The (0,1) band of the atmospheric system of O2(b1&Sgr;g-X3&Sgr;g) at 8640 ¿ exhibits a large enhancement at twilight; the morning enhancement greatly exceeds that in the evening. Some contribution to the enhancement arises from fluorescent scattering of sunlight by O2 in the 80- to 100-km region. Of comparable strength in the evening, but wholly dominant in the morning, is production of O2(1&Sgr;) by energy transfer from O(1D) produced in ozone photolysis within the same altitude region. The mesospheric ozone density can thereby de determined up to at least 100 km; at dawn the ozone density at 100 km approaches 108 cm-3, well above that predicted in some recent model calculations.