46 research outputs found
Horizontal Magnetic Fields in the Solar Photosphere
The results of 2D MHD simulations of solar magnetogranulation are used to
analyze the horizontal magnetic fields and the response of the synthesized
Stokes profiles of the FeI 1564.85 nm line to the magnetic fields. Selected
1.5-h series of the 2D MHD models reproduces a region of the network fields
with their immediate surrounding on the solar surface with the unsigned
magnetic flux density of 192 G. According to the magnetic field distribution
obtained, the most probable absolute strength of the horizontal magnetic field
at an optical depth of tau_5 = 1 (tau_5 denotes tau at lambda = 500 nm) is 50
G, while the mean value is 244 G. On average, the horizontal magnetic fields
are stronger than the vertical fields to heights of about 400 km in the
photosphere due to their higher density and the larger area they occupy. The
maximum factor by which the horizontal fields are greater is 1.5. Strong
horizontal magnetic flux tubes emerge at the surface as spots with field
strengths of more than 500 G. These are smaller than granules in size, and have
lifetimes of 3.6 min. They form in the photosphere due to the expulsion of
magnetic fields by convective flows coming from deep subphotospheric layers.
The data obtained qualitatively agree with observations with the Hinode space
observatory.Comment: 15 pages, 7 figures, published by Astronomicheskii Zhurnal (in
Russian) and Astronomy Reports (in English
Detection of the Li I 6104 Angstroem transition in the Population II star HD 140283
Lithium is one of the few primordially produced elements. The value of the
primordial Li is taken to be that observed in metal--poor dwarfs, where it is
not contaminated by stellar Li sources which act on longer time scales. The
atmospheric abundance is currently derived from the LiI 6707 Angstroem
resonance transition and the validity of the models employed has been
questioned (Kurucz 1995). In this letter we report the first detection of the
Li I 6104 Angstroem 2^2P - 3^2D subordinate transition in the prototype
population II star HD 140283. The same Li abundance of (Li/H) =1.4x10^{-10} is
found consistent with both the resonance and subordinate lines. The two lines
form at different depths in the atmosphere implying that the 1-D homogeneous
atmospheric models used in the abundance determination are essentially correct.
When coupled with the standard big bang yields, the Li in the halo dwarfs
provides two solutions for the baryon-to-photon ratio \eta_{10}=
n_{b}/n_{\gamma} x 10^{10}and for the present baryon density \Omega_b
h_{70}^2=0.0748\eta_{10}: a) a first solution at \eta_{10} about 1.8,
consistent with the \eta_{10} implied by the high deuterium values D/H\approx 2
x 10^{-4}observed in some quasar absorption systems (Webb et al 1997) and b) a
second solution at \eta_{10} about 4 which is consistent, within the errors,
with the low deuterium D/H =3.4x10^{-5} measured in other quasar absorption
systems (Burles & Tytler 1998).Comment: To appear in ApJ Letters, 11 pages 3 figures, uses aasms4.st
Improved SOT (Hinode mission) high resolution solar imaging observations
We consider the best today available observations of the Sun free of
turbulent Earth atmospheric effects, taken with the Solar Optical Telescope
(SOT) onboard the Hinode spacecraft. Both the instrumental smearing and the
observed stray light are analyzed in order to improve the resolution. The Point
Spread Function (PSF) corresponding to the blue continuum Broadband Filter
Imager (BFI) near 450 nm is deduced by analyzing i/ the limb of the Sun and ii/
images taken during the transit of the planet Venus in 2012. A combination of
Gaussian and Lorentzian functions is selected to construct a PSF in order to
remove both smearing due to the instrumental diffraction effects (PSF core) and
the large-angle stray light due to the spiders and central obscuration (wings
of the PSF) that are responsible for the parasitic stray light. A
Max-likelihood deconvolution procedure based on an optimum number of iterations
is discussed. It is applied to several solar field images, including the
granulation near the limb. The normal non-magnetic granulation is compared to
the abnormal granulation which we call magnetic. A new feature appearing for
the first time at the extreme- limb of the disk (the last 100 km) is discussed
in the context of the definition of the solar edge and of the solar diameter. A
single sunspot is considered in order to illustrate how effectively the
restoration works on the sunspot core. A set of 125 consecutive deconvolved
images is assembled in a 45 min long movie illustrating the complexity of the
dynamical behavior inside and around the sunspot.Comment: 15 pages, 22 figures, 1 movi
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
The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 active stars
observed with the High Energy Transmission Grating Spectrometer on {\em
Chandra} in order to investigate their coronal plasma density. Densities where
investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII.
While Si XIII lines in all stars of the sample are compatible with the
low-density limit, Mg XI lines betray the presence of high plasma densities ( cm) for most of the sources with higher X-ray luminosity ( erg/s); stars with higher and tend to have higher
densities at high temperatures. Ratios of O VII lines yield much lower
densities of a few cm, indicating that the ``hot'' and
``cool'' plasma resides in physically different structures. Our findings imply
remarkably compact coronal structures, especially for the hotter plasma
emitting the Mg XI lines characterized by coronal surface filling factor,
, ranging from to , while we find
values from a few up to for the cooler plasma emitting the O
VII lines. We find that approaches unity at the same stellar surface
X-ray flux level as solar active regions, suggesting that these stars become
completely covered by active regions. At the same surface flux level,
is seen to increase more sharply with increasing surface flux. These
results appear to support earlier suggestions that hot K plasma in
active coronae arises from flaring activity, and that this flaring activity
increases markedly once the stellar surface becomes covered with active
regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical
Journal. A version of the paper with higher quality figures is available from
http://www.astropa.unipa.it/Library/preprint.htm