Measurements of the solar soft X-ray irradiance by the Student Nitric Oxide Explorer: First analysis and underflight calibrations

Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft X-ray irradiance. These measurements are carried out by a multichannel photometer system. The spectral range between 2 and 20 nm is covered by three channels with bandpasses of 2-7 nm, 6-19 nm, and 17-20 nm respectively. Absolute sensitivities were measured preflight using the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. The results of the first 1.5 years of SNOE solar measurements are presented. During this time period the F_10.7 solar index varied between 80 and 250 × 10^-22 W m^-2 Hz^-1 and the 81-day average of the F_10.7 solar index varied between 100 and 175 × 10^-22 W m^-2 Hz^-1. The solar irradiances in the 2-7 nm interval varied between 0.3 and 2.5 mW m^-2, while the irradiances in the 6-19 and 17-20 nm intervals varied between 0.5 and 3.5 and 1.0 and 3.5 mW m^-2, respectively. The measured irradiances are correlated with the F_10.7 solar index with a correlation coefficient of ∼0.9 in all three bandpasses. For the levels of activity observed so far the SNOE measurements are typically a factor of 4.0 larger than the irradiances predicted by the Hinteregger et al. [1981] empirical model (hereafter the Hinteregger model). This fact and a long-term trend in the ratio of SNOE measurements to Hinteregger model predictions show that the Hinteregger model underpredicts the long-term variability in the solar soft X-ray irradiance. It is shown that other empirical models provide a reasonable representation of the 27-day variability but also underpredict the magnitude and long term variability. A sounding rocket measurement made on November 2, 1998, by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment prototype instrument using the same technique measured the solar irradiance in similar wavelength bands and produced results that are in good agreement with the SNOE measurements.