Mean shear flows generated by nonlinear resonant Alfvén waves 

Istvan Ballai Submitted: 20090715 09:52
In the context of resonant absorption, nonlinearity has two
different manifestations. The first is the reduction in amplitude
of perturbations around the resonant point (wave energy
absorption). The second is the generation of mean shear flows
outside the dissipative layer surrounding the resonant point.
Ruderman et al. [Phys. Plasmas, 4, 75 (1997)] studied
both these effects at the slow resonance in isotropic plasmas.
Clack et al. [Astron. Astrophys., 494, 317 (2009)]
investigated nonlinearity at the Alfvén resonance, however,
they did not include the generation of mean shear flow. In this
present paper, we investigate the mean shear flow, analytically,
and study its properties. We find that the flow generated is
parallel to the magnetic surfaces and has a characteristic
velocity proportional to epsilon^{1/2}, where epsilon is the
dimensionless amplitude of perturbations far away from the
resonance. This is, qualitatively, similar to the flow generated
at the slow resonance. The jumps in the derivatives of the
parallel and perpendicular components of mean shear flow across
the dissipative layer are derived. We estimate the generated mean
shear flow to be of the order of 10 km s^{1} in both the
solar upper chromosphere and solar corona, however, this value
strongly depends on the choice of boundary conditions. It is
proposed that the generated mean shear flow can produce a
KelvinHelmholtz instability at the dissipative layer which can
create turbulent motions. This instability would be an additional
effect, as a KelvinHelmholtz instability may already exist due
to the velocity field of the resonant Alfvén waves. This flow can
also be superimposed onto existing large scale motions in the
solar upper atmosphere.
Authors: Christopher Clack, Istvan Ballai
Projects: None

Publication Status: accepted
Last Modified: 20090715 10:25


