Radiative and magnetic properties of solar active regions

Free access article

Issue		A&A Volume 483, Number 2, May IV 2008


Page(s)		609 - 621
Section		The Sun
DOI		http://dx.doi.org/10.1051/0004-6361:20078183
Published online		11 March 2008

A&A 483, 609-621 (2008)
DOI: 10.1051/0004-6361:20078183

I. Global magnetic field and EUV line intensities

A. Fludra¹ and J. Ireland²

¹ STFC Rutherford Appleton Laboratory, Space Science and Technology Department, Chilton, Didcot, OX11 0QX, UK
e-mail: A.Fludra@rl.ac.uk
² ADNET Systems, Inc., NASA Goddard Space Flight Center, Mail Code 671.1, Greenbelt, MD 20771, USA
e-mail: Jack.Ireland@nasa.gov

(Received 28 June 2007 / Accepted 29 January 2008)

Abstract
Context. The relationships between the photospheric magnetic flux and either the X-ray or extreme ultraviolet emission from the solar atmosphere have been studied by several authors. Power-law relations have been found between the total magnetic flux and X-ray flux or intensities of the chromospheric, transition region, and coronal emission lines in solar active regions. These relations were then used to infer the mechanism of the coronal heating.
Aims. We derive accurate power laws between EUV line intensities and the total magnetic flux in solar active regions and discuss their applications. We examine whether these global power laws are capable of providing the diagnostics of the coronal heating mechanism.
Methods. This analysis is based on EUV lines recorded by the Coronal Diagnostic Spectrometer (CDS) on SOHO for 48 solar active regions, as they crossed the central meridian in years 1996-1998. Four spectral lines are used: He I 584.3 Å (3 $\times$ 10⁴ K), O V 629.7 Å (2.2 $\times$ 10⁵ K), Mg IX 368.06 Å (9.5 $\times$ 10⁵ K), and Fe XVI 360.76 Å (2.0 $\times$ 10⁶ K). In particular, the Fe XVI 360.76 Å line, seen only in areas of enhanced heating in active regions or bright points, has not been used before for this analysis.
Results. Empirical power laws are established between the total active region intensity in the lines listed above and the total magnetic flux. We demonstrate the usefulness of some spatially integrated EUV line intensities, $I_{\rm T}$ , as a proxy for the total magnetic flux, $\Phi$ , in active regions. We point out the approximate, empirical nature of the $I_{\rm T}-\Phi$ relationships and discuss the interpretation of the global power index. Different power index values for transition region and coronal lines are explained by their different dependence on pressure under the assumption of hydrostatic loop models. However, the global power laws are dominated by the size of the active regions, and we demonstrate for the first time the difficulties in uniquely relating the power index in the global $I_{\rm T}-\Phi$ relationship to the power index for individual loops and comment on results obtained by other authors. We caution against using global power laws to infer the coronal heating mechanism without a detailed knowledge of the distributions of the magnetic flux densities and instrumental response as a function of temperature. Despite these uncertainties, we show that the intensities of the transition region lines in individual loops depend on the photospheric magnetic flux density, $\phi$ , through $I_{\rm tr} \propto \phi^{\delta_{\rm t}}$ , $\delta_{\rm t} < 1$ , and the coronal line Fe XVI, $I_{\rm Fe} \propto \phi^{\delta_{\rm c}}$ , $\delta_{\rm c} > 1$ , and under the assumption of hydrostatic loops we can place a constraint on the coronal heating models, obtaining the volumetric heating rate, $E_{\rm H}$ (erg cm^-3 s^-1), $E_{\rm H} \propto \phi^\gamma$ , where 0.6 < $\gamma$ < 1.1.

Key words: Sun: UV radiation -- Sun: magnetic fields -- Sun: corona -- Sun: transition region

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