Origin of coronal heating and activity
It has been well accepted by scientists that the origin of coronal heating and activity should exist in the solar surface (photosphere) and the convection zone below the photosphere. Since convection causes the gases to move intensively in the photosphere (Fig. 2), the magnetic-field lines emerging from the surface are bent or mixed by the intensive convection. As a result, the magneto-hydro dynamic (MHD) waves may be excited on the magnetic-field lines, or the magnetic-field lines may be much mixed in the corona above the surface to form a number of magnetic discontinuities. We believe that further understanding of these mechanisms will help to resolve the mystery of coronal heating.
Our understanding of the physical processes for heating would advance greatly if we were able to understand observationally details of the relation between the heated coronal loops and the motion and behavior of the magnetic field on the surface, where the loops’ magnetic-field lines are rooted. Since the coronal loops with considerably large diameter become into magnetic flux bundles with diameter of about 100km at the photosphere surface, it is essential to observe them as precisely as 0.2 arc second in angular resolution. The concentrated flux bundles are called thin magnetic flux tubes where the magnetic field is intensified to about 1.3k gauss.
Observations of the magnetic field on the solar surface
The Zeeman effect is mostly used to measure the surface magnetic field. Light emitted from or transmitting through an atmosphere with a magnetic field is affected by the magnetic field and, as a result, becomes polarized. In spectra data, an absorption line, which was a single line with no magnetic field, is split into several lines, if the strong magnetic field exists. By confirming how the line is split and to what magnitude it is polarized, we can estimate the magnitude and direction of the magnetic field at the surface. By using a retardation plate and a polarization plate, we measure the Stokes vector (I, Q, U, V), which represents the full state of polarization. Q and U components show linear polarization while V component shows circular polarization. Fig. 3 shows an example where two absorption lines sensitive to magnetic fields are measured as spectra.