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In the absence of magnetic storms the 6300 A airglow is almost entirely due to F region ionic recombination when the local solar zenith angle exceeds 105 deg. Under certain circumstances there may be some enhancement from magnetically conjugate photoelectrons. The authors intend here to discuss their twilight observations quantitatively in terms of a mechanism based upon the photodissociation of O2 and to indicate how such measurements provide a simple and inexpensive means for monitoring the thermospheric abundance of O2. The discussion reveals (a) a large seasonal variation in the photodissociation (SR) component of twilight 6300 A, (b) enhancements in the SR component at 27 day intervals following a major magnetic storm, (c) appreciable changes in the SR component from day to day and (d) a means for isolating a fourth source for O (singlet D).