Bibcode
Beck, C.; Rezaei, R.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 502, Issue 3, 2009, pp.969-979
Advertised on:
8
2009
Revista
Número de citas
42
Número de citas referidas
34
Descripción
Context: Observations made with the spectropolarimeter onboard the
HINODE satellite have detected abundant horizontal magnetic fields in
the internetwork quiet Sun. Aims: We compare the results for the
horizontal fields obtained at 630 nm with ground-based observations at
1.56 μm, where the sensitivity to magnetic fields is higher than in
the visible. Methods: We obtained 30-s integrated
spectropolarimetric data of the quiet Sun on disc centre during a period
of extremely stable and good seeing. The data have a rms noise in
polarization of around 2 × 10-4 of the continuum
intensity. The low noise level allows the spectra to be inverted with
the SIR code. We compare the inversion results with proxies to determine
the magnetic flux. Results: We confirm the presence of the
horizontal fields in the quiet Sun internetwork as reported for the
satellite data, including voids without linear polarization signal that
extend over an area of a few granules. Voids in the circular
polarization signal are only of granular scale. More than 60% of the
surface show polarization signals of above four times the rms noise
level. We find that the total magnetic flux contained in the more
inclined to horizontal fields (γ > 45°) is lower by a
factor of around 2 than that of the less inclined fields. The proxies
for flux determination are strongly affected by the thermodynamic state
of the atmosphere, and hence, seem to be unreliable. Conclusions:
During spells of good seeing conditions, adaptive optics can render
ground-based slit-spectrograph observations at a 70-cm telescope
equivalent to the seeing-free space-based data of half-meter class
telescopes. We suggest that the difference in the ratio of horizontal to
transversal flux between the ground-based infrared data and the
satellite-based visible data is due to the different formation heights
of the respective spectral lines. We emphasize that the true amount of
magnetic flux cannot be derived directly from the spectra. For purely
horizontal flux, one would need its vertical extension that has to be
estimated by explicit modeling, using the observed spectra as boundary
conditions, or be taken from MHD simulations. Time-series of the
evolution of the magnetic flux and chromospheric diagnostics are needed
to address its possible contribution to chromospheric heating.
Appendices A and B are only available in electronic form at
http://www.aanda.org
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