Transcript Alfven wave

Magnetic Activity
Physical Astronomy
Professor Lee Carkner
Lecture 11
Questions
 A brown dwarf has L = 4X1022 W, M =
1X1029 kg, and R = 7X107 m. How many
years could it shine from gravitational
energy?
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E ~ -(3/10)(GM2/R)
E ~ -(3/10)(6.67X10-11)(1X1029)2/(7X107)
E ~ 2.86X1039 J
T = E/L = 2.86X1039/4X1022 = 7.15X1016 s
T = 2.27X109 years = 2.3 billion years
Magnetism
In the regions above the photosphere,
magnetic fields provide:
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Responsible for:
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Corona
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Magnetic field given as B
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Often use Gauss (1 T = 10000 G)
The Corona
 Above the photosphere is
the corona
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 Strongly ionized
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 Can only be seen by eye
during total eclipse
 Produces radio and X-ray
emission
Emission Lines
Corona produces
forbidden lines
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Also produces X-ray
line emission from
strongly ionized
heavy elements
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Free-Free Emission
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Will emit a photon
Sometimes called
braking radiation or
Bremsstrahlung
Produces radio and Xray emission in the
sun
Synchrotron Emission
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 As the electron spirals
around it changes velocity
and thus emits photons
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 Produces radio emission
from corona
Zeeman Effect
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The energy levels are slightly altered by the
presence of the B field
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Dn = eB/4pme
Where Dn is the difference between the
normal line the magnetically altered one
MHD
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Need to apply magnetohydrodynamics (MHD)
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vs = (gP/r)½ = (gkT/mmH)½
Where P, T and r are pressure, temperature
and density and g is the ratio of specific heats for
the gas
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vs is the “normal” wave speed
Waves traveling faster than this will create a shock
wave
Magnetic Pressure
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um = B2/2m0
Where m0 is the permeability of free space = 4p X 10-7
N A-2
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They don’t want to be compressed
Pm =B2/2m0
This pressure is like the tension in a string
Alfven Waves
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like a wave on a string
The velocity of the Alfven wave is given by
vm
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vm = B/(m0r)½
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As they change the B field, they create currents
that produce Joule heating
Mass Ejections
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 Open magnetic fields
produce the solar wind
 Particles spiral out away from
the sun and into the solar
system
 Speed ~750 km/s
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 Large, random ejections of
material
Sunspots
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 Regions where a strong
magnetic field is inhibiting
convection
 B ~ 1/10 T
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 T ~4000 K
 Up to 5 times fainter
Starspots
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 But we can see variations
in luminosity due to spots
rotating in and out of our
field of view
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 Very magnetically active
stars can have spots
covering half the star
Sunspot Cycles
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 Takes 11 years for Sun’s
magnetic field to get wound
up and then break and
reconnect
 There is also a longer term
cycle where the number of
spots at peak is low for
many cycles
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 Temperatures were also
anomalously low
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Next Time
Read 12.1-12.2
Homework: 11.13, 11.14, 11.16, 12.1,
12.10