Space Weather and the Upper Atmosphere

James Hecht

Aerospace researchers have been helping to develop satellite-based instruments for studying the outer reaches of the atmosphere.

The advent of meteorological satellites has significantly improved scientific knowledge of Earth's weather and climate. But while models of the troposphere have grown quickly during the past few decades, models of the upper atmosphere (above 100 kilometers) remain relatively undeveloped. One class of research, for example, involves how—and how much—does the input of auroral energy affect this region.

When energetic electrons and protons from the sun reach Earth, they interact with components of the upper atmosphere. In addition to producing the luminescent aurora borealis, this interaction can generate winds and push atmospheric constituents to higher altitudes, where they can increase the drag on satellites. The energetic particles can also ionize the molecular nitrogen and atomic oxygen in the upper atmosphere, which can radically alter the ionosphere and interfere with satellite-based communications.

Operational civilian and military systems can benefit from fast information on auroral heating effects, but ground-based systems can't supply this information on a global scale. Optical sensors are limited by weather or daylight considerations, while radar systems may be constrained by expense and geography. Ground-based systems can certainly be useful in developing and testing models and instruments, but only satellite-based remote-sensing systems can provide the breadth and depth of data needed for operational use. Aerospace has recently been involved in building satellite instruments to measure auroral energy inputs into the atmosphere. These instrument concepts have been adopted for NASA research missions and will play a part in future versions of the Defense Meteorological Satellite Program (DMSP) and the National Polar-orbiting Operational Environmental Satellite System (NPOESS).

Probing the Aurora

The intensity of colors in the aurora borealis can reveal characteristics about auroral energy and heating. When auroral electrons collide with atmospheric nitrogen molecules or oxygen atoms, their energy gets absorbed, and photons are emitted. The more auroral particles there are, the more photons are emitted. Precipitating auroral electrons whose energies are more than 1000 electron volts typically result in significant green emission (at 5577 angstroms) originating near 100 kilometers in altitude, while lower-energy auroral electrons produce significant red emission (at 6300 angstroms) above 200 kilometers. Thus, measuring the brightness of the green emission can help determine total energy input, and calculating the ratio of red to green can identify the average energy of the auroral particles. Emissions at other wavelengths depend on the densities of atmospheric components such as nitrogen or atomic oxygen as well as on total and average energy input.

Nighttime ratio of atomic oxygen to molecular nitrogen during a period of extreme auroral activity. The arrow indicates satellite track. Note the considerable reduction of atomic oxygen over Alaska. Less than 24 hours earlier, levels were more consistent with model predictions. (Doug Strickland, CPI, Inc.)

Based on this phenomenon, Aerospace scientists identified a set of auroral colors that are bright enough to be measured in a few seconds and that can be used to calculate the total and average energy input, as well as the ratio of the atmospheric molecular nitrogen to atomic oxygen density. Researchers then developed a simple filter camera system, consisting of a telescope with a 1degree field of view of the sky, a filter wheel with four filters to separate auroral colors, and a photomultiplier tube to detect the auroral photons. This computer-controlled system is situated at Poker Flat, Alaska, and operates every night from October to April.

Global Ultraviolet Imager

The ideas behind the ground-based instrument at Poker Flat also provided the basis for a satellite-based instrument known as the Global Ultraviolet Imager. Built jointly by the Johns Hopkins Applied Physics Laboratory and The Aerospace Corporation, the Global Ultraviolet Imager is a far-ultraviolet (115–180 nanometer) scanning imaging spectrograph. It provides horizon-to-horizon images in five selectable ultraviolet wavelengths, or "colors."

Ratio of atomic oxygen to molecular nitrogen. The top image was compiled during a period of low auroral activity. A bulge of oxygen-depleted air can be seen over the Indian Ocean at low latitudes and throughout the high southern latitudes. The northern latitudes show a ratio consistent with predictions. The lower image, taken three days earlier, shows a period of moderate auroral activity. In the southern hemisphere, the region of oxygen depletion extends to somewhat lower latitudes than in the top image; however, the most dramatic changes occur in the northern hemisphere. The data over Alaska, for example, were taken after a full night of auroral electron precipitation. Note the dramatic oxygen depletions that extend down through the northern United States. (Doug Strickland, CPI, Inc.)

The Global Ultraviolet Imager operates on much the same principle as the ground-based instrument in Poker Flat, although the underlying physics is somewhat different. For example, average energy is obtained by comparing the intensity of two ultraviolet colors. However, one of these colors is absorbed more by molecular oxygen, which increases at lower altitudes. A greater concentration of high-energy electrons, which penetrate to lower altitudes, produces a different color ratio than a mix with more low-energy particles.

The Global Ultraviolet Imager was launched in 2002 aboard NASA's TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics) satellite, which was designed to characterize the energy inputs to the upper atmosphere below 200 kilometers. TIMED flies at a relatively low altitude (roughly 620 kilometers) and has a revisit time of 97 minutes. Thus, the imager can provide a detailed snapshot of the effects of auroral heating in a particular region of auroral precipitation. These images provide useful information on parameters such as the ratio of atomic oxygen to molecular nitrogen at spatial sizes of several tens of kilometers.

The instrument can also obtain atomic oxygen and molecular nitrogen from dayglow emissions—nonthermal emissions produced by the interaction of solar radiation and Earth's atmosphere that appear during daytime. Thus, observations from the imager can provide a global picture of composition change over a restricted local time on the sunlit portion of Earth. This means that composition change can be monitored at middle and low latitudes not accessible by auroral sensors operating in the polar regions.

Data obtained from the Global Ultraviolet Imager have raised many interesting questions. For example, ground-based measurements have shown that the ratio of atomic oxygen to molecular nitrogen can fall dramatically during periods of heightened auroral activity. Such composition changes can influence the ionosphere, a knowledge of which is important for communications. The Global Ultraviolet Imager has also recorded this phenomenon, tracking oxygen-depleted regions of the upper atmosphere as they expand to cover more of the globe and following them as they shift in response to changing upper-atmosphere winds. Still, the mechanism for the rapid drop in atomic oxygen has not yet been identified.

Future Applications

Aerospace work with the Global Ultraviolet Imager has had direct implications for defense and civil satellite programs. For example, the Air Force recently launched the DMSP F-16 satellite, which included a number of new space weather sensors. One of these, the Special Sensor Ultraviolet Spectrographic Imager (SSUSI), is almost identical to the Global Ultraviolet Imager. The experience Aerospace gained with the Global Ultraviolet Imager is being used to help integrate SSUSI into an operational sensor. In addition to information on global atomic oxygen and molecular nitrogen, SSUSI will provide information on the extent of the aurora and the height and strength of the ionosphere using information from the same five colors used on Global Ultraviolet Imager.

NPOESS will also include an instrument similar to SSUSI. The data from DMSP and NPOESS sensors will eventually feed large assimilative computer models that could ultimately describe space weather much the same way that large-scale meteorological models describe tropospheric weather today.

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