The high temperature of the corona, 2 x 10⁶ K, recognized by the presence of highly ionized atoms, such as Fe^XIV, is caused by the ionization of accelerated electrons by a double layer which tends to form along magnetic field-aligned currents; ions could also be accelerated. It is shown that electric currents of intensity of about 10 mA, consisting of 10 Kev electrons, accelerated by the double layer along magnetic field-aligned currents, could provide the ionization rate of 6.2 x 10⁹/cm³s in the middle level of the corona, where the density of hydrogen atoms is about 10¹⁰/cm³; the ionization is thus about 60 %, as is known. A dynamo process of the photosphere may be responsible for generating the field-aligned currents. A double layer might be formed in a prominent discontinuity of the electron density at about the height of 2 x 10³ km, where the above estimates are made.