Title: The Magnetosphere
Podcaster: Terry and Stephen Whelan
Description: This is a short non mathematical introduction to some important elements of space plasma physics. Stephen plays the role of a confused student to allow a pedagogical dialog to progress.
Bio: Terry did graduate research in space physics at the University of Iowa in the 1980s. Stephen is a high school drama student.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Clockwork.
What is a magnetosphere? This is the region of space around a planet where the magnetic field of the planet is the dominant feature. Important elements of this discussion are the solar wind, plasma physics, and of course the planetary magnetic field.
It follows from the definition that only planets with magnetic fields can have a magnetosphere. Earth has a magnetic field as do all of the gas and ice giants as well as Mercury.
Mercury is the enigma, the commonly proposed mechanism for magnetic field creation is a dynamo effect relying on a liquid core and planet rotation. Mercury rotates very slowly. Mars has no magnetic field but is only a little smaller than earth and rotates at about the same speed. Venus has not magnetic field and spins slowly. The ice giants Neptune and Uranus are pretty weird as well, their magnetic axis is very offset from the rotation axis of the plant.
All of these have been studied by various spacecraft over the years. Each of these had an instrument called a magnetometer that measures the magnitude and direction of the magnetic field at the location in space. These planets magnetic fields vary greatly in strength, this has little impact on the processes of magnetosphereic physics it does however impact the length and time scales of the various processes. It is customary in magnetosphereic physics to scale the size of the magnetosphere by the radius of the planet. A good reference on planetary physics is always the unfortunately named ‘nineplanets.org’. Hard numbers and other data on planets are available on a Goddard web site referenced in the show transcript (http://nssdc.gsfc.nasa.gov/planetary/factsheet)
Plasma Physics. Most gasses in space are in the form of plasma. A plasma is a gas where one or more of the electrons in an atom have left the atom resulting in a mixture gas of positively charged atomic ions and negatively charged electrons. Since a hydrogen atom consists of one proton and one electron so a hydrogen plasma consists of electrons and protons.
Charged particles create electric and magnetic fields; these fields also exert forces upon the particles. Changing magnetic fields cause electric fields, and vice versa. These relationships are expressed in Maxwell’s equations and the Lorentz force. You can learn more about these equations along with some very intimidating partial differential equations on Wikipedia. These interactions get complex very quickly. Magnetic and electric fields known as vector fields: at each point in space they have both a strength (magnitude) and direction.
Protons are about 2000 times heavier than electrons, however the magnitude of their electric charges are identical. It follows that the forces on electrons and protons are about the same size but the accelerations are much different.
As well as the electrodynamic influences a plasma is a gas, it has temperature, density, and pressure. In a space application the electrons and protons can have different temperatures, and can be considered two inter mixed gasses.
The solar wind. The gas (plasma) at the surface of the sun can escape away from the sun forming a flow radially out from the sun. We call this flow the solar wind. The theory of star formation predicts that all stars will have some kind of stellar wind. The density and speed of the solar wind varies greatly, causing what are known as solar storms. Compared to any gas we are familiar with on earth the solar wind has a very low density, typically 5 to 20 protons per cubic centimeter. The equivalent number for the earth’s atmosphere would have about 15 digits.
The particles of the solar wind are deflected by the earth’s magnetic field. This deflection of the charged particles causes an electric current which modified the magnetic field. The solar wind flows round this obstacle but compresses the magnetic field. The imaginary surface in space that separates the solar wind from the earth’s magnetic field is called the magnetopause. The region inside the magnetopause is the magnetosphere. Under typical solar conditions the magnetopause is about 10Re from the planet. Since the solar wind is flowing at a supersonic speed a shock wave called the bow shock forms in front of the planet. The magnetosphere forms a long cylinder behind the planet cause the magneto tail. Above the poles of the magnetic field the solar wind deflection is much less than at the equator this results in ‘holes’ in the magnetopause called the cusp regions.
It has been claimed that without the magnetosphere life could not form on earth. If the solar wind were to impinge directly on the planetary atmosphere it would over the billions of years blow away the atmosphere. No atmosphere no life. Modern communications satellites all orbit within the magnetosphere, the earth’s magnetic field protects the electronics from radiation damage. High enough intensity solar storms compress the magnetosphere enough that the geostationary orbits of communications satellites would be inside the solar wind. This alludes to the subject of space weather. Observations of the sun tell us when solar storms may impinge on the earth, magnetosphereic physics tells us something about the impact those storms may have.
End of podcast:
365 Days of Astronomy
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