CSI 769/ASTR 769 Topics in Space Weather Fall 2005 Lecture 03 Sep. 20, 2005 Surface Magnetic Field Aschwanden, “Physics of the Solar Corona” Chap. 5, P , Magnetic Fields

Magnetic Field Effects determine the structure of the outer atmosphere No magnetic field, possibly no corona produce the solar activities that drive space weather modulate the heliospheric structure

Magnetogram versus continuum

Magnetic Field Measurement Zeeman effect The splitting of a spectral line because of the presence of magnetic field.

Magnetic Field Measurement (cont.) A magnetogram measures the wavelength shift of certain spectral lines; the shift is proportional to the wavelength as Δλ = 4.7 x g λ 2 B λ: wavelength g: Lande factor, e.g., FeI 6173Å (g=2.5) B: magnetic field strength

Magnetic Field: Hale’s Polarity Law Hale’s polarity law 1.Sunspots are grouped in pairs of opposite polarities 2.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 3.The leading polarity of sunspots is the same as the polarity in the polar region of the same hemisphere 4.from one sunspot cycle to the next, the magnetic polarities of sunspot pairs undergo a reversal in each hemisphere. + -

Magnetic Field: Hale’s Polarity Law (cont.)

Solar Cycle Solar Cycle of Sunspot: 11 year

Solar Cycle (cont.) Butterfly diagram of Sunspot

Solar Cycle: Butterfly Diagram (cont.) 1.Butterfly diagram of Sunspot shows position of sunspots as well as their numbers with time 1.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 2.At the start of a new cycle, these bands form at mid- latitudes, widen 3.They then move toward the equator as each cycle progresses 4.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 (cont.) 22 year magnetic cycle versus 11 year sunspot number cycle

Solar Cycle (ctnl.) Magnetogram: 1995 versus 2001 Minimum Maximum

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

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

Coronal Magnetic Field Almost no direct observation Extrapolation from surface observations: numerically solving Maxwell equations with boundary condition Details see Aschwanden book, Chap 5, P , “Magnetic Field”