Properties of a Polar Coronal Hole During the Solar Minimum in 2007

TL;DR

This study analyzes the physical properties of a polar coronal hole during the 2007 solar minimum, comparing measurements of temperature, density, and elemental abundances with previous minima and models.

Contribution

It provides detailed measurements of electron and ion temperatures, densities, and elemental abundances in a polar coronal hole during the 2007 solar minimum, extending previous observations and testing model predictions.

Findings

Te, Ne, and Ti are similar in both minima.

No FIP effect observed in elemental abundances.

Ti decreases with charge-to-mass ratio, no upturn for q/M > 0.25.

Abstract

We report measurements of a polar coronal hole during the recent solar minimum using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Five observations are analyzed that span the polar coronal hole from the central meridian to the boundary with the quiet Sun corona. We study the observations above the solar limb in the height range of 1.03 - 1.20 solar radii. The electron temperature Te and emission measure EM are found using the Geometric mean Emission Measure (GEM) method. The EM derived from the elements Fe, Si, S, and Al are compared in order to measure relative coronal-to-photospheric abundance enhancement factors. We also studied the ion temperature Ti and the non-thermal velocity Vnt using the line profiles. All these measurements are compared to polar coronal hole observations from the previous (1996-1997) solar minimum and to model predictions for relative abundances. There are many similarities in the physical properties of the polar coronal holes between the two minima at these low heights. We find that Te, electron density Ne, and Ti are comparable in both minima. Te shows a comparable gradient with height. Both minima show a decreasing Ti with increasing charge-to-mass ratio q/M. A previously observed upturn of Ti for ions above q/M > 0.25 was not found here. We also compared relative coronal-to-photospheric elemental abundance enhancement factors for a number of elements. These ratios were about 1 for both the low first ionization potential (FIP) elements Si and Al and the marginally high FIP element S relative to the low FIP element Fe, as is expected based on earlier observations and models for a polar coronal hole. These results are consistent with no FIP effect in a polar coronal hole.