The Continuous Presence of Gravity Waves in the Upper Atmosphere above Arecibo Observatory, Puerto Rico – a resolved Mystery?
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.
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 (63 miles) altitude during the daytime and above 200 km 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 ~3-12 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:
Examples of radar measurements of GWs above Arecibo are provided below. The data in the first figure below were obtained with an ultra high-resolution photoelectron technique two. The electron density fluctuation caused by the GW is shown as a percentage relative to background for the high-resolution results. The second figure below shows data obtained with the first technique at night. There is a one-to-one correspondence between GW structures observed with the two techniques, but the resolution achievable with the center line technique at night is not as good as that obtained with photoelectrons during the daytime. 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 are present.
On the basis of past measurements of gravity waves at Arecibo made with rudimentary techniques and the present high-resolution wave signatures, one arrives at the conclusion that these waves have been continuously present over Arecibo for at least 50 years and most likely millions of years. The wave 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. At present there is only one viable explanation, and it involves the generation of the gravity waves by the ocean in the vicinity of the mid-Atlantic ridge. Internal ocean gravity waves generated as a result of the dominant semidiurnal internal tides in this region are the most likely source of the atmospheric gravity waves seen at Arecibo. Essentially the ocean tide hundreds of meters below the surface pushes ocean water against the steep underwater topography created primarily by volcanism. The internal ocean waves are highly selective in that they generate surface perturbations having harmonics that are locked to the 12 h tide. Only waves having intrinsic periods of 2 hours or less as observed at Arecibo have the properly oriented phase front necessary to excite atmospheric gravity waves. This source process is dependent only on the lunar tide and mid-Atlantic ridge topography; it is not subject to the large variability and impulsiveness encountered with the standard atmospheric/ionospheric weather.
We have learned the following from our investigation. Without the moon there would be no continuum of gravity waves over Arecibo. Because of the stability of the wave field, we can now estimate a constant delay versus altitude for satellite paths over the mid-Atlantic region in North America.
This work was funded by the National Science Foundation under grants: ATM-9202658, ATM-0456085, ATM-0946636, and AGS-1656898.
High-Resolution Measurements of Electron-Landau Damped Photoelectrons in the Ionosphere above Arecibo.
First results from wideband (electron phase energies of 5 eV – 51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory have been obtained. This spectrum is presented below. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys measured with the incoherent scatter radar at Arecibo, Puerto Rico. The leading edge of the photoelectron spectral line produces the plasma line peak via electron Landau damping. The trailing edge of the photoelectron spectral line reduces Langmuir wave energy, which results in the plasma line valleys, also via the electron Landau damping process. The peaks and other structures occur within an enhancement zone extending from electron phase energies of 14 eV to 27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14 eV to 27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas. Obviously, there is much to be learned about the photoelectron plasma line at Arecbo.
Details of the above observations may be found in: Djuth, F. T., H. C. Carlson, L. D. Zhang (2018) Incoherent Scatter Radar Studies of Daytime Plasma Lines, Earth Moon Planets 121:13–43 https://doi.org/10.1007/s11038-018-9513-5
This work was funded by the National Science Foundation under grant AGS-1012006.