Observations of the solar soft X-ray irradiance by the student nitric oxide explorer

The student nitric oxide explorer (SNOE) satellite made daily solar observations of the solar soft X-ray irradiance over most of the period from March 10, 1998 through March 16, 2002. Wavelengths below 30 nm, referred to as soft X-rays, were measured in broadband channels consisting of photodiodes with thin film filters deposited directly on the diode surfaces. SNOE had three such channels measuring in the bands 0.1-7, 6-19 , and 17-20 nm. The solar rotational (∼27-day) variability in these bands is 44%, 28%, and 14%, respectively, and the solar cycle (11-year) variability is approximately factors of 11, 6 and 5, respectively. The SNOE observations are compared to more recent observations by the TIMED SEE instrument which uses the same technique at similar levels of solar activity and are found to be larger by a factor nearing two. The SNOE observations are shown however to be in excellent agreement with the EUVAC empirical model of solar irradiance.