X-ray bright points (XBPs) were first observed in rocket X-ray telescope images in 1969 by the AS&E group. They are small, compact, magnetically bipolar regions and are found in great numbers. These rockets and the Skylab mission permitted studies of the behavior of the XBP population as a function of time, and Davis (1983) found an anticorrelation between the XBP numbers and the sunspot number; however, Strong and Harvey noted that the XBPs detected by Yohkoh did not show such an effect.

In order to study this and other questions relating to the nature of XBPs, K. Nakakubo (Master's thesis, Gakugei Daigaku) has created software capable of finding XBPs in Yohkoh images automatically, taking into account the statistical significance of the bright-point detection and its morphology. This work was directed by H. Hara of the National Astronomical Observatory of Japan. An illustration of the results appears in the image below (click to enlarge).

The images on the left side of the figure above show SXT images from different times, and the images on the right have the automatically-detected XBP locations superimposed as contours. The plot at the bottom shows the counts vs time, with dotted lines showing the times of the images above; the images with large coronal bright regions have fewer XBP counts.

In its first application, this software has been used to study the time variation of XBP numbers across a good fraction of the solar cycle. The figure below illustrates this:

Here the (numerous) points identified with dots represent the XBP counts from 1993 onwards, with no corrections for sensitivity. The dotted line shows the sunspot number, which clearly has a drastically different time variation. The explanation of the number variation can be found in the histograms of XBP counts, as shown below:

The histograms on the left represent XBP brightnesses, while those on the right show the background signal level. The dotted lines (showing the 1993 histograms) can be compared with the histograms for subsequent years, and one can see that as the background gets higher, the faint XBPs become less visible. The XBP brightness histogram does not vary appreciably at the bright end of the range. The actual observed variability of the XBP number therefore probably can be explained by the variation in sensitive area on the disk - it is simply harder to see XBPs projected against brighter regions. This is the explanation put forward by Strong and Harvey. These results now confirm this quantitatively, and we will be able to do many other things with the automatically generated XBP statistics. We speculate that the very different behavior of XBPs and active regions suggests fundamentally different sources of magnetism - a "photospheric dynamo" for the XBPs, and a deep-seated traditional dynamo for the ARs. In this picture the XBPs result from swept-up weak field, reconnecting impulsively in the network boundaries and vertices, whereas the active-region field (including the "active network") results directly from emerging flux generated deep in the convection zone.

ISAS: June 5, 1998