arXiv subjects

T. V. Zaqarashvili

Publications and source records attributed to T. V. Zaqarashvili.

83 records · Page 5Linked to original sources

arXiv2007

Resonant energy conversion of 3-minute intensity oscillations into Alfven waves in the solar atmosphere

Nonlinear coupling between 3-minute oscillations and Alfven waves in the solar lower atmosphere is studied. 3-minute oscillations are considered as acoustic waves trapped in a chromospheric cavity and oscillating along transversally inhomogeneous vertical magnetic field. It is shown that under the action of the oscillations the temporal dynamics of Alfven waves is governed by Mathieu equation. Consequently, the harmonics of Alfven waves with twice period and wavelength of 3-minute oscillations grow exponentially in time near the layer where the sound and Alfven speeds equal. Thus the 3-minute oscillations are resonantly absorbed by pure Alfven waves near this resonant layer. The resonant Alfven waves may penetrate into the solar corona taking energy from the chromosphere. Therefore the layer c_s=v_A may play a role of energy channel for otherwise trapped acoustic oscillations.

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arXiv2007

Spectral line width decrease in the solar corona: resonant energy conversion from Alfv{é}n to acoustic waves

Observations reveal an increase with height of the line width of several coronal spectral lines probably caused by outwardly propagating Alfv{é}n waves. However, the spectral line width sometimes shows a sudden decrease at a height 0.1-0.2 R, where the ratio of sound to Alfven speeds may approach unity. Qualitative analysis shows that the resonant energy conversion from Alfven to acoustic waves near the region of the corona where the plasma $β$ approaches unity may explain the observed spectral line width reduction.

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arXiv2007

Coupling between radial and torsional oscillations in a magnetized plasma and possible stellar applications

The weakly non-linear interaction of slow magnetoacoustic and torsional Alfven waves propagating along an unperturbed poloidal magnetic field in a stellar interior is studied in the high plasma beta limit. It is shown that slow magnetoacoustic waves parametrically drive torsional modes with the half frequency and wave number. Therefore global slow magnetoacoustic waves which have a radial velocity polarization, may amplify the torsional oscillations. Possible applications of this mechanism to the Sun and binary stars are briefly discussed.

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arXiv2006

Two-wave interaction in ideal magnetohydrodynamics

The weakly nonlinear interaction of sound and linearly polarised Alfv{é}n waves propagating in the same direction along an applied magnetic field is studied. It is found that a sound wave is coupled to the Alfv{é}n wave with double period and wavelength when the sound and Alfv{é}n speeds are equal. The Alfv{é}n wave drives the sound wave through the ponderomotive force, while the sound wave returns energy back to the Alfv{é}n wave through the parametric (swing) influence. As a result, the two waves alternately exchange their energy during propagation. The process of energy exchange is faster for waves with stronger amplitudes. The phenomenon can be of importance in astrophysical plasmas, including the solar atmosphere and solar wind.

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arXiv2006

Observation of kink waves in solar spicules

Height series of H$α$ spectra in solar limb spicules obtained with the 53 cm coronagraph of the Abastumani Astrophysical Observatory are analyzed. Each height series covered 8 different heights beginning at 3800 km above the photosphere. The spatial difference between neighboring heights was 1$^{\prime \prime}$, consequently $\sim$ 3800 - 8700 km distance above the photosphere has been covered. The total time duration of each height series was 7 s. We found that nearly 20% of measured height series show a periodic spatial distribution of Doppler velocities. We suggest that this spatial periodicity in Doppler velocity is caused by propagating kink waves in spicules. The wave length is found to be $\sim$ 3500 km. However the wave length tends to be $\sim$ 1000 km at the photosphere due to the height variation of the kink speed. This probably indicates to a granular origin for the waves. The period of waves is estimated to be in the range of 35-70 s. These waves may carry photospheric energy into the corona, therefore can be of importance in coronal heating.

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arXiv2005

Kink waves in solar spicules: observation and modelling

Height series of Doppler observation at the solar limb (covering 3800 - 8700 km distance above the photosphere) in $H_α$ spectral line obtained by big coronagraph of Abastumani Astrophysical Observatory \citep{khu} show the periodic spatial distribution of Doppler velocities in spicules. We suggest that the periodic spatial distribution is caused by propagating kink waves in spicule. The wave length is found to be $\sim$ 3500 km. Numerical modelling of kink wave propagation from the photosphere to observed heights gives the wave length of kink waves at the photosphere to be $\sim$ 1000 km, which indicates to the granular origin of the waves. The period of waves is estimated to be in the range of 35-70 s.

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arXiv2005

Resonant conversion of standing acoustic oscillations into Alfv{é}n waves in the $β~ 1$ region of the solar atmosphere

We show that 5-minute acoustic oscillations may resonantly convert into Alfv{é}n waves in the $β{\sim}1$ region of the solar atmosphere. Considering the 5-minute oscillations as pumping standing acoustic waves oscillating along unperturbed vertical magnetic field, we find on solving the ideal MHD equations that amplitudes of Alfv{é}n waves with twice the period and wavelength of acoustic waves exponentially grow in time when the sound and Alfv{é}n speeds are equal, i.e. $c_s \approx v_A$. The region of the solar atmosphere where this equality takes place we call a {\it swing layer}. The amplified Alfv{é}n waves may easily pass through the chromosphere and transition region carrying the energy of p-modes into the corona.

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arXiv2005

Origin of long-period Alfv{é}n waves in the solar wind

We suggest that the observed long-period Alfv{é}n waves in the solar wind may be generated in the solar interior due to the pulsation of the Sun in the fundamental radial mode. The period of this pulsation is about 1 hour. The pulsation causes a periodical variation of density and large-scale magnetic field, this affecting the Alfv{é}n speed in the solar interior. Consequently the Alfv{é}n waves with the half frequency of pulsation (i.e. with the double period) can be parametrically amplified in the interior below the convection zone due to the recently suggested swing wave-wave interaction. Therefore the amplified Alfv{é}n waves have periods of several hours. The waves can propagate upwards through the convection zone to the solar atmosphere and cause the observed long-period Alfv{é}n oscillations in the solar wind.

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arXiv2004

The mechanism of Swing Absorption of fast magnetosonic waves in inhomogeneous media

The recently suggested swing interaction between fast magnetosonic and Alfvén waves (2002) is generalized to inhomogeneous media. We show that the fast magnetosonic waves propagating across an applied non-uniform magnetic field can parametrically amplify the Alfvén waves propagating along the field through the periodical variation of the Alfvén speed. The resonant Alfvén waves have half the frequency and the perpendicular velocity polarization of the fast waves. The wavelengths of the resonant waves have different values across the magnetic field, due to the inhomogeneity in the Alfvén speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfvén waves, which satisfy the condition for onset of a standing pattern, have stronger growth rates. In these regions the fast magnetosonic waves can be strongly 'absorbed', their energy going in transversal Alfvén waves. We refer to this phenomenon as 'Swing Absorption'. This mechanism can be of importance in various astrophysical situations.

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arXiv2004

'Swing Absorption' of fast magnetosonic waves in inhomogeneous media

The recently suggested swing interaction between fast magnetosonic and Alfven waves (Zaqarashvili and Roberts 2002) is generalized to inhomogeneous media. We show that the fast magnetosonic waves propagating across an applied non-uniform magnetic field can parametrically amplify the Alfven waves propagating along the field through the periodical variation of the Alfven speed. The resonant Alfven waves have half the frequency and the perpendicular velocity polarization of the fast waves. The wave lengths of the resonant waves have different values across the magnetic field, due to the inhomogeneity in the Alfven speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfven waves, which satisfy the condition for onset of a standing pattern, have stronger growth rates. In these regions the fast magnetosonic waves can be strongly 'absorbed', their energy going in transversal Alfven waves. We refer to this phenomenon as 'Swing Absorption'. This mechanism can be of importance in various astrophysical situations.

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arXiv2003

Observation of coronal loop torsional oscillation

We suggest that the global torsional oscillation of solar coronal loop may be observed by the periodical variation of a spectral line width. The amplitude of the variation must be maximal at the velocity antinodes and minimal at the nodes of the torsional oscillation. Then the spectroscopic observation as a time series at different heights above the active region at the solar limb may allow to determine the period and wavelength of global torsional oscillation and consequently the Alfv{é}n speed in corona. From the analysis of early observation (Egan & Schneeberger \cite{egan}) we suggest the value of coronal Alfv{é}n speed as $\sim 500 {\rm km}{\cdot}{\rm s}^{-1}$.

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