Exploring the Atmosphere Where Satellites Fly

Satellites are critical to many of the technological capabilities that global society has come to depend on. Global positioning systems provide essential navigation for land, sea, and air transportation, weather satellites collect data critical for predicting severe weather on Earth, and communications satellites broadcast television and radio transmissions into living rooms worldwide. However, society often takes satellite capability for granted. Characteristics of the atmosphere – for example, atmospheric mass – in regions where satellites orbit can have notable effects on satellite function. Scientists in the National Center for Atmospheric Research’s High Altitude Observatory (HAO) are using models to improve their understanding and ability to predict abrupt changes in the upper atmosphere that can affect satellites’ ability to perform as expected.

Satellites achieve a stable orbit by maintaining a velocity that allows the vehicle, pulled by the planet’s gravitational force, to fall toward Earth at a rate equivalent to that at which the Earth’s curvature moves away from the forward-moving satellite. But changes in atmospheric density affect satellite velocity. Moreover, changes in density are not uniform across regions of the atmosphere. Because these changes vary in space and time, those on the ground must sometimes manually adjust satellite velocity, as well as manage internal satellite warning systems related to avoiding collisions, and re-entry predictions, says Liying Qian, a scientist in HAO studying this problem.

“The atmosphere in the Earth’s thermosphere, which exists roughly 90 to 600 kilometers above the Earth, is nearly a vacuum. Many space vehicles fly in the upper thermosphere and encounter drag from this very thin atmosphere. Perturbations on their orbits occur when mass density changes in this region,” explains Qian. “Understanding changes in mass density in this region is critical, particularly because density changes can crop up unexpectedly, having important and often undesired outcomes on satellite flight.”

mass density image

The figure above compares modeled and measured global mean mass density at 400 km. Each dot shows a daily value of the mean density. Red dots show simulations by the NCAR TIE-GCM, while the black shows measurement through analysis of atmospheric drag on orbiting objects. The red and black lines show the 365-day centered running mean of global mean mass density.

At or below 150 kilometers, the Earth’s gravitational pull requires that satellites have some means of constant propulsion to maintain their orbit, while above 600 kilometers atmospheric drag is minimal. In between these regions, the ability to predict changes in mass density would improve Earth-based satellite control during times when the thermosphere’s neutral density experiences disruptions.

Disruptions occur because of changes in the amount of incoming solar radiation, for example from solar flares, or from geomagnetic disturbances caused by the effects of solar winds, as well as the natural dynamics occurring within the thermosphere. Increased solar activity may, for example, increase the amount of extreme-ultraviolet radiation in the upper atmosphere, exciting or ionizing the thermosphere’s constituents. Some of the fast moving, charged particles expelled from the Sun’s upper atmosphere in solar winds, can break through the Earth’s protective magnetosphere, which sits above the thermosphere, causing regional changes. All of these events can affect the density of the thermosphere. In addition, the thermosphere is affected by processes occurring in lower atmospheric regions. For example, gases such as carbon dioxide, methane, water vapor, and ozone from the troposphere or stratosphere, the layers of atmosphere closest to Earth, can affect the density and dynamics of the overlying atmosphere.

To both gain an improved understanding of the atmospheric dynamics at this altitude and to better predict changes due to space weather or effects of changing climate and chemical composition of the lower atmosphere, among other questions, Qian and her colleagues turned to NCAR’s Thermosphere Ionosphere Electrodynamic General Circulation Model (TIE-GCM). The TIE-GCM models the Earth’s atmosphere from approximately 97 to 600 kilometers, but the scientists wanted to know how well the model performed as compared to reality, information that is critical for estimating expected and actual satellite orbits and necessary for improving scientists’ ability to predict density changes.

Qian and the team generated model runs, replicating commonly occurring external forcing events – that is, events that influence or force changes to occur to the characteristics of the thermosphere, such as the effects of solar flares and geomagnetic storms. They then compared this output to observations collected from the CHAMP (Challenging Minisatellite Payload) satellite. Among the instruments loaded onto CHAMP are accelerometers, which provide information that can be used to derive effects of satellite drag.

By comparing the model output with observed atmospheric phenomena, the scientists gained information about how well the model did at predicting changes in the thermosphere’s density. In addition, the comparison provided insights into the variability and driving mechanisms related to density changes, such as time of day, effects of space weather and solar cycle variations, and changes at varying altitudes.

“By assessing the observations and the modeled output, we gained new knowledge about thermospheric dynamics and factors that affect density changes, which provides valuable knowledge for improving satellite operations,” says Qian. “A number of challenges still await us, such as how lower atmospheric forcings affect annual and semi-annual variation of the thermosphere’s density, among other questions.”