The ultraviolet dayglow at solar maximum: 1. Far UV spectroscopy at 3.5 Å resolution

The earth's far ultraviolet dayglow (1080–1515 Å) was observed at ∼3.5 Å resolution during a period of high solar activity near solar maximum on June 27, 1980. The observations were made at local noon by rocket‐borne spectrometers viewing toward the earth's northern limb at 90° zenith angle (ZA) at altitudes between 100 and 245 km, and at 98° ZA between 245 and 260 km. The solar zenith angle was 8.9°. These spectra are compared with earlier lower‐resolution dayglow data obtained during a period of lower solar activity and with auroral spectra. The brightness ratio of O I λ1356 to the N2 Lyman‐Birge‐Hopfield (LBH) system, an indicator of the O to N2 density ratio, is lower than that previously measured at mid‐latitudes and closer to the value found in aurorae. In the LBH system a depletion of the bands originating on the υ′ = 3 vibrational level of the excited state is found. Some weak N2 Birge‐Hopfield bands and N I lines only marginally detected previously in the dayglow are confirmed.