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Exploring Solar Magnetism: From Emission Lines to Sunspots

Delve into the intriguing world of solar magnetism with Professor Lee Carkner's lecture. Discover how magnetic fields impact the corona, synchrotron emission, and mass ejections. Unravel the secrets of sunspots and starspots, and explore the fascinating phenomena of magnetic pressure and Alfven waves. Dive deep into the mysteries of brown dwarfs and magnetic effects on emission lines. Engage with 11 thought-provoking questions and explore the dynamics of magnetic fields in astronomy.

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Exploring Solar Magnetism: From Emission Lines to Sunspots

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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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