30,293 research outputs found
Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionized prominence plasma: Effect of helium
Prominences are partially ionized, magnetized plasmas embedded in the solar
corona. Damped oscillations and propagating waves are commonly observed. These
oscillations have been interpreted in terms of magnetohydrodynamic (MHD) waves.
Ion-neutral collisions and non-adiabatic effects (radiation losses and thermal
conduction) have been proposed as damping mechanisms. We study the effect of
the presence of helium on the time damping of non-adiabatic MHD waves in a
plasma composed by electrons, protons, neutral hydrogen, neutral helium (He I),
and singly ionized helium (He II) in the single-fluid approximation. The
dispersion relation of linear non-adiabatic MHD waves in a homogeneous,
unbounded, and partially ionized prominence medium is derived. The period and
the damping time of Alfven, slow, fast, and thermal waves are computed. A
parametric study of the ratio of the damping time to the period with respect to
the helium abundance is performed. The efficiency of ion-neutral collisions as
well as thermal conduction is increased by the presence of helium. However, if
realistic abundances of helium in prominences (~10%) are considered, this
effect has a minor influence on the wave damping. The presence of helium can be
safely neglected in studies of MHD waves in partially ionized prominence
plasmas.Comment: Research note submitted in A&
Note on new interesting baryon channels to measure the photon polarization in b -> s gamma
At LHC a large number of b-flavored baryons will be produced. In this note we
propose new baryon modes to determine the photon helicity of the penguin
transition . The decay has the
drawback that the , being neutral and long-lived, will escape
detection most of the time. To overcome this difficulty, transitions of the
type have been proposed, where
denotes an excited state decaying strongly within the detector into the clean
mode . The doublet , that decays weakly, has a number of good
features. The charged baryon will decay into the mode ,
where the ground state hyperon , although it will decay most of the time
outside the detector, can be detected because it is charged. We consider also
the decay of into , where a higher mass state
can decay strongly within the detector. We point out that the initial
transverse polarization of has to be known in all cases. To determine
this parameter through the transition , we distinguish
between different cases, and underline that in some situations one needs {\it
theoretical input} on the asymmetry parameter of the primary
decay. {\it A fortiori} the same considerations apply to the case of the
The spatial damping of magnetohydrodynamic waves in a flowing partially ionised prominence plasma
Solar prominences are partially ionised plasmas displaying flows and
oscillations. These oscillations show time and spatial damping and, commonly,
have been explained in terms of magnetohydrodynamic (MHD) waves. We study the
spatial damping of linear non-adiabatic MHD waves in a flowing partially
ionised plasma, having prominence-like physical properties. We consider single
fluid equations for a partially ionised hydrogen plasma including in the energy
equation optically thin radiation, thermal conduction by electrons and
neutrals, and heating. Keeping the frequency real and fixed, we have solved the
obtained dispersion relations for the complex wavenumber, k, and have analysed
the behaviour of the damping length, wavelength and the ratio of the damping
length to the wavelength, versus period, for Alfven, fast, slow and thermal
waves.Comment: 28 pages, 9 figure
The statistical significance of the N-S asymmetry of solar activity revisited
The main aim of this study is to point out the difficulties found when trying
to assess the statistical significance of the North-South asymmetry (hereafter
SSNSA) of the most usually considered time series of solar activity. First of
all, we distinguish between solar activity time series composed by integer or
non-integer and dimensionless data, or composed by non-integer and dimensional
data. For each of these cases, we discuss the most suitable statistical tests
which can be applied and highlight the difficulties to obtain valid information
about the statistical significance of solar activity time series. Our results
suggest that, apart from the need to apply the suitable statistical tests,
other effects such as the data binning, the considered units and the need, in
some tests, to consider groups of data, affect substantially the determination
of the statistical significance of the asymmetry. Our main conclusion is that
the assessment of the statistical significance of the N-S asymmetry of solar
activity is a difficult matter and that an absolute answer cannot be given,
since many different effects influence the results given by the statistical
tests. In summary, the quantitative results about the statistical significance
of the N-S asymmetry of solar activity provided by different authors, as well
as the studies about its behaviour, must be considered with care because they
depend from the chosen values of different parameters or from the considered
units.Comment: Astronomy and Astrophysics Latex, 9 pages, 4 figure
Transverse oscillations of two coronal loops
We study transverse fast magnetohydrodynamic waves in a system of two coronal
loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field.
The collective oscillatory properties of the system due to the interaction
between the individual loops are investigated from two points of view. Firstly,
the frequency and spatial structure of the normal modes are studied. The system
supports four trapped normal modes in which the loops move rigidly in the
transverse direction. The direction of the motions is either parallel or
perpendicular to the plane containing the axes of the loops. Two of these modes
correspond to oscillations of the loops in phase, while in the other two they
move in antiphase. Thus, these solutions are the generalization of the kink
mode of a single cylinder to the double cylinder case. Secondly, we analyze the
time-dependent problem of the excitation of the pair of tubes. We find that
depending on the shape and location of the initial disturbance, different
normal modes can be excited. The frequencies of normal modes are accurately
recovered from the numerical simulations. In some cases, because of the
simultaneous excitation of several eigenmodes, the system shows beating and the
phase lag between the loops is .Comment: Accepted for publication in The Astrophysical Journa
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