Thermo-field dynamics and one-loop order QED at finite temperature

Thermo Field Dynamics is used to evaluate the temperature corrections to: i) the electron mass, ii) the electric and magnetic correlation lengths, and iii) the electron magnetic moment. Calculations are carried out in a covariant way. The results for i) and ii) are consistent with calculations by other authors using different frameworks, while our temperature-dependent correction to the electron magnetic moment (iii) turns out to differ from the one previously evaluated. Ward-Takahashi identities are obtained and verified as a consistency proof. (orig.)