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Since the discovery of X-ray bright points (XBP, Vaiana et al. 1970; Krieger et al. 1971), extensive studies have been performed and many theoretical models have been proposed (e.g., Priest et al. 1994; Yokoyama et al. 1996; Longcope et al. 1998). Approximately 1500 such small-scale point-like brightenings are born over the entire Sun every day. The average lifetime of XBPs reaches up to 8 hours (Golub et al. 1974). Magnetic reconnection is widely accepted as the heating mechanism of XBPs. If so, it is expected to observe large Doppler velocity, comparable to the coronal Alfven speed, in the reconnection outflow region. However, reported Doppler velocities of XBPs are typically ~10 km/s (e.g., Vilhu et al. 2002; Madjarska et al. 2003; Popesou et al. 2004; Xia et al. 2004), with the maximum of 35 km/s (Brosius et al. 2007), which is contradictory to the magnetic reconnection model. We intend to carry out a thorough investigation on the magnetic field, density and Doppler velocity of XBPs using HINODE observations in order to clarify the working mechanism of XBPs and solve the big puzzle. |
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