경희대학교 BK21

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● 날짜 : 2018년 04월 17일 (화요일)
● 시간 : 16시 30분
● 장소 : 천문대 영상실
● 강사 : Valeri M. Nakariakov (Kyung Hee University)
 
Title : Nonlinear evolution of torsional Alfven waves in magnetic flux tubes

 
Abstract : In the solar corona, the perpendicular spatial scale of Alfven waves is typically much shorter than the parallel scale, and hence the waves appear in the torsional form. Similarly to the linearly polarised plane Alfven waves, torsional waves nonlinearly induce compressive perturbations by the ponderomotive force. The modification of the local plasma density and magnetic field strengths, and hence the Alfven speed, leads to the nonlinear self-interaction of Alfven waves, accompanied by the nonlinear cascade and enhanced damping. In the case of torsional waves, the ponderomotive effect is much reacher than in the case of plane Alfven waves, as torsional waves are essentially oblique due to their intrinsic dependence on the radial coordinate. The ponderomotive force leads to the induction of compressive flows in the perpendicular direction. Also, the radial non-uniformity of the torsional wave amplitude makes the wave speed dependent on the radial coordinate too, which causes phase mixing. In waveguiding magnetic flux tubes, nonlinearly induced compressive perturbations are found to be of three types: the longitudinal flows propagating at the Alfven and tube speeds, respectively, and transverse flows propagating at the Alfven speed outside the flux tube. The efficiency of the nonlinear cascade in torsional wave could be significantly weaker than in plane Alfven waves.