Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds

Takeru K. Suzuki(1), Huirong Yan(2,3) Alex Lazarian(2) & Joseph P. Cassinelli(2)

1 : Dept. of Physics, Kyoto University, Kyoto 606-8502, Japan
2 : Dept. of Astronomy, University of Wisconsin, 475 N. Charter St., Madison, WI 53706
3 : Present Address : CITA, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 3H8, Canada

We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars.
Stellar rotation causes magnetic field lines anchored at the surface
to form a spiral pattern and magneto-rotational winds can be driven.
If the structure is a magnetically dominated, fast MHD waves
generated at the surface can propagate almost radially outward and
cross the field lines. The propagating waves undergo collisionless damping
owing to interactions with particles surfing on magnetic mirrors that are
formed by the waves themselves. The energy damping rate is especially
effective where the angle between the wave propagation and the field
lines becomes moderately large ($\sim 20$ to $80^{\circ}$). The angle tends
naturally to increase into this range because the field in magneto-rotational
winds develops an increasingly large azimuthal component.
The dissipation of the wave energy produces heating and acceleration of the
We show using specified wind structures that this damping process can be
important in both solar-type stars and massive stars that have moderately
large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which
are rapidly rotating and show X-ray luminosities much larger than the sun.
The mechanism could also be important for producing the
extended X-ray emitting regions inferred to exist in massive stars of
spectral type middle B and later.

Reference: ApJ, in press
Status: Manuscript has been accepted


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