Introduction to Space Weather

Jie Zhang CSI 662 / PHYS 660Spring, 2012Copyright ©

The Sun: MagnetismFeb. 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, 2012

2.1. Sunspots and Solar Cycle2.2. Magnetic Field Measurements2.3. Laws of Solar Magnetism2.4. Solar Dynamo

Plasma-2: Magnetohydrodynamic (MHD) EquationsPlasma-3: MagnetohydrokinematicsPlasma-4: MHD Dynamo

CH2: Solar MagnetismReferences 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

http://galileo.rice.edu/sci/observations/sunspot_drawings.html

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, 1683-84,

By Thomas Wyke

Sunspot and ClimateRiver 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.

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.

http://en.wikipedia.org/wiki/River_Thames_frost_fairs

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 10-13 λ02 gB

•λ0: 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 time3. 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 wider6. At the end of cycle, sunspots are close to equator and then

disappear7. 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 3.135 in Kallenrode)

CH2.4. Solar Dynamo

Differential rotationand meridional circulation

α effect of turbulence twisting the field

()() BBBUB

t

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 αΩ 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