198 research outputs found
Strength distribution of solar magnetic fields in photospheric quiet Sun regions
The magnetic topology of the solar photosphere in its quietest regions is
hidden by the difficulties to disentangle magnetic flux through the resolution
element from the field strength of unresolved structures. The observation of
spectral lines with strong coupling with hyperfine structure, like the observed
MnI line at 553.7 nm, allows such differentiation.
The main aim is to analyse the distribution of field strengths in the network
and intranetwork of the solar photosphere through inversion of the MnI line at
553.7 nm.
An inversion code for the magnetic field using the Principal Component
Analysis (PCA) has been developed. Statistical tests are run on the code to
validate it. The code has to draw information from the small-amplitude spectral
feature oppearing in the core of the Stokes V profile of the observed line for
field strengths below a certain threshold, coinciding with lower limit of the
Paschen-Back effect in the fine structure of the involved atomic levels.
The inversion of the observed profiles, using the circular polarization (V)
and the intensity (I), shows the presence of magnetic fields strengths in a
range from 0 to 2 kG, with predominant weak strength values. Mixed regions with
mean strength field values of 1130 and 435 Gauss are found associated with the
network and intranetwork respectively.
The MnI line at 553 nm probes the field strength distribution in the quiet
sun and shows the predominance of weak, hectoGauss fields in the intranetwork,
and strong, kiloGauss fields in the network. It also shows that both network
and intranetwork are to be understood at our present spatial resolutions as
field distributions of which we hint the mean properties.Comment: 10 pages, 6 figure
Multi-line Stokes inversion for prominence magnetic-field diagnostics
We present test results on the simultaneous inversion of the Stokes profiles
of the He I lines at 587.6 nm (D_3) and 1083.0 nm in prominences (90-deg
scattering). We created datasets of synthetic Stokes profiles for the case of
quiescent prominences (B<200 G), assuming a conservative value of 10^-3 of the
peak intensity for the polarimetric sensitivity of the simulated observations.
In this work, we focus on the error analysis for the inference of the magnetic
field vector, under the usual assumption that the prominence can be assimilated
to a slab of finite optical thickness with uniform magnetic and thermodynamic
properties. We find that the simultaneous inversion of the two lines
significantly reduces the errors on the inference of the magnetic field vector,
with respect to the case of single-line inversion. These results provide a
solid justification for current and future instrumental efforts with multi-line
capabilities for the observations of solar prominences and filaments.Comment: 14 pages, 5 figures, 1 tabl
Structure of prominence legs: Plasma and magnetic field
We investigate the properties of a `solar tornado' observed on 15 July 2014,
and aim to link the behaviour of the plasma to the internal magnetic field
structure of the associated prominence. We made multi-wavelength observations
with high spatial resolution and high cadence using SDO/AIA, the IRIS
spectrograph and the Hinode/SOT instrument. Along with spectropolarimetry
provided by the THEMIS telescope we have coverage of both optically thick
emission lines and magnetic field information. AIA reveals that the two legs of
the prominence are strongly absorbing structures which look like they are
rotating, or oscillating in the plane of the sky. The two prominence legs,
which are both very bright in Ca II (SOT), are not visible in the IRIS Mg II
slit-jaw images. This is explained by the large optical thickness of the
structures in Mg II which leads to reversed profiles, and hence to lower
integrated intensities at these locations than in the surroundings. Using lines
formed at temperatures lower than 1 MK, we measure relatively low Doppler
shifts on the order of +/- 10 km/s in the tornado-like structure. Between the
two legs we see loops in Mg II, with material flowing from one leg to the
other, as well as counterstreaming. It is difficult to interpret our data as
showing two rotating, vertical structures which are unrelated to the loops.
This kind of `tornado' scenario does not fit with our observations. The
magnetic field in the two legs of the prominence is found to be preferentially
horizontal.Comment: 13 pages, 14 figures, one tabl
On the nature of transverse coronal waves revealed by wavefront dislocations
Coronal waves are an important aspect of the dynamics of the plasma in the
corona. Wavefront dislocations are topological features of most waves in nature
and also of magnetohydrodynamic waves. Are there dislocations in coronal waves?
The finding and explanation of dislocations may shed light on the nature and
characteristics of the propagating waves, their interaction in the corona and
in general on the plasma dynamics. We positively identify dislocations in
coronal waves observed by the Coronal Multi-channel Polarimeter (CoMP) as
singularities in the Doppler shifts of emission coronal lines. We study the
possible singularities that can be expected in coronal waves and try to
reproduce the observed dislocations in terms of localization and frequency of
appearance. The observed dislocations can only be explained by the interference
of a kink and a sausage wave modes propagating with different frequencies along
the coronal magnetic field. In the plane transverse to the propagation, the
cross-section of the oscillating plasma must be smaller than the spatial
resolution, and the two waves result in net longitudinal and transverse
velocity components that are mixed through projection onto the line of sight.
Alfv\'en waves can be responsible of the kink mode, but a magnetoacoustic
sausage mode is necessary in all cases. Higher (flute) modes are excluded. The
kink mode has a pressure amplitude that is smaller than the pressure amplitude
of the sausage mode, though its observed velocity is larger. This concentrates
dislocations on the top of the loop. To explain dislocations, any model of
coronal waves must include the simultaneous propagation and interference of
kink and sausage wave modes of comparable but different frequencies, with a
sausage wave amplitude much smaller than the kink one.Comment: 11 pages. 5 figures. Accepted for publication in A&
Spectral Characteristics of the He I D3 Line in a Quiescent Prominence Observed by THEMIS
We analyze the observations of a quiescent prominence acquired by the
Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites
Solaires (THEMIS) in the He I 5876 A (He I D3) multiplet aiming to measure the
spectral characteristics of the He I D3 profiles and to find for them an
adequate fitting model. The component characteristics of the He I D3 Stokes I
profiles are measured by the fitting system approximating them with a double
Gaussian. This model yields an He I D3 component peak intensity ratio of
, which differs from the value of 8 expected in the optically thin
limit. Most of the measured Doppler velocities lie in the interval km/s,
with a standard deviation of km/s around the peak value of 0.4 km/s.
The wide distribution of the full-width at half maximum has two maxima at 0.25
A and 0.30 A for the He I D3 blue component and two maxima at 0.22 A and 0.31 A
for the red component. The width ratio of the components is . We
show that the double-Gaussian model systematically underestimates the blue wing
intensities. To solve this problem, we invoke a two-temperature multi-Gaussian
model, consisting of two double-Gaussians, which provides a better
representation of He I D3 that is free of the wing intensity deficit. This
model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool
and the hot component of the target prominence. The cool and hot components of
a typical He I D3 profile have component peak intensity ratios of 6.6 and 8,
implying a prominence geometrical width of 17 Mm and an optical thickness of
0.3 for the cool component, while the optical thickness of the hot component is
negligible. These prominence parameters seem to be realistic, suggesting the
physical adequacy of the multi-Gaussian model with important implications for
interpreting He I D3 spectropolarimetry by current inversion codes.Comment: 25 pages,1 movie, 10 figures, 2 tables, 2 equations. The final
publication is available at Springer via
http://dx.doi.org/10.1007/s11207-017-1118-z The supplementary movie is
available for viewing and download at
https://www.dropbox.com/s/7tskvnc593tlbyv/Prominence_HeID3_GONG_AIA.mpg?dl=
Tangled Magnetic Fields in Solar Prominences
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called "hedgerow"
prominences, which consist of thin vertical threads. We explore the possibility
that such prominences are supported by tangled magnetic fields. A variety of
different approaches are used. First, the dynamics of plasma within a tangled
field is considered. We find that the contorted shape of the flux tubes
significantly reduces the flow velocity compared to the supersonic free fall
that would occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such fields to
gravitational forces is considered. We demonstrate that the prominence plasma
can be supported by the magnetic pressure of a tangled field that pervades not
only the observed dense threads but also their local surroundings. Tangled
fields with field strengths of about 10 G are able to support prominence
threads with observed hydrogen density of the order of 10^(11) cm^(-3).
Finally, we suggest that the observed vertical threads are the result of
Rayleigh-Taylor instability. Simulations of the density distribution within a
prominence thread indicate that the peak density is much larger than the
average density. We conclude that tangled fields provide a viable mechanism for
magnetic support of hedgerow prominences.Comment: 14 pages (emulateapj style), 10 figures, ApJ, in pres
Magnetic field in atypical prominence structures: Bubble, tornado and eruption
Spectropolarimetric observations of prominences have been obtained with the
THEMIS telescope during four years of coordinated campaigns. Our aim is now to
understand the conditions of the cool plasma and magnetism in `atypical'
prominences, namely when the measured inclination of the magnetic field
departs, to some extent, from the predominantly horizontal field found in
`typical' prominences. What is the role of the magnetic field in these
prominence types? Are plasma dynamics more important in these cases than the
magnetic support? We focus our study on three types of `atypical' prominences
(tornadoes, bubbles and jet-like prominence eruptions) that have all been
observed by THEMIS in the He I D_3 line, from which the Stokes parameters can
be derived. The magnetic field strength, inclination and azimuth in each pixel
are obtained by using the Principal Component Analysis inversion method on a
model of single scattering in the presence of the Hanle effect. The magnetic
field in tornadoes is found to be more or less horizontal, whereas for the
eruptive prominence it is mostly vertical. We estimate a tendency towards
higher values of magnetic field strength inside the bubbles than outside in the
surrounding prominence. In all of the models in our database, only one magnetic
field orientation is considered for each pixel. While sufficient for most of
the main prominence body, this assumption appears to be oversimplified in
atypical prominence structures. We should consider these observations as the
result of superposition of multiple magnetic fields, possibly even with a
turbulent field component.Comment: 13 pages, 9 figure
Low-lying magnetic loops in the solar internetwork
The aim of this work is to study the structure of the magnetic field vector
in the internetwork and search for the presence of small-scale loops. We invert
1.56 micron spectropolarimetric observations of internetwork regions at disc
centre by applying the SIR code. This allows us to recover the atmospheric
parameters that play a role in the formation of these spectral lines. We are
mainly interested in the structure of the magnetic field vector. We find that
many opposite polarity elements of the internetwork are connected by short
(2-6''), low-lying (photospheric) loops. These loops connect at least the 10-20
% of the internetwork flux visible in our data. Also we have some evidence that
points towards a dynamic scenario which can be produced by the emergence of
internetwork magnetic flux.Comment: Accepted for publication in A&A letter
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