The Purple Mountain Observatory has made progress in the study of the trigger mechanism of solar flares

Recently, the “Multi-band Observation Study of Solar Activity” at the Purple Mountain Observatory of the Chinese Academy of Sciences has made progress in the study of the trigger mechanism of solar flare, providing new and powerful evidence for the photosphere shear motion trigger flare and magnetic implosion conjecture, and the research results were published in The Astrophysical Journal Letters.

Solar flares, as well as coronal mass ejections and dark strip eruptions, are thought to be different manifestations of the same explosive phenomenon in the Solar atmosphere, releasing high-energy particles and plasmas that are the main cause of catastrophic space weather. Therefore, the study of the energy accumulation process and trigger mechanism of these phenomena is a cutting-edge topic in contemporary solar physics. Over the decades, researchers have proposed many mechanisms to explain the specific processes of flare triggering, such as the Tether-cutting model, the Magnetic flux cancellation model, and the Magnetic breakout model. These models use the shear motion of the photosphere, especially the shear motion of sunspots, as an important link in the triggering process. However, in the observational reports of the flare triggered by the shear motion of the photosphere, pure photosphere shear motion is relatively rare, mostly accompanied by other forms of photosphere motion such as magnetic emergence, magnetic offset elimination, and rotational motion.

Recently, researchers based on the Solar Dynamic Observatory’s (SDO) atmospheric imager (AIA) and heliostat (HMI) data to study an M3.3-magnitude flare at 11818 in the active zone on August 17, 2013. Observations suggest that prior to the outbreak, a sunspot in this active region was abruptly activated by a quasi-stationary state, undergoing a shear motion along the magnetic reversal line for about ten hours, eventually causing this flare to occur. During this period, the flow curves of different bands in this active zone showed significant spikes compared with the quasi-stationary stage. These spikes are thought to be precursors to the outbreak of this M3.3-magnitude flare.

In addition, near the peak moment of the flare (18:24 UT), the sunspot completed a 180-degree shift in the direction of motion within a few minutes and remained in motion until 18:48 UT. After ruling out the possibility of convective motion of the photosphere itself, through directional comparisons and magnitude calculations, the researchers confirmed that the phenomenon was caused by the Lorentz force exerted on the sunspots produced by the rapid reconstitution of the coronal magnetic field.

Figure 1.First row: Continuous spectral imaging of the photosphere recorded by SDO/HMI, where S1 is the target sunspot. Second line: Magnetic map of the vertical direction of the photosphere recorded by the SDO/HMI. The red (blue) arrow represents the horizontal velocity of the photosphere at a vertical magnetic field of less than -1000 G (greater than 1000 G).

Figure 2.(a) Displacement curve of the target sunspot. (b) Flow curves for each band of SDO/AIA

The event demonstrates the mutual magnetism of different heights in the Solar atmosphere, i.e. the reaction of the photosphere to the corona by the sudden activation of the sunspot shear motion before the flare occurs, and during the flare, by the reversal of the direction of sunspot motion, the corona reacts to the photosphere. The study provides evidence for the photosphere shear motion trigger flares and the magnetic implosion conjecture, which has been published in The Astrophysical Journal Letters. The research work has been funded by the National Natural Science Foundation of China, the key research and development project of the Ministry of Science and Technology, and the strategic leading science and technology project of the Chinese Academy of Sciences.

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