Measurement of metastable N/+/1S/ 5755-A emission in the twilight thermosphere

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