114 research outputs found
Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae
The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot
On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs
We investigate the detectability of Zeeman broadening in optical Stokes I
spectra of slowly rotating sun-like stars. To this end, we apply the LTE
spectral line inversion package SPINOR to very-high quality CES data and
explore how fit quality depends on the average magnetic field, Bf .
One-component (OC) and two-component (TC) models are adopted. In OC models, the
entire surface is assumed to be magnetic. Under this assumption, we determine
formal 3{\sigma} upper limits on the average magnetic field of 200 G for the
Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of ~
500 G is found for 59 Vir (G0V), and of ~ 1000 G for HD 68456 (F6V). A
distinction between magnetic and non-magnetic regions is made in TC models,
while assuming a homogeneous distribution of both components. In our TC
inversions of 59 Vir, we investigate three cases: both components have equal
temperatures; warm magnetic regions; cool magnetic regions. Our TC model with
equal temperatures does not yield significant improvement over OC inversions
for 59 Vir. The resulting Bf values are consistent for both. Fit quality is
significantly improved, however, by using two components of different
temperatures. The inversions for 59 Vir that assume different temperatures for
the two components yield results consistent with 0 - 450 G at the formal
3{\sigma} confidence level. We thus find a model dependence of our analysis and
demonstrate that the influence of an additional temperature component can
dominate over the Zeeman broadening signature, at least in optical data.
Previous comparable analyses that neglected effects due to multiple temperature
components may be prone to the same ambiguities.Comment: 18 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Structural Invariance of Sunspot Umbrae Over the Solar Cycle: 1993-2004
Measurements of maximum magnetic flux, minimum intensity, and size are
presented for 12 967 sunspot umbrae detected on the NASA/NSO
spectromagnetograms between 1993 and 2004 to study umbral structure and
strength during the solar cycle. The umbrae are selected using an automated
thresholding technique. Measured umbral intensities are first corrected for a
confirming observation of umbral limb-darkening. Log-normal fits to the
observed size distribution confirm that the size spectrum shape does not vary
with time. The intensity-magnetic flux relationship is found to be steady over
the solar cycle. The dependence of umbral size on the magnetic flux and minimum
intensity are also independent of cycle phase and give linear and quadratic
relations, respectively. While the large sample size does show a low amplitude
oscillation in the mean minimum intensity and maximum magnetic flux correlated
with the solar cycle, this can be explained in terms of variations in the mean
umbral size. These size variations, however, are small and do not substantiate
a meaningful change in the size spectrum of the umbrae generated by the Sun.
Thus, in contrast to previous reports, the observations suggest the equilibrium
structure, as testified by the invariant size-magnetic field relationship, as
well as the mean size (i.e. strength) of sunspot umbrae do not significantly
depend on solar cycle phase.Comment: 17 pages, 6 figures. Published in Solar Physic
Measuring the Hidden Aspects of Solar Magnetism
2008 marks the 100th anniversary of the discovery of astrophysical magnetic
fields, when George Ellery Hale recorded the Zeeman splitting of spectral lines
in sunspots. With the introduction of Babcock's photoelectric magnetograph it
soon became clear that the Sun's magnetic field outside sunspots is extremely
structured. The field strengths that were measured were found to get larger
when the spatial resolution was improved. It was therefore necessary to come up
with methods to go beyond the spatial resolution limit and diagnose the
intrinsic magnetic-field properties without dependence on the quality of the
telescope used. The line-ratio technique that was developed in the early 1970s
revealed a picture where most flux that we see in magnetograms originates in
highly bundled, kG fields with a tiny volume filling factor. This led to
interpretations in terms of discrete, strong-field magnetic flux tubes embedded
in a rather field-free medium, and a whole industry of flux tube models at
increasing levels of sophistication. This magnetic-field paradigm has now been
shattered with the advent of high-precision imaging polarimeters that allow us
to apply the so-called "Second Solar Spectrum" to diagnose aspects of solar
magnetism that have been hidden to Zeeman diagnostics. It is found that the
bulk of the photospheric volume is seething with intermediately strong, tangled
fields. In the new paradigm the field behaves like a fractal with a high degree
of self-similarity, spanning about 8 orders of magnitude in scale size, down to
scales of order 10 m.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Non-linear numerical simulations of magneto-acoustic wave propagation in small-scale flux tubes
We present results of non-linear, 2D, numerical simulations of
magneto-acoustic wave propagation in the photosphere and chromosphere of
small-scale flux tubes with internal structure. Waves with realistic periods of
three to five minutes are studied, after applying horizontal and vertical
oscillatory perturbations to the equilibrium model. Spurious reflections of
shock waves from the upper boundary are minimized thanks to a special boundary
condition. This has allowed us to increase the duration of the simulations and
to make it long enough to perform a statistical analysis of oscillations. The
simulations show that deep horizontal motions of the flux tube generate a slow
(magnetic) mode and a surface mode. These modes are efficiently transformed
into a slow (acoustic) mode in the vA < cS atmosphere. The slow (acoustic) mode
propagates vertically along the field lines, forms shocks and remains always
within the flux tube. It might deposit effectively the energy of the driver
into the chromosphere. When the driver oscillates with a high frequency, above
the cut-off, non-linear wave propagation occurs with the same dominant driver
period at all heights. At low frequencies, below the cut-off, the dominant
period of oscillations changes with height from that of the driver in the
photosphere to its first harmonic (half period) in the chromosphere. Depending
on the period and on the type of the driver, different shock patterns are
observed.Comment: 22 pages 6 color figures, submitted to Solar Physics, proceeding of
SOHO 19/ GONG 2007 meeting, Melbourne, Australi
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Small-scale solar magnetic fields
As we resolve ever smaller structures in the solar atmosphere, it has become
clear that magnetism is an important component of those small structures.
Small-scale magnetism holds the key to many poorly understood facets of solar
magnetism on all scales, such as the existence of a local dynamo, chromospheric
heating, and flux emergence, to name a few. Here, we review our knowledge of
small-scale photospheric fields, with particular emphasis on quiet-sun field,
and discuss the implications of several results obtained recently using new
instruments, as well as future prospects in this field of research.Comment: 43 pages, 18 figure
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Propagating spectropolarimetric disturbances in a large sunspot
We present results derived from the analysis of spectropolarimetric measurements of active region AR12546, which represents one of the largest sunspots to have emerged onto the solar surface over the last 20 years. The region was observed with full-Stokes scans of the Fe I 617.3 nm and Ca II 854.2 nm lines with the Interferometric BIdimensional Spectrometer (IBIS) instrument at the Dunn Solar Telescope over an uncommon, extremely long time interval exceeding three hours. Clear circular polarization (CP) oscillations localized at the umbra-penumbra boundary of the observed region were detected. Furthermore, the multi-height data allowed us to detect the downward propagation of both CP and intensity disturbances at 2.5−3~mHz, which was identified by a phase delay between these two quantities. These results are interpreted as a propagating magneto-hydrodynamic surface mode in the observed sunspot
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