The mystery of coronal heating

The problem of coronal heating is one the eight key mysteries in modern astronomy and astrophysics. Since the discovery of extremely hot corona more than 70 years ago,many theoretical models have attempted to explain this mystery. However, so far no model can explain the coronal heating process perfectly. Recently, there have been several new discoveries,such as the observation of hot plasma ejections along the ultrafine magnetic channels from the solar surface upward to the corona, hot upflows of type II spicules, magnetic tornados, and EUV cyclones,which imply that the hot corona may be heated by hot plasma upflows directly from the underlying atmosphere. But how do these hot upflows form? We notice the fact that the magnetic gradient is ubiquitous in the whole solar atmosphere, based on which we have proposed a new theoretical model, the magnetic-gradient pumping mechanism, to explain the coronal heating process.In this mechanism, each magnetic flux tube acts as a pump to drive the high energy particles to move upward, accumulate in the upper atmosphere, and finally form the hot corona. This model may provide a new approach to explain the mystery of coronal heating.