The plasma filling factor of coronal bright points

Abstract

Aims. We determine the volumetric plasma filling factor of coronal bright points.Methods. Rastered spectra of the regions of the quiet Sun were recorded by the Extreme ultraviolet Imaging Spectrometer (EIS) during operations with the Hinode satellite. Calibrated intensities of Fe XII lines were obtained and images of the quiet corona were constructed. From the imaged spectra, the emission measures and geometrical widths of coronal bright points were obtained. Electron densities were determined from density-sensitive line ratios. A comparison of the emission measure and bright point widths with the electron densities yielded the plasma-filling factor.Results. The median electron density of coronal bright points is 4 × 10cm-3at a temperature of 1.6 ×106 K. The volumetric plasma-filling factor of coronal bright points was found to vary from 4 × 10-5 to 0.2 with a median value of 0.015.Conclusions. The current set of EIS coronal bright point observations indicates that the median value of their plasma filling-factor is 0.015. This can be interpreted as evidence of a considerable subresolution structure in coronal bright points or as the result of a single completely-filled plasma loop with a width on the order of 1-2 arcsec that has not been spatially resolved in these measurements. © 2008 ESO.