Volume 523, November-December 2010
|Number of page(s)||12|
|Published online||16 November 2010|
Radiative and magnetic properties of solar active regions
II. Spatially resolved analysis of O V 62.97 nm transition region emission
Space Science and Technology DepartmentSTFC Rutherford Appleton
Laboratory, Chilton, Didcot
OX11 0QX, UK
2 Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
Accepted: 11 August 2010
Context. Global relationships between the photospheric magnetic flux and the extreme ultraviolet emission integrated over active region area have been studied in a previous paper by Fludra & Ireland (2008, A&A, 483, 609). Spatially integrated EUV line intensities are tightly correlated with the total unsigned magnetic flux, and yet these global power laws have been shown to be insufficient for accurately determining the coronal heating mechanism owing to the mathematical ill-conditioning of the inverse problem.
Aims. Our aim is to establish a relationship between the EUV line intensities and the photospheric magnetic flux density on small spatial scales in active regions and investigate whether it provides a way of identifying the process that heats the coronal loops.
Methods. We compare spatially resolved EUV transition region emission and the photospheric magnetic flux density. This analysis is based on the O V 62.97 nm line recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) and SOHO MDI magnetograms for six solar active regions. The magnetic flux density φ is converted to a simulated O V intensity using a model relationship I(φ,L) = CφδLλ, where the loop length L is obtained from potential magnetic field extrapolations. This simulated spatial distribution of O V intensities is convolved with the CDS instrument’s point spread function and compared pixel by pixel with the observed O V line intensity. Parameters δ and λ are derived to give the best fit for the observed and simulated intensities.
Results. Spatially-resolved analysis of the transition region emission reveals the complex nature of the heating processes in active regions. In some active regions, particularly large, local intensity enhancements up to a factor of five are present. When areas with O V intensities above 3000 erg cm-2 s-1 sr-1 are ignored, a power law has been fitted to the relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. The average power index δ from all regions is 0.4 ± 0.1 and λ = − 0.15 ± 0.07. However, the scatter of intensities in all regions is significantly greater than ± 3σ from the fitted model. We therefore determine for the first time an empirical lower boundary for the IOV − φ relationship that is the same for five active regions. We postulate that it represents a basal heating. Because this boundary is present in the spatially-resolved data, this is compelling proof that the magnetic field is one of the major factors contributing to the basal component of the heating of the coronal plasma. We discuss the implications for the diagnostics of the coronal heating mechanism.
Key words: Sun: UV radiation / Sun: surface magnetism / Sun: transition region / Sun: corona / magnetic fields
© ESO, 2010
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