The temperature structure of solar coronal plasmas

In the early 1940s it was at last accepted that the temperature of the solar corona is at least 1 MK and varies considerably from region to region throughout the solar activity cycle. It was recognized that during solar minimum periods the electron temperatures of plasmas in polar regions do not exceed 1 MK, but during solar maximum periods the plasma temperatures of highly active regions could be as high as 3 MK. Nevertheless, until recently the consensus among the solar physics community was that coronal temperatures vary among the different regions in a continuous manner. In the present paper we review the evidence showing that solar coronal plasmas (T_e>0.7 MK) are isothermal and their temperature can have only a small set of fixed values.