Enhanced vibration of molecular nitrogen is produced in SAR arcs by collisions with hot thermal electrons. The rate coefficient for the reaction between oxygen ions and molecular nitrogen is increased as a result of the vibrational excitation, and this leads to an increase in the rate of loss of F region ionization. By solving the F region continuity equation for conditions appropriate to SAR arcs both with and without allowance for the effects of vibrational excitation we show that vibrational excitation may be the cause of the F region electron density depression that is observed in SAR arcs. Our solutions also show than the observed depression cannot be caused by redistribution of ionization due to enhanced electron temperatures alone.