Rare "double-decker" filament eruption

Traditional model represents the chromospheric filaments as dense, cold (relative to its surroundings) material seating in the lower part of horizontal magnetic flux tubes. In this model, the magnetic forces support the filament material against gravity, keeping it from falling down to the solar surface. The model explains many properties of filaments including their relation with the coronal mass ejections. However, the magnetic flux tube is continuous structure, and when the filament erupts, it is expected that the entire flux tube erupts with it. There are, however, instances when only a portion of filament material erupts leaving other part intact. Such filament splitting was explained in the framework of so-called double-decker filaments, when the magnetic reconnection results in splitting of magnetic flux tube into two separate tubes. A filament eruption on 6 September 2025 at 14:51 UT observed by GONG at Cerro Tololo, Chile  captured the filament splitting into two separate filaments. There are some small flare-like brightennings, but no X-ray class flare.  Also, no CME was associated with this event, and so, this is a failed filament eruption. These observations provide a strong support for the double-decker filament model. See for yourself.

What is a double-decker filament?

It refers to two separate filaments situated above the same polarity inversion line. Rui Liu et al (2012, doi:10.1088/0004-637X/756/1/59) proposed two magnetic configuration of a double-decker filament: (a) the upper and lower branches are flux ropes, (b) the upper branch is the flux rope and the lower one is the sheared arcade. Such configuration allows the eruption of one of the filaments, without impacting the other filament.  The filament eruption could be triggered by a loss of equilibrium due to flux imbalance, the torus or kink instabilities. Sunspot rotation was observed prior to some double-decker filament eruptions. More recent studies (e.g., Dongxu Liu et al 2025, doi: 10.3847/2041-8213/addfca) provide support to torus instability as a trigger for the filament eruption.