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Showing papers by "Michael P. Hickey published in 2011"


Journal ArticleDOI
TL;DR: In this article, ground-based Global Positioning System (GPS) measurements of ionospheric total electron content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis following two recent seismic events: the Samoa earthquake of 29 September 2009 and the Chile earthquake of 27 February 2010.
Abstract: [1] Ground-based Global Positioning System (GPS) measurements of ionospheric total electron content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis following two recent seismic events: the Samoa earthquake of 29 September 2009 and the Chile earthquake of 27 February 2010. Both earthquakes produced ocean tsunamis that were destructive to coastal communities near the epicenters, and both were observed in tidal gauge and buoy measurements throughout the Pacific Ocean. We observe fluctuations in TEC correlated in time, space, and wave properties with these tsunamis using the Jet Propulsion Laboratory's Global Ionospheric Mapping software. These TEC measurements were band-pass filtered to remove ionospheric TEC variations with wavelengths and periods outside the typical range for tsunamis. Observable variations in TEC appear correlated with the tsunamis in some locations (Hawaii and Japan), but not in others (Southern California or near the epicenters). Where variations are observed, the typical amplitude tends to be ∼0.1–0.2 TEC units for these events, on the order of ∼1% of the background TEC value. These observations are compared to estimates of expected tsunami-driven TEC variations produced by Embry Riddle Aeronautical University's Spectral Full Wave Model, an atmosphere-ionosphere coupled model, and are found to be in good agreement. Significant TEC variations are not always seen when a tsunami is present, but in these two events the regions where a strong ocean tsunami was observed coincided with clear TEC observations, while a lack of clear TEC observations coincided with smaller sea surface height amplitudes. There exists the potential to apply these detection techniques to real-time GPS TEC data, providing estimates of tsunami speed and amplitude that may be useful for early warning systems.

97 citations


Journal ArticleDOI
TL;DR: In this article, the authors used a full-wave model of the viscous damping of atmospheric gravity waves propagating in a nonisothermal atmosphere to explore the relative contributions of these sources of wave heating as a function of wave properties and altitude.
Abstract: [1] Total wave heating is the sum of the convergence of the sensible heat flux and the divergence of the viscous flux of wave kinetic energy. Numerical simulations, using a full-wave model of the viscous damping of atmospheric gravity waves propagating in a nonisothermal atmosphere, are carried out to explore the relative contributions of these sources of wave heating as a function of wave properties and altitude. It is shown that the sensible heat flux always dominates in the lower thermosphere, giving a lower region of heating and an upper stronger region of cooling. The heating due to the divergence of the viscous flux of kinetic energy is significant only for fast waves (horizontal phase speed greater than about 120 m s−1). The faster the wave is, the greater the heating in the upper thermosphere can be. The viscous heat source in per unit mass terms can greatly exceed the sensible heat source for fast waves and might be a significant heat source for the middle and upper thermosphere.

52 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined vertical group velocities as a measure of vertical energy flow velocity for gravity and acoustic waves propagating into the dissipative lower thermosphere.
Abstract: [1] The response to wave forcing of finite duration comprises a transient forerunner and the steady state signal (or simply the signal). It is the latter that carries information on the spectral content of the forcing, and the signal velocity is the velocity at which wave energy flows. To the extent that group velocity is a good measure of the energy flow velocity, the ray-tracing formalism is a valid description of signal propagation. We have examined vertical group velocities as a measure of vertical energy flow velocity for gravity and acoustic waves propagating into the dissipative lower thermosphere. We find that the effects of dissipation on gravity waves can cause group velocity to become a meaningless measure of the energy flow velocity. When certain terms originating in the diffusion of heat and momentum are neglected, the validity of group velocity can be extended to F region altitudes. For acoustic waves, group velocity can be a good measure of energy flow velocity throughout the lower thermosphere because acoustic waves are far less subject to dissipation.

30 citations


Book ChapterDOI
29 Jan 2011
TL;DR: Tsunamis propagate at the surface of the deep ocean horizontal phase speeds of approximately 200 m/s, which is about two-thirds of the lower atmospheric sound speed as discussed by the authors.
Abstract: Tsunamis propagate at the surface of the deep ocean horizontal phase speeds of approximately 200 m/s, which is about two-thirds of the lower atmospheric sound speed. They have large horizontal wavelengths that are typically of a few hundred kilometers, and they remain coherent over large propagation distances. They also have large horizontal extents (sometimes a few thousand kilometers) parallel to their wave fronts. They can traverse great distances over a span of several hours, so that large areas of the oceanatmosphere interface are impacted. Typical dominant wave periods associated with tsunamis are a few tens of minutes. In the deep ocean their amplitudes are usually quite small with surface displacements being only a few centimeters, but occasional large events can have amplitudes of a few tens of cm. The speeds, wavelengths and periods of tsunamis lie within the range of those of atmospheric gravity waves. These are vertically transverse waves with motions of air parcels mainly influenced by gravity and buoyancy. The vertical displacement of the water acts like a moving corrugation at the base of the atmosphere and so very effectively generates atmospheric gravity waves. In general a spectrum of waves will be produced by a tsunami. Most of the power in the spectrum resides in internal gravity waves, with acoustic waves and evanescent waves being less efficiently generated. Internal waves can transport energy and momentum vertically through the atmosphere. Due to the decrease of mean atmospheric density with increasing altitude, the amplitude of these waves increases as they propagate upward in order to conserve wave energy. At sufficiently high altitudes molecular viscosity and thermal conductivity damp the waves, and their amplitudes then decrease with increasing altitude. Because the waves have high phase speeds (commensurate with the tsunami speed), they are deep waves with vertical wavelengths of ~ 100 km. This allows them to reach the middle thermosphere (~ 250 km altitude) before the molecular dissipation becomes severe. Atmospheric winds also influence the upward propagation of atmospheric gravity waves. Because the winds vary with height the waves may be propagating with the wind at some heights and against the wind at other heights. In the former case the vertical wavelengths are shortened, which increases the velocity shears and thereby increases the viscous damping rate. In the latter case the vertical wavelengths are increased, which decreases the velocity shears and decreases the viscous damping rate. At these heights the tsunami-driven atmospheric gravity waves have large amplitudes so that their interaction with the ionosphere is likely to produce detectable traveling

7 citations


20 Sep 2011
TL;DR: In this article, the authors analyzed fluctuations correlated in time, space, and wave properties with the Tohoku tsunami in TEC estimates processed using JPL's Global Ionospheric Mapping Software.
Abstract: Ground-based Global Positioning System (GPS) measurements of ionospheric Total Electron Content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis. We have observed such traveling ionospheric disturbances (TIDs) following recent seismic events, including the Tohoku tsunami of March 11, 2011. We analyze fluctuations correlated in time, space, and wave properties with this tsunami in TEC estimates processed using JPL’s Global Ionospheric Mapping Software. The TEC estimates were band-pass filtered to remove ionospheric TEC variations with periods outside the typical range of internal gravity waves caused by tsunamis. Observable variations in TEC appear correlated with the Tohoku tsunami near the epicenter, at Hawaii, and near the west coast of North America. Disturbance magnitudes are 1-10% of the background TEC value. Observations near the epicenter are compared to estimates of expected tsunami-driven TEC variations produced by Embry Riddle Aeronautical University’s Spectral Full Wave Model, an atmosphere-ionosphere coupling model, and found to be in good agreement. The potential exists to apply these detection techniques to real-time GPS TEC data, providing estimates of tsunami speed and amplitude that may be useful for future early warning systems.

1 citations