Through studying three homologous eruptive events in an active region, the authors conclude that shearing motions and magnetic flux cancellation play a dominant role leading to the recurrent eruptions, and are key processes forming the eruptive structures.
A large sample statistical study of normalized Lorentz force and torques in emerging magnetic field shows that the photospheric magnetic field has a rather small Lorentz forces and torques, close to a force-free state.
This study explores the magnetic triggers of recurrent active region jets. Both widely debated triggers, namely, flux cancellation and flux emergence, are associated alternatively to the apparently homologous jets.
In an MHD simulation of flux emergence, a δ-sunspot is formed spontaneously by a collision of areas with opposite polarities. Driven by convective flows and counter-streaming flows, sheared polarity inversion lines form and flux ropes are created above.
Helicity injection by the continued shear and converging flows contributes to a sigmoid’s sustenance, its core field twist, ans its eventual eruption.
Magnetic flux of opposite polarities belonging to two different emerging/emerged bipoles inside multipolar magnetic regions, can experience “collisional shearing”, a process resulting in strong shearing and fast cancellation of magnetic flux near the polarity inversion line. This type of flux cancellation is found to be the cause of a succession of major flares and CMEs in complex active regions.
Both magnetic flux emergence and shearing flows occurred before the X9.3 flare on 2017 September 6. This analysis shows that shearing flows played a more significant role in leading to the helicity and electric currents buildup before the major eruption.
The early phase of a flux emergence was observed by IRIS, and spectra of the accompanying UV bursts are analyzed. Many bursts appear to be associated with the magnetic flux cancellation, and almost all of them are located in regions with large squashing factors.
Two homologous circular-ribbon flares associated with two filament eruptions were observed and analyzed. The emergence of magnetic flux ropes helped to inject free energy into the region and drive the magnetic reconnection above it.
A comparison of the surface flow patterns in observation and numerical simulation suggests that the flux tube emerging speed has been overestimated in theories.