The solar soft X-ray background flux and its relation to flare occurrence

The soft X-ray background flux (XBF) based on GOES 1–8 Å measurements for the period 1975–2003 is studied. There is strong evidence that in the XBF the flare contribution is not eliminated but the XBF is dominated by flare and post-flare emission of intense events. The significant delay (∼ 2 years) of the peak of the X-ray background flux with regard to sunspot numbers reported for cycle 21 recurs in the present cycle 23. The relation between monthly XBF and sunspot numbers can be well represented by a power law. For cycles 21 and 23 the derived fit values are the same within the uncertainties, whereas the values for cycle 22 are significantly different. It is suggested that the lag of the XBF in cycles 21 and 23 is a secondary effect related to the substantial contribution of energetic flares which is not fully subtracted out by the actual XBF definition.