Free-Free Emission in the Far-Ultraviolet Spectral Range: A Resource for Diagnosing Solar and Stellar Flare Plasmas
We report the detection of free-free (bremsstrahlung) emission near 1200 Å from a flare at the solar limb observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The observations consist of a time series of slit spectra at a fixed pointing that lasted almost 2 hr, during which the observed solar region produced a C8 flare. Using the free-free continuum intensities in conjunction with intensities of high-temperature (106-107 K) emission lines that appear in the same wavelength range, we derive the flare plasma electron density, electron temperature, emission measure, and nonthermal mass motions before, during, and after the flare. We describe a new diagnostic method for determining the temperature of cooling plasmas. Because the free-free radiation is emitted primarily by the interaction of electrons with nuclei of H and He atoms, we are also able to derive the Fe/H, Al/H, and Ca/H abundance ratios from the line intensities of highly ionized Fe, Al, and Ca lines and the intensities of the free-free emission, assuming a He abundance. The present work demonstrates the exceptional plasma diagnostic potential of ultraviolet free-free continuum radiation when coupled with emission-line intensities. We demonstrate that a similar technique could be employed to diagnose plasma properties of stellar flares using a high-resolution spectrometer with a sufficiently large effective collecting area.