Observations of transition region plasma

Recent space missions have changed our view of the solar transition region. In particular the ESA/NASA Solar and Heliospheric Observatory (SOHO) and NASA's TRACE satellite have provided a unique opportunity to explore the solar atmosphere in detail. The combination of high spatial, spectral and temporal observations has made it possible to derive three dimensional images of the emission and velocity structures of solar features. Active region loop structures at transition region temperatures appear to be extremely time variable and dynamic, a result with profound implications for our understanding and modeling of the upper solar atmosphere. Large Dopplershifts have also been observed in these structures. A 3-minute transition region oscillation has been observed above sunspots suggesting upward-propagating acoustic waves. Clear evidence of velocity oscillations in the internetwork regions has also been observed in both the chromosphere and the transition region. The longstanding and puzzling problem of the apparent net red shift of emission lines from the transition region has been revisited. The extensive wavelength coverage of the SOHO spectrometers has made it possible to extend the measurements to much higher temperatures compared to previous instruments. The combination of magnetograms, EUV spectral imaging and the high resolution broad-band images from TRACE has also given us new insight concerning the structure of the transition region and its relation with the photospheric magnetic field.