69 research outputs found
Convective intensification of magnetic fields in the quiet Sun
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field B_e, corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field B_p that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealised numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and is characterised by a pattern of vigorous, time-dependent, “granular” motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localised concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than B_e, and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contain ultra-intense magnetic fields that are significantly greater than B_p. Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultra-intense fields develop owing to nonlinear interactions between magnetic fields and convection; they cannot be explained in terms of “convective collapse” within a thin flux tube that remains in overall pressure equilibrium with its surroundings
Observational Evidence for Small-Scale Mixture of Weak and Strong Fields in the Quiet Sun
Three different maps of the quiet Sun, observed with the Advanced Stokes
Polarimeter (ASP) and the Diffraction-Limited Stokes Polarimeter (DLSP), show
evidence of strong (1700 G) and weak (500 G) fields coexisting
within the resolution element both at network and internetwork locations. The
angular resolution of the observations is of 1\arcsec (ASP) and 0.\arcsec6
(DLSP), respectively. Even at the higher DLSP resolution, a significant
fraction of the network magnetic patches harbor a mixture of strong and weak
fields. Internetwork elements that exhibit kG fields when analyzed with a
single-component atmosphere are also shown to harbor considerable amounts of
weak fields. Only those patches for which a single-component analysis yields
weak fields do not show this mixture of field strengths. Finally, there is a
larger fractional area of weak fields in the convective upflows than in the
downflows.Comment: ApJ, in pres
Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun
The Ca II infrared triplet is one of the few magnetically sensitive
chromospheric lines available for ground-based observations. We present
spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun
region near a decaying active region and compare the results with a simulation
of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile
pairs shows that the two lines, despite arguably being formed fairly close,
often do not have similar shapes. In the network, the local magnetic topology
is more important in determining the shapes of the Stokes V profiles than the
phase of the wave, contrary to what our simulations show. We also find that
Stokes V asymmetries are very common in the network, and the histograms of the
observed amplitude and area asymmetries differ significantly from the
simulation. Both the network and internetwork show oscillatory behavior in the
Ca II lines. It is stronger in the network, where shocking waves, similar to
those in the high-beta simulation, are seen and large self-reversals in the
intensity profiles are common.Comment: 23 pages, 17 figures, accepted to ApJ some figures are low-res, for
high-res email [email protected]
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
Numerical Simulations of Shock Wave-Driven Jets
We present the results of numerical simulations of shock wave-driven jets in
the solar atmosphere. The dependence of observable quantities like maximum
velocity and deceleration on parameters such as the period and amplitude of
initial disturbances and the inclination of the magnetic field is investigated.
Our simulations show excellent agreement with observations, and shed new light
on the correlation between velocity and deceleration and on the regional
differences found in observations.Comment: 7 pages, 11 figures, submitted to Ap
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
Measurements of plasma motions in dynamic fibrils
We present a 40 minute time series of filtergrams from the red and the blue
wing of the \halpha line in an active region near the solar disk center. From
these filtergrams we construct both Dopplergrams and summed ``line center''
images. Several dynamic fibrils (DFs) are identified in the summed images. The
data is used to simultaneously measure the proper motion and the Doppler
signals in DFs. For calibration of the Doppler signals we use spatially
resolved spectrograms of a similar active region. Significant variations in the
calibration constant for different solar features are observed, and only
regions containing DFs have been used in order to reduce calibration errors. We
find a coherent behavior of the Doppler velocity and the proper motion which
clearly demonstrates that the evolution of DFs involve plasma motion. The
Doppler velocities are found to be a factor 2--3 smaller than velocities
derived form proper motions in the image plane. The difference can be explained
by the radiative processes involved, the Doppler velocity is a result of the
local atmospheric velocity weighted with the response function. As a result the
Doppler velocity originates from a wide range in heights in the atmosphere.
This is contrasted by the proper motion velocity which is measured from the
sharply defined bright tops of the DFs and is therefore a very local velocity
measure. The Doppler signal originates from well below the top of the DF.
Finally we discuss how this difference together with the lacking spatial
resolution of older observations have contributed to some of the confusion
about the identity of DFs, spicules and mottles.Comment: 8 pages, 7 figures, Accepted in ApJ, see
http://www.astro.uio.no/~oysteol for better quality figures and mpg movi
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
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