A laboratory study of the λ2145 Å auroral mystery feature

The prominent emission feature near λ2145 Å in the ultraviolet spectrum of an aurora has been tentatively identified by Dick [1978] as the doublet lines, λ2139.68 Å and λ2143.55 Å, emitted by metastable N + ( 5 S) ions, and dissociative excitation of N 2 by electron impact with a cross section ≥2 × 10 −18 cm² has been proposed as the source of this species. A detailed laboratory study of dissociative excitation is described in this letter which suggests two alternative viewpoints of this process: (1) If the calculated radiative lifetime for the N + ( 5 S) state [4.4 µsec] is correct, then the N + ( 5 S) dissociative excitation cross section is <3 × 10 −21 cm². Thus, a new N + ( 5 S) source mechanism would have to be found in order to account for the auroral data. (2) If dissociative excitation does form N + ( 5 S) ions efficiently, then the laboratory and field observations imply a radiative lifetime for this state >10 msec, thus suggesting that there are major errors in the lifetime computation.