Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Fall, 2009 Copyright © The Heliosphere: Solar Wind Oct. 08, 2009

Roadmap Part 1: The Sun Part 2: The Heliosphere Part 3: The Magnetosphere Part 4: The Ionsophere Part 5: Space Weather Effects Part 2: The Heliosphere 1.Solar wind (steady state) 2.Solar wind transients (ICMEs, shocks), and energetic particles

The Heliosphere: Solar Wind CSI 662 / PHYS 660 October References: Kallenrode: Chap. 6 Prolss: Chap. 6

Plasma Physics 1. MHD waves, Alfven waves –Kallenrode: Chap 4.1, Chap 4.2

What is Solar Wind? Solar wind is the continuous flow of plasma outward from the Sun through the solar system It was recognized in 1950s by observing the tails of comets: the straight blue tail is driven by plasma flow

What Causes Solar Wind? High thermal pressure in hot corona overcomes the gravitational constraints, resulting in coronal expansion Extremely low pressure (10 orders of magnitude smaller) in interstellar medium can not contain the coronal pressure Expansion becomes supersonic

Problems with Static corona Solar Wind Solution 1. Hydrostatic, isothermal

Problems with Static corona Solar Wind Solution 2. Chapman’s solution: Hydrostatic, but non-isothermal

Problems with Static corona Solar Wind Solution 3. Parker’s solution: Hydrodynamic (steady state), isothermal

The family of solutions: A: solar wind B: solar breeze C: captured wind D: always super-sonic E: no solar origin F: solar breeze Parker’s Solar Wind Solution

lachzor page 239 T. Gombosi Parker’s solution for different coronal temperatures For example, for T=10 6 K, and coronal density of 2x10 8 cm -3, r c =6R s. The solar wind accelerates to up to 40R S, and afterwards propagates to a nearly constant speed of 500km/s Parker’s Solar Wind Solution

Archimedean Spiral of interplanetary magnetic field Garden Sprinkler Analogy Jetlines that connect the flow elements from the same differentially small source region form an Archimedean spiral However, the streamline, along which an individual flow element is flowing, is strictly radial IMF Structure

φ:the azimuth angle of magnetic field line at r φ 0 : the azimuth angle of magnetic field footpoint at the surce surface r 0 : source surface, e.g., 2.5 R sun u_sw: solar wind speed, e.g., 400 km/s ω_sun:solar rotation, 2.7 X radians/sec IMF Structure

IMF structure Frozen-in theorem: Plasma elements connected at any point in time by a common magnetic field line remain connected by a common field line Since the plasma flow in a single spiral jet-line originates from a common source, the IMF should follow the jet-line, thus also display an Archimedean spiral pattern. At certain height (e.g., 2.5 Rs), all magnetic field lines open and point radial. A surface of this is called source surface The spiral pattern is traced back to the source surface

IMF fall more slowly! Part of solar dynamo As we go outward in the solar system the magnetic field becomes more and more azimuthal

IMF

Heliospheric Current Sheet In a global sense, there is a huge current system flowing in a circumsolar disk, separating the two magnetic polarities The current sheet is inclined with respect to the ecliptic plan Solar rotation axis is 7° tilted Solar magnetic dipole axis is further tilted from the rotation axis

Magnetic Sector Structure The Earth at one time above the current sheet, but at other times below the current sheet During solar minima, current sheet is rather simple, resulting two magnetic sectors as seen from the Earth

Magnetic Sector Structure The IMF sector structure is determined by the pattern of magnetic field at the source surface The magnetic field at the source surface is determined by the photospheric magnetic field through potential field calculation The section pattern can be complex at solar maximum

Fast and Slow Wind

Solar Wind: Bimodal Fast wind originates from coronal hole, Slow wind originates from regions close to streamer belts or heliospheric current sheet SW heliographic latitudinal Distribution (Ulysses observation)

Slow Solar Wind: Speeds between km/s Average density is ~ 8 ions/cm 3 (1AU) Solar Minimum -slow wind originates from regions close to the heliospheric current sheet Solar Maxima - slow wind originates above the active regions in the streamer belt Fast Solar Wind: originates in coronal holes Has flow speeds between km/s; average density is low ~ 3 ions/cm 3 (1AU) The proton temperature is about 2x10 5 K The electron temperature is about 1x10 5 K Fast and Slow Wind Solar Wind: Bimodal slow wind is denser and cooler fast wind is thinner and hotter

Solar Wind in 3-D Fast solar wind at high latitude At low latitude, fast and solar wind alternating, and superposed with transients (ICMEs, CIRs)

Plasma Waves in Solar Wind Solar wind plasma and magnetic field is highly variable on different temporal and spatial scale, caused by waves and turbulences in the supersonic flow. Alvfen wave: δu= δB

Alfven Wave MHD waves, Alfven wave –Kallenrode: Chap 4.1, Chap 4.2 Alfven wave is caused by magnetic tension force. It propagates along the magnetic field. It’s disturbance is perpendicular to the magnetic field Alfven wave speed

The End