First 3D Reconstructions of Coronal Loops with the STEREO A+B Spacecraft. II. Electron Density and Temperature Measurements

Abstract

Using the stereoscopically derived three-dimensional (3D) geometry of 30 loops observed with STEREO EUVI (described in Paper I) we determine here the electron density profiles ne(s) and electron temperature profiles Te(s) from a triple-filter analysis of the stereoscopic images taken in the wavelengths of λ=171, 195, and 284 Å. The statistical results of our analysis of seven complete loops are: observed loop widths wobs=2.6+/-0.1 Mm, corresponding to effective loop widths of w=1.1+/-0.3 Mm if corrected for the instrumental point-spread function; loop flux ratios floop/ftotal=0.11+/-0.04 mean loop (DEM peak) temperatures Tp=1.1+/-0.2 MK; DEM temperature Gaussian widths σDEM=0.35+/-0.04 MK; temperature variations along loops σT/Tp=0.24+/-0.05 (resolution-corrected) loop base densities ne=(2.2+/-0.5)×109 cm-3 loop lengths of L=130+/-67 Mm; and all quantities are found to agree between STEREO A and B within a few percent. The temperature profiles T(s) along loops are found to be nearly constant, within the uncertainties of the background subtraction. The density profiles ne(s) are consistent with the gravitational stratification of hydrostatic loops, ne(h)=nbaseexp(-h/λT), defined by the temperature scale heights λT and stereoscopically measured from the height profiles h(s). The stereoscopic 3D reconstruction allows us for the first time to accurately measure the loop length L and to test loop scaling laws. We find that the observations are not consistent with equilibrium solutions, but rather display the typical overpressures of loops that have been previously heated to higher temperatures and cool down in a nonequilibrium state, similar to earlier EIT and TRACE measurements.