Observations of 1–8 Å solar X-ray variability during solar cycle 21

Abstract

One of the goals of the Solar Electromagnetic Radiation Flux Study (SERFS) for the World Ionospheric-Thermospheric Study (WITS) is to provide a more complete record of the variation of the daily 1–8 Å solar soft X-ray flux. As part of this study, I will calculate the daily mean and the daily background X-ray fluxes. The background fluxes, which I present in this paper, should be indicators of the quiescent X-ray flux principally from active regions. In contrast to this daily background flux, the daily mean flux includes the variable emissions from flares and coronal mass ejections. Although the present study uses 15 years of data from the NOAA Geostationary Operational Satellites (GOES) for 1974 to 1988, the inclusion of Solar Radiation (SOLRAD) data could extend this type of analysis back to 1964. In this paper, daily background X-ray fluxes in the form of monthly averages and annually-smoothed (13-month) values are presented for Solar Cycle 21. Intermediate term variations (on the scale of months) of the 1–8 Å background flux roughly mimic those of other chromospheric and coronal indices such as Ca K, Helium 10830 Å, and 10-cm radio flux. The annually-smoothed daily background X-ray flux was unique in peaking the latest and in showing no broad maximum. This annually-smoothed behavior is best matched by photospheric white-light facular areas. I also discuss anticipated future developments in soft X-ray monitoring. These include a new series of full-Sun soft X-ray photometers on the non-spinning GOES I-M. Also on GOES will be the operational USAF-NOAA Solar X-ray Imagers measuring the 8–20 Å, the 20–60 Å, and the thermospherically-crucial 255–300 Å bands. Nevertheless, this report and that of /1/ both note the continuing existence of an “XUV flux measurement gap,” with no long-term monitoring being planned between the soft X-ray region and the Lyman-alpha wavelengths.