What is CINDI?
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
http://cindispace.utdallas.edu/
CINDI Educational and Public Outreach website
http://cindispace.utdallas.edu/education/
C/NOFS website
http://www.vs.afrl.af.mil/Factsheets/cnofs.swf
Center for Space Sciences—University of Texas at Dallas—spring 2005