Speaker
Munehito Shoda
(The University of Tokyo)
Description
Classically, the solar chromosphere was thought to be a dissipative medium for Alfvén
waves, in that those generated by the convective motions were thought to dissipate
most of their energy there. However, recent high-resolution observations near the
coronal base strongly suggest that the upward Alfvén waves are generated or amplified
in the chromosphere by the large amplitude acoustic (shock) waves. This means that
the classical wave-turbulence modelings of the solar corona and the solar wind need
some modification, because most of such modelings neglect the interaction between the
acoustic waves and the Alfvén waves. Understanding the Alfvén wave generation in the
solar chromosphere is essential for more realistic modeling. We have performed both
numerical simulations of one-dimensional MHD systems and the observations of the
transverse Doppler velocities using off-limb spicule observation by IRIS. We first
have investigated numerically the nonlinear interaction between large-amplitude
acoustic (shock) waves and small-amplitude Alfvén waves in the chromosphere. We have
found that, if the plasma beta is around unity, Alfvén waves are trapped and
amplified around the shock front, leading to the strong amplification of the Alfvén
waves. From this process, only upward Alfvén waves are generated. Since this process
is likely to occur for strong acoustic shock waves propagating in a beta-unity
plasma, the solar chromosphere is a favorable place for this process. We have
compared the power-spectrum of the transverse velocity between the simulations and
observations. Mg II k line is used for deriving the observational transverse
velocities in the chromosphere. The similarity of the power spectrum strongly
suggests that the generation of upward Alfvénic waves occur in the middle
chromosphere under the formation height of Mg II k line.
Primary author
Munehito Shoda
(The University of Tokyo)
