Measurements are reported of the 5755-¿ emission arising from the transition N+(1D-1S) made at Sutherland, South Africa (32.4 ¿S, 20.8 ¿E). The surface brightness of the emission in mid-November 1977 decayed from ~5 R at 10 ¿ solar depression angle (SDA) to ~1 R at 15 ¿ SDA. Using these data we have determined that less than 5% of all N2 ionizations result in the production of N+ ions in the 1S state. We find quenching of N+(1S) by neutral constituents and electrons to be negligible above 220 km in the twilight thermosphere in November and December 1977. Measurements of the 5755-¿ emission therefore potentially provide a ground based means of measuring the N2 density in the twilight thermosphere.