203 research outputs found
Differences between Doppler velocities of ions and neutral atoms in a solar prominence
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.Context. In astrophysical systems with partially ionized plasma the motion of ions is governed by the magnetic field while the neutral
particles can only feel the magnetic field’s Lorentz force indirectly through collisions with ions. The drift in the velocity between
ionized and neutral species plays a key role in modifying important physical processes like magnetic reconnection, damping of
magnetohydrodynamic waves, transport of angular momentum in plasma through the magnetic field, and heating.
Aims. This paper investigates the differences between Doppler velocities of calcium ions and neutral hydrogen in a solar prominence
to look for velocity differences between the neutral and ionized species.
Methods. We simultaneously observed spectra of a prominence over an active region in H I 397 nm, H I 434 nm, Ca II 397 nm, and
Ca II 854 nm using a high dispersion spectrograph of the Domeless Solar Telescope at Hida observatory, and compared the Doppler
velocities, derived from the shift of the peak of the spectral lines presumably emitted from optically-thin plasma.
Results. There are instances when the difference in velocities between neutral atoms and ions is significant, e.g. 1433 events (∼ 3 %
of sets of compared profiles) with a difference in velocity between neutral hydrogen atoms and calcium ions greater than 3σ of the
measurement error. However, we also found significant differences between the Doppler velocities of two spectral lines emitted from
the same species, and the probability density functions of velocity difference between the same species is not significantly different
from those between neutral atoms and ions.
Conclusions. We interpreted the difference of Doppler velocities as a result of motions of different components in the prominence
along the line of sight, rather than the decoupling of neutral atoms from plasma.This work was supported by a Grant-in-Aid for Scientific
Research (No. 22244013, P.I. K. Ichimoto; No. 15K17609, P.I. T. Anan; No.
16H01177, P.I. T. Anan) from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. A.H. is supported by his STFC Emest Rutherford
Fellowship grant number ST/L00397X/2
Magnetic Flux Loss and Flux Transport in a Decaying Active Region
We estimate the temporal change of magnetic flux perpendicular to the solar
surface in a decaying active region by using a time series of the spatial
distribution of vector magnetic fields in the photosphere. The vector magnetic
fields are derived from full spectropolarimetric measurements with the Solar
Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux
transport rate in a decaying sunspot and its surrounding moat region. The
amount of magnetic flux that decreases in the sunspot and moat region is very
similar to magnetic flux transported to the outer boundary of the moat region.
The flux loss rates [] of magnetic elements with positive and
negative polarities are balanced each other around the outer boundary of the
moat region. These results suggest that most of the magnetic flux in the
sunspot is transported to the outer boundary of the moat region as moving
magnetic features, and then removed from the photosphere by flux cancellation
around the outer boundary of the moat region.Comment: 16 pages, 7 figures, Accepted for publication in Ap
Magnetic Structure of Sunspots
In this review we give an overview about the current state-of-knowledge of
the magnetic field in sunspots from an observational point of view. We start by
offering a brief description of tools that are most commonly employed to infer
the magnetic field in the solar atmosphere with emphasis in the photosphere of
sunspots. We then address separately the global and local magnetic structure of
sunspots, focusing on the implications of the current observations for the
different sunspots models, energy transport mechanisms, extrapolations of the
magnetic field towards the Corona and other issues.Comment: Published version (including high resolution figures):
http://www.livingreviews.org/lrsp-2011-
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
A comparison of measured and simulated solar network contrast
Long-term trends in the solar spectral irradiance are important to determine
the impact on Earth's climate. These long-term changes are thought to be caused
mainly by changes in the surface area covered by small-scale magnetic elements.
The direct measurement of the contrast to determine the impact of these
small-scale magnetic elements is, however, limited to a few wavelengths, and
is, even for space instruments, affected by scattered light and instrument
defocus. In this work we calculate emergent intensities from 3-D simulations of
solar magneto-convection and validate the outcome by comparing with
observations from Hinode/SOT. In this manner we aim to construct the contrast
at wavelengths ranging from the NUV to the FIR.Comment: Proceedings paper, IAU XXVII, Symposium 264, 3 page
Properties of Umbral Dots from Stray Light Corrected Hinode Filtergrams
High resolution blue continuum filtergrams from Hinode are employed to study
the umbral fine structure of a regular unipolar sunspot. The removal of
scattered light from the images increases the rms contrast by a factor of 1.45
on average. Improvement in image contrast renders identification of short
filamentary structures resembling penumbrae that are well separated from the
umbra-penumbra boundary and comprise bright filaments/grains flanking dark
filaments. Such fine structures were recently detected from ground based
telescopes and have now been observed with Hinode. A multi-level tracking
algorithm was used to identify umbral dots in both the uncorrected and
corrected images and to track them in time. The distribution of the values
describing the photometric and geometric properties of umbral dots are more
easily affected by the presence of stray light while it is less severe in the
case of kinematic properties. Statistically, umbral dots exhibit a peak
intensity, effective diameter, lifetime, horizontal speed and a trajectory
length of 0.29 I_QS, 272 km, 8.4 min, 0.45 km/s and 221 km respectively. The 2
hr 20 min time sequence depicts several locations where umbral dots tend to
appear and disappear repeatedly with various time intervals. The correction for
scattered light in the Hinode filtergrams facilitates photometry of umbral fine
structure which can be related to results obtained from larger telescopes and
numerical simulations.Comment: Accepted for publication in ApJ : 10 pages, 10 figures, 3 table
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