ADAS analysis of the differential emission measure structure of the inner solar corona^*

II. A study of the “quiet Sun” inhomogeneities from SOHO CDS-NIS spectra

¹ Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: alanzafame@ct.astro.it
² Kwasan Observatory, Kyoto University, Yamashina, Kyoto 607-8471, Japan
³ Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

Received: 4 October 2004
Accepted: 20 November 2004

Abstract

We present a study of the differential emission measure (DEM) of a “quiet Sun” area observed in the extreme ultraviolet at normal incidence by the Coronal Diagnostic Spectrometer (CDS) on the SOHO spacecraft. The data used for this work were taken using the NISAT_S observing sequence. This takes the full wavelength ranges from both the NIS channels (308–381 Å and 513–633 Å) with the 2 arcsec by 240 arcsec slit, which is the narrowest slit available, yielding the best spectral resolution. In this work we contrast the DEM from subregions of 2  80 arcsec² with that obtained from the mean spectrum of the whole raster (20  240 arcsec²). We find that the DEM maintains essentially the same shape in the subregions, differing by a constant factor between 0.5 and 2 from the mean DEM, except in areas were the electron density is below  cm^-3 and downflow velocities of 50 km s^-1 are found in the transition region. Such areas are likely to contain plasma departing from ionisation equilibrium, violating the basic assumptions underlying the DEM method. The comparison between lines of Li-like and Be-like ions may provide further evidence of departure from ionisation equilibrium. We find also that line intensities tend to be lower where velocities of the order of 30 km s^-1 or higher are measured in transition region lines. The DEM analysis is also exploited to improve the line identification performed by [CITE] and to investigate possible elemental abundance variations from region to region. We find that the plasma has composition close to photospheric in all the subregions examined.