migratory fish and birds see a world about which we have little awareness. Various species can detect and use electric and magnetic fields—both steady and wavelike—as well as chemical tracers, starlight, and many other natural phenomena.
South Sea Islanders have well-known navigational skills that are, in fact, attuned to just these sorts of wave phenomena, the eventual subject of this talk. They describe part of their technique as “ feeling” the ocean; some navigators literally lie down in the bottom of a vessel and feel the pattern of currents and waves, which, it turns out, are not random, but reflect the pattern of islands and, quite possibly, other underwater features. This set of patterns forms a map that is passed on from generation to generation (but that may soon be lost to the human population). Another thread here is the extent to which increasing technology and remote sensing have allowed the human race to detect and study atmospheric waves, which are not obvious to the casual observer, and what this knowledge reveals about our environment.
My main interest here lies in atmospheric waves that are so low in frequency that their wavelengths range from the size of small countries to entire continents. The Earth's gravitational field plays such an important role in the propagation of these waves that they are usually referred to as gravity waves, although “buoyancy waves” may be a more appropriate term to avoid confusion with gravitational waves generated by black holes, and the like. An important distinction is that these waves carry energy and momentum upward, not just horizontally as exhibited in the normal weather. They travel up into space itself.
Wave activity in the Earth's oceans is a common sight. Ocean tides themselves are a wave phenomenon that propagates once around the Earth in a day's time and has a wavelength comparable in size to its circumference. Superposed on this slow waxing and waning of the tides are waves that break on the shoreline and come from distant storm systems. The misnamed tidal wave or tsunami is an extreme example of an ocean wave phenomenon that can carry energy from a massive earthquake or a volcanic eruption horizontally for vast distances, sometimes with devastating consequences. As we shall see, earthquakes and tornadoes also send enormous pulses of waves upward into space.
Surface ocean waves and tsunamis, which break on the shore, involve horizontal propagation. However, the ocean and the atmosphere can support internal waves as well, waves that were hardly noticed until recently. Mariners used to talk about slow water, a region of the ocean in which a ship traveled more slowly than normal. This phenomenon is now thought to be due to waves propagating in the water itself, not on its surface, internal waves invisible from above that oppose a ship's progress. The Earth's atmosphere has no palpable surface; hence, if it is to have waves of any importance to space weather, they must in some sense also be internal. In the next section we present some observations of internal waves in the Earth's atmosphere.
Almost everyone has seen a visual manifestation of internal atmospheric buoyancy waves, most likely without recognizing them. They appear as parallel bands of clouds, each representing a certain phase of a wave as it travels through the atmosphere. As we shall see, these waves have intrinsic oscillations of virtually every parameter characterizing the atmosphere: fluctuating winds, pressure, density, and temperature all occur. It is the temperature variation that yields the cloud patterns as the air parcel periodically changes temperature from above to below the dew point. These patterns in the ice crystals can long outlive the wave packet that formed them and drift for long distances with the background wind.
The atmosphere has no surface, but if one remembers to look for these patterns in the clouds, either from the ground or from an airplane, it becomes very clear that there is as much wave activity in the air as there is in the oceans. An even better vantage point is the Space Shuttle (see Figure 1 ).
In mountainous regions, such as those viewed from the Shuttle, very interesting clouds often form upwind from the peaks, sometimes creating a string of clouds that do not move relative to the mountain. These orographic clouds (see Figure 2 ) form when the wind lifts up to go over the mountain. This launches a wave moving backward against the flow direction. A stationary wave pattern is created when the speed of the wave is equal to and opposite that of the wind and a fixed pattern of high and low temperatures arises. The result is a regular pattern of cloud puffs that may remain fixed for hours.
If an exact matching of wind speed and wave speed seems surprising or even unlikely, the next time you are on a boat stare at the water behind it without looking at the shoreline. The water exhibits a pattern of deep depression just behind the boat and a rise in the water level just aft. Then glance to the right and left, and imagine you are not moving at all; there are fixed patterns of high and low water levels radiating away from the boat. As far as you—the observer —are concerned, these patterns do not move; they simply are there! Yet we all know that an observer on shore sees a wave coming onto the beach with a definite frequency and wavelength. The water “cleverly ” picks out exactly the right wave (e.g., the right wavelength and frequency match) moving with the boat. Standing on solid earth (as is the mountain) is just like a vantage point on the ship. The waves are simply there.
Other planetary atmospheres support waves as well. The Viking Probe camera has provided some spectacular examples when trained on Mars, where the winds are very fierce and any orography is bound to show wavelike behavior in its environs. Figure 3 shows a craterlike feature ringed by regular wave patterns, most likely mirrored in the dust kicked up by the wind.
Prior to this century the only way to detect upper atmospheric disturbances was through changes in the Earth's magnetic field. This method showed that tides existed in the air as well as in the ocean, but
since the upper atmosphere was not even thought to exist, it was quite a problem to explain the observations. It was not until the Space Age allowed us to measure the thermal structure of the atmosphere that much progress was made in explaining tides in the atmosphere in any detail. The infamous V-2 rocket, taken as a spoil of World War II, was outfitted with a thermometer and launched over the New Mexico desert. Now we know that it gets cold on a mountaintop, and one might guess this trend would continue. But no, it turns out that the temperature begins to rise again due to absorption of UV light by the ozone layer. This discovery changed everything for theorists studying atmospheric tides. They were finally able to show that solar heating dominates atmospheric tides, as opposed to the well-established dominant role of the Moon and the Sun's gravitational pull on the oceans. A very interesting work by Sydney Chapman and R.S. Lindzen (see reading list) explains the history of tidal theory, including the dominant influence of Lord Kelvin, who espoused an incorrect theory with such authority that it was accepted for decades. The first hint about upward-traveling waves came out of tidal theory, and the regular pattern of magnetic fluctuations began to be understood. Electric currents must be flowing in space, driven by tidal surges. Space weather exists.
During the spectacular Leonids meteor shower of 1866, and possibly the equally bountiful one of 1833, strange distortions of meteor contrails were seen. The sketch in Figure 4 was drawn by an observer in Cardiff, England, for an event on November 14, 1866, an event that was visible for 10 minutes (Trowbridge, 1907). Illustrations were published and analyzed, but again, the explanations were not easy. At the time the atmosphere was not known to extend as high as the observations indicated, let alone thought to have internal wave activity. Such long-lived trails are very rare except during showers. Indeed, during the November 1996 Leonids shower the author observed five such displays, one lasting several minutes.
Buoyancy waves were first detected in the aftermath of the great volcanic explosion of Krakatau in 1885. This was indeed the first blast heard round the world, at least by instruments. Once the reports