Introduction: the nature of the solar chromosphere

The solar chromosphere owes its name to the brilliant red emission seen from the region just above the limb at times of total solar eclipse. The red emission is due to the overwhelming contribution of the Hα hydrogen line at the wavelength 6563 Å. This line emission is produced in large part by scattering of photospheric radiation from hydrogen atoms in the chromosphere, and gives little information about the chromospheric temperature. However, other emission lines such as the D3 line at 5876 Å (the discovery of which gave the name ‘helium’ to the responsible element) indicate a chromospheric temperature considerably higher than the temperature of the underlying photosphere.

Radiative transfer theory, however, indicates that in an atmosphere in radiative equilibrium the temperature generally decreases with height, reaching a surface value near 4300 K in the case of the Sun. Since the temperature in the chromosphere is substantially higher than this value, there must be a source of non-radiative energy to heat it. The height of the chromosphere seen above the solar limb is many times greater than the density scale height appropriate to chromospheric temperatures; this extension and observed rapid motions both indicate that the chromosphere is in a state of intense dynamic activity. The total energy fed into the chromosphere as heat and kinetic energy is about 4 × 106 erg cm-2 s-1, or about 10-4 of the solar luminosity. A small fraction of this large flux of energy penetrates even higher and heats the corona.