Alfvén instability of steady state flux tubes. Isothermal flow |
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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
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Publication Status: A&A 533, A68 (2011)
Last Modified: 2011-09-16 08:42
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Analysis of power spectra of Doppler shift time series as a diagnostic tool for coronal loops |
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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
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Publication Status: A&A, in press
Last Modified: 2006-10-27 07:59
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