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) satel lite has made daily observations of the solar soft X-ray irradiance. These measurements are carried out by a multichannel photometer system. The spect ral 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 Facili ty of the National Institute of Standards and Technology. The results of th e first 1.5 years of SNOE solar measurements are presented. During this tim e period the F-10.7 solar index varied between 80 and 250 x 10(-22) W m(-2) Hz(-1) and the 81-day average of the F-10.7 solar index varied between 100 and 175 x 10(-22) W m(-2) Hz(-1). The solar irradiances in the 2 - 7 nm in terval 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 F10.7 solar index with a correlation coefficient of similar to0.9 in all three ba ndpasses. 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 thereafter the Hinteregger model) . This fact and a long-term trend in the ratio of SNOE measurements to Hint eregger 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 v ariability 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 prototyp e instrument using the same technique measured the solar irradiance in simi lar wavelength bands and produced results that are in good agreement with t he SNOE measurements.