Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Heliosphere: The Solar Wind March 01, 2012

Roadmap Part 1: Sun Part 2: Heliosphere Part 3: Magnetosphere Part 4: Ionosphere Part 5: Space Weather Effects CH5: The Solar Wind CH6: Interplanetary Transients

CH5: The Solar Wind CSI 662 / PHYS 660 Mar. 01, Solar Wind Properties 5.2. Solar Wind Models 5.3. Interplanetary Magnetic Field and Models Plasma-8: Solar Wind Models Plasma-9: Interplanetary Magnetic Field Models

CH5: The Solar Wind References and Reading Assignment: KAL CH 6.2 (on Solar Wind) KAL CH 6.3 (on Interplanetary Magnetic Field)

CH 5.1 Solar Wind

CH 5.1. Solar Wind Solar wind is the continuous flow of plasma outward from the Sun through the solar system Its existence was recognized in 1940s by observing the tails of comets: –The white tail is made of dusts pushed away by solar photons, or radiation pressure –The blue tail is made of charged ions dragged away by the solar wind

Fast and Slow Wind Bimodal Distribution Fast wind from high latitudes Slow wind from low latitudes Solar wind 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 Bimodal Distribution slow wind is denser and cooler fast wind is thinner and hotter

CH 5.1. Solar Wind Solar wind velocity is always in the radial direction

What Causes Solar Wind?

High pressure in the corona overcomes the gravitational pull, causing the corona to expand and plasma to accelerate, forming solar wind Extremely low pressure (10 orders of magnitude smaller) in the interstellar medium can not constrain a stationary corona, also resulting in solar wind CH 5.2. Solar Wind Models

Problems with Static corona CH 5.2. Solar Wind Models 1. Hydrostatic model with isothermal assumption

Problems with Static corona 2. Chapman’s model: Hydrostatic model with thermal conduction CH 5.2. Solar Wind Models

Problems with Static corona 3. Parker’s model: Hydrodynamic, and with isothermal assumption CH 5.2. Solar Wind Models

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

Interplanetary magnetic field IMF is generated by the solar wind Magnetic field in the corona is dragged onto the heliosphere through the frozen-in effect, producing the IMF The upper end follows the flow field The lower end roots at the source surface, which rotates together with the Sun The rotation of footpoints produces curved magnetic field lines in the heliosphere: the Archimedean curve CH5.3. IMF

Garden Sprinkler Analogy In a velocity field: Streakline: the locus of points of all the fluid particles that have passed continuously through a particular spatial point in the past Pathline: the trajectory that individual fluid particles follow Streamline: the curves that are instantaneously tangent to the vector flow field (or other vector field, such as B field) CH5.3. IMF

Streamline of solar wind velocity field: similar to pathline Streamline of solar wind magnetic field: similar to streakline, because magnetic field is frozen into plasma parcels IMF has a spiral pattern, or Archimedean pattern. At a certain height (e.g., 2.5 Rs), all magnetic field lines open and point out along radial direction. The surface of this height is called the source surface The interplanetary spiral pattern is traced back to the source surface Below the source surface, magnetic field has a mixture of closed loops, cusps, and open lines. CH5.3. IMF

The Archimedean Line of IMF, as seen in the rotation frame: φ:the azimuth angle of plasma parcel at time t r: the distance from Sun center of plasma parcel at time t φ 0 : the azimuth angle of magnetic field footpoint at the source 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 CH 5.3. IMF Model

As we go outward in the solar system the magnetic field becomes more and more azimuthal CH 5.3. IMF Model

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, a current sheet is rather simple, resulting two magnetic sectors as seen from the Earth for one solar rotation

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

The End