370 research outputs found
Diagnostics of active and eruptive prominences through hydrogen and helium lines modelling
In this study we show how hydrogen and helium lines modelling can be used to
make a diagnostic of active and eruptive prominences. One motivation for this
work is to identify the physical conditions during prominence activation and
eruption. Hydrogen and helium lines are key in probing different parts of the
prominence structure and inferring the plasma parameters. However, the
interpretation of observations, being either spectroscopic or obtained with
imaging, is not straightforward. Their resonance lines are optically thick, and
the prominence plasma is out of local thermodynamic equilibrium due to the
strong incident radiation coming from the solar disk. In view of the shift of
the incident radiation occurring when the prominence plasma flows radially, it
is essential to take into account velocity fields in the prominence diagnostic.
Therefore we need to investigate the effects of the radial motion of the
prominence plasma on hydrogen and helium lines. The method that we use is the
resolution of the radiative transfer problem in the hydrogen and helium lines
out of local thermodynamic equilibrium. We study the variation of the computed
integrated intensities in H and He lines with the radial velocity of the
prominence plasma. We can confirm that there exist suitable lines which can be
used to make a diagnostic of the plasma in active and eruptive prominences in
the presence of velocity fields.Comment: 5 pages, 4 colour figure
Effect of motions in prominences on the helium resonance lines in the extreme ultraviolet
<b>Context</b>: Extreme ultraviolet resonance lines of neutral and ionised helium observed in prominences are difficult to interpret as the prominence plasma is optically thick at these wavelengths. If mass motions are taking place, as is the case in active and eruptive prominences, the diagnostic is even more complex.
<b>Aims</b>: We aim at studying the effect of radial motions on the spectrum emitted by moving prominences in the helium resonance lines and at facilitating the interpretation of observations, in order to improve our understanding of these dynamic structures.
<b>Methods</b>: We develop our non-local thermodynamic equilibrium radiative transfer code formerly used for the study of quiescent prominences. The new numerical code is now able to solve the statistical equilibrium and radiative transfer equations in the non-static case by using velocity-dependent boundary conditions for the solution of the radiative transfer problem. This first study investigates the effects of different physical conditions (temperature, pressure, geometrical thickness) on the emergent helium radiation.
<b>Results</b>: The motion of the prominence plasma induces a Doppler dimming effect on the resonance lines of HE i and HE ii. The velocity effects are particularly important for the HE ii λ 304 Å line as it is mostly formed by resonant diffusion of incident radiation under prominence conditions. The HE i resonance lines at 584 and 537 Å also show some sensitivity to the motion of the plasma, all the more when thermal emission is not too important in these lines. We also show that it is necessary to consider partial redistribution in frequency for the scattering of the incident radiation.
Conclusions.This set of helium lines offers strong diagnostic possibilities that can be exploited with the SOHO spectrometers and with the EIS spectrometer on board the Hinode satellite. The addition of other helium lines and of lines from other elements (in particular hydrogen) in the diagnostics will further enhance the strength of the method
Plasma diagnostic of a solar prominence from hydrogen and helium resonance lines
We present the first comparison of profiles of H et He resonance lines
observed by SUMER with theoretical profiles computed with our non-LTE radiative
transfer code. We use the H I Lyman-beta, H I Lyman-epsilon, and He I 584 A
lines. Our code allows us to obtain the plasma parameters in prominences in
conjunction with a multi-line, multi-element set of observations. The plasma
temperature in the prominence core is ~ 8600 K and the pressure is 0.03
dyn/cm^2. The Ly-beta line is formed in a higher temperature region (more than
11000 K).Comment: 2 pages, 2 color figures. Proceedings of SF2A, Semaine de
l'Astrophysique Francaise, Journees de la SF2A 2006, Pari
Sensitivity of the liquid bridge hydrodynamics to local capillary contributions
International audienceIn the usual models of thermocapillary flows, a vorticity singularity occurs at the contact free surface-solid boundaries. The steady axisymmetric hydrodynamics of the side-heated liquid bridge of molten metal is addressed here for its sensitivity to the size delta of a length scale explicitly introduced to regularize the problem. By linear stability analysis of the flows, various stable steady states are predicted: The already known steady states which are reflection-symmetric about the mid-plane, but also others which do not possess this property. The thresholds in Ma of the associated bifurcations are strongly dependent on delta, and converge with delta-->0 towards low values. Published data give these results some physical relevance. (C) 2002 American Institute of Physics
Line profiles and intensity ratios in prominence models with a prominence to corona interface
In this work we study the hydrogen, helium and calcium spectra emitted by a one-dimensional prominence model in magneto-hydrostatic equilibrium. The prominence slab consists of two parts: a cool core where the plasma is optically thick for some lines, and a prominence-to-corona transition region (PCTR) with a strong temperature gradient. The models are defined by 5 parameters: temperature, pressure, slab thickness, microturbulent velocity and altitude. We solve the NLTE radiative transfer equations for all optically thick transitions. We present line ratios between infrared, optical and EUV lines, as well as line profiles.
We show that the presence of a PCTR, where both collisional and radiative excitations are important, affects H, He, and Ca populations and emergent lines in different manners
Kinematics and helicity evolution of a loop-like eruptive prominence
We aim at investigating the morphology, kinematic and helicity evolution of a
loop-like prominence during its eruption. We use multi-instrument observations
from AIA/SDO, EUVI/STEREO and LASCO/SoHO. The kinematic, morphological,
geometrical, and helicity evolution of a loop-like eruptive prominence are
studied in the context of the magnetic flux rope model of solar prominences.
The prominence eruption evolved as a height expanding twisted loop with both
legs anchored in the chromosphere of a plage area. The eruption process
consists of a prominence activation, acceleration, and a phase of constant
velocity. The prominence body was composed of left-hand (counter-clockwise)
twisted threads around the main prominence axis. The twist during the eruption
was estimated at 6pi (3 turns). The prominence reached a maximum height of 526
Mm before contracting to its primary location and partially reformed in the
same place two days after the eruption. This ejection, however, triggered a CME
seen in LASCO C2. The prominence was located in the northern periphery of the
CME magnetic field configuration and, therefore, the background magnetic field
was asymmetric with respect to the filament position. The physical conditions
of the falling plasma blobs were analysed with respect to the prominence
kinematics. The same sign of the prominence body twist and writhe, as well as
the amount of twisting above the critical value of 2pi after the activation
phase indicate that possibly conditions for kink instability were present. No
signature of magnetic reconnection was observed anywhere in the prominence body
and its surroundings. The filament/prominence descent following the eruption
and its partial reformation at the same place two days later suggest a confined
type of eruption. The asymmetric background magnetic field possibly played an
important role in the failed eruption.Comment: 9 pages, 8 figures, in press in A&
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