The sub-arcsec bright points (BP) associated with the small scale magnetic fields in the lower solar atmosphere are advected by the evolution of the photospheric granules. We measure various quantities related to the horizontal motions of the BPs observed in two wavelengths, including the velocity auto-correlation function. A 1 hr time sequence of wideband Hα observations conducted at the Swedish 1-m Solar Telescope (SST ), and a 4 hr Hinode G-band time sequence observed with the Solar Optical telescope are used in this work. We follow 97 SST and 212 Hinode BPs with 3800 and 1950 individual velocity measurements respectively. For its high cadence of 5 s as compared to 30 s for Hinode data, we emphasize more on the results from SST data. The BP positional uncertainty achieved by SST is as low as 3 km. The position errors contribute 0.75 km 2 s −2 to the variance of the observed velocities. The raw and corrected velocity measurements in both directions, i.e., (v x , v y ), have Gaussian distributions with standard deviations of (1.32, 1.22) and (1.00, 0.86) km s −1 respectively. The BP motions have correlation times of about 22 − 30 s. We construct the power spectrum of the horizontal motions as a function of frequency, a quantity that is useful and relevant to the studies of generation of Alfvén waves. Photospheric turbulent diffusion at time scales less than 200 s is found to satisfy a power law with an index of 1.59.
Aims. Our aim is to identify and trace the X-ray bright points (XBPs) over the disc and use them as tracers to determine the coronal rotation. This investigation will help to clarify and understand several issues: (i) whether the corona rotates differentially; (ii) whether the rotation depends on the sizes of the XBPs; and (iii) whether there is a dependence on phases of the solar magnetic cycle. Methods. We analysed the daily full-disc soft X-ray images observed with (i) X-Ray Telescope (XRT) onboard the Hinode mission during January, March, and April 2007; and (ii) Soft X-ray Telescope (SXT) onboard the Yohkoh from 1992 to 2001 using SSW in IDL. We used the tracer method to trace the passage of XBPs over the solar disc with the help of overlaying grids and derived the sidereal angular rotation velocity and the coordinates (latitude and longitude) of the XBPs. Results. We have determined the position of a large number of XBPs, both in Hinode/XRT and Yohkoh/SXT images, and followed them over the solar disc as a function of time. We derived the coronal sidereal angular rotation velocity and compared it with heliocentric latitude and as a function of solar activity cycle. In addition, we measured the sizes of all the XBPs and related them to the coronal rotation. The important results derived from these investigations are: (i) the solar corona rotates differentially like the photosphere and chromosphere; (ii) the sidereal angular rotation velocity is independent of the sizes of the XBPs; (iii) the sidereal angular rotation velocity does not depend on phases of the solar magnetic cycle; and (iv) the differential rotation of the corona is present throughout the solar magnetic cycle.
We have digitized the Ca II K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the MgIl h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg I1 h and k c/w ratio.
Aims. We investigate the variability in temperature as a function of time among a sample of coronal X-ray bright points (XBPs). Methods. We analysed a 7-h (17:00-24:00 UT) long time sequence of soft X-ray images observed almost simultaneously in two filters (Ti_poly and Al_mesh) on April 14, 2007 with X-ray telescope (XRT) onboard the Hinode mission. We identified and selected 14 XBPs for a detailed analysis. The light curves of XBPs were derived using the SolarSoft library in IDL. The temperature of XBPs was determined using the calibrated temperature response curves of the two filters by means of the intensity ratio method. Results. We find that the XBPs show a high variability in their temperature and that the average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in average X-ray emission. It is evident from the results of time series that the XBP heating rate can be highly variable on short timescales, suggesting that it has a reconnection origin.
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