Variation of the X-ray bright point number over the solar activity

We have counted the number of X-ray bright points (XBPs) in the quiet-Sun region from Yohkoh soft X-ray images during a period of 1993-2000. Since we define XBPs as a small region that is less than similar to60" with a significantly enhanced emission in soft X-ray intensity compared with the adjacent background corona, the number of XBPs in the whole Sun area is affected by the soft X-ray intensity of the background corona and the presence of the solar active regions. Under these conditions, the number of XBPs in the whole Sun area is anticorrelated with the sunspot number owing to the change of background X-ray intensity and the occultation by active regions during the 11 yr activity cycle. In order to estimate the real number variation with little artifact, we have calculated the number of XBPs in a unit area by limiting the X-ray intensity range of the background corona and by removing a bias effect of a selected intensity threshold for the XBP counting. The evaluated change of the XBP number density in the quiet Sun is less than a factor of 2 over the period in the present study, and there is no clear enhancement in XBP number near the solar minimum. We conclude that the number density of XBPs is nearly independent of the 11 yr solar activity cycle. Since an XBP found in the quiet Sun is believed to be an event that is produced through an interacting process of opposite-polarity magnetic fields in the quiet Sun, the low-amplitude variation of the XBP number density suggests that there is a mechanism to create solar magnetic fields, which are different from those associated with active regions, irrespective of the 11 yr solar activity cycle.