CINDI stands for Coupled Ion Neutral Dynamic Investigation and is a space science project sponsored jointly by NASA and the Air Force. It involves building two instruments to be flown on the C/NOFS satellite to study the earth’s ionosphere. The instruments will measure the concentration and kinetic energy of the ions and neutral particles in space as the satellite passes through them. This information will be used in building models to understand the various structures in the ionosphere, like bubbles of different densities of ions. These structures can interfere with radio signals between the Earth and spacecraft in orbit, thus causing errors in tracking and loss of communication. The C/NOFS spacecraft (shown above) will be launched in 2005 into an equatorial orbit that ranges from 375 to 750 kilometers in altitude. It is scheduled to take data for two years.
Even though the region of space above 375 km is called a vacuum, it is not a
complete vacuum. There are still between10 to 100 million particles of air in
every cubic centimeter up there (though that is still about one-one hundred
billionth the density of air down at the surface of Earth.) CINDI is composed
of two separate instruments: the ion velocity meter (IVM) and the neutral wind
meter (NWM). As the names suggest they separately measure the ionized and neutral
particles that exist in the ionosphere at these altitudes. Each instrument is
mounted on the front of the spacecraft looking forward into the direction of
the satellite’s orbital motion. The satellite is traveling much faster
than the particles are, so as the ions stream into the openings for the IVM,
the instrument measures how fast they are going in the direction of the satellite’s
travel. The IVM also measures how fast the ions are traveling at right angles
to the satellite’s orbital motion. Subtracting out the satellite’s
speed from the first measurement and adding in the speeds in the two directions
at right angles gives us a full three-dimensional measurement of the ions’
velocity Measuring the total number of ions entering the IVM tells us what the
ion density is at that point in space. Measuring the variations of the ions’
velocities over a short period of time tells us both the temperature of the
ions and also their relative composition by elements. The NWM measures the velocity
of the neutral particles at the same time. Even though the ions and neutral
particles are in the same place, they are not necessarily moving in the same
direction at the same speed. The neutral particles are affected mostly by the
Earth’s gravity pulling them down, and also the pressure from the surrounding
particles pushing them up and sideways. The ions are affected by these same
forces, but since they are electrically charged, they are also being pushed
around by the local magnetic and electric fields. The only time the ions and
the neutrals interact is when they collide with each other, and this further
changes the directions of both their motions. When the ions and neutrals collide
a lot, then they are said to be strongly “coupled” to one another,
and that’s where the word “coupled” comes from in the acronym
CINDI.
By measuring the
motions of the ions and neutrals we can study the variations or the structure
of the ionosphere. Studying these structures we can see how they change at different
local times of the day, or how they change from day to day, or change with season,
or how they change after solar storms. Eventually we hope that we will have
enough information to understand and predict the formation and movements of
these structures. And since these structures can interfere with the radio signals
traveling between the Earth and satellites further out, we hope that we can
begin to forecast when these outages will occur and how severe they will be.
This is just like predicting the weather down here at the surface, which is
why this field of study is called “space weather.”
Websites for more information:
CINDI website at the University of
Texas at Dallas
C/NOFS website
Center for Space Sciences—University of Texas at Dallas—spring 2005
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