As is well known, magnetic field plays a key role in solar eruptions. In this study, the researchers used the line-of-sight magnetograms observed by Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic Imager (HMI) to investigate the evolution of magnetic field.

They found that the convergence motion of two opposite polarities occurred in active region NOAA 12790. When two opposite polarities approached together, the continuous magnetic cancellation occurred at the polarity inversion line (PIL) in this active region.

Due to the magnetic cancellation, a unidirectional and a bidirectional jet were found in the higher atmosphere above the site of magnetic cancellation by using the observations of SDO and New Vacuum Solar Telescope (NVST), and a hot channel was formed. With ongoing magnetic cancellation, the large-scale magnetic loops were formed, and several bidirectional jets occurred along the large-scale arched magnetic loops connecting the two opposite polarities.

The magnetic cancellation at the PIL resulted in the increase of the axial magnetic flux and the instability of the hot channel. The eruption of the hot channel produced a C-class flare, a cusp structure, and a halo CME. The speed of the CME reached 1400 kilometers per second.

It is worth pointing out that the observational evidence of magnetic reconnection were also observed during the occurrence of the CME. The magnetic reconnection may play a vital role in the acceleration of the CME.

Studying the formation of CMEs is very important for forecasting the occurrence of disastrous space weather. This study reveals a clear physical process of the Halo CME formation and provides important observational basis for the forecast of solar eruptions.