1. Physical Astronomy Professor Lee Carkner Lecture 11
Magnetic Activity
Physical Astronomy
Professor Lee Carkner
Lecture 11

2. 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

3. Magnetism
In the regions above the photosphere, magnetic fields provide:
Responsible for:
Corona
Magnetic field given as B
Often use Gauss (1 T = G)

4. 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

5. Emission Lines Corona produces forbidden lines
Also produces X-ray line emission from strongly ionized heavy elements

6. Free-Free Emission
Will emit a photon
Sometimes called braking radiation or Bremsstrahlung
Produces radio and X-ray emission in the sun

7. Synchrotron Emission
As the electron spirals around it changes velocity and thus emits photons
Produces radio emission from corona

8. Zeeman Effect Dn = eB/4pme
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

9. vs = (gP/r)½ = (gkT/mmH)½
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

10. Magnetic Pressure um = B2/2m0 Pm =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

11. Alfven Waves The velocity of the Alfven wave is given by vm
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

12. 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

13. Sunspots
Regions where a strong magnetic field is inhibiting convection
B ~ 1/10 T
T ~4000 K
Up to 5 times fainter

14. 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

15. 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

16. Next Time
Read
Homework: 11.13, 11.14, 11.16, 12.1, 12.10