The relative vibrational population of N2+ (B2 &Sgr;u+) in a rare proton aurora, observed on March 8 (UT), 1978, was twice as high as the values found in almost all nighttime auroras. On the basis of various laboratory measurements the proton aurora observations point to precipitation of ~1.2-keV protons, carrying an energy flux of ~14 ergs cm-2 s-1. Simultaneous measurements of N2+ 1 NG (0, 1), (1, 2), (0, 2), and (1, 3) bands as well as H&bgr; and [O I> 6300-¿ emissions from this rare, low-energy proton aurora yield H&bgr;/N2+ 1 NG (0, 2) ?3 and [O I> 6300/N2+ 1 NG (0, 1) ?2.4. The corresponding values in the auroras excited predominantly by electrons are generally low (e.g., <0.7 and 0.6, respectively). Most of the N2+ 1 NG emissions from proton auroras are excited by protons (and not hydrogen atoms which constitute the bulk of the charge equilibrated auroral particle beam) interacting with N2.