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Alfvén instability of steady state flux tubes. Isothermal flow  

Youra Taroyan   Submitted: 2011-09-15 05:24

Context. MHD instabilities are expected to play an important role in the dynamics and energetics of the solar atmosphere.

Aims. One application of a recently discovered MHD instability to an open magnetic flux tube is investigated. The flux tube is gravitationally stratified and permeated by a smooth isothermal flow.

Methods. The equilibrium structure is derived, and the stability of the tube with respect to small amplitude torsional perturbations generated at the footpoint by random convective motions analysed.

Results. It is shown that torsional perturbations are exponentially amplified in time if a narrow region exists where the flux tube rapidly expands while the plasma is flowing away from the footpoint. No high flow speeds are required for the instability to set in.

Conclusions. These obtained results may account for the nonthermal broadenings associated with upflows in magnetic regions of the lower solar atmosphere. However, additional studies that incorporate temperature variations are needed for more robust conclusions.


Authors: Taroyan, Y.
Projects: None

Publication Status: A&A 533, A68 (2011)
Last Modified: 2011-09-16 08:42
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Alfvén instability in coronal loops with siphon flows  

Youra Taroyan   Submitted: 2009-04-09 02:41

A new magnetohydrodynamic (MHD) instability in coronal loops is presented. It is demonstrated that small-amplitude torsional Alfvénic disturbances generated by random photospheric motions are inevitably amplified in asymmetrically stratified loops with siphon flows. The growth rates linked with the instability increase with increasing flow speeds. The instability is caused by over-reflection of Alfvén waves at the transition region where steep variations in density exist.

Authors: Y. Taroyan
Projects: None

Publication Status: ApJ, 694, 69-75
Last Modified: 2009-04-09 13:12
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Subject will be restored when possible  

Youra Taroyan   Submitted: 2008-01-31 03:27

The upward propagation of linear acoustic waves in a gravitationally stratified solar atmosphere is studied. The wave motion is governed by the Klein-Gordon equation which contains a cut-off frequency introduced by stratification. The acoustic cut-off may act as a potential barrier when the temperature decreases with height. It is shown that waves trapped below the barrier could be subject to a resonance which extends into the entire unbounded atmosphere of the Sun. The parameter space characterizing the resonance is explored.

Authors: Y. Taroyan, R. Erdelyi
Projects: None

Publication Status: Solar Physics 2008, in press
Last Modified: 2008-01-31 09:11
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Subject will be restored when possible  

Youra Taroyan   Submitted: 2008-01-31 03:24

Standing slow (acoustic) waves commonly observed in hot coronal loops offer a unique opportunity to understand the properties of the coronal plasma. The lack of evidence for similar oscillations in cooler loops is still a puzzle. The high cadence EIS instrument on board recently launched Hinode has the capability to detect wave motion in EUV lines both in the imaging and spectroscopy modes. The paper aims to establish the distinct characteristics of standing and propagating acoustic waves and to predict their footprints in EIS data. A 1D hydrodynamic loop model is used and the consequences of various types of heating pulses are examined. In each case, the resulting hydrodynamic evolution of the loop is converted into observables using a selection of available EIS spectral lines and windows. Propagating/standing acoustic waves are a natural response of the loop plasma to impulsive heating. Synthetic EIS observations of such waves are presented both in the imaging and spectroscopy modes. The waves are best seen and identified in spectroscopy mode observations. It is shown that the intensity oscillations, unlike the Doppler shift oscillations, continuously suffer phase shifts due to heating and cooling of the plasma. It is therefore important to be aware of this effect when interpreting the nature of the observed waves.

Authors: Y. Taroyan, S. Bradshaw
Projects: Hinode/EIS

Publication Status: Astron. Astrophys. 2008, in press
Last Modified: 2008-01-31 09:11
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Forward modeling of hot loop oscillations observed by SUMER and SXT  

Youra Taroyan   Submitted: 2007-04-04 07:43

An example of hot active region loop oscillations observed by SUMER and SXT is presented. The hypothesis that a fundamental mode standing slow sausage (acoustic) wave is initiated by a footpoint microflare is tested and confirmed using a foward modelling approach. The oscillation is set up immediately after the heating pulse. The duration, temporal behaviour and total heat input of the microflare are estimated using the oscillation parameters. The rapid energy release is followed by cooling. The time-distance profile of the heating rate along the loop is recovered using the intensity and Doppler shift time series. Hot loop oscillations are mainly observed in the Doppler shift. The absence of intensity oscillations in this and similar cases is explained. It is also found that the intensity oscillation, unlike the Doppler shift oscillation, undergoes half a period phase variation when the background intensity passes through its maximum, thus making it more difficult to detect.

Authors: Y. Taroyan, R. Erdelyi, T. J. Wang, S. J. Bradshaw
Projects: SoHO-SUMER

Publication Status: ApJ 659, L173-L176, 2007
Last Modified: 2007-04-06 12:26
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Analysis of power spectra of Doppler shift time series as a diagnostic tool for coronal loops  

Youra Taroyan   Submitted: 2006-10-27 04:05

Context. Analysing the structure of solar coronal loops is crucial to our understanding of the processes which heat and maintain the coronal plasma at multimillion degree temperatures. The determination of the physical parameters of the loops remains both an observational and theoretical challenge. Aims. The present work is aimed at tackling some of these challenges such as the determination of the average loop temperature and its distribution along a given loop. Methods. A new diagnostic technique for quiescent coronal loops based on the analysis of power spectra of Doppler shift time series is proposed. It is assumed that the loop is heated randomly both in space and time by small-scale discrete impulsive events of unspecified nature. The loop evolution is characterised by longitudinal motions caused by the random heating events. These random motions can be represented as a superposition of the normal modes of the loop, i.e., its standing acoustic wave harmonics. The idea is borrowed from helioseismology where a similar approach resulted in a deep understanding of the solar interior. Results. It is shown that depending on the heliographic position of the loop and its geometrical orientation, various harmonics can be identified in the power spectra of the line shift time series. The highest power peak corresponds to the fundamental mode. The peaks become smaller as the frequency increases. The frequencies of the harmonics are determined by the loop length and temperature and thus are suggested to be used as a temperature diagnostic tool. It is demonstrated that the analysis of the power spectra allows the distinction between uniformly heated loops from loops heated near their footpoints and to estimate the average energy of a single heating event. The proposed new method could in principle be used to study the multithermal structure of coronal loops. Conclusions. The power spectrum analysis is a potentially powerful technique for coronal loop diagnostics.

Authors: Taroyan, Y., Erdelyi, R., Doyle, J. G., Bradshaw, S. J.
Projects: None

Publication Status: A&A, in press
Last Modified: 2006-10-27 07:59
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Footpoint excitation of standing acoustic waves in coronal loops  

Youra Taroyan   Submitted: 2005-02-23 13:07

A new theoretical model for the study of slow standing sausage mode oscillations in hot (T > 6 MK) active region coronal loops is presented. These oscillations are observed by the SUMER spectrometer on board the SoHO satellite. The model contains the transition region and the upper chromosphere which enables us to study the entire process of hot loop oscillations - from the impulsive footpoint excitation phase to the rapid damping phase. It is shown that standing acoustic waves can be excited by an impulsive heat deposition at the chromospheric footpoint of a loop if the duration of the pulse matches the fundamental mode period. The pulse is immediately followed by a standing wave consistent with the SUMER observations in hot loops. The amount of released energy determines the oscillation amplitude. The combined effects of thermal conduction and radiation on the behaviour of the standing acoustic waves in hot gravitationally stratified loops are investigated. In addition to damping, these effects lead to downflows which are superimposed on the oscillations. The implications of the results in coronal seismology are discussed.

Authors: Taroyan, Y., Erdelyi, R., Doyle, J. G., Bradshaw, S. J.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2005-04-12 11:35
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Abstracts by Author
Alfven instability of steady state flux tubes. Isothermal flow
Alfven instability in coronal loops with siphon flows
Subject will be restored when possible
Subject will be restored when possible
Forward modeling of hot loop oscillations observed by SUMER and SXT
Analysis of power spectra of Doppler shift time series as a diagnostic tool for coronal loops
Footpoint excitation of standing acoustic waves in coronal loops

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