The Continuous Presence of Gravity Waves in the Upper Atmosphere above Arecibo Observatory, Puerto Rico – an Unsolved Mystery
Atmospheric Gravity Waves – What are they?
In the earth’s atmosphere, compressional and gravitational forces (i.e., buoyancy forces) combine to produce upwardly propagating waves known as atmospheric gravity waves (GWs). Because of their compressional component, these waves are sometimes referred to as acoustic-gravity waves. However, they are distinct from acoustic waves, which have much smaller spatial dimensions. Waves with only gravity as the restoring force occur on the surface of the ocean (ocean gravity waves), but in the atmosphere where density varies gradually with altitude, the waves exist in the medium and have a compressional component. In the troposphere/stratosphere, GWs generate relatively small perturbations in wind speed, temperature, and density. However, the vertical flux of wave energy is proportional to the product of atmospheric density and the wind speed-squared. Thus, as the waves propagate to greater heights where atmospheric density is lower, wind speeds become greater. Classic sources of GWs in the lower atmosphere include strong thunderstorms and mesoscale convective systems, air flow over mountains and other orographic features, perturbations in the jet stream, tsunamis, earthquakes, and volcanoes. Sources at high altitudes include variations in Joule and particle heating rates in the auroral region, nonlinearities in upwardly propagating atmospheric tides, and the passing of the terminator during a solar eclipse. GW periods range from 5 min to several hours, and their horizontal wavelengths extend from several kilometers to roughly 1000 km. A pictorial representation of a GW is presented below.
Atmospheric Gravity Wave Observations at Arecibo Observatory
As a GW travels to heights of ~100 km (about 63 miles) and greater, the wave couples to the background ionization (i.e., the ionosphere). At Arecibo Observatory (18.24° N, 66.75° W) significant ionization is present above
~100 km altitude during the daytime and above ~200 km altitude during the night. At these altitudes, the gravity waves generate imprints on the background ionization and can readily be detected with the powerful Arecibo 430 MHz radar. This is the most sensitive radar in the world. It is commonly referred to as an incoherent scatter radar or Thomson scatter radar. At 430 MHz, the backscatter echo near the radar center frequency originates from groups of electrons that shield ions in the charge-neutral ionospheric plasma. The motion of the electrons is therefore tied to the movement of the ions, and the echo bandwidth is only about 10 kHz. This center line backscatter provides a direct measurement of electron density versus altitude. A second technique used to monitor the electron density profile at Arecibo entails measurements of plasma wakes that occur when photoelectrons speed through the ionosphere. These echoes occur only during the daytime and are offset by 3-8 MHz from 430 MHz. This second technique yields the best possible altitude and temporal resolution. A more detailed discussion of the two measurement techniques is available in the following journal references:
 Djuth, F. T., M. P. Sulzer, and J. H. Elder, Application of the coded long pulse technique to plasma line studies of the ionosphere, Geophys. Res. Lett., 21, 2725-2728, 1994.
 Djuth, F. T., M. P. Sulzer, J. H. Elder, and V. B. Wickwar, High-resolution studies of atmosphere-ionosphere coupling at Arecibo Observatory, Puerto Rico, Radio Sci., 32, 2321-2344, 1997.
 Djuth, F. T., M. P. Sulzer, S. A. Gonzáles, J. D. Mathews, J. H. Elder, and R. L. Walterscheid, A Continuum of gravity waves in the Arecibo thermosphere?, Geophys. Res. Lett., 31(16), doi:2003GL019376, 2004.
 Livneh, D., I. Seker, F. T. Djuth, and J. D. Mathews, Continuous quasi-periodic thermospheric waves over Arecibo, JGR, in press, 2007.
Examples of radar measurements of GWs above Arecibo are provided below. The data in Figures 2 and 3 were obtained with the high-resolution photoelectron technique and are presented as waterfall plots. Figures 4 and 5 compare the high-resolution technique with lightly filtered center line observations recorded simultaneously. In this case, the data are shown in range-time-intensity format. The electron density fluctuation caused by the GW is shown as a percentage relative to background for the high-resolution results, and a modified signal-to-noise ratio that is proportional to electron density variation is used for the center line measurements. There is a one-to-one correspondence between GW structures observed with the two techniques, but the resolution achievable with the center line technique is not quite as good as that obtained with photoelectrons. Figure 6 shows more recent center line data that are optimally filtered. The heavier filtering allows GWs to be clearly seen day and night. The GW imprints curve toward increasing time at altitudes below about 150 km because the GW vertical wavelength is shorter at these heights and therefore the vertical phase velocity is smaller. The short wavelength waves arrive after the faster long wavelength waves.
What is the Source of the Atmospheric Gravity Waves?
It turns out that GWs are continuously observed in the upper atmosphere above Arecibo, and their characteristics do not change significantly between day and night. In addition, seasonal changes are not evident, and there is no correlation with geomagnetic activity. The waves appear to be consistently arriving at Arecibo from the northeast. A recent investigation described in  seems to rule out all classic sources for the Arecibo GWs.
At auroral latitudes, atmospheric disturbances are generated even under quiet geomagnetic conditions and the associated GWs propagate in the
southward direction. Thus, it is appealing to invoke the propagation of GWs from the polar region down to Puerto Rico as an explanation for the Arecibo observations. However, model results indicate that these waves should dissipate well before reaching Arecibo. Nevertheless, it is conceivable that there is a missing element in our concept of long-distance gravity wave propagation. A new round of experiments is planned to test the auroral propagation hypothesis.
Alternatively, it appears that natural far infragravity ocean waves (100 – 200 km wavelength) observed in the open ocean can generate GWs that propagate into the upper atmosphere. In the present case the direction of ocean wave propagation would have to be toward Arecibo. On the other hand, surface winds could blow over a standing ocean wave produced by far infragravity waves or ocean currents and thereby generate atmospheric gravity waves. It is noteworthy that even small amplitude (2 cm) tsunamis on the open ocean generate significant GWs in the upper atmosphere and that a small amplitude tsunami is very similar to a far infragravity wave. This mechanism preserves the concept that the Arecibo waves are produced at midlatitudes and directly propagate into the upper atmosphere. GWs must be generated at distances between 650 km and 1000 km from Puerto Rico if they are to be observed in the atmosphere above Arecibo. The role of ocean waves in the production of the Arecibo GWs remains an important avenue of investigation.
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