121 research outputs found
Tensorial depolarization of alkali atoms by isotropic collisions with neutral hydrogen
Results. We consider the problem of isotropic collisions between an alkali
atom and neutral hydrogen. We calculate the collisional tensorial components of
general p and s-states, characterized by their effective principal quantum
number . It is found that the behaviour of the tensorial components obey
simple power laws allowing quick calculations of the depolarizing collisional
rates. As application, our results should allow a rigorous treatment of the
atomic polarization profiles of the D1 -D2 lines of alkali atoms.
Conclusions. Close coupling treatments of atomic collisions are needed to
decipher the information encoded in the polarized radiation from the Sun.
Important problems remain unresolved like the role of collisions in the
Paschen-Back conditions.Comment: Accepted for publication in A&
Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?
The physical interpretation of scattering line polarization offers a novel
diagnostic window for exploring the thermal and magnetic structure of the quiet
regions of the solar atmosphere. Here we evaluate the impact of isotropic
collisions with neutral hydrogen atoms on the scattering polarization signals
of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the
IR triplet of Ca II, with emphasis on the collisional transfer rates between
nearby J-levels. To this end, we calculate the linear polarization produced by
scattering processes considering realistic multilevel models and solving the
statistical equilibrium equations for the multipolar components of the atomic
density matrix. We confirm that the lower levels of the 13 lines of multiplet
42 of Ti I are completely depolarized by elastic collisions. We find that
upper-level collisional depolarization turns out to have an unnoticeable impact
on the emergent linear polarization amplitudes, except for the {\lambda 4536
line for which it is possible to notice a rather small depolarization caused by
the collisional transfer rates. Concerning the Ca II lines, we show that the
collisional rates play no role on the polarization of the upper level of the K
line, while they have a rather small depolarizing effect on the atomic
polarization of the metastable lower levels of the Ca II IR triplet.Comment: Accepted for publication in Astronomy and Astrophysic
A unified numerical model of collisional depolarization and broadening rates due to hydrogen atom collisions
Interpretation of solar polarization spectra accounting for partial or
complete frequency redistribution requires data on various collisional
processes. Data for depolarization and polarization transfer are needed but
often missing, while data for collisional broadening are usually more readily
available. Recent work by Sahal-Br\'echot and Bommier concluded that despite
underlying similarities in the physics of collisional broadening and
depolarization processes, relationships between them are not possible to derive
purely analytically.
We aim to derive accurate numerical relationships between the collisional
broadening rates and the collisional depolarization and polarization transfer
rates due to hydrogen atom collisions. Such relationships would enable accurate
and efficient estimation of collisional data for solar applications.
Using earlier results for broadening and depolarization processes based on
general (i.e. not specific to a given atom), semi-classical calculations
employing interaction potentials from perturbation theory, genetic programming
(GP) has been used to fit the available data and generate analytical functions
describing the relationships between them. The predicted relationships from the
GP-based model are compared with the original data to estimate the accuracy of
the method.Comment: 10 pages, 7 figures, Accepted for publication in Astronomy &
Astrophysic
Evidence for collisional depolarization of the \ion{Ba}{ii} line in the low chromosphere
Context. Rigorous modeling of the \ion{Ba}{ii} formation is
potentially interesting since this strongly polarized line forms in the solar
chromosphere where the magnetic field is rather poorly known. Aims. To
investigate the role of isotropic collisions with neutral hydrogen in the
formation of the polarized \ion{Ba}{ii} line and, thus, in the
determination of the magnetic field. Methods. Multipole relaxation and transfer
rates of the and p-states of \ion{Ba}{ii} by isotropic collisions with
neutral hydrogen are calculated. We consider a plane parallel layer of
\ion{Ba}{ii} situated at the low chromosphere and anisotropically illuminated
from below which produces linear polarization in the line by
scattering processes. To compute that polarization, we solve the statistical
equilibrium equations for \ion{Ba}{ii} levels including collisions, radiation
and magnetic field effects. Results. Variation laws of the relaxation and
transfer rates with hydrogen number density and
temperature are deduced. The polarization of the line is
clearly affected due to isotropic collisions with neutral hydrogen although the
collisional depolarization of its upper level is negligible. This
is because the alignment of the metastable levels and
of the \ion{Ba}{ii} are vulnerable to collisions. At the height of formation of
the line where cm, we find that the neglecting of the collisions induces
inaccuracy of 25% on the calculation of the polarization and 35 %
inaccuracy on microturbulent magnetic field determination.Comment: 12 pages, 4 figures, accepted for publication in A&
Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields
The physics of the solar chromosphere depends in a crucial way on its
magnetic structure. However there are presently very few direct magnetic field
diagnostics available for this region. Here we investigate the diagnostic
potential of the Hanle effect on the Ba II D2 line resonance polarization for
the determination of weak chromospheric turbulent magnetic fields......Comment: In press in astronomy and astrophysic
Collisional depolarization and transfer rates of spectral lines by atomic hydrogen. II: application to d states of neutral atoms
The theory of collisional depolarization of spectral lines by atomic hydrogen
(Derouich et al. \cite{derouich1}) is extended to =2) atomic levels.
Depolarization rates, polarization and population transfer rates are calculated
and results are given as a function of the temperature. Cross sections as a
function of the effective quantum number for a relative velocity of 10
are also given together with velocity exponents ,
if \textbf{they exist}, on the assumption that the cross section varies with
velocity as . A discussion of our results is presented.Comment: 11 pages including 3 figures and 2 tables, accepted in A&
Collisional depolarization and transfer rates of spectral lines by atomic hydrogen. IV: application to ionised atoms
The semi-classical theory of collisional depolarization of spectral lines of
neutral atoms by atomic hydrogen (Derouich et al. 2003a; Derouich et al. 2003b;
Derouich et al. 2004 and references therein) is extended to spectral lines of
singly ionised atoms. In this paper we apply our general method to the
particular cases of the 3d and states of the CaII ion and to
the state of the SrII ion. Analytical expressions of all rates as a
function of local temperature are given.
Our results for the CaII ion are compared to recent quantum chemistry
calculations. A discussion of our results is presented.Comment: 19 pages, 14 figures, submitted to Astronomy and Astrophysic
A model of dengue fever
BACKGROUND: Dengue is a disease which is now endemic in more than 100 countries of Africa, America, Asia and the Western Pacific. It is transmitted to the man by mosquitoes (Aedes) and exists in two forms: Dengue Fever and Dengue Haemorrhagic Fever. The disease can be contracted by one of the four different viruses. Moreover, immunity is acquired only to the serotype contracted and a contact with a second serotype becomes more dangerous. METHODS: The present paper deals with a succession of two epidemics caused by two different viruses. The dynamics of the disease is studied by a compartmental model involving ordinary differential equations for the human and the mosquito populations. RESULTS: Stability of the equilibrium points is given and a simulation is carried out with different values of the parameters. The epidemic dynamics is discussed and illustration is given by figures for different values of the parameters. CONCLUSION: The proposed model allows for better understanding of the disease dynamics. Environment and vaccination strategies are discussed especially in the case of the succession of two epidemics with two different viruses
The Effect of Physical Exercise on the Dynamics of Glucose and Insulin
Abstract Regular physical activity is indicated either to prevent and delay the onset of non-insulin-dependent diabetes or to assure a good control of diabetes by increasing insulin sensitivity and ameliorating the metabolism of glucose disappearance. Many studies and experiments have dealt with this subject. In this paper, we introduce the effect of physical activity via parameters of a mathematical model which allows us to compare the behaviour of blood glucose in normal, non-insulin-dependent diabetes and insulin-dependent diabetes people, with and without physical effort. Extreme cases of physical activity leading to hypoglycaemia or aggravating hyperglycaemia are also underlined.
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