The EUV spectral irradiance of the Sun from 1997 to date

We present measurements of the EUV spectral irradiance we have obtained from radiance measurements with the SOHO Coronal Diagnostic Spectrometer, from 1997 to date. We discuss the contribution of the various regions of the Sun to the total EUV irradiance, and how they varied dramatically between the last two solar minima. These observations allow us to interpret spectral irradiance measurements in the EUV which have been available since 2002 with the TIMED mission. We also briefly discuss how changes in various activity indices compare with the EUV spectral variability, and the limitations of spectral modelling.

Diagnostics of Coronal Heating in Solar Active Regions

We study the relationship between EUV spectral line intensities and the photospheric magnetic field in solar active regions, using magnetograms from SOHO-MDI and EUV spectra of the Fe XVI 360.8 Â line (2 × 106 K) and the O V 629.7 A line (220,000 K) from the Coronal Diagnostic Spectrometer on SOHO, recorded for several active regions. We overlay and compare spatial patterns of the O V emission and the magnetic flux concentrations, with a 4″ x 4″ spatial resolution, and search for a relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. While this dependence exhibits a certain amount of scatter, it can be represented by a power law fit. The average power index from all regions is 0.7 ± 0.2. Applying static loop models, we derive the dependence of the heating rate on the magnetic flux density, Eh ∝ B0.8, and compare it to the dependence predicted by the coronal heating models.