J/ApJ/738/119 Conversion from magnetoacoustic to Alfven waves (Cally+, 2011)
Benchmarking fast-to-Alfven mode conversion in a cold magnetohydrodynamic
plasma.
Cally P.S., Hansen S.C.
<Astrophys. J., 738, 119 (2011)>
=2011ApJ...738..119C 2011ApJ...738..119C
ADC_Keywords: Sun ; Magnetic fields ; Polarization ; Models
Keywords: magnetohydrodynamics (MHD) - Sun: oscillations - sunspots
Abstract:
Alfven waves may be generated via mode conversion from fast
magnetoacoustic waves near their reflection level in the solar
atmosphere, with implications both for coronal oscillations and for
active region helioseismology. In active regions this reflection
typically occurs high enough that the Alfven speed a greatly exceeds
the sound speed c, well above the a=c level where the fast and slow
modes interact. In order to focus on the fundamental characteristics
of fast/Alfven conversion, stripped of unnecessary detail, it is
therefore useful to freeze out the slow mode by adopting the
gravitationally stratified cold magnetohydrodynamic model c->0. This
provides a benchmark for fast-to-Alfven mode conversion in more
complex atmospheres. Assuming a uniform inclined magnetic field and an
exponential Alfven speed profile with density scale height h, the
Alfven conversion coefficient depends on three variables only: the
dimensionless transverse-to-the-stratification wavenumber κ=kh,
the magnetic field inclination from the stratification direction
θ, and the polarization angle φ of the wavevector relative
to the plane containing the stratification and magnetic field
directions. We present an extensive exploration of mode conversion in
this parameter space and conclude that near-total conversion to
outward-propagating Alfven waves typically occurs for small θ
and large φ (80-90°), though it is absent entirely when
θ is exactly zero (vertical field). For wavenumbers of
helioseismic interest, the conversion region is broad enough to
encompass the whole chromosphere.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 98 162 Forward Alfven conversion coefficient A+ for
a range of κ, φ, and magnetic field
inclinations θ
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See also:
J/ApJ/737/L35 : Pulsed Alfven waves in the solar wind (Gosling+, 2011)
J/MNRAS/410/735 : MAG waves in magnetic and rotating stars (Campos, 2011)
J/ApJS/169/439 : Models of solar magnetic structures (Socas-Navarro, 2007)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 deg theta [10/80] Magnetic field inclination from the
stratification direction θ
4- 8 F5.3 --- kappa [0/5] Transverse-to-the-stratification
wavenumber κ
10- 14 F5.3 --- A+5 Alfven conversion coefficient at φ=5°
polarization angle
16- 20 F5.3 --- A+15 Alfven conversion coefficient at φ=15°
polarization angle
22- 26 F5.3 --- A+25 Alfven conversion coefficient at φ=25°
polarization angle
28- 32 F5.3 --- A+35 Alfven conversion coefficient at φ=35°
polarization angle
34- 38 F5.3 --- A+45 Alfven conversion coefficient at φ=45°
polarization angle
40- 44 F5.3 --- A+55 Alfven conversion coefficient at φ=55°
polarization angle
46- 50 F5.3 --- A+65 Alfven conversion coefficient at φ=65°
polarization angle
52- 56 F5.3 --- A+75 Alfven conversion coefficient at φ=75°
polarization angle
58- 62 F5.3 --- A+85 Alfven conversion coefficient at φ=85°
polarization angle
64- 68 F5.3 --- A+95 Alfven conversion coefficient at φ=95°
polarization angle
70- 74 F5.3 --- A+105 Alfven conversion coefficient at φ=105°
polarization angle
76- 80 F5.3 --- A+115 Alfven conversion coefficient at φ=115°
polarization angle
82- 86 F5.3 --- A+125 Alfven conversion coefficient at φ=125°
polarization angle
88- 92 F5.3 --- A+135 Alfven conversion coefficient at φ=135°
polarization angle
94- 98 F5.3 --- A+145 Alfven conversion coefficient at φ=145°
polarization angle
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 16-Jan-2013