On April 16, 2008, a suite of NASA instruments was launched into space to study a unique region of Earth's upper atmosphere: the electrically charged region called the ionosphere. The instruments, known collectively as CINDI (Coupled Ion-Neutral Dynamics Investigation), fly aboard an Air Force Research Laboratory satellite called C/NOFS (Communications/Navigation Outage Forecasting System) to study this region that hovers some 60 to 400 miles above Earth.

The ionosphere is crucial for modern communications. Low-frequency radio waves bounce off it to travel from one part of Earth to another. Various satellites, including the global positioning system (GPS), send high-frequency radio waves through the ionosphere down to receivers on Earth.

In this region the right conditions exist to allow incoming energy from the sun to knock electrons off the atoms. So the area seethes with charged particles moving under forces of both conventional winds and of electric fields that drive the particles perpendicular to the magnetic field lines surrounding Earth.

The ionosphere changes constantly: between night and day, with the seasons, between the equator and the poles, and with every incoming burst of radiation from the sun. Small changes in the ionosphere, at night, for example, can simply garble the radio waves - a phenomenon known as scintillation. But at worst, an event such as a giant solar flare can black out radio transmissions completely.

"All the space assets we have come to rely on, in one way or another propagate radio waves through this region of Earth's atmosphere so we need to understand it better," says Rob Pfaff, project scientist for CINDI at NASA's Goddard Space Flight Center in Greenbelt, Md. "Our goals with CINDI are to determine why the region becomes irregular, and, ultimately, to be able to forecast when and where it will be irregular."

Studying the ionosphere is like trying to understand a very complicated lava lamp, in which blobs of different materials move up and down in response to changes in temperature. In the case of the ionosphere, scientists want to understand the kinds of heat and energy affecting particle movement and how these motions interact and rely on each other.

Every piece of data, such as where certain particles appear and what causes areas of lower and higher density, represents a significant advance toward predicting change in the region. Over the last five years, CINDI has gleaned information about the distribution in height of different kinds of particles, about how winds sweep through the atmosphere in response to events on the sun, and what causes density changes in both the charged and neutral particles in the ionosphere.

One of the early observations by CINDI was of the top of the ionosphere layer, which is dominated by hydrogen ions near dawn. The middle layer of the area is dominated by oxygen ions. In 2008, CINDI found that the transition region, where there is an equal number of both particles, was located about 370 miles up, much closer to Earth than expected.

Since CINDI launched at a time of low solar activity -- a period of the sun's approximate 11-year cycle known as solar minimum - the mission has had the chance to observe how the ionosphere changes as the sun's activity ramps up to solar maximum, currently expected in late 2013.

Over five years of watching, this oxygen/hydrogen transition region has now moved up in space to over 430 miles in altitude, providing an indicator of how Earth's atmosphere swells and expands in response to increased energy coming in from the sun.

The measurements of charged oxygen and hydrogen were made by an instrument called the Ion Velocity Meter, similar to previous instruments that have measured the ionosphere. However, CINDI also carries a first-of-its-kind instrument to measure the neutral winds, called the Neutral Wind Meter.

Neutral winds are particularly difficult to detect with direct instruments and they can be measured remotely at only certain altitudes and at certain times of day. Funded by NASA and developed at the University of Texas in Dallas, the Neutral Wind Meter has provided the first in-situ measurements in the upper thermosphere of daytime and nighttime winds in the north-south direction.

"One of the key questions we are trying to unravel is how the charged and neutral gases move in response to each other," says Rod Heelis, principal investigator for CINDI at the University of Texas at Dallas.

"The charged gases affect radio propagation, but the neutral winds, much like ones we experience down on Earth, affect how the charged gases move around, so we need to understand both."

Previous studies had observed how charged particles react to geomagnetic storms - another space weather phenomenon that occurs when energy from the sun causes near-Earth space to change rapidly and repeatedly, which can also interrupt radio signals.

CINDI showed that the neutral particles also react to such storms. The density increases and the perturbed winds flow toward the equator at night. This represents critical information to place into models of how the ionosphere moves and changes.

CINDI observations have also helped map out areas of higher and lower ion densities - known as blobs and bubbles, respectively - another characteristic of the ionosphere that affects radio propagation.

Depending on geographical location and altitude, scientists can now categorize which blobs and bubbles are likely to be more intense and whether they'll drift up or down. These categorizations also help scientists identify what creates the disturbances, a process that often starts some ways away from the observed effect.

Another important result from CINDI involves observations of how charged particles move up and down within the ionosphere. The observations show that vertical movement at sunset correlates to changes in the upper ionosphere about an hour and a half later. Finding advance indicators such as these that point to later disturbances is exactly the kind of research for which CINDI was designed.

"It's an exciting time for space weather science with numerous spacecraft able to track events on the sun all the way toward Earth," says Pfaff.

"But CINDI is the only one gathering direct measurements at the peak of the ionosphere."

Originally scheduled to collect data for two years, the satellite has continued to provide crucial observations for five years running. CINDI has had the opportunity to watch as the sun has increased in activity, and with its direct measurements of how neutral and charged particles interact, the instruments continue to help improve predictions of just when disturbances in the ionosphere will be at their worst.

CINDI is a NASA-sponsored Mission of Opportunity conducted by the University of Texas at Dallas. NASA's Explorer Program at Goddard manages the CINDI mission. The Explorer Program provides frequent flight opportunities for world-class scientific investigations from space within heliophysics and astrophysics.