Unusual spectral absorption observed in the 16 August 1989 limb flare

Abstract

A relative complete set of He I 10830 Angstrom profiles and their coincident slit-jaw Ha images of the large limb flare (2N/X20) of 16 August 1989 were observed by the solar spectrograph at Purple Mountain Observatory. In addition to the unusually broadened spectral profiles observed in the impulsive phase, more than half of the observed He I 10830 Angstrom profiles are characterized by central reversals, which were detected not only in the impulsive phase but also in the late decaying phase. The central-reversed profiles may exist at different heights, ranging from the solar limb to (3-4) x 10(4) km above. The absorption varies with time and position, with a typical lifetime and size of several minutes and 5-6 arc sec, respectively. Depths of the absorption profiles also change clearly. The absorptions are usually deeper at the loop footpoint near the solar limb and shallower at loop-top. However, the most unusual feature is that all the line-center wavelengths of them show no shift relative to that of the quiet chromosphere near the limb, implying the apparent velocities are zero while the associated emission profiles have different apparent velocities. Theoretical simulations demonstrate that the Doppler widths of the absorptions are in the range of (0.35-0.5) Angstrom and increase with height, and the source functions are (0.11-0.3) times the disk center intensity. However, the absorptions have a relative large range of optical thickness (0.1-1.3) in the I-3 component of the He I 10830 triplet. We have not observed such absorption in other limb flares, including the SB/X2.9 flare of 17 August 1989 that occurred in the same active region as the studied one (NOAA 5629). Our studies show that the absorption could not result from the scattering by the telluric atmosphere or from normal chromospheric absorption. This unique phenomenon may be related to extra intense X-ray flux and caused by diffuse and non uniform materials dissociated from the flare instead of self-absorption of the flare.