The statistical properties of the solar soft X-ray fluence during 1997–2008

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The statistical properties of the solar soft X-ray (SXR) flare fluence, i.e. the time integral of SXR flux of a flare, $F_{\mathit{SXR}}$ , and sum of the $F_{\mathit{SXR}}$ of all flares produced by a solar active region (AR), $F_{\mathit{AR}}$ , during 1997–2008 have been investigated. The results show that $F_{\mathit{SXR}}$ has moderate correlation with the area of the associated AR, while the correlation between $F_{\mathit{AR}}$ and the largest area of the associated AR is also moderate. The total number of ARs that can produce at least one SXR flare during 1997–2008, $N_{t}$ , is 1408. The sum of $F_{\mathit{AR}}$ produced by 1408 ARs, $\sum F_{\mathit{AR}}$ , is 89 585.81 (erg s cm−2) and the average value of $F_{\mathit{AR}}$ is 63.6 (erg s cm−2). 34 ARs ( $F_{\mathit{AR}}\ge 500\ \mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}$ ) contributed 55.72 % of $\sum F_{\mathit{AR}}$ . 111 ARs $(100\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2})\le F_{\mathit{AR}}<500\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}))$ contributed 24.33 % of $\sum F_{\mathit{AR}}$ . 437 ARs $(10\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2} )\le F_{\mathit{AR}}< 100\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}))$ contributed 17.48 % of $\sum F_{\mathit{AR}}$ . The rest 826 ARs only contributed 2.52 % of $\sum F_{\mathit{AR}}$ . The number of ARs decreases dramatically with $F_{\mathit{AR}}$ and the distribution function of $F_{\mathit{AR}}$ is $N(F_{\mathit{AR}})=2840e^{-0.1286F_{\mathit{AR}}}$ . Finally, the north-south asymmetry of $F_{\mathit{AR}}$ has been investigated. The results show that the solar flare activities dominated in the northern hemisphere during 1997–2000 and the southern hemisphere during 2001–2008 respectively. During 1997–2008, the solar flare activities are stronger in the southern hemisphere than in the northern hemisphere.