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Magnetic Activity

Magnetic Activity. Physical Astronomy Professor Lee Carkner Lecture 11. Questions. A brown dwarf has L = 4X10 22 W, M = 1X10 29 kg, and R = 7X10 7 m. How many years could it shine from gravitational energy? E ~ -(3/10)(GM 2 /R) E ~ -(3/10)(6.67X10 -11 )(1X10 29 ) 2 /(7X10 7 )

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Magnetic Activity

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  1. 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 = 10000 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 • 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. 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 • 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 • 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 12.1-12.2 • Homework: 11.13, 11.14, 11.16, 12.1, 12.10

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