Solar Coronal X-ray Bright Points from Hinode/XRT Obsercations

Solar coronal X-ray bright points (XBPs) have been an enigma since their discovery in late 1960’s. XBPs have been studied in great detail using Skylab and Yohkoh X-ray images.Their correspondence with small bipolar magnetic regions was discovered by combining ground-based magnetic field measurements with simultaneous space-born X-ray imaging observations. The number of XBPs (daily) found on the Sun varies from several hundreds up to a few thousands. It is known that the observed XBP number is anti-correlated with the solar cycle, but this is an observational bias and the number density of XBPs is nearly independent of the 11-yr solar activity cycle. It is found that the diameters of the XBPs are around 10-20 arc sec and their life times range from 2 hours to 2 days. Studies have indicated the temperatures to be fairly low, T = 2 × 10⁶ K, and electron densities n_e = 5 ×10⁹cm⁻³ , although cooler XBPs exist. XBPs are also useful as tracers of coronal rotation (Kariyappa 2008, A & A, in press) and contribute to the solar X-ray irradiance variability. Assuming that almost all XBPs represent new magnetic flux emerging at the solar surface, their overall contribution to the solar magnetic flux would exceed that of the active regions. Since a statistical interaction of the magnetic field is associated with the production of XBPs, the variation of the XBP number on the Sun will be a measure of the magnetic activity of its origin.

Bright points are also observed in the chromosphere using high resolution CaII H and K spectroheliograms and filtergrams. Extensive studies have been conducted to determine their dynamical evolution, the contribution to chromospheric oscillations and heating, and to UV irradiance variability. The oscillations of the bright points at the higher chromosphere have been investigated using SOHO/SUMER Lyman series observations. It is known from these studies that the chromospheric bright points are associated with 3-min periods in their intensity variations. The study of the spatial and temporal relationship between the solar coronal XBPs and the photospheric and chromospheric magnetic features is an important issue in physics of the Sun.

The Hinode/XRT observations provide an opportunity to investigate and understand more deeply the dynamical evolution and nature of the XBP than has been possible to date and to determine their relation to the large-scale magnetic features. Such high resolution observations and investigations would be helpful in understanding the role of oscillations and the nature of the waves associated with XBPs to heat the corona.

A 7-hour time sequence of soft X-ray images obtained on April 14, 2007 from X-Ray Tele- scope (XRT) on-board the Hinode mission have been analysed. From the images, 14 XBPs (xbp1, xbp2,............, xbp14) and 2 background regions (xbp15 and xbp16) have been identified and selected in a quiet region near the center of the solar disk for analysis. The images have been calibrated using the routine xrt prep.pro in IDL under SSW and this routine performs many corrections. On the calibrated images the rectangular boxes, covering the selected XBPs, have been placed and derived the cumulative intensity values of the XBPs by

Figure 1: A sample of an image from the time series obtained by Hinode/XRT on April 14, 2007 at 17:05:17 UT. Where xbp1, xbp2,...........xbp14 are the X-ray bright points and xbp15 and xbp16 are the background coronal regions selected for the analysis.

adding all the pixel intensity values. After correcting for background coronal emission, the light curves of all the XBPs have been derived for further analysis. For the first time, the power spectrum analysis on XBP data has been used to determine the periods of intensity oscillations.

The XRT data shows that XBPs tend to produce small and large time scale fluctuations in their intensity and some periods of intensity oscillation are similar in all the XBPs. The periods observed with XRT data ranges from a few minutes to hours and these findings arein good agreement with the results derived from the analysis of full-disk images obtained by the Yohkoh/SXT experiment. Although at first sight the light curves of 14 XBPs seem to be very diverse in their pattern during evolution, the XBPs can broadly be grouped into three classes depending on their emission level. The class I XBPs show a very large intensity enhancement, whereas the class II XBPs show moderate brightness enhancement and the class III XBPs show only a marginal intensity enhancement during their dynamical evolution. Since the periods of intensity oscillation in all the three cases of XBPs seem to be similar, this can be taken as an evidence that heating mechanisms in the three cases of XBPs are similar. XBPs exhibit a wide variety of time scales ranging from a few minutes to hours in their intensity variations and the periods are almost similar in all the cases of XBPs and thus seems to be independent of the differences in the brightness enhancement. The XBPs are the sites where intense brightness enhancement is seen, and the brightness oscillates with different periods. It suggests that the regions of intense vertical magnetic field strength coincide with regions that are bright, indicating non-radiative heating, irrespective of the

sizes of these structures. A comparison between the XBPs and underlying photosphericmagnetic features has suggested that the horizontal component of the magnetic field may be playing an important role in driving the brightening of an XBP. Therefore the explanation of the existence of different classes of XBPs with similar periods among all the XBPs may be related to the different strengths of the magnetic field with which they have been associated. For further details refer Kariyappa & Varghese 2008, A & A, 485, 289.