Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Sun: Magnetism Feb. 09, 2012

Roadmap Part 1: Sun Part 2: Heliosphere Part 3: Magnetosphere Part 4: Ionosphere Part 5: Space Weather Effects CH1: Structure CH2: Magnetism and Dynamo CH3: Magnetic Structure CH4: Solar Eruptions

CH2: Solar Magnetism CSI 662 / PHYS 660 Feb. 07, Sunspots and Solar Cycle 2.2. Magnetic Field Measurements 2.3. Laws of Solar Magnetism 2.4. Solar Dynamo Plasma-2: Magnetohydrodynamic (MHD) Equations Plasma-3: Magnetohydrokinematics Plasma-4: MHD Dynamo

CH2: Solar Magnetism References and Reading Assignment: KAL CH 3.1 and 3.2 (on MHD) KAL CH 3.4 (on Magnetohydrokinematics) KAL CH 3.6 (on MHD Dynamo) KAL CH 6.6 (on Solar Cycle)

CH2.1 Sunspot and Solar Cycle Galileo Sunspot Drawing: June 02, 1613

CH2.1 Sunspot and Solar Cycle SDO HMI Sunspot Picture Feb. 01, 2012

CH2.1 Sunspot and Solar Cycle 11-year cycle of sunspot numbers (SSN) SSN is historically a good index of solar activity. Correlate well with geomagnetic activities Maunder Minimum

Sunspot and Climate Thames Frost Fair, , By Thomas Wyke

Sunspot and Climate River Thames frost fairs were held on the Tideway of the River Thames at London between the 15th and 19th centuries, during the period known as the Little Ice Age, when the river froze over. During that time the British winter was more severe than now, and the river was wider and slower.TidewayRiver ThamesLondonLittle Ice Age During the Great Frost of 1683–84, the worst frost recorded in England, [1][2][3] the Thames was completely frozen for two months, with the ice reaching a thickness of 11 inches (28 cm) in London. Solid ice was reported extending for miles off the coasts of the southern North Sea (England, France and the Low Countries), causing severe problems for shipping and preventing the use of many harbours. [4] Near Manchester, the ground was frozen to 27 inches; in Somerset, to more than four feet. [1][2][3]North SeaEnglandFranceLow Countries [4]ManchesterSomerset

CH2.2. Magnetic Field Measurements SDO HMI Magnetogram Image Feb. 01, 2012

CH2.2. Magnetic Field Measurements Nature of sunspot: areas of concentration of strong magnetic field The strength is about ~ 500 – 3000 Gauss As a comparison, the Earth magnetic field is about 0.5 Gauss Magnetogram Continuum Image

Photospheric measurement is based on Zeeman effect: the splitting of a spectral line because of the presence of magnetic field. Zeeman Effect Δλ = 4.7 x λ 0 2 gB λ 0 : original wavelength g: Lande factor, e.g., FeI 6173Å (g=2.5) B: magnetic field strength

Longitudinal magnetic field: circular polarization SOHO (1995) / MDI (Michelson Doppler Imager) Transverse magnetic field: linear polarization SDO (2010) / HMI (Helioseismic and Magnetic Imager) Zeeman Effect

CH2.3. Laws of Solar Magnetism 1. Sporer’s Law: Sunspot emerge at relatively high latitudes and move towards the equator 3. Joy’s Law: The tilt angle of the active regions is proportional to the latitude 2. Hale’s Law: rules of magnetic polarity

A diagram shows the position (latitude) of sunspot with time It describe the movement of sunspot in the time scale of solar cycle -- Sporer’s Law Butterfly Diagram of Sunspot

1.Sunspots do not appear at random over the surface of the sun. 2.At any time, they are concentrated in two latitude bands on either side of the equator. But these bands move with time 3.At the start of a cycle, these bands form at mid-latitudes (~30°) 4.As cycle progresses, they move toward the equator. 5.As cycle progresses, sunspot bands becomes wider 6.At the end of cycle, sunspots are close to equator and then disappear 7.At the minimum of the cycle, old cycle spots near the equator overlaps in time with new cycle spots at high latitudes Butterfly Diagram of Sunspot

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. The Hale cycle is 22 years, while the sunspot cycle is 11 years Hale’s Polarity Law

22 years Butterfly diagram of Magnetic Field Global dipole field most of the time Polar field reversal during the solar maximum Solar Magnetic Cycle

22 year magnetic cycle 11 year sunspot number cycle Solar Magnetic Cycle

The Evolution Solar Magnetic Cycle

CH2.4. Solar Dynamo Solar magnetic field is generated through a dynamo process The dynamo is driven by the differential rotation of the Sun Electric Dynamo: DC Generator

Solar dynamo is a process by which the magnetic field in an electrically conducting fluid is maintained against Ohmic dissipation It is mathematically described by the magnetic induction equation (see Eq in Kallenrode) CH2.4. Solar Dynamo Differential rotation and meridional circulation α effect of turbulence twisting the field Diffusion caused by electric resistivity Diffusion caused by turbulance

Surface Latitudinal Differential Rotation: rotation at equator (25 days) is faster than the higher latitudes, progressively slower, at poles (35 days) Radial Differential Rotation At equatorial region, interior rotates slower than surface At polar region, interior rotates faster than surface Tachocline: at the bottom of convection zone, have the largest shear motion in the radial direction, the location of the generation of strong magnetic field Solar Differential Rotation

The flow of material along meridian lines from the equator toward the poles at the surface and from the poles to the equator deep insid Dynamo cycle primarily governed by meridional flow speed Solar Meridional Flow (Dikpati, de Toma, Gilman, Arge & White, 2004, ApJ, 601, 1136)

(i)Generation of toroidal field by shearing a pre-existing poloidal field by differential rotation (Ω-effect ) Solar α Ω Dynamo: Ω-effect Proposed by Parker (1955) Mathematically formulated by Steenbeck, Krause & Radler (1969)

(ii) Re-generation of poloidal field by lifting and twisting a toroidal flux tube by helical turbulence (α- effect) Solar Dynamo: α-effect

The End