20 research outputs found
Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope
CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter at the
Swedish 1-m Solar Telescope, opens a new perspective in solar polarimetry. With
better spatial resolution (0.13") than Hinode in the Fe I 6302 A line and
similar polarimetric sensitivity reached through postprocessing, CRISP
complements the SP spectropolarimeter onboard Hinode. We present some of the
data which we obtained in our June 2008 campaign and preliminary results from
LTE inversions of a pore containing umbral dots.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
The Evershed Effect with SOT/Hinode
The Solar Optical Telescope onboard Hinode revealed the fine-scale structure
of the Evershed flow and its relation to the filamentary structures of the
sunspot penumbra. The Evershed flow is confined in narrow channels with nearly
horizontal magnetic fields, embedded in a deep layer of the penumbral
atmosphere. It is a dynamic phenomenon with flow velocity close to the
photospheric sound speed. Individual flow channels are associated with tiny
upflows of hot gas (sources) at the inner end and downflows (sinks) at the
outer end. SOT/Hinode also discovered ``twisting'' motions of penumbral
filaments, which may be attributed to the convective nature of the Evershed
flow. The Evershed effect may be understood as a natural consequence of thermal
convection under a strong, inclined magnetic field. Current penumbral models
are discussed in the lights of these new Hinode observations.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
Theoretical Models of Sunspot Structure and Dynamics
Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.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
The Evershed Flow and the Brightness of the Penumbra
The Evershed flow is a systematic motion of gas that occurs in the penumbra
of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation.
We know that the flow is magnetized, often supersonic, and that it shows
conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin
remains unclear. The hope is that a good observational understanding of the
relation between the flow and the penumbral magnetic field will help us
determine its nature. Here I review advances in the characterization of the
Evershed flow and sunspot magnetic fields from high-resolution spectroscopic
and spectropolarimetric measurements. Using this information as input for 2D
heat transfer simulations, it has been demonstrated that hot Evershed upflows
along nearly horizontal field lines are capable of explaining one of the most
intriguing aspects of sunspots: the surplus brightness of the penumbra relative
to the umbra. They also explain the existence of penumbral filaments with dark
cores. These results support the idea that the Evershed flow is largely
responsible for the transport of energy in the penumbra.Comment: 18 pages, 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, Heidelberg, 200
Weather in stellar atmosphere: the dynamics of mercury clouds in alpha Andromedae
The formation of long-lasting structures at the surfaces of stars is commonly
ascribed to the action of strong magnetic fields. This paradigm is supported by
observations of evolving cool spots in the Sun and active late-type stars, and
stationary chemical spots in the early-type magnetic stars. However, results of
our seven-year monitoring of mercury spots in non-magnetic early-type star
alpha Andromedae show that the picture of magnetically-driven structure
formation is fundamentally incomplete. Using an indirect stellar surface
mapping technique, we construct a series of 2-D images of starspots and
discover a secular evolution of the mercury cloud cover in this star. This
remarkable structure formation process, observed for the first time in any
star, is plausibly attributed to a non-equilibrium, dynamical evolution of the
heavy-element clouds created by atomic diffusion and may have the same
underlying physics as the weather patterns on terrestrial and giant planets.Comment: 10 pages, 2 figures; to be published in Nature Physic
Convection and the Origin of Evershed Flows
Numerical simulations have by now revealed that the fine scale structure of
the penumbra in general and the Evershed effect in particular is due to
overturning convection, mainly confined to gaps with strongly reduced magnetic
field strength. The Evershed flow is the radial component of the overturning
convective flow visible at the surface. It is directed outwards -- away from
the umbra -- because of the broken symmetry due to the inclined magnetic field.
The dark penumbral filament cores visible at high resolution are caused by the
'cusps' in the magnetic field that form above the gaps. Still remaining to be
established are the details of what determines the average luminosity of
penumbrae, the widths, lengths, and filling factors of penumbral filaments, and
the amplitudes and filling factors of the Evershed flow. These are likely to
depend at least partially also on numerical aspects such as limited resolution
and model size, but mainly on physical properties that have not yet been
adequately determined or calibrated, such as the plasma beta profile inside
sunspots at depth and its horizontal profile, the entropy of ascending flows in
the penumbra, etc.Comment: 13 pages, 7 figures. 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
Solar Spectroscopy and (Pseudo-)Diagnostics of the Solar Chromosphere
I first review trends in current solar spectrometry and then concentrate on
comparing various spectroscopic diagnostics of the solar chromosphere. Some are
actually not at all chromospheric but just photospheric or clapotispheric and
do not convey information on chromospheric heating, even though this is often
assumed. Balmer Halpha is the principal displayer of the closed-field
chromosphere, but it is unclear how chromospheric fibrils gain their large
Halpha opacity. The open-field chromosphere seems to harbor most if not all
coronal heating and solar wind driving, but is hardly seen in optical
diagnostics.Comment: To appear in "Recent Advances in Spectroscopy: Astrophysical,
Theoretical and Experimental Perspectives", eds. R.K. Chaudhuri, M.V.
Mekkaden, A.V. Raveendran and A. Satya Narayanan, Astrophysics and Space
Science Proceedings, Springer, Heidelberg, 2009. Revision: references
corrected, new references added, minor text correction