other space telescopes, conducted a detailed study on the precursors and eruption process of solar filament eruptions, which explores the onset mechanisms of filament eruptions in solar active regions. The findings were recently published in Astronomy and Astrophysics.

Solar filaments, consisting of dense and cool plasma material suspended in the extremely hot corona, are common and spectacular features on the Sun. Their eruptions often lead to the generation of solar storms such as solar flares, coronal mass ejections, and large-scale solar oscillations. Understanding the filaments or prominences including their formation, stability, eruption, and magnetic field structures in the corona is a main subject in solar physics. How solar filaments become destabilized leading to intense eruptions has been a question without a certain answer.

In this study, researchers focused on the eruption process of the inverted U-shaped filament in active region NOAA 12680 on September 12, 2017. Utilizing observational data from the NVST telescope, the Solar Dynamics Observatory (SDO), and the Solar Terrestrial Relations Observatory-Ahead (STEREO-A), they studied the relevant physical processes preceding the eruption and explore the onset mechanism from two observational perspectives.

Two filament-like jet activities were found to occur before the eruption of the inverted U-shaped filament, both exhibiting characteristics of untwisting motion. During these activities, Y-shaped brightenings, newly emerging magnetic flux accompanied by magnetic cancellation, and the formation of newly moving fibrils were identified. Researchers concluded that the two filament-like jet activities were caused by magnetic reconnection between the newly emerging magnetic flux and the background open magnetic field, resulting in the release of the filament-confined magnetic field.

Approaching the eruption of the filament, outflows were observed being ejected from the brightenings associated with the two filament-like jet activities, indicating that these outflows underwent similar physical processes and led to the release of the filament-confined magnetic field. Nonlinear force-free field and potential extrapolations revealed that the filament was in a state of magnetic decay with a decay index below 1.0 before the eruption, but its twist number reached 2.4 turns. Additionally, a decrease was found in the magnetic field strength (magnetic pressure) above the filament.

Based on the above findings, researchers proposed a scenario for the onset process of the inverted U-shaped filament. The emergence of new magnetic flux triggers disturbances or uplifts in the confined magnetic field. Magnetic reconnection occurs between the disturbed confined magnetic field and the background open magnetic field, leading to the release of the confined magnetic field. The release of the confined magnetic field results in a decrease in magnetic pressure above the filament. When the confined magnetic field decreases to a critical value, the filament with strong twist numbers will uplift, leading to an eruption.

This study advances the understanding of solar filament eruptions, and offers insights into the onset mechanism of solar storms. It also enhances our ability to predict space weather.