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| Paper: |
Variation of the X-ray Bright Point Number over the Solar Activity Cycle |
| Volume: |
325, The Solar-B Mission and the Forefront of Solar Physics |
| Page: |
307 |
| Authors: |
Hara, H.; Nakakubo-Morimoto, K. |
| Abstract: |
We have counted the number of X-ray bright points (XBPs) in the quiet-Sun region from Yohkoh soft X-ray images during the period of 1993-2000. Since we define an XBP as a small region that is less than 60 arcsec 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 active regions. Under these conditions, the number of XBPs in the whole Sun area is anti-correlated with the sunspot number due to the change of background X-ray intensity and the occultation by active regions during the 11-year 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 two 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 XBP is nearly independent of the 11-year solar activity cycle. Since an XBP found in the quiet Sun is recognized to be an event which 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-year solar activity cycle. |
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