Evidence for Non-Maxwellian Electron Energy Distributions in the Solar Transition Region: Si III Line Ratios from SUMER

Recent calculations of Si III emission-line strengths are compared with SUMER observations from a quiet solar region, a coronal hole, and an active region. Diagnostic line ratios are used to derive Te and Ne for the emitting plasma in each region, and good agreement between theory and observations is found. A major enhancement in the intensity of the 1313 Å emission line is observed, as well as some evidence of a small decrease in the temperature of maximum ionization fraction, as one moves from the coronal hole to the active region. Possible explanations for these effects are discussed, and it is concluded that they may be caused by the presence of nonthermal electrons in the transition region.

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