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Numerical Simulations of Loops Heated to Solar Flare Temperatures - Part Two - X-Ray and Ultraviolet Spectroscopy
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1983
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In the first paper in this series, numerical simulation techniques were used to investigate the fluid dynamics of plasma that is confined to a magnetic flux tube and is heated to solar flare temperatures of about 2 × 10<SUP>7</SUP> K. The temperature, density, and velocity of the plasma were derived as functions of position and time in the flux tube or loop, after deposition of flare energy at the top of the loop. In this paper the results of the dynamical calculations described in the first paper in this series are used to predict the spectral line intensities, profiles, and wavelengths of several X-ray lines and the UV line of Fe XXI at 1354.1 Å. The distribution of emission from these lines within the loop is computed, as well as the profiles of the lines that would be recorded by a spectrometer that viewed the entire loop. Three different viewing orientations of the loop are considered. The computed spectra are compared with recent observations obtained from orbiting spacecraft. The computed differential emission measure is flatter than observed, and this result is similar to previous analytical and numerical calculations. <P />The computed X-ray profiles of Fe xxv and Ca xix lines show a stationary component, i.e., no shift in wavelength due to the Doppler effect, and shifted components produced by ablated chromospheric plasma. These profiles qualitatively resemble some of the X-ray observations. A large Doppler shift of about 0.7 Å is predicted for the Fe XXI line. Such a shift is unobserved in spectra obtained from the Naval Research Laboratory spectrograph on Skylab. <P />Physically different flare models can apparently produce markedly different spectroscopic results. Differences between computed and observed spectra suggest modifications of the model that might produce better agreement between these quantities and hence result in a better understanding of flare morphology and heating mechanisms.