Fast and Alfvén waves driven by azimuthal footpoint motions - I. Periodic driver
Fast and Alfvén waves driven by azimuthal footpoint motions - I. Periodic driver
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由方位点运动驱动的快波和阿尔文波 - I. 周期驱动器
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
M. Goossens
中科院分区：
文献类型：
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作者：
A. Groof;K. Paes;M. Goossens
本文研究了在线性理想磁流体力学中，由足尖运动驱动的日冕环中Alfven波和快磁声波的激发。分析仅限于方位角极化的足点运动，使只有阿尔芬波直接激发耦合到快速磁声波在以后的时间。在本研究中，周期性的驱动器应用在循环的一端。在De Groof & Goossens（2002）的配套论文（以下称为论文II）中研究了更现实的随机驱动器的影响。本文的第一部分研究了无耦合时（方位波数ky = 0）的共振吸收和相位混合。由于密度在整个回路中变化，因此在驱动频率等于局部Alfven频率的磁性表面处发生共振。在第二部分中，考虑了与快波耦合的ky 6 0的Alfven波，我们发现MHD波的行为强烈地依赖于驱动频率。D.特别是驱动频率等于准模频率似乎使差异。在这些情况下，激发的快波是系统的全局振荡，并形成准模式，因为它们通过与Alfven模式的共振耦合而被阻尼。由于这些共振发生在原始Alfven波峰值的相同位置，因此共振峰值被进一步放大。虽然在大多数情况下耦合对直接激发的Alfven波的增长有负面影响，但与非耦合情况相比，用准模频率驱动会导致共振峰的更快增长和更明显的长度尺度减小。
The excitation of Alfv en and fast magneto-acoustic waves in coronal loops driven by footpoint motions is studied in linear, ideal MHD. The analysis is restricted to azimuthally polarized footpoint motions so that only Alfv en waves are directly excited which couple to fast magneto-acoustic waves at later times. In the present study a periodic driver is applied at one end of the loop. The eects of a more realistic random driver are studied in the companion paper De Groof & Goossens (2002) (hereafter referred to as Paper II). The rst part of the paper is devoted to the study of resonant absorption and phase-mixing in the absence of coupling (azimuthal wavenumber ky = 0). Since the density varies across the loop, resonances occur at the magnetic surfaces where the driving frequency equals the local Alfv en frequency. In a second part where Alfv en waves with ky 6 0 coupling to fast waves are taken into account, we nd that the behaviour of the MHD waves is strongly dependent on the driving frequency !d. Especially driving frequencies equal to a quasi-mode frequency seem to make the dierence. The fast waves excited in these cases are global oscillations of the system and form quasi-modes as they are damped through the resonant coupling with Alfv en modes. Since these resonances occur at the same location where the original Alfv en wave peaks, the resonant peak is further amplied. While in most cases coupling has a negative eect on the growth of the directly excited Alfv en waves, driving with a quasi-mode frequency leads to a faster growth of the resonant peaks and a more ecient decrease in length scales than in the uncoupled case.