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 N ASA's TRACE satellite have provided a unique opportunity to explore the sol ar atmosphere in detail. The combination of high spatial, spectral and temp oral observations has made it possible to derive three dimensional images o f the emission and velocity structures of solar features. Active region loo p structures at transition region temperatures appear to be extremely time variable and dynamic, a result with profound implications for our understan ding and modeling of the upper solar atmosphere. Large Dopplershifts have a lso been observed in these structures. A 3-minute transition region oscilla tion has been observed above sunspots suggesting upward-propagating acousti c waves. Clear evidence of velocity oscillations in the internetwork region s has also been observed in both the chromosphere and the transition region . The longstanding and puzzling problem of the apparent net red shift of em ission lines from the transition region has been revisited. The extensive w avelength coverage of the SOHO spectrometers has made it possible to extend the measurements to much higher temperatures compared to previous instrume nts. The combination of magnetograms, EUV spectral imaging and the high res olution broad-band images from TRACE has also given us new insight concerni ng the structure of the transition region and its relation with the photosp heric magnetic field.