59 research outputs found
Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm
Judge et al. (2021) recently argued that a region of the solar spectrum in
the near-UV between about 250 and 290 nm is optimal for studying magnetism in
the solar chromosphere due to an abundance of Mg II, Fe II, and Fe I lines that
sample various heights in the solar atmosphere. In this paper we derive
requirements for spectropolarimetric instruments to observe these lines. We
derive a relationship between the desired sensitivity to magnetic field and the
signal-to-noise of the measurement from the weak-field approximation of the
Zeeman effect. We find that many lines will exhibit observable polarization
signals for both longitudinal and transverse magnetic field with reasonable
amplitudes
Scales of the magnetic fields in the quiet Sun
Context: The presence of a turbulent magnetic field in the quiet Sun has been
unveiled observationally using different techniques. The magnetic field is
quasi-isotropic and has field strengths weaker than 100G. It is pervasive and
may host a local dynamo. Aims: We aim to determine the length scale of the
turbulent magnetic field in the quiet Sun. Methods: The Stokes V area asymmetry
is sensitive to minute variations in the magnetic topology along the line of
sight. Using data provided by Hinode-SOT/SP instrument, we performed a
statistical study of this quantity.We classified the different magnetic regimes
and infer properties of the turbulent magnetic regime. In particular we
measured the correlation length associated to these fields for the first time.
Results: The histograms of Stokes V area asymmetries reveal three different
regimes: one organized, quasi-vertical and strong field (flux tubes or other
structures of the like); a strongly asymmetric group of profiles found around
field concentrations; and a turbulent isotropic field. For the last, we confirm
its isotropy and measure correlation lengths from hundreds of kilometers down
to 10km, at which point we lost sensitivity. A crude attempt to measure the
power spectra of these turbulent fields is made. Conclusions: In addition to
confirming the existence of a turbulent field in the quiet Sun, we give further
prove of its isotropy.We also measure correlation lengths down to 10km. The
combined results show magnetic fields with a large span of length scales, as
expected from a turbulent cascade.Comment: 11 pages, 6 figures. Accepted for publication in A&
Characterization of horizontal flows around solar pores from high-resolution time series of images
Though there is increasing evidence linking the moat flow and the Evershed
flow along the penumbral filaments, there is not a clear consensus regarding
the existence of a moat flow around umbral cores and pores, and the debate is
still open. Solar pores appear to be a suitable scenario to test the
moat-penumbra relation as evidencing the direct interaction between the umbra
and the convective plasma in the surrounding photosphere, without any
intermediate structure in between. The present work studies solar pores based
on high resolution ground-based and satellite observations. Local correlation
tracking techniques have been applied to different-duration time series to
analyze the horizontal flows around several solar pores. Our results establish
that the flows calculated from different solar pore observations are coherent
among each other and show the determinant and overall influence of exploding
events in the granulation around the pores. We do not find any sign of
moat-like flows surrounding solar pores but a clearly defined region of inflows
surrounding them. The connection between moat flows and flows associated to
penumbral filaments is hereby reinforced by this work.Comment: 10 pages, 10 figures, Accepted for publication in Astronomy and
Astrophysics
Analysis of a Fragmenting Sunspot using Hinode Observations
We employ high resolution filtergrams and polarimetric measurements from
Hinode to follow the evolution of a sunspot for eight days starting on June 28,
2007. The imaging data were corrected for intensity gradients, projection
effects, and instrumental stray light prior to the analysis. The observations
show the formation of a light bridge at one corner of the sunspot by a slow
intrusion of neighbouring penumbral filaments. This divided the umbra into two
individual umbral cores. During the light bridge formation, there was a steep
increase in its intensity from 0.28 to 0.7 I_QS in nearly 4 hr, followed by a
gradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity
was accompanied by a large reduction in the field strength from 1800 G to 300
G. The smaller umbral core gradually broke away from the parent sunspot nearly
2 days after the formation of the light bridge rendering the parent spot
without a penumbra at the location of fragmentation. The penumbra in the
fragment disappeared first within 34 hr, followed by the fragment whose area
decayed exponentially with a time constant of 22 hr. The depleted penumbra in
the parent sunspot regenerated when the inclination of the magnetic field at
the penumbra-QS boundary became within 40 deg. from being completely horizontal
and this occurred near the end of the fragment's lifetime. After the
disappearance of the fragment, another light bridge formed in the parent which
had similar properties as the fragmenting one, but did not divide the sunspot.
The significant weakening in field strength in the light bridge along with the
presence of granulation is suggestive of strong convection in the sunspot which
might have triggered the expulsion and fragmentation of the smaller spot.
Although the presence of QS photospheric conditions in sunspot umbrae could be
a necessary condition for fragmentation, it is not a sufficient one.Comment: Accepted for publication in ApJ; 15 pages, 15 figures, 1 tabl
Study of single-lobed circular polarization profiles in the quiet Sun
The existence of asymmetries in the circular polarization (Stokes V) profiles
emerging from the solar photosphere is known since the 1970s. These profiles
require the presence of a velocity gradient along the line of sight, possibly
associated with gradients of magnetic field strength, inclination and/or
azimuth. We have focused our study on the Stokes V profiles showing extreme
asymmetry in the from of only one lobe. Using Hinode spectropolarimetric
measurements we have performed a statistical study of the properties of these
profiles in the quiet sun. We show their spatial distribution, their main
physical properties, how they are related with several physical observables and
their behavior with respect to their position on the solar disk. The single
lobed Stokes V profiles occupy roughly 2% of the solar surface. For the first
time, we have observed their temporal evolution and have retrieved the physical
conditions of the atmospheres from which they emerged using an inversion code
implementing discontinuities of the atmospheric parameters along the line of
sight. In addition, we use synthetic Stokes profiles from 3D magnetoconvection
simulations to complement the results of the inversion. The main features of
the synthetic single-lobed profiles are in general agreement with the observed
ones, lending support to the magnetic and dynamic topologies inferred from the
inversion. The combination of all these different analysis suggests that most
of the single-lobed Stokes V profiles are signals coming from magnetic flux
emergence and/or submergence processes taking place in small patches in the
photospheric of the quiet sun.Comment: 21 pages, 26 figures, 1 tabl
Spectropolarimetery of umbral fine structures from Hinode: Evidence for magnetoconvection
We present spectropolarimetric analysis of umbral dots and a light bridge
fragment that show dark lanes in G-band images. Umbral dots show upflow as well
as associated positive Stokes V area asymmetry in their central parts. Larger
umbral dots show down flow patches in their surrounding parts that are
associated with negative Stokes V area asymmetry. Umbral dots show weaker
magnetic field in central part and higher magnetic field in peripheral area.
Umbral fine structures are much better visible in total circularly polarized
light than in continuum intensity. Umbral dots show a temperature deficit above
dark lanes. The magnetic field inclination show a cusp structure above umbral
dots and a light bridge fragment. We compare our observational findings with 3D
magnetohydrodynamic simulations.Comment: Accepted for publication in MNRAS, 6 pages, 6 figure
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
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
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