Pressure diagnostics of coronal loops observed by NIXT

Abstract

The Normal Incidence X-ray Telescope (NIXT sounding rocket payload — a set of multilayer telescopes of novel design — provides images of the corona at sub-arcsec angular resolution in narrow X-ray spectral bands centered at the wavelengths of particular coronal emission lines. The NIXT 63.5 angstrom coronal images show the well-known and ubiquitous coronal loops but also, mostly in active regions, shallow and bright areas of intense emission not resembling loops. We have explained such areas within the traditional physics of coronal loop models as intense emission in the NIXT band coming from a narrow region at the base of high pressure loops; the particular nature of the NIXT temperature sensitivity, with its bimodal temperature response, is the key to detecting such a feature. We discuss the implications of this finding and, in particular, we show the possibility of new diagnostics of plasma pressure independent of the traditional one based on the determination of emission measure. In addition, we show that there is very little dependence of the spatial distribution of the X-ray emissivity in the NIXT passband on the details of the spatial distribution of the heating function.