Long-term intensity variations in the solar helium II Lyman alpha line

A small grazing incidence monochromator on the Oso 4 spacecraft has monitored variations in the absolute intensity of a 10-A band of the solar EUV spectrum centered on the helium II Lyman α line at 304-A during the periods October to December 1967 and June 1968 to December 1969. Measurements were also made during these periods of the absolute intensity of the solar hydrogen Lyman α line at 1216 A. The instrument was recalibrated after eighteen months in orbit by comparison with sounding rocket spectrometers and showed no loss of efficiency in the helium II mode of operation. The mean observed value of the helium II Lyman α line intensity was 8.8×109 photons/cm²/sec (estimated photomeric accuracy ±30%) with long-term variations of up to 20% from the mean value and short-term flux increases of up to 25%. The observed value of the hydrogen Lyman α line intensity remained essentially constant at 3.6×1011 photons/cm²/ sec (±30%). Ground-based measurements of solar activity are seen to be poor indicators of the level of the solar EUV flux with the general level of the observed helium II Lyman α line intensity following most closely variations in the relative sunspot number.