A wave cannot grow forever, and Hines' small-amplitude approximation must break down. In the case of a surface water wave, it breaks when the velocity of the water “inside the wave” is faster than the speed of the wave itself. As the wave approaches the beach and necessarily slows down to zero, this condition must occur, and the wave steepens, overturns, and breaks on the shore. In the case of an internal wave, it does not have to slow down to break since the velocity of the air “inside the wave” is getting higher and higher as the amplitude increases with altitude. Eventually, the internal oscillating wave velocity exceeds the wave speed and it must break. Most of the energy and momentum of the wave, in both cases, is then deposited in the background medium. This has important implications for the atmosphere since the turbulence generated by breaking waves contributes to complete mixing of the air up to the very base of space.

NEW REMOTE SENSING SCHEMES

Making these wave phenomena manifest by using incoherent scatter radar techniques or using TMA and meteor trails is not the only way to detect upper atmospheric waves ( Figure 12 ). Modern technology has made the study of these waves possible in many ways. The vapor debris from burned-out meteors allows these waves to be detected not only visually, but also as a target for powerful lasers on the Earth's surface. Such lasers can be tuned to the resonant frequency of, say, sodium or iron from meteors, and when reradiated, the metal atom layers can become visible to photomultipliers. The strength of the returned signal is proportional to the density of the atoms in the region and can be used to track their response to passing waves.

The atom layers display complicated height changes in response to passing waves. Some very sophisticated systems can also measure the Doppler spread of the returning signal, yielding the temperature fluctuations in the wave, a more meaningful and direct measure of the wave amplitude than the height change. Remarkably, even the wind can be determined by the Doppler shift effect, a change in frequency of one part in 108 or better.

Sometimes the layers appear to grow out of the background with remarkable speed and strength. First dubbed Sudden Sodium Layers ( Figure 13 ), these rapid changes in space and/or time are quite remarkable and not yet well understood. They are found in iron and potassium (K) as well as sodium (Na) and are now called Sudden Atom Layers (SAL). There seems to be some relationship between atoms and electrons in the ionosphere, but as yet we do not know what it is.

The properties of these metal atom layers have become of sufficient interest to warrant National Aeronautics and Space Administration (NASA) rocket flights through them. One of the planned payloads is shown schematically in Figure 14 . The plan is to use ground-based lidars at the Arecibo Observatory in Puerto Rico to detect the SAL and then to fly payload instruments through them.

Instruments on board will provide vertical profiles of the atom layers using their natural optical emissions as well as resonant scatter from an Na-K lamp on board; will measure the plasma density and electric field profiles as well as the density of positive and negative ions; and will measure the small dust particles thought to exist in the upper atmosphere. We hope that these flights will yield more information on the source of atom layers.

Although very powerful, these methods result only in vertical cuts through the region. With modern cameras, however, it has also become possible to take two-dimensional pictures of the region using photons emitted in the chemical reactions that occur in thin layers of the atmosphere. For example, sequences of images such as the ones shown in Figure 15 can be used to trace the motion of waves across the sky. Here, a wall of light is seen to propagate across the night sky with ripples trailing behind, just like waves behind a boat; they move at the same speed as the disturbance.

The direction from which the waves arrive can yield some information on their sources, which by and large remain a mystery. Are the waves caused by weather fronts? Are their sources isotropic, and



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ASPECTS OF WEATHER AND SPACE WEATHER IN THE EARTH'S UPPER ATMOSPHERE: THE ROLE OF INTERNAL ATMOSPHERIC WAVES A wave cannot grow forever, and Hines' small-amplitude approximation must break down. In the case of a surface water wave, it breaks when the velocity of the water “inside the wave” is faster than the speed of the wave itself. As the wave approaches the beach and necessarily slows down to zero, this condition must occur, and the wave steepens, overturns, and breaks on the shore. In the case of an internal wave, it does not have to slow down to break since the velocity of the air “inside the wave” is getting higher and higher as the amplitude increases with altitude. Eventually, the internal oscillating wave velocity exceeds the wave speed and it must break. Most of the energy and momentum of the wave, in both cases, is then deposited in the background medium. This has important implications for the atmosphere since the turbulence generated by breaking waves contributes to complete mixing of the air up to the very base of space. NEW REMOTE SENSING SCHEMES Making these wave phenomena manifest by using incoherent scatter radar techniques or using TMA and meteor trails is not the only way to detect upper atmospheric waves ( Figure 12 ). Modern technology has made the study of these waves possible in many ways. The vapor debris from burned-out meteors allows these waves to be detected not only visually, but also as a target for powerful lasers on the Earth's surface. Such lasers can be tuned to the resonant frequency of, say, sodium or iron from meteors, and when reradiated, the metal atom layers can become visible to photomultipliers. The strength of the returned signal is proportional to the density of the atoms in the region and can be used to track their response to passing waves. The atom layers display complicated height changes in response to passing waves. Some very sophisticated systems can also measure the Doppler spread of the returning signal, yielding the temperature fluctuations in the wave, a more meaningful and direct measure of the wave amplitude than the height change. Remarkably, even the wind can be determined by the Doppler shift effect, a change in frequency of one part in 108 or better. Sometimes the layers appear to grow out of the background with remarkable speed and strength. First dubbed Sudden Sodium Layers ( Figure 13 ), these rapid changes in space and/or time are quite remarkable and not yet well understood. They are found in iron and potassium (K) as well as sodium (Na) and are now called Sudden Atom Layers (SAL). There seems to be some relationship between atoms and electrons in the ionosphere, but as yet we do not know what it is. The properties of these metal atom layers have become of sufficient interest to warrant National Aeronautics and Space Administration (NASA) rocket flights through them. One of the planned payloads is shown schematically in Figure 14 . The plan is to use ground-based lidars at the Arecibo Observatory in Puerto Rico to detect the SAL and then to fly payload instruments through them. Instruments on board will provide vertical profiles of the atom layers using their natural optical emissions as well as resonant scatter from an Na-K lamp on board; will measure the plasma density and electric field profiles as well as the density of positive and negative ions; and will measure the small dust particles thought to exist in the upper atmosphere. We hope that these flights will yield more information on the source of atom layers. Although very powerful, these methods result only in vertical cuts through the region. With modern cameras, however, it has also become possible to take two-dimensional pictures of the region using photons emitted in the chemical reactions that occur in thin layers of the atmosphere. For example, sequences of images such as the ones shown in Figure 15 can be used to trace the motion of waves across the sky. Here, a wall of light is seen to propagate across the night sky with ripples trailing behind, just like waves behind a boat; they move at the same speed as the disturbance. The direction from which the waves arrive can yield some information on their sources, which by and large remain a mystery. Are the waves caused by weather fronts? Are their sources isotropic, and

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ASPECTS OF WEATHER AND SPACE WEATHER IN THE EARTH'S UPPER ATMOSPHERE: THE ROLE OF INTERNAL ATMOSPHERIC WAVES FIGURE 14 A schematic diagram of Cornell University's Sudden Atom Layer rocket. Coinvestigators from the University of New Hampshire, Utah State University, Goddard Space Flight Center, Aerospace Corporation, and the Naval Research Laboratory have contributed instruments. do only some get through to the upper atmosphere? Or do they have a preferred direction from the beginning? We simply do not know. We also do not fully understand how the waves break and return their energy and momentum to the atmosphere, although this is perhaps the most important question of all. Since waves grow exponentially with height, even the most insignificant wave must break eventually or be absorbed back into the flow when its phase velocity matches the wind, but in either case the wave energy and momentum become part of the local budget of these important parameters. A number of analysis schemes are being applied to this problem, many of which involve spectral analysis of data sets. One interesting approach has been to make two-dimensional spectra of the waves detected in the airglow images. An example in which two monochromatic waves cross is given in Figure 16 . The analysis clearly shows two separate peaks corresponding to two different directions and wavelengths (Taylor and Garcia, 1995). Once the peaks are identified they can be emphasized by digital filters centered on the peaks and the crossing pattern reconstructed, as shown at the bottom right. Sometimes such an analysis reveals more complicated patterns involving what seem to be nonlinear wave behavior that may shed light on wave-breaking processes.

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ASPECTS OF WEATHER AND SPACE WEATHER IN THE EARTH'S UPPER ATMOSPHERE: THE ROLE OF INTERNAL ATMOSPHERIC WAVES FIGURE 15 Sequential photos of emission from chemical reactions in the mesosphere make waves visible. Courtesy of Francisco J. Garcia, graduate student, School of Electrical Engineering, Cornell University.

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ASPECTS OF WEATHER AND SPACE WEATHER IN THE EARTH'S UPPER ATMOSPHERE: THE ROLE OF INTERNAL ATMOSPHERIC WAVES FIGURE 16 Crossing waves revealed by light emissions from the mesosphere. Reproduced from Taylor and Garcia (1995) with permission of the American Geophysical Union. A sequence of photos thus allows not only the horizontal wavelength to be found but also the horizontal phase velocity and, of course, the period. This leaves only the vertical wavelength as an unknown. Since airglow emissions come from a layer where conditions are just right for the chemistry, the only information we obtain is that the wavelength cannot be much smaller than the layer thickness or the image would be smeared out. So, there is still plenty of experimental work to be done, but some progress has been made. Most advances are likely to involve measurements from more than one instrument at a time since each has its own limitations. In the next decade, enormous progress will occur in remote sensing of the atmosphere. NASA plans a Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) mission, which will look down on the Earth's atmosphere from above, in addition to several Department of Defense (DOD) satellites such as the Advanced Resolution and Global Observation Satellite (ARGOS) and those in the Defense Meteorological Satellite Program (DMSP). Already, signals from the global positioning satellites, which must traverse the ionosphere on their way to the Earth, are being used for space weather observations as part of the National Space Weather Program. More observation points are being built on the Earth. The National Science Foundation and Canada are jointly building an observatory in the Canadian Arctic, the Japanese and Indonesians are planning an equatorial station, the Indians have a world-class facility, and the Europeans have extended their high-latitude Scandinavian observatory system as far north as Spitzbergen. This will increase the number of major Earthbound space weather observatories to 10, giving Spaceship Earth a reasonable number of portholes through which to view its surrounding environment. Two important specific consequences of internal waves are treated next, one dealing with global change and the upper atmosphere, and one with space weather.