The dynamical and time variable solar transition region observed with the coronal diagnostic spectrometer (CDS) on SOHO

Abstract

CDS on SOHO has detected an upper solar atmosphere that is more dynamic and time variable than previously expected. Exciting and unexpected is the strongly time variable and dynamic active region loops at transition region temperatures. Loops in the temperature range of 1–5 × 10 5 K, extending 20 000 – 90 000 km above the solar surface, are seen to appear and disappear in less than one hour. Spectral lines from the same loops are frequently Doppler shifted corresponding to plasma velocities of ±50–100 km s −1 in sections of the loops. The dynamic active region loops may be connected to the down-flows seen near sunspots, frequently in sunspot plumes. Other new, time variable and dynamic phenomena are also seen, i.e. the so called “blinkers”, and rotating transition region features. The blinkers occur in the quiet Sun network, with similar phenomena also being seen in active regions. They consist of strong temporal variations in the EUV line emission at transition region temperatures and last a few minutes. The rotating features are macro-spicule like structures, seen both on the limb and on the disk. They show blue- and red-shifted emission on either side of an axis extending above the foot-points. The appearance of variable loops and blinkers show that we need to revise our concepts of the structure of the upper solar atmosphere and the processes going on in these regions, perhaps in the direction of a more fine structured atmosphere.