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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
