Solar Magnetic Field, Solar Cycle, and Solar Dynamo

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CSI 769-001/PHYS 590-001 Solar Atmosphere Fall 2004 Lecture 04 Sep. 22, 2004. Solar Magnetic Field, Solar Cycle, and Solar Dynamo. Magnetic Field is the governing force. Govern the structure of the outer atmosphere - PowerPoint PPT Presentation

Transcript of Solar Magnetic Field, Solar Cycle, and Solar Dynamo

  • CSI 769-001/PHYS 590-001 Solar Atmosphere Fall 2004 Lecture 04 Sep. 22, 2004

    Solar Magnetic Field, Solar Cycle, and Solar Dynamo

  • Magnetic Field is the governing forceGovern the structure of the outer atmosphereNo magnetic field, no corona and possibly no chromosphere

    Govern the solar activity

    Govern the heliospheric structure and space weather

  • SunspotObserved in continuum visible light as Galileo did

  • Photosphere: Sunspot (cont.)A phenomenological description of something we see

    Been noticed in ancient time

    Since 1700, systematic record of sunspot number

    Sunspot was found to be a magnetic feature in 1930

    Sunspot is half the brightness. B = T4 ,Or T ~ B 1/4Tspot/Tsun=(Lspot/Lsun)1/4=(0.5)1/4 = 0.84Tsun = 5700 KTspot = 5700 * 0.84 = 4788 K

    Sunspot is about 1000 K cooler than surrounding

  • Sunspot: balancing (ctnl.) Pext = Pint + Pmag

    Pext: external thermal pressurePext = Next * K *TextN: particle densityK: Boltzmann;s constantT: temperature

    Pint: internal thermal pressurePint= Nint * K * Tint

    Pmag: magnetic pressure inside sunspotPmag = B2/8B: magnetic field strength in the sunspot

    Pext > P int, because Text > T int, but B helps the balance

  • Sunspot: structure (ctnl.)Sunspots show two main structures:

    Umbra: a central dark region,Penumbra: surrounding region of a less darker and filamentary zoneSOHO/MDI 2004/10/24

  • Mangetic Field of SunspotObserved by a magnetogram

  • Magnetic Field: Measurement (ctnl.) Zeeman effectThe splitting of a spectral line because the presence of magnetic field. The electrons moving along different magnetic direction may have different energy

  • Magnetic Field: Measurement (ctnl.) A magnetograph measures the wavelength shift of certain spectral lines; the shift is proportional to the wavelength as

    = 4.7 x 10-13 g 2 H

    : wavelengthg: Lande factorH: magnetic field strengthReference: Zirin (1966), P. 368

  • Magnetic Field: (ctnl.) Hales polarity law

    Sunspots are grouped in pairs of opposite polarities

    The ordering of leading polarity/trailing polarity with respect to the east-west direction (direction of rotation) is the same in a given hemisphere, but is reversed from northern to southern hemisphere

    The leading polarity of sunspots is the same as the polarity in the polar region of the same hemisphere

    from one sunspot cycle to the next, the magnetic polarities of sunspot pairs undergo a reversal in each hemisphere.

  • Magnetic Field: (ctnl.) Hales polarity law+-- +- ++ -+ -

  • Solar CycleSolar Cycle of Sunspot: 11 year

  • Solar Cycle (ctnl.)Butterfly diagram of Sunspot

  • Solar Cycle: Butterfly Diagram (ctnl.)Butterfly diagram of Sunspot shows position of sunspots as well as their numbers

    Sunspots do not appear at random over the surface of the sun but are concentrated in two latitude bands on either side of the equator

    these bands first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses

    The cycles overlap at the time of sunspot cycle minimum with old cycle spots near the equator and new cycle spots at high latitudes

  • Solar Cycle (ctnl.)

  • Solar Cycle (ctnl.)22 year magnetic cycle versus 11 year sunspot number cycle

  • Solar Cycle (ctnl.)Mixed Hale polarity during solar minimumAlso see textbookFigure 4.5, P102

  • Solar Cycle (ctnl.)Maunder Minimum from 1645 to 1715 Associated with Little Ice AgePossible connection between solar activity and terrestrial climate

  • Solar Cycle (ctnl.)Solar Cycle of X-ray corona

  • Solar Cycle (ctnl.)Magnetogram: 1995 versus 2001

  • Solar Cycle (ctnl.)Coronal at EUV (EIT 195 , 1.5 MK): 1995 versus 2001

  • Solar Cycle (ctnl.)Chromosphere (EIT 304 , 0.08 MK): 1995 versus 2001

  • Solar Cycle (ctnl.)Outer corona (LASCO C2): 1995 versus 2001

  • Solar Cycle (ctnl.)Solar activities strongly follow the 11-year solar cycleFlare: 1/day at minimum, 10/day at maximumCME: 0.4/day at minimum, 4/day at maximum3. As well as Geomagnetic Storms and Solar Energetic Particle events

  • Solar DynamoGeneration of Solar Magnetic Field: magnetic dynamo theory

  • Solar Dynamo (ctnl.)Maxwell EquationsFaradays law of InductionAmperes LawGausss LawLaw of no magnetic Charges

    Generalized Ohms Law

    Equation of Magnetic Induction

  • Solar Dynamo (ctnl.)Solar Differential RotationSurface Latitudinal Differential Rotation:rotation at equator (25 days) is faster than the higher latitudes, progressively slower, at poles (35 days)Radial Differential RotationAt equatorial region, interior(27 days) rotates slower than surface (25 days)At polar region, interior (27 days) rotates faster than surface (35 days)

    (See textbook Figure 4.14 at P. 122)

  • Solar Dynamo (ctnl.)Poloidal field stretched to toroidal field and strenghened(also See textbook Figure 4.15 at P. 123)

  • Solar Dynamo (ctnl.)Poloidal field rises and turns into toroidal field(also See textbook Figure 4.16 at P. 124)

  • Solar DynamoSummary of the - dynamo (P121P126)Initial poloidal field is stretched becoming toroidal field, wound up and strengthened, at the bottom of convection zoneToroidal field, which becomes sufficiently strong, rises through the convection zone because of magnetic buoyancy forceAs the magnetic flux tube rises, the Coriolis force has the so-called helical convection effect that take some E-W oriented field and turn it into N-S oriented poloidal field (FiThis process forms the cycle of poloidal field toroidal field and poloidal field again. More importantly, the new poloidal field is directed opposite to the field which started the cycle