The Interplanetary Medium
Facts
- The space between the planets is far from empty. It contains:
electromagnetic radiation (photons);
hot plasma (electrons, protons and other ions)
a.k.a. the solar wind;
cosmic rays;
microscopic dust particles; and
magnetic fields (primarily the Sun's).
- While the Sun's radiation is obvious,
the other components of
the interplanetary medium were not discovered until very recently.
- The temperature of the interplanetary medium is about 100,000 K.
Its density is about 5 particles/cm3 near the Earth and decreases by an
inverse square law farther from the Sun. However, the density is highly
variable, it can be as much as 100 particles/cm3.
- Though very tenuous, it has measurable effects on the paths of spacecraft.
- Except near some of the planets, interplanetary space is filled with the
Sun's magnetic field. Its interactions with the solar wind are very
complicated. Within a few solar radii of the Sun the magnetic field
determines the flow of the solar wind; much of the flow is trapped in
magnetic loops. But some regions of the Sun's magnetic field are open
allowing the solar wind to escape. Farther out the plasma dominates
and the magnetic field is entrained in the particle flow.
- Some planets (e.g. Earth,
Jupiter) have
their own magnetic fields. These create smaller
magnetospheres that dominate the Sun's
influence within their boundaries. Jupiter's magnetosphere is very large,
extending over a million km in all directions and as far as the orbit of
Saturn in the direction away from the Sun. The Earth's much smaller,
extending only a few thousand km, but protects us from
the otherwise very dangerous effects of the solar wind.
- For non-magnetic bodies, such as the Moon,
the solar wind impacts the surface directly.
-
As the solar wind moves out into space, it
creates a magnetized bubble of hot plasma around the Sun, called
the heliosphere. Eventually, the expanding solar wind encounters
the charged particles and magnetic field in the interstellar gas.
The boundary created between the solar wind and interstellar gas
is the heliopause. The precise shape and location of the heliopause is
not known but it is probably similar in shape to the Earth's magnetosphere and
the bow shock is probably about 110 - 160 AU
from the Sun.
The Voyager
and Pioneer spacecraft will probably reach
the heliopause in another decade or so.
- The Ulysses spacecraft is conducting an
extensive study of the Sun and the solar wind.
- The highest energy particles in the interplanetary medium are called
cosmic rays. Some are of solar origin; the most energetic, however,
originate in some other unknown and very energetic processes outside our
solar system.
- The interaction of the solar wind, the Earth's magnetic field
and the Earth's upper atmosphere causes the
auroras.
Other planets with significant magnetic fields (esp. Jupiter) have similar effects.
- The zodical light and the
gegenschein are caused by interplanetary dust.
Pictures
- (above) Aurora from the Shuttle
120k gif
-
aurora borealis
44k jpg
- more aurora images from space
- more aurora images from the ground
aurora from Alaska
16k jpg
- more aurora images from Alaska
- images of Saturn's aurora from HST
More about the Interplanetary Medium
Open Issues
- The nature of the heliopause remains one of the great
unanswered questions in space physics. Will
the Voyager spacecraft last long
enough to reach it?
... Sun
... Small Bodies
... Meteorites
... Medium
... Other Systems
... 
Bill Arnett; last updated:
1996 April 30
