Solar active regions: SOHO/CDS and TRACE observations of quiescent coronal loops

Abstract

This study focuses on the diagnostic capabilities of the TRACE and SOHO-CDS instruments to describe the physical characteristics of quiescent solar active regions. In particular we concentrate on the 1 MK loops, best seen in the TRACE EUV 173 Å passband. These are high-lying structures which can remain stable over timescales of hours to days. We present simultaneous observations of off-limb active region 1 MK loops performed with TRACE (173 Å and 195 Å bands) and the SOHO-CDS. The CDS spectral capabilities complement the TRACE observations well and allow an in-depth study of the plasma properties. Some important new results are presented here. In terms of cross-field characteristics, we find that these loops are isothermal and much cooler (T ≃ 0.7-1.1 × 106 K) than the surrounding diffuse high-lying active region emission (T ≃ 2 × 106 K). We show that the diffuse background-foreground constitutes the bulk of the high-lying quiescent active region emission. We explain some of the discrepancies found by various authors (based on SOHO and TRACE observations) with an overestimation of loop temperatures and densities. This was caused by inaccurate or neglected background subtraction in the data analysis. We studied the leg of a 1 MK loop and found that along its length temperatures and densities have steeper profiles compared to those derived from previous studies based on TRACE observations. The observations are consistent with an element abundance FIP effect of about 4, filling factors close to unity, and a constant density across the loop. We find a remarkable consistency between the CDS and TRACE observations, although we also find severe limitations to the use of TRACE diagnostic techniques.