Plasma diagnostics of active-region evolution and implications for coronal heating

Abstract

A detailed study is presented of the decaying solar-active region NOAA 10103 observed with the Coronal Diagnostic Spectrometer (CDS), the Michelson Doppler Imager (MDI) and the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). Electron-density maps formed using Si x (356.03 Å/347.41 Å) show that the density varies from ∼1010 cm-3 in the active-region core to ∼7 × 108 cm-3 at the region boundaries. Over the 5 d of observations, the average electron density fell by ∼30 per cent. Temperature maps formed using Fe XVI (335.41 Å)/Fe XIV (334.18 Å) show electron temperatures of ∼2.34 × 106 K in the active-region core and ∼2.10 × 106 K at the region boundaries. Similarly to the electron density, there was a small decrease in the average electron temperature over the 5-d period. The radiative, conductive and mass-flow losses were calculated and used to determine the resultant heating rate (P H). Radiative losses were found to dominate the active-region cooling process. As the region decayed, the heating rate decreased by almost a factor of 5 between the first and last day of observations. The heating rate was then compared to the total unsigned magnetic flux (φ tot = ∫ dA|Bz|), yielding a power law of the form PH ∼ φtot0.81±0.32 This result suggests that waves rather than nanoflares may be the dominant heating mechanism in this active region. © 2005 RAS.