Solar flares are sudden rapid releases of energy. They occur when the magnetic fields on the Sun rearrange themselves into a state of lower energy giving up the tremendous amount of extra energy they had before rearrangement in the form of electromagnetic radiation and streams of high energy atomic particles. They are the most energetic explosions in our solar system. The amount of energy released can be huge, about the same as would be released by a 10 million volcanic explosions on Earth, and it is all released in just a few minutes. Huge flares occur only a few times during the solar cycle maximum, but many smaller flares occur much more frequently.
The left hand picture above is of the Sun as seen in xrays. The bright area with the circle around it is a place where a solar flare is occuring. The Sun is producing more x-rays there than usual. If you look closer at that spot (right hand image above) you find that most of the high energy x-rays (hard x-rays) are coming from three smaller areas. These are at the top of the magnetic field loops where magnetic reconnection is occuring and the energy is being released (changing form from magnetic energy into electromagnetic energy and particle motion) and at the bottom of the magnetic loops where the energetic particles produced at the top are hitting the denser plasma below and heating it up.
The high energy particle produced during magnetic reconnection at the top of the magnetic loop follow the magnetic field downward hitting the the solar surface and heating it up. Since the manetic field changes during reconnection, the place where the energy is deposited changes with time and so does the appearance of the solar flare. The images below are links to movie files evolution of two different flares with time.
Why do the high energy particles produced at the top of the magnetic field loops follow the magnetic field lines and end up at the bottom?