1,694 research outputs found
The Solar Radius in the EUV during the Cycle XXIII
Aims. To determine the solar transition region and coronal radius at EUV
wavelengths and its time evolution during Solar Cycle XXIII.
Methods. We use daily 30.4 and 17.1 nm images obtained by the Extreme
Ultraviolet Imager (EIT) aboard the SoHO satellite and derive the solar radius
by fitting a circle to the limb brightness ring.
Results. The weighted mean of the temporal series gives (967''.56 +/- 0''.04)
and (969''.54 +/- 0''.02) at 30.4 and 17.1 nm respectively. No significant
correlation was found with the solar cycle at any of the two wavelengths.
Conclusions. Since the temperature formation of the 30.4 nm line is between
(60 - 80) 10^3 K (Transition Region), the obtained result is bigger than that
derived from present atmospheric models. On the contrary this height is
compatible with radio models.Comment: accepted for publication in Astronomy & Astrophysics minor changes
introduced during review proces
A Contrast-Based Approach to the Identification of Texture Faults
Texture analysis based on the extraction of contrast features is very effective in terms of both computational complexity and discrimination capability. In this framework, max-min approaches have been proposed in the past as a simple and powerful tool to characterize a statistical texture. In the present work, a method is proposed that allows exploiting the potential of max -min approaches to efficiently solve the problem of detecting local alterations in a uniform statistical texture. Experimental results show a high defect discrimination capability and a good attitude to real-time applications, which make it particularly attractive for the development of industrial visual inspection systems
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
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
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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
Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
<p><b>Aims:</b> A novel method of using hard X-rays as a diagnostic for chromospheric density and magnetic structures is developed to infer sub-arcsecond vertical variation of magnetic flux tube size and neutral gas density.</p>
<p><b>Methods:</b> Using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data and the newly developed X-ray visibilities forward fitting technique we find the FWHM and centroid positions of hard X-ray sources with sub-arcsecond resolution (~0.2'') for a solar limb flare. We show that the height variations of the chromospheric density and the magnetic flux densities can be found with an unprecedented vertical resolution of ~150 km by mapping 18-250 keV X-ray emission of energetic electrons propagating in the loop at chromospheric heights of 400-1500 km.</p>
<p><b>Results:</b> Our observations suggest that the density of the neutral gas is in good agreement with hydrostatic models with a scale height of around 140 30 km. FWHM sizes of the X-ray sources decrease with energy suggesting the expansion (fanning out) of magnetic flux tubes in the chromosphere with height. The magnetic scale height B(z)(dB/dz)-1 is found to be of the order of 300 km and a strong horizontal magnetic field is associated with noticeable flux tube expansion at a height of ~900 km.</p>
On the observability of T Tauri accretion shocks in the X-ray band
Context. High resolution X-ray observations of classical T Tauri stars
(CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13
cm^-3). This emission has been attributed to shock-heated accreting material
impacting onto the stellar surface. Aims. We investigate the observability of
the shock-heated accreting material in the X-ray band as a function of the
accretion stream properties (velocity, density, and metal abundance) in the
case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D
hydrodynamic model describing the impact of an accretion stream onto the
chromosphere, including the effects of radiative cooling, gravity and thermal
conduction. We explore the space of relevant parameters and synthesize from the
model results the X-ray emission in the [0.5-8.0] keV band and in the resonance
lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the
absorption from the chromosphere. Results. The accretion stream properties
influence the temperature and the stand-off height of the shocked slab and its
sinking in the chromosphere, determining the observability of the shocked
plasma. Our model predicts that X-ray observations preferentially detect
emission from low density and high velocity shocked accretion streams due to
the large absorption of dense post-shock plasma. In all the cases examined, the
post-shock zone exhibits quasi-periodic oscillations due to thermal
instabilities, but in the case of inhomogeneous streams and beta<<1, the shock
oscillations are hardly detectable. Conclusions. We suggest that, if accretion
streams are inhomogeneous, the selection effect introduced by the absorption on
observable plasma components may explain the discrepancy between the accretion
rate measured by optical and X-ray data as well as the different densities
measured using different He-like triplets in the X-ray band.Comment: 12 pages, 7 figures. Accepted for publication on A&
Effect of longitudinal magnetic and density inhomogeneity on transversal coronal loop oscillations
Context. Observations of post-flare transversal coronal loop oscillations by TRACE have given us an excellent opportunity to implement magneto-seismological techniques for probing the plasma fine structure of the Sun's upper atmosphere.
Aims. We investigate the combined effect of magnetic and density stratification on transversal coronal loop oscillations.
Methods. A coronal loop will be modelled as an expanding magnetic flux tube with arbitrary longitudinal plasma density. The governing equation of the fast kink body mode is derived and solved by analytical approximation and numerical methods.
Results. It is found that even a relatively small coronal loop expansion can have a significant and pronounced effect on the accuracy of the plasma density scale height measurements derived from observation of loop oscillations.
Conclusions. To conduct more accurate and realistic magneto-seismology of coronal loops, the magnetic field divergence should be taken into account
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