Solar Wind Interactions

Will Barley and Ray Greenwell

I. Intro to Plasma

A. 4 states of matter: solid, liquid, gas, plasma

1. As gas is heated, it dissociates into an atomic gas that is ionized as the collisions between atoms are able to free the outermost electrons.
2. Plasma is a mix of neutral particles, positive ions, and electrons.

B. Because some or all of the particles are charged and capable of creating and interacting with electromagnetic fields, many phenomena not present in ordinary states are found in plasmas.

C. Most of the universe exists in a plasma state.

II. Solar Wind

A. The first indication that the sun had "wind" came from the tails of comets.

1. They were observed to always point away from the sun, whether the comet was approaching or moving away from the sun.
2. Dust tails pushed away by the pressure from the sunlight.
3. Ion tails point away also, and sometimes appeared kinked or bent. Sunlight pressure cannot explain this.
4. A "solar corpuscular radiation" proposed to explain this.

B. In 1958, Eugene Parker tried to derive the equilibrium structure of the corona.

1. He found that topmost layers of corona flowed away from the sun.
2. Named "solar wind".
3. Its existence was later confirmed by experiments aboard spacecraft.

C. Solar wind facts

1. Density at earth's orbit: about 6 ions per cubic centimeter
2. Ions generally resemble distribution of elements in the sun: mostly protons, 5% helium, traces of oxygen and other elements.
3. Flows away with a mean speed of about 400km/sec
4. Flows out more than 30 times more distant from the sun than the earth.

III. Interaction with the Earth

A. Interplanetary Magnetic Field

1. Solar wind carries solar field lines, filling interplanetary space.
2. IMF allows solar wind to pick up ions (producing comet tails).
3. IMF plays a major role in linking the magnetosphere to the solar wind.

a. drives magnetospheric convection system
b. creates magnetic storms and substorms
c. powers the aurora
d. heats polar upper atmosphere
e. drives large neutral atmospheric winds
f. energizes much of the plasma on the Earth's magnetic field lines

B. Magnetosphere

1. Pressure of solar wind modifies the form of the magnetosphere.

a. On dayside it is about 10 Earth radii.
b. On nightside, it is about 100 Earth radii.

2. Magnetosphere is filled with plasma that originates from ionosphere and solar wind.
3. Solar wind cannot enter Earth's magnetic field.

C. Geomagnetic storms

1. Result of major variations in IMF intensity and direction.
2. Effects are of practical importance to us:

a. Current surges: causing flickering lights & blackouts.
b. Static interference and interrupted transmission of radio, television, and telephone signals.
c. Erratic behavior of navigation instruments
d. Disruption of defense communications: Early Warning radar system.
e. Alterations in ozone layer.

IV. Ulysses Mission

A. Designed to explore interplanetary space at high solar latitudes.

1. No man made launch vehicle could provide the needed velocity to achieve high latitudes.
2. Ulysses was aimed close to Jupiter so that the large gravitational field would accelerate Ulysses out of the elliptic plane.
3. Ulysses' mission timed to coincide with quiet portion of 11 year cycle.

B. Ulysses spacecraft was designed to characterize the heliosphere as a function of solar latitude.

1. Regions higher than 70 degrees latitude were its main interest.
2. No previous spacecraft have reached solar latitudes higher than 32 degrees.

C. Scientists have long been aware that there are major differences in the sun between the polar regions and the lower (middle) latitudes.

1. Sun spots are only seen at lower latitudes.
2. Photos of the solar corona taken during eclipses often show dark regions over the poles.
3. Magnetic field lines that leave the solar surface usually return to it, but some field lines, particularly those over the poles extend deep into interplanetary space.
4. Solar wind expands into interplanetary space along these field lines.
5. The dark areas are where hot gas escapes because of low gas density.

V. Ulysses Results

A. Solar wind has variations in both speed and density.

1. At high latitudes the velocity is high and the density low.
2. Near the equator, the velocity is low and the density is high.
3. Near the poles, the solar wind escapes at a steady speed, about 740km/sec.

B. Ulysses found that the amount of magnetic flux in the solar wind did not vary greatly with latitude. This is an indication of the importance of pressure forces near the sun for evenly distributing magnetic flux.

C. New findings on composition and temperature.

1. Low speed streams have a higher ratio of Magnesium to Oxygen.
2. Low speed solar wind has much higher temperature: over 1.6 million degrees, indicative of a hot coronal source.
3. The temperature/composition differences turn out to be a much better way to differentiate solar winds than speed.

D. Diffusion of cosmic rays in the heliosphere.

1. Magnetic field lines of the sun are "wound up" in a spiral pattern due to the rotation of the sun.
2. Near the poles the rotation velocity is lower due to the rotation of the sun.
3. So the azimuthal magnetic fields are weaker.
4. The length of the magnetic field to the boundary of the heliosphere where cosmic rays enter the solar system is less.

VI. Conclusion

A. Shape and character of gases controlled by magnetic fields.

B. Major differences in location of solar wind origins.

C. By learning more about the solar wind, we can understand our own atmosphere and weather.

Back to