The relationship between UV and X-ray active region structures

Yohkoh and the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) jointly observed two brightenings in active region NOAA 7981 on 6 August 1996. Combining the UV data from CDS with information from the high time resolution coronal images obtained with the Soft X-ray Telescope (SXT) on Yohkoh, provides us with important information on the relationship between the transition region and corona. Our observations show that cool plasma (T_e = 2.2 x 10^-5 K) can lie at the same altitude as the hot coronal plasma (T_e = 1–4 x 10⁶ K). The lower temperature structure is not formed from the cooling of the hotter coronal loop. We are also able to observe a low temperature cut-off of T_e = 1–4 x 10⁶ K for a loop which repeatedly brightened over the period of approximately one day.