Issue |
A&A
Volume 483, Number 2, May IV 2008
|
|
---|---|---|
Page(s) | 609 - 621 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20078183 | |
Published online | 11 March 2008 |
Radiative and magnetic properties of solar active regions
I. Global magnetic field and EUV line intensities
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
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 (3104 K), O V 629.7 Å
(2.2
105 K), Mg IX 368.06 Å (9.5
105 K), and Fe XVI 360.76 Å
(2.0
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, IT, as a proxy for the total magnetic flux, Φ,
in active regions. We point out the approximate, empirical nature of
the 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
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, ϕ, through
,
, and the coronal
line Fe XVI,
,
, and
under the assumption of hydrostatic loops we can place a constraint
on the coronal heating models, obtaining the volumetric heating
rate, EH (erg cm-3 s-1),
,
where 0.6 < γ < 1.1.
Key words: Sun: UV radiation / Sun: magnetic fields / Sun: corona / Sun: transition region
© ESO, 2008
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