Observations of Active Region Loops with the EUV Imaging Spectrometer on Hinode

TL;DR

This study uses Hinode's EUV Imaging Spectrometer to analyze active region coronal loops, revealing narrow temperature distributions and low filling factors that support filament-based heating models.

Contribution

First detailed spectroscopic analysis showing that active region loops have narrow temperature distributions and low filling factors, supporting filament models of coronal heating.

Findings

Coronal loops have narrow temperature distributions ($\sigma_T extless 3 imes10^5$ K).

Filling factors in these loops are approximately 10%.

Results support filament-based models of coronal heating.

Abstract

Previous solar observations have shown that coronal loops near 1 MK are difficult to reconcile with simple heating models. These loops have lifetimes that are long relative to a radiative cooling time, suggesting quasi-steady heating. The electron densities in these loops, however, are too high to be consistent with thermodynamic equilibrium. Models proposed to explain these properties generally rely on the existence of smaller scale filaments within the loop that are in various stages of heating and cooling. Such a framework implies that there should be a distribution of temperatures within a coronal loop. In this paper we analyze new observations from the EUV Imaging Spectrometer (EIS) on \textit{Hinode}. EIS is capable of observing active regions over a wide range of temperatures (\ion{Fe}{8}--\ion{Fe}{17}) at relatively high spatial resolution (1\arcsec). We find that most isolated coronal loops that are bright in \ion{Fe}{12} generally have very narrow temperature distributions ($\sigma_T \lesssim 3\times10^5$ K), but are not isothermal. We also derive volumetric filling factors in these loops of approximately 10%. Both results lend support to the filament models.