Hα counterparts of X-ray bright points in the solar atmosphere

X-ray bright points (XBPs) are small-scale brightenings in the solar corona. Their counterparts in the lower atmosphere, however, are poorly investigated. In this paper, we study the counterparts of XBPs in the upper chromosphere where the H alpha line center is formed. The XBPs were observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft during the observing plan (HOP0124) in August 2009, coordinated with the Solar Magnetic Activity Research Telescope (SMART) in the Kwasan and Hida Observatory, Kyoto University. It is found that there are 77 H alpha brightenings in the same field of view of XRT, and among 57 XBPs, 29 have counterparts in the H alpha channel. We found three types of relationship: Types a, b and c, corresponding to XBPs appearing first, H alpha brightenings occurring first and no respective correspondence between them. Most of the strong XBPs belong to Type a. The H alpha counterparts generally have double-kernel structures associated with magnetic bipoles and are cospatial with the footpoints of the XBP loops. The average lag time is similar to 3 minutes. This implies that for Type a the heating, presumably through magnetic reconnection, occurs first in the solar upper atmosphere and then goes downwards along the small-scale magnetic loops that comprise the XBPs. In this case, the thermal conduction plays a dominant role over the non-thermal heating. Only a few events belong to Type b, which could happen when magnetic reconnection occurs in the chromosphere and produces an upward jet which heats the upper atmosphere and causes the XBP. About half of the XBPs belong to Type c. Generally they have weak emission in SXR. About 62% H alpha brightenings have no corresponding XBPs. Most of them are weak and have single structures.