H_α spectra on 2017 December 28 and Doppler-shifted absorption features in the profiles. (a) Observed H_α spectra overlaid for comparison. (b) Observed (solid lines) and synthesized (dash-dotted gray lines) H_α spectra. For the last four observed spectra (solid colored lines), the first observed spectrum is superimposed for comparison. Arrows indicate the absorption features that appeared in the H_α profiles. (c) Subtracted H_α profiles and Gaussian fitting (dashed lines) for the subtraction at phase 0.703. (d) Differences between the first observed H_α spectrum and the subsequent four observed spectra (black solid lines), together with the Gaussian fittings (dashed lines). Phases and flare times are indicated in the panels, and all spectra are corrected to the rest velocity frame of the K0 IV primary star of UX Ari. (e) Temporal variation of the bulk velocities of the absorption feature. The blue and red dashed lines represent the linear fittings for the blueshifted and redshifted velocities, respectively. The gray dotted line indicates the zero-point of the velocity. (f) Temporal variation of the EWs of the absorption feature.