The chromospheric double hard X-ray (HXR) sources generally appear at the conjugate footpoints of flaring loops with asymmetric flux distributions. The behavior of such HXR footpoint asymmetry should be affected by several effects simultaneously and cannot be attributed to a single effect easily. In this study, we attempt to address the properties of photospheric magnetic fields in the areas coinciding with asymmetric HXR footpoints based on RHESSI observations during 2002-2009. A total of 172 time intervals in 22 flares closed to the solar disk center with available pre-flare MDI magnetograms are investigated. The strong HXR footpoint is found to preferentially (75%) locate at the region with weak magnetic field strength, which is qualitatively consistent with the asymmetric magnetic mirror scenario. The HXR footpoint fluxes become more asymmetric when the footpoints move to the areas with more asymmetric field strength. A feature of asymmetry reversal between different energy ranges is observed in some flares, although no significant energy dependence of footpoint asymmetry is found in our statistical results. We also investigated the possible causes of time-dependent HXR footpoint asymmetry by examining the 2004 November 4?M5.4 flare and the 2004 November 6 M3.6 flare. By comparing the estimated asymmetry quantities with the HXR light curves, the asymmetry reversal in the late period of the M5.4 flare is mainly attributed to the difference of coronal energy release or acceleration processes in different periods, while it is associated with the location changes of HXR footpoints moving to different magnetic field regions in the M3.6 flare.
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