142 research outputs found
Linear dynamics of the solar convection zone: excitation of waves in unstably stratified shear flows
In this paper we report on the nonresonant conversion of convectively
unstable linear gravity modes into acoustic oscillation modes in shear flows.
The convectively unstable linear gravity modes can excite acoustic modes with
similar wave-numbers. The frequencies of the excited oscillations may be
qualitatively higher than the temporal variation scales of the source flow,
while the frequency spectra of the generated oscillations should be
intrinsically correlated to the velocity field of the source flow. We
anticipate that this nonresonant phenomenon can significantly contribute to the
production of sound waves in the solar convection zone.Comment: 8 pages. To appear in the proceedings of the conference "Waves in
Dusty, Solar and Space Plasmas", Leuven, Belgium 21-26 May 200
Linear coupling of modes in 2D radially stratified astrophysical discs
We investigate mode coupling in a two dimensional compressible disc with
radial stratification and differential rotation. We employ the global radial
scaling of linear perturbations and study the linear modes in the local
shearing sheet approximation. We employ a three-mode formalism and study the
vorticity (W), entropy (S) and compressional (P) modes and their coupling
properties. The system exhibits asymmetric three-mode coupling: these include
mutual coupling of S and P-modes, S and W-modes, and asymmetric coupling
between the W and P-modes. P-mode perturbations are able to generate potential
vorticity through indirect three-mode coupling. This process indicates that
compressional perturbations can lead to the development of vortical structures
and influence the dynamics of radially stratified hydrodynamic accretion and
protoplanetary discs.Comment: 10 pages, 10 figures, MNRAS (accepted
Fast magnetohydrodynamic oscillation of longitudinally inhomogeneous prominence threads: an analogue with quantum harmonic oscillator
Previous works indicate that the frequency ratio of second and first
harmonics of kink oscillations has tendency towards 3 in the case of prominence
threads. We aim to study the magnetohydrodynamic oscillations of longitudinally
inhomogeneous prominence threads and to shed light on the problem of frequency
ratio. Classical Sturm--Liouville problem is used for the threads with
longitudinally inhomogeneous plasma density. We show that the spatial variation
of total pressure perturbations along the thread is governed by the stationary
Schr\"{o}dinger equation, where the longitudinal inhomogeneity of plasma
density stands for the potential energy. Consequently, the equation has bounded
solutions in terms of Hermite polynomials. Boundary conditions at the thread
surface lead to transcendental dispersion equation with Bessel functions. Thin
flux tube approximation of the dispersion equation shows that the frequency of
kink waves is proportional to the expression \alpha(2n+1), where \alpha is the
density inhomogeneity parameter and n is the longitudinal mode number.
Consequently, the ratio of the frequencies of second and first harmonics tends
to 3 in prominence threads. Numerical solution of the dispersion equation shows
that the ratio only slightly decreases for thicker tubes in the case of smaller
longitudinal inhomogeneity of external density, therefore the thin flux tube
limit is a good approximation for prominence oscillations. However, stronger
longitudinal inhomogeneity of external density may lead to the significant
shift of frequency ratio for wider tubes and therefore the thin tube
approximation may fail. The tendency of frequency ratio of second and first
harmonics towards 3 in prominence threads is explained by the analogy of the
oscillations with quantum harmonic oscillator, where the density inhomogeneity
of the threads plays a role of potential energy.Comment: 8 pages, 7 figures (accepted in A&A
Primordial magnetic field limits from cosmological data
We study limits on a primordial magnetic field arising from cosmological
data, including that from big bang nucleosynthesis, cosmic microwave background
polarization plane Faraday rotation limits, and large-scale structure
formation. We show that the physically-relevant quantity is the value of the
effective magnetic field, and limits on it are independent of how the magnetic
field was generated.Comment: 7 pages, 6 figure
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