Are Coronal Loops Isothermal or Multithermal? Yes!

TL;DR

This study uses simultaneous TRACE and SOHO/CDS observations to analyze the temperature structure of solar coronal loops, finding some are isothermal while others are multithermal, highlighting the complexity of coronal loop thermal properties.

Contribution

It provides the first detailed comparison of coronal loop temperature structures using simultaneous multi-instrument observations, clarifying the isothermal versus multithermal nature.

Findings

Loop 1 is isothermal at Log T = 5.85

Loop 2 is multithermal with emission between Log T 6.2 and 6.4

Loop 3 appears multithermal but may be affected by blending

Abstract

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!