130 research outputs found
Direct Imaging of Fine Structure in the Chromosphere of a Sunspot Umbra
High-resolution imaging observations from the Hinode spacecraft in the CaII H
line are employed to study the dynamics of the chromosphere above a sunspot. We
find that umbral flashes and other brightenings produced by the oscillation are
extremely rich in fine structure, even beyond the resolving limit of our
observations (0.22"). The umbra is tremendously dynamic, to the point that our
time cadence of 20 s does not suffice to resolve the fast lateral (probably
apparent) motion of the emission source. Some bright elements in our dataset
move with horizontal propagation speeds of 30 km/s. We have detected
filamentary structures inside the umbra (some of which have a horizontal
extension of ~1500 km) which, to our best knowledge, had not been reported
before. The power spectra of the intensity fluctuations reveals a few distinct
areas with different properties within the umbra that seem to correspond with
the umbral cores that form it. Inside each one of these areas the dominant
frequencies of the oscillation are coherent, but they vary considerably from
one core to another.Comment: Accepted for publication in Ap
Space efficient opposed-anvil high-pressure cell and its application to optical and NMR measurements up to 9 GPa
We have developed a new type of opposed-anvil high pressure cell with
substantially improved space efficiency. The clamp cell and the gasket are made
of non-magnetic Ni-Cr-Al alloy. Non-magnetic tungsten carbide (NMWC) is used
for the anvils. The assembled cell with the dimension \phi 29mm \times 41mm is
capable of generating pressure up to 9 GPa over a relatively large volume of 7
mm3. Our cell is particularly suitable for those experiments which require
large sample space to achieve good signal-to-noise ratio, such as the nuclear
magnetic resonance (NMR) experiment. Argon is used as the pressure transmitting
medium to obtain good hydrostaticity. The pressure was calibrated in situ by
measuring the fluorescence from ruby through a transparent moissanite (6H-SiC)
window. We have measured the pressure and temperature dependences of the 63Cu
nuclear-quadrupole-resonance (NQR) frequency of Cu2O, the in-plane Knight shift
of metallic tin, and the Knight shift of platinum. These quantities can be used
as reliable manometers to determine the pressure values in situ during the
NMR/NQR experiments up to 9 GPa.Comment: 9 pages, 5 figures, 3 tables, accepted for publication in J. Phys.
Soc. Jp
The Structure and Dynamics of the Upper Chromosphere and Lower Transition Region as Revealed by the Subarcsecond VAULT Observations
The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond (0.5") images of this region with high
sensitivity and cadence. Detailed analyses of those observations have
contributed significantly to new ideas about the nature of the transition
region. Here, we present a broad overview of the Ly-a atmosphere as revealed by
the VAULT observations, and bring together past results and new analyses from
the second VAULT flight to create a synthesis of our current knowledge of the
high-resolution Ly-a Sun. We hope that this work will serve as a good reference
for the design of upcoming Ly-a telescopes and observing plans.Comment: 28 pages, 11 figure
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
Stokes Diagnostis of 2D MHD-simulated Solar Magnetogranulation
We study the properties of solar magnetic fields on scales less than the
spatial resolution of solar telescopes. A synthetic infrared
spectropolarimetric diagnostics based on a 2D MHD simulation of
magnetoconvection is used for this. We analyze two time sequences of snapshots
that likely represent two regions of the network fields with their immediate
surrounding on the solar surface with the unsigned magnetic flux density of 300
and 140 G. In the first region we find from probability density functions of
the magnetic field strength that the most probable field strength at logtau_5=0
is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface,
while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic
flux is -28 G and its imbalance is -0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10.
We estimate the distribution of line-of-sight velocities on the surface of log
tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region.
The averaged velocity in the granules is -1.2 km/s and in the intergranules is
2.5 km/s. In the second region, the corresponding values of the mean velocities
are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of
synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the
amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude
asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 figure
Magnetic Coupling in the Quiet Solar Atmosphere
Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz force.
First the coupling of the convecting and overshooting fluid in the surface
layers of the Sun with the magnetic field. Here, the plasma motion provides the
dominant force, which shapes the magnetic field and drives the surface dynamo.
Progress in the understanding of the horizontal magnetic field is summarized
and discussed. Second, the coupling between acoustic waves and the magnetic
field, in particular the phenomenon of wave conversion and wave refraction. It
is described how measurements of wave travel times in the atmosphere can
provide information about the topography of the wave conversion zone, i.e., the
surface of equal Alfv\'en and sound speed. In quiet regions, this surface
separates a highly dynamic magnetic field with fast moving magnetosonic waves
and shocks around and above it from the more slowly evolving field of high-beta
plasma below it. Third, the magnetic field also couples to the radiation field,
which leads to radiative flux channeling and increased anisotropy in the
radiation field. It is shown how faculae can be understood in terms of this
effect. The article starts with an introduction to the magnetic field of the
quiet Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field with the
help of the Hanle effect.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
Relation between photospheric magnetic field and chromospheric emission
We simultaneously observed the Stokes parameters of the photospheric iron
line pair at 630.2 nm and the intensity profile of the chromospheric Ca II H
line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53 deg. We
perform a statistical analysis of network and inter-network properties.The
H-index is the integrated emission in a 0.1 nm band around the Ca core. We
separate a non-magnetically, H_non, and a magnetically, H_mag, heated component
from a non-heated component, H_co in the H-index. The average network and
inter-network H-indices are equal to 12 and 10 pm, respectively. The emission
in the network is correlated with the magnetic flux density, approaching a
value of H 10 pm for vanishing flux. The inter-network magnetic field is
dominated by weak field strengths with values down to 200 G and its mean
absolute flux density is 11 Mx cm. We find that a dominant fraction of
the calcium emission caused by the heated atmosphere in the magnetic network,
has non-magnetic origin (H_mag = 2pm, H_non = 3pm). Considering the effect of
straylight, the contribution from an atmosphere with no temperature rise to the
H-index (H_co = 6pm) is about half of the observed H-index in the
inter-network. The H-index in the inter-network is not correlated to any
property of the photospheric magnetic field, suggesting that magnetic flux
concentrations have a negligible role in the chromospheric heating in this
region. The height range of the thermal coupling between the photosphere and
low/mid chromosphere increases in presence of magnetic field. In addition, we
demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a
significant over-estimation of the magnetic field strength.Comment: 15 pages, accepted in Astron. Astrophys., 200
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