FAR-OUT FORECAST:COMMUNICATIONS AND RADAR WILL BENEFIT FROM SPACE-BASED SENSORS
ABLE TO WARN OF IONOSPHERIC CHANGES

Partly cloudy, chance of showers. The daily weather reports help us
decide what to wear and what to carry-an umbrella or sunglasses. Soon,
however, we'll watch reports on space weather just as eagerly to see where to
tune in our communications systems: Solar flare sparks severe storm in
the ionosphere; switch to alternate satellite link.

The U.S. Navy and Air Force are working on space-based systems to
monitor the weather in the ionosphere, the layer of the earth's
atmosphere that affects satellite communications. Tuned to ultraviolet and
infrared wavelengths and clamped to the undersides of weather satellites,
optical sensors will orbit the earth above the ionosphere. The Navy's
system, scheduled for tests in 1992, will vertically scan the atmosphere
with spectrographs, spectrometers, and photometers to create a cross
section of the ionosphere. The Air Force's instruments will look straight
down to give a picture over a broad geographic location. When used
together, the two systems will generate a three-dimensional view. About
60 to 1,000 kilometers above the earth's surface, the ionosphere experiences
weather changes just like our atmosphere. however, violent "storms" in the
ionosphere do a different type of damage: They can knock out
communications satellites, disrupt radio transmissions, and even cripple the
military's over-the-horizon radar. Such storms often travel from the earth's
poles toward the equator, but surprises can happen. For example, a
solar flare blocked communication with Air Force One in 1984.

The ionosphere is created by high-energy rays of ultraviolet radiation from
the sun that disrupt the energy balance holding together molecules of
oxygen and other atmospheric gases. The radiation knocks electrons away
from the protons and neutrons that make up the dense nuclei ot the gas
atoms. The electrons leave behind ions. These separated electrons and
positively charged ions make up the ionosphere. The thickness of the
ionosphere and the number of free electrons vary as the sun emits
different amounts of ultraviolet radiation during its 27-day rotations. The
density of electrons can become so great that satellite signals cannot
penetrate the ionosphere.

The military's optical sensors will pick up the radiant energy (photons
scattered by the ions), calculating the number of positive ions and
thus the number of free electrons. Noticed when electrons begin building
up in a certain area of the ionosphere, those using a satellite link can
switch to a satellite in an unaffected part of the sky or to ground-based
communications facilities.

Rather than passing through the ionosphere, as satellite signals do, the
signals used in over-the-horizon radar are reflected by the
ionosphere-twice, in fact, as they bounce off the ionosphere to the
radar's target, reflect once again off the ionosphere, and are finally
captured by a radar ground station. The angle of reflection, determined
by the number ot free electrons in the ionosphere, must be known in
order to pinpoint the target's location. "That's the rub," says Robert
Meier, head of the Upper Atmospheric Physics Branch of the Naval
Research Laboratory in Washington, DC. "You have to know what the
ionosphere is doing way out there."

Systems that help warn of the arrival of ionospheric storms are already
in place. For example, after a solar flare, alerts are sent to defense and
civilian communications systems.

But the ionosphere's reactions to solar flares can vary widely from day
to day. With the space-based sensors, we'll be able to keep up with
them.