1,839 research outputs found
Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results
An approximate method for solving the rate equations for the hydrogen
populations was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with
six energy levels and fixed radiative rates. It has been tested extensively in
one-dimensional simulations. The extended method has been used to create a
three-dimensional model that extends from the upper convection zone to the
chromosphere. The ionisation degree of hydrogen in our time-dependent
simulation is comparable to the corresponding equilibrium value up to 500 km
above optical depth unity. Above this height, the non-equilibrium ionisation
degree is fairly constant over time and space, and tends to be at a value set
by hot propagating shock waves. The hydrogen level populations and electron
density are much more constant than the corresponding values for statistical
equilibrium, too. In contrast, the equilibrium ionisation degree varies by more
than 20 orders of magnitude between hot, shocked regions and cool, non-shocked
regions. The simulation shows for the first time in 3D that the chromospheric
hydrogen ionisation degree and electron density cannot be calculated in
equilibrium. Our simulation can provide realistic values of those quantities
for detailed radiative transfer computations.Comment: 8 pages, 7 figure
The Relation between the Radial Temperature Profile in the Chromosphere and the Solar Spectrum at Centimeter, Millimeter, Sub-millimeter, and Infrared Wavelengths
Solar observations from millimeter to ultraviolet wavelengths show that there
is a temperature minimum between photosphere and chromosphere. Analysis based
on semi-empirical models locate this point at about 500 km over the
photosphere. The consistency of these models has been tested by means of
millimeter to infrared observations.
In the present work, we show that variations of the theoretical radial
temperature profile near the temperature minimum impacts the brightness
temperature at centimeter, submillimeter, and infrared wavelengths, but the
millimeter wavelength emission remains unchanged. We found a region between 500
and 1000 km over the photosphere that remains hidden to observations at the
frequencies under study in this work.Comment: Accepted in Solar Physic
Observation of a short-lived pattern in the solar chromosphere
In this work we investigate the dynamic behavior of inter-network regions of
the solar chromosphere. We observed the chromosphere of the quiet Sun using a
narrow-band Lyot filter centered at the Ca II K 2v emission peak with a
bandpass of 0.3A. We achieved a spatial resolution of on average 0.7" at a
cadence of 10s. In the inter-network we find a mesh-like pattern that features
bright grains at the vertices. The pattern has a typical spatial scale of 1.95"
and a mean evolution time scale of 53s with a standard deviation of 10s. A
comparison of our results with a recent three-dimensional radiation
hydrodynamical model implies that the observed pattern is of chromospheric
origin. The measured time scales are not compatible with those of reversed
granulation in the photosphere although the appearance is similar. A direct
comparison between network and inter-network structure shows that their typical
time scales differ by at least a factor of two. The existence of a rapidly
evolving small-scale pattern in the inter-network regions supports the picture
of the lower chromosphere as a highly dynamical and intermittent phenomenon.Comment: Letter A&A 4 pages 5 figure
Inhomogeneities on the surface of 21 Lutetia, the asteroid target of the Rosetta mission
CONTEXT: In July 2010 the ESA spacecraft Rosetta will fly-by the main belt
asteroid 21 Lutetia. Several observations of this asteroid have been so far
performed, but its surface composition and nature are still a matter of debate.
For long time Lutetia was supposed to have a metallic nature due to its high
IRAS albedo. Later on it has been suggested to have a surface composition
similar to primitive carbonaceous chondrite meteorites, while further
observations proposed a possible genetic link with more evolved enstatite
chondrite meteorites. AIMS: In order to give an important contribution in
solving the conundrum of the nature of Lutetia, in November 2008 we performed
visible spectroscopic observations of this asteroid at the Telescopio Nazionale
Galileo (TNG, La Palma, Spain). METHODS: Thirteen visible spectra have been
acquired at different rotational phases. RESULTS: We confirm the presence of a
narrow spectral feature at about 0.47-0.48 micron already found by Lazzarin et
al. (2009) on the spectra of Lutetia. We also find a spectral feature at about
0.6 micron, detected by Lazzarin et al. (2004) on one of their Lutetia's
spectra. More importantly, our spectra exhibit different spectral slopes
between 0.6 and 0.75 micron and, in particular, we found that up to 20% of the
Lutetia surface could have flatter spectra. CONCLUSIONS: We detected a
variation of the spectral slopes at different rotational phases that could be
interpreted as possibly due to differences in the chemical/mineralogical
composition, as well as to inhomogeneities of the structure of the Lutetia's
surface (e.g., the presence of craters or albedo spots) in the southern
hemisphere.Comment: 3 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics. Updated on 25 March 2010
Signatures of Emerging Subsurface Structures in Acoustic Power Maps
We show that under certain conditions, subsurface structures in the solar
interior can alter the average acoustic power observed at the photosphere above
them. By using numerical simulations of wave propagation, we show that this
effect is large enough for it to be potentially used for detecting emerging
active regions before they appear on the surface. In our simulations,
simplified subsurface structures are modeled as regions with enhanced or
reduced acoustic wave speed. We investigate the dependence of the acoustic
power above a subsurface region on the sign, depth, and strength of the wave
speed perturbation. Observations from the Solar and Heliospheric
Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the emergence
of NOAA active region 10488 are used to test the use of acoustic power as a
potential precursor of magnetic flux emergence.Comment: 7 pages, 5 figures, accepted for publication in Solar Physics on 21
March 201
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Characterization of the Subsurface of 67P/Churyumov-Gerasimenko's Abydos Site
We investigate the structure of the subsurface of the Abydos site using a cometary nucleus model with parameters adapted to comet 67P/Churyumov-Gerasimenko and the Abydos landing site. We aim to compare the production rates derived from our model with those of the main molecules measured by Ptolemy. This will allow us to retrieve the depths at which the different molecules still exist in solid form
Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope
The asteroid (21) Lutetia is the target of a planned close encounter by the
Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been
extensively observed by a variety of astronomical facilities. We used the
Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide
wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety
of HST filters and a ground-based visible light spectrum, we employed synthetic
photometry techniques to derive absolute fluxes for Lutetia. New results from
ground-based measurements of Lutetia's size and shape were used to convert the
absolute fluxes into albedos. We present our best model for the spectral energy
distribution of Lutetia over the wavelength range 120-800 nm. There appears to
be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than
~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is
considerably larger than that of typical C-chondrite material (~4%). The
geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not
consistent with a metal-dominated surface at infrared or radar wavelengths, and
its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed
for typical primitive, chondritic material. We derive a relatively high FUV
albedo of ~10%, a result that will be tested by observations with the Alice
spectrograph during the Rosetta flyby of Lutetia in July 2010.Comment: 14 pages, 2 tables, 8 figure
Positions and sizes of X-ray solar flare sources
<p><b>Aims:</b> The positions and source sizes of X-ray sources taking into account Compton backscattering (albedo) are investigated.</p>
<p><b>Methods:</b> Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source.</p>
<p><b>Results:</b> We show that the albedo effect can alter the true source positions and substantially increase the measured source sizes. The source positions are shifted by up to ~0.5” radially towards the disk centre and 5 arcsec source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. The X-ray sources therefore should have minimum observed sizes, and thus their FWHM source size (2.35 times second-moment) will be as large as ~7” in the 20-50 keV range for a disk-centered point source at a height of 1 Mm (~1.4”) above the photosphere. The source size and position change is greater for flatter primary X-ray spectra, a stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV.</p>
<p><b>Conclusions:</b> Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and we suggest that footpoint sources should be larger in X-rays than in either optical or EUV ranges.</p>
X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock
High spectral resolution X-ray observations of CTTSs demonstrate the presence
of plasma at T~2-3X10^6 K and n_e~10^11-10^13 cm^-3, unobserved in
non-accreting stars. Stationary models suggest that this emission is due to
shock-heated accreting material, but they do not allow to analyze the stability
of such material and its position in the stellar atmosphere. We investigate the
dynamics and the stability of shock-heated accreting material in CTTSs and the
role of the stellar chromosphere in determining the position and the thickness
of the shocked region. We perform 1-D HD simulations of the impact of the
accretion flow onto chromosphere of a CTTS, including the effects of gravity,
radiative losses from optically thin plasma, thermal conduction and a well
tested detailed model of the stellar chromosphere. Here we present the results
of a simulation based on the parameters of the CTTS MP Mus. We find that the
accretion shock generates an hot slab of material above the chromosphere with a
maximum thickness of 1.8X10^9 cm, density n_e~10^11-10^2 cm^-3, temperature
T~3X10^6 K and uniform pressure equal to the ram pressure of the accretion flow
(~450 dyn cm^-2). The base of the shocked region penetrates the chromosphere
and stays where the ram pressure is equal to the thermal pressure. The system
evolves with quasi-periodic instabilities of the material in the slab leading
to cyclic disappearance and re-formation of the slab. For an accretion rate of
~10^-10 M_sun yr^-1, the shocked region emits a time-averaged X-ray luminosity
L_X~7X10^29 erg s^-1, which is comparable to the X-ray luminosity observed in
CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the
simulation matches in detail all the main features of the O VIII and O VII
lines of the star MP Mus.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic
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
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