A Yohkoh BCS Search for Hot Prograde Flows
摘要
Recent spectroscopic analyses of observations from the EUV Variability Experiment (EVE) appear to show the presence of hot high-speed prograde flows in the solar atmosphere associated with solar active regions. However, the existence of these flows has not yet been confirmed in observations from other instruments or at other wavelengths. In this work we attempt to determine whether similar prograde flows can also be detected in soft X-ray spectroscopic data. To examine this, we analyze soft X-ray spectroscopic data from the Yohkoh Bragg Crystal Spectrometer (BCS). Since BCS was a whole-Sun instrument, in order to ensure clear spectroscopic results from a single active region, we restrict consideration to intervals when only a single active region moved across the Sun from limb to limb. We found three suitable intervals. Our analysis of the data for these intervals do not indicate the presence of prograde flows in soft X-rays, and we establish an upper limit of about 30 km s−1 for such a pattern. Three possibilities may account for this difference with the EVE observations: (a) the hot prograde flows may not exist, and the EVE result is an artifact; (b) the hot prograde flows may not occur at the higher temperatures observed by soft X-rays; (c) Yohkoh BCS may only have been capable of observing flare emissions and lacked the sensitivity necessary to detect quiescent active regions, suggesting that the physics of flaring loops may differ from that of slowly-varying active-region loops. We favor explanation (a) but have not identified the exact nature of the artifact.