Spatial Distribution and Temporal Evolution of Coronal Bright Points

Abstract

We present a statistical study of the spatial distribution and temporal evolution of coronal bright points (BPs) by analyzing a continuous set of observations of a quiet-Sun region of size 780'' × 780'' over a period of 55 hours. The main data set consists of observations taken by EIT (the Extreme-ultraviolet Imaging Telescope on board the SOHO spacecraft) in its Fe xii 195 Å channel which is sensitive to coronal plasma of temperature ∼ 1.5 MK; we also use soft X-ray observations by SXT (Soft X-ray Telescope on the Yohkoh spacecraft) which is sensitive to coronal plasma of temperature > 2.5 MK. The flux histogram for all pixels in EIT 195 Å images indicates that BPs have a power law flux distribution extending down to a level of 3σ (σ, root mean square deviation) above the average flux of the quiet Sun, while the bulk quiet Sun has a Gaussian-like flux distribution. Using a 3σ intensity threshold, we find a spatial density of one BP per 90 Mm × 90 Mm area, or equivalently 800 BPs for the entire solar surface at any moment. The average size of a BP is 110 Mm². About 1.4% of the quiet-Sun area is covered by bright points and the radiation from all BPs is only about 5% of that from the whole quiet Sun. Thus, the atmosphere above quiet-Sun regions is not energetically dominated by BPs. During the 55-hour period of EIT observations, we identify 48 full-life-cycle BPs which can be tracked from their initial appearance to final disappearance. The average lifetime of these BPs is 20 hours, which is much longer than the previously reported 8 hours based on Skylab X-ray observations (Golub et al., 1974). We also see shorter life times and smaller numbers of BPs in the soft X-ray images than in the EIT 195 Å observations, suggesting that the temperature of BPs is generally below 2 MK.