DIFFERENTIAL ROTATION RATE OF X-RAY BRIGHT POINTS AND SOURCE REGION OF THEIR MAGNETIC FIELDS

The differential rotation rate of the solar corona has statistically been estimated from the motion of X-ray bright points (XBPs) that were observed with the Yohkoh soft X-ray telescope and the source region of magnetic fields is discussed from the evaluated rotation rate. The rotation rate of a pair of XBPs in a given latitude band is calculated from the position of an XBP in an X-ray image and that of an arbitrary-chosen XBP in a different X-ray image obtained with a time lag Δt. We have made the histogram of rotation rate from such samples in each latitude band for many X-ray images, and it shows a symmetric distribution that is well fitted by a Voigt function, not a Gaussian function, with a background. The modal rates at various longitudes in a given latitude band become the same value when a height is adopted as a characteristic height of XBPs, and we define the modal value against latitude as the differential rotation rate of XBPs in the present study. The differential rotation rate shows a similar trend as that of photospheric magnetic fields evaluated from the local correlation technique. We find that the differential rotation rate changes with a parameter Δt that is associated with the lifetime of XBPs, and that it becomes smaller in height for shorter Δt with the lower end that approaches to the rotation rate determined from the photospheric Doppler measurements. This trend suggests that magnetic fields associated with XBPs with a short lifetime are rooted just below the surface of the Sun at the top of the convection zone, and that they have a different origin from active regions.