59 research outputs found
Polarization of photospheric lines from turbulent dynamo simulations
We employ the magnetic and velocity fields from turbulent dynamo simulations
to synthesize the polarization of a typical photospheric line. The synthetic
Stokes profiles have properties in common with those observed in the quiet Sun.
The simulated magnetograms present a level of signal similar to that of the
Inter-Network regions. Asymmetric Stokes V profiles with two, three and more
lobes appear in a natural way. The intensity profiles are broadened by the
magnetic fields in fair agreement with observational limits. Furthermore, the
Hanle depolarization signals of the Sr I 4607 A line turn out to be within the
solar values. Differences between synthetic and observed polarized spectra can
also be found. There is a shortage of Stokes V asymmetries, that we attribute
to a deficit of structuring in the magnetic and velocity fields from the
simulations as compared to the Sun This deficit may reflect the fact that the
Reynolds numbers of the numerical data are still far from solar values. We
consider the possibility that intense and tangled magnetic fields, like those
in the simulations, exist in the Sun. This scenario has several important
consequences. For example, less than 10% of the existing unsigned magnetic flux
would be detected in present magnetograms. The existing flux would exceed by
far that carried by active regions during the maximum of the solar cycle.
Detecting these magnetic fields would involve improving the angular resolution,
the techniques to interpret the polarization signals, and to a less extent, the
polarimetric sensitivity.Comment: Accepted for publication in ApJ. 20 pag. 11 fig
Fluxtube model atmospheres and Stokes V zero-crossing wavelengths
First results of the inversion of Stokes I and V profiles from plage regions
near disk center are presented. Both low and high spatial resolution spectra of
FeI 6301.5 and FeI 6302.5 A obtained with the Advanced Stokes Polarimeter (ASP)
have been considered for analysis. The thin flux tube approximation,
implemented in an LTE inversion code based on response functions, is used to
describe unresolved magnetic elements. The code allows the simultaneous and
consistent inference of all atmospheric quantities determining the radiative
transfer with the sole assumption of hydrostatic equilibrium. By considering
velocity gradients within the tubes we are able to match the full ASP Stokes
profiles. The magnetic atmospheres derived from the inversion are characterized
by the absence of significant motions in high layers and strong velocity
gradients in deeper layers. These are essential to reproduce the asymmetries of
the observed profiles. Our scenario predicts a shift of the Stokes V
zero-crossing wavelengths which is indeed present in observations made with the
Fourier Transform Spectrometer.Comment: To appear in ApJ Letters (1997) (in press
Hinode Observations of Magnetic Elements in Internetwork Areas
We use sequences of images and magnetograms from Hinode to study magnetic
elements in internetwork parts of the quiet solar photosphere. Visual
inspection shows the existence of many long-lived (several hours) structures
that interact frequently, and may migrate over distances ~7 Mm over a period of
a few hours. About a fifth of the elements have an associated bright point in
G-band or Ca II H intensity. We apply a hysteresis-based algorithm to identify
elements. The algorithm is able to track elements for about 10 min on average.
Elements intermittently drop below the detection limit, though the associated
flux apparently persists and often reappears some time later. We infer proper
motions of elements from their successive positions, and find that they obey a
Gaussian distribution with an rms of 1.57+-0.08 km/s. The apparent flows
indicate a bias of about 0.2 km/s toward the network boundary. Elements of
negative polarity show a higher bias than elements of positive polarity,
perhaps as a result of to the dominant positive polarity of the network in the
field of view, or because of increased mobility due to their smaller size. A
preference for motions in X is likely explained by higher supergranular flow in
that direction. We search for emerging bipoles by grouping elements of opposite
polarity that appear close together in space and time. We find no evidence
supporting Joy's law at arcsecond scales.Comment: 22 pages, 12 figure
Magnetic properties of photospheric regions having very low magnetic flux
The magnetic properties of the quiet Sun are investigated using a novel
inversion code, FATIMA, based on the Principal Component Analysis of the
observed Stokes profiles. The stability and relatively low noise sensitivity of
this inversion procedure allows for the systematic inversion of large data sets
with very weak polarization signal. Its application to quiet Sun observations
of network and internetwork regions reveals that a significant fraction of the
quiet Sun contains kilogauss fields (usually with very small filling factors)
and confirms that the pixels with weak polarization account for most of the
magnetic flux. Mixed polarities in the resolution element are also found to
occur more likely as the polarization weakens.Comment: To apapear in ApJ. 39 pages, 12 figures (2 of them are color figures
Magnetoacoustic shocks as driver of quiet Sun mottles
We present high spatial and high temporal resolution observations of the
quiet Sun in H-alpha obtained with the Swedish 1-m Solar Telescope on La Palma.
We observe that many mottles, jet-like features in the quiet Sun, display clear
up- and downward motions along their main axis. In addition, many mottles show
vigorous transverse displacements. Unique identification of the mottles
throughout their lifetime is much harder than for their active region
counterpart, dynamic fibrils. This is because many seem to lack a sharply
defined edge at their top, and significant fading often occurs throughout their
lifetime. For those mottles that can be reliably tracked, we find that the
mottle tops often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they reach,
similar to what was found earlier for dynamic fibrils. Combined with an
analysis of oscillatory properties, we conclude that at least part of the quiet
Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed
polarity environment and vigorous dynamics suggest that reconnection may play a
significant role in the formation of some quiet Sun jets.Comment: 12 pages, 4 figures. ApJ Letters, in pres
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 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
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