2,885 research outputs found
ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996?
A gap in the total solar irradiance (TSI) measurements between ACRIM-1 and
ACRIM-2 led to the ongoing debate on the presence or not of a secular trend
between the minima preceding cycles 22 (in 1986) and 23 (1996). It was recently
proposed to use the SATIRE model of solar irradiance variations to bridge this
gap. When doing this, it is important to use the appropriate SATIRE-based
reconstruction, which we do here, employing a reconstruction based on
magnetograms. The accuracy of this model on months to years timescales is
significantly higher than that of a model developed for long-term
reconstructions used by the ACRIM team for such an analysis. The constructed
`mixed' ACRIM - SATIRE composite shows no increase in the TSI from 1986 to
1996, in contrast to the ACRIM TSI composite.Comment: 4 figure
Milne-Eddington inversions of the He I 10830 {\AA} Stokes profiles: Influence of the Paschen-Back effect
The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet
at 10830 {\AA}, leading to changes in strength and in position of the Zeeman
components of these lines. We illustrate the relevance of this effect using
synthetic Stokes profiles of the He I 10830 {\AA} multiplet lines and
investigate its influence on the inversion of polarimetric data. We invert data
obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
Tower Telescope (VTT). We compare the results of inversions based on synthetic
profiles calculated with and without the Paschen-Back effect being included. We
find that when taking into account the incomplete Paschen-Back effect, on
average 16% higher field strength values are obtained. We also show that this
effect is not the main cause for the area asymmetry exhibited by many He I
10830 Stokes V-profiles. This points to the importance of velocity and magnetic
field gradients over the formation height range of these lines.Comment: Accepted for publication in A&A on Jun 12th 200
Stokes diagnostics of simulated solar magneto-convection
We present results of synthetic spectro-polarimetric diagnostics of radiative
MHD simulations of solar surface convection with magnetic fields. Stokes
profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared
spectral ranges emerging from the simulated atmosphere have been calculated in
order to study their relation to the relevant physical quantities and compare
with observational results. We have analyzed the dependence of the Stokes-I
line strength and width as well as of the Stokes-V signal and asymmetries on
the magnetic field strength. Furthermore, we have evaluated the correspondence
between the actual velocities in the simulation with values determined from the
Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles
(zero-crossing shift). We confirm that the line weakening in strong magnetic
fields results from a higher temperature (at equal optical depth) in the
magnetic flux concentrations. We also confirm that considerable Stokes-V
asymmetries originate in the peripheral parts of strong magnetic flux
concentrations, where the line of sight cuts through the magnetopause of the
expanding flux concentration into the surrounding convective donwflow.Comment: Astronomy & Astrophysics, in pres
Quiet Sun magnetic fields observed by Hinode: Support for a local dynamo
The Hinode mission has revealed copious amounts of horizontal flux covering
the quiet Sun. Local dynamo action has been proposed to explain the presence of
this flux. We sought to test whether the quiet Sun flux detected by Hinode is
due to a local or the global dynamo by studying long-term variations in the
polarisation signals detectable at the disc centre of the quiet Sun between
November 2006 and May 2012, with particular emphasis on weak signals in the
internetwork. The investigation focusses on line-integrated circular
polarisation V_tot and linear polarisation LP_tot profiles obtained from the Fe
I 6302.5 \AA absorption line in Hinode SOT/SP. Both circular and linear
polarisation signals show no overall variation in the fraction of selected
pixels from 2006 until 2012. There is also no variation in the magnetic flux in
this interval of time. The probability density functions (PDF) of the
line-of-sight magnetic flux can be fitted with a power law from 1.17 x 10^17 Mx
to 8.53 x 10^18 Mx with index \alpha=-1.82 \pm 0.02 in 2007. The variation of
\alpha 's across all years does not exceed a significance of 1\sigma. Linearly
polarised features are also fitted with a power law, with index \alpha=-2.60
\pm 0.06 in 2007. Indices derived from linear polarisation PDFs of other years
also show no significant variation. Our results show that the ubiquitous
horizontal polarisation on the edges of bright granules seen by Hinode are
invariant during the minimum of cycle 23. This supports the notion that the
weak circular and linear polarisation is primarily caused by an independent
local dynamo
On the intensity contrast of solar photospheric faculae and network elements
Sunspots, faculae and the magnetic network contribute to solar irradiance
variations. The contribution due to faculae and the network is of basic
importance, but suffers from considerable uncertainty. We determine the
contrasts of active region faculae and the network, both as a function of
heliocentric angle and magnetogram signal. To achieve this, we analyze
near-simultaneous full disk images of photospheric continuum intensity and
line-of-sight magnetic field provided by the Michelson Doppler Interferometer
(MDI) on board the SOHO spacecraft. Starting from the surface distribution of
the solar magnetic field we first construct a mask, which is then used to
determine the brightness of magnetic features, and the relatively field-free
part of the photosphere separately. By sorting the magnetogram signal into
different bins we are able to distinguish between the contrasts of different
concentrations of magnetic field. We find that the contrasts of active region
faculae (large magnetogram signal) and the network (small signal) exhibit a
very different CLV, showing that the populations of magnetic flux tubes are
different. This implies that these elements need to be treated separately when
reconstructing variations of the total solar irradiance with high precision. We
have obtained an analytical expression for the contrast of photospheric
magnetic features as a function of both position on the disk and magnetic field
strength, by performing a 2-dimensional fit to the observations.Comment: 12 pages, 8 figures, uses aa.cl
The relationship between chromospheric emissions and magnetic field strength
Aims. We analyze observational data from 4 instruments to study the
correlations between chromospheric emission, spanning the heights from the
temperature minimum region to the middle chromosphere, and photospheric
magnetic field. Methods: The data consist of radio images at 3.5 mm from the
Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 A from TRACE, Ca II
K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric
magnetograms. For the first time interferometric millimeter data with the
highest currently available resolution are included in such an analysis. We
determine various parameters of the intensity maps and correlate the
intensities with each other and with the magnetic field. Results: The
chromospheric diagnostics studied here show a pronounced similarity in their
brightness structures and map out the underlying photospheric magnetic field
relatively well. We find a power law to be a good representation of the
relationship between photospheric magnetic field and emission from
chromospheric diagnostics at all wavelengths. The dependence of chromospheric
brightness on magnetic field is found to be different for network and
internetwork regions.Comment: 13 pages, 14 figures, 3 table
Solar Irradiance Variability and Climate
The brightness of the Sun varies on all time scales on which it has been
observed, and there is increasing evidence that it has an influence on climate.
The amplitudes of such variations depend on the wavelength and possibly on the
time scale. Although many aspects of this variability are well established, the
exact magnitude of secular variations (going beyond a solar cycle) and the
spectral dependence of variations are under discussion. The main drivers of
solar variability are thought to be magnetic features at the solar surface. The
climate reponse can be, on a global scale, largely accounted for by simple
energetic considerations, but understanding the regional climate effects is
more difficult. Promising mechanisms for such a driving have been identified,
including through the influence of UV irradiance on the stratosphere and
dynamical coupling to the surface. Here we provide an overview of the current
state of our knowledge, as well as of the main open questions
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