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?
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:
Responsible for:
Corona
Magnetic field given as B
Often use Gauss (1 T = 10000 G)
The Corona
Above the photosphere is
the corona
Strongly ionized
Can only be seen by eye
during total eclipse
Produces radio and X-ray
emission
Emission Lines
Corona produces
forbidden lines
Also produces X-ray
line emission from
strongly ionized
heavy elements
Free-Free Emission
Will emit a photon
Sometimes called
braking radiation or
Bremsstrahlung
Produces radio and Xray emission in the
sun
Synchrotron Emission
As the electron spirals
around it changes velocity
and thus emits photons
Produces radio emission
from corona
Zeeman Effect
The energy levels are slightly altered by the
presence of the B field
Dn = eB/4pme
Where Dn is the difference between the
normal line the magnetically altered one
MHD
Need to apply magnetohydrodynamics (MHD)
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
vs is the “normal” wave speed
Waves traveling faster than this will create a shock
wave
Magnetic Pressure
um = B2/2m0
Where m0 is the permeability of free space = 4p X 10-7
N A-2
They don’t want to be compressed
Pm =B2/2m0
This pressure is like the tension in a string
Alfven Waves
like a wave on a string
The velocity of the Alfven wave is given by
vm
vm = B/(m0r)½
As they change the B field, they create currents
that produce Joule heating
Mass Ejections
Open magnetic fields
produce the solar wind
Particles spiral out away from
the sun and into the solar
system
Speed ~750 km/s
Large, random ejections of
material
Sunspots
Regions where a strong
magnetic field is inhibiting
convection
B ~ 1/10 T
T ~4000 K
Up to 5 times fainter
Starspots
But we can see variations
in luminosity due to spots
rotating in and out of our
field of view
Very magnetically active
stars can have spots
covering half the star
Sunspot Cycles
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
Temperatures were also
anomalously low
Next Time
Read 12.1-12.2
Homework: 11.13, 11.14, 11.16, 12.1,
12.10