Are Coronal Loops Isothermal or Multithermal? Yes!

Surprisingly few solar coronal loops have been observed simultaneously with TRACE and SOHO/CDS, and even fewer analyses of these loops have been conducted and published. The SOHO Joint Observing Program 146 was designed in part to provide the simultaneous observations required for in-depth temperature analysis of active region loops and determine whether these loops are isothermal or multithermal. The data analyzed in this paper were taken on 2003 January 17 of AR 10250. We used TRACE filter ratios, emission measure loci, and two methods of differential emission measure analysis to examine the temperature structure of three different loops. TRACE and CDS observations agree that Loop 1 is isothermal with Log T $=$ 5.85, both along the line of sight as well as along the length of the loop leg that is visible in the CDS field of view. Loop 2 is hotter than Loop 1. It is multithermal along the line of sight, with significant emission between 6.2 $<$ Log T $<$ 6.4, but the loop apex region is out of the CDS field of view so it is not possible to determine the temperature distribution as a function of loop height. Loop 3 also appears to be multithermal, but a blended loop that is just barely resolved with CDS may be adding cool emission to the Loop 3 intensities and complicating our results. So, are coronal loops isothermal or multithermal? The answer appears to be yes!

💡 Research Summary

The paper presents a detailed temperature diagnostics study of three coronal loops observed simultaneously with TRACE and SOHO/CDS on 2003 January 17 (active region AR 10250). The authors combine three complementary techniques: TRACE 171 Å/195 Å filter‑ratio analysis, emission‑measure (EM) loci curves derived from multiple CDS spectral lines, and two independent differential emission‑measure (DEM) inversion methods (regularized least‑squares and maximum‑entropy). Loop 1 shows a consistent TRACE ratio corresponding to Log T ≈ 5.85, and its CDS EM loci intersect at the same temperature. Both DEM reconstructions produce a narrow peak (ΔLog T ≈ 0.1), confirming that Loop 1 is essentially isothermal along the line of sight and throughout the visible leg. Loop 2 is hotter; TRACE ratios indicate Log T ≈ 6.2–6.4, while CDS EM loci do not converge at a single point but rather overlap over a broad temperature range. The DEM solutions reveal multiple peaks or a flat distribution between 6.2 < Log T < 6.4, indicating a multithermal plasma mixture along the line of sight. The loop apex lies outside the CDS field of view, preventing a height‑dependent temperature profile. Loop 3 exhibits the most complex behavior. TRACE ratios are highly variable, and CDS detects lines formed from 5.8 < Log T < 6.3. EM loci show several crossing points, and DEM analysis yields a broad, relatively flat temperature distribution. The authors suggest that unresolved neighboring structures, blended within the CDS resolution, may contribute cool emission and artificially broaden the temperature profile. Overall, the study demonstrates that coronal loops cannot be classified as universally isothermal or multithermal; the temperature structure depends on the specific loop, its geometry, and the observational limitations. By integrating imaging (TRACE) with spectroscopic (CDS) diagnostics and employing both EM loci and DEM techniques, the authors provide a robust framework for future investigations, especially with upcoming high‑resolution EUV instruments that will better resolve fine‑scale thermal structuring in the solar corona.