Propagation Of Atmospheric Gravity Waves Research Articles

The Role of Neutral Wind Velocity and Its Vertical Component on Predictability of Formation and Localization of Sporadic E (Es)

The formation and localization of sporadic E (Es) layers predicted by the ion vertical drift velocity and its vertical change in the lower thermosphere during nighttime are shown analytically and by numerical simulations. The consideration of the existence of a minimum negative value of the vertical change of the ion vertical drift velocity as a necessary condition of formation of the Es layer and determining ion convergence rate into this layer is extended in case of the presence of vertical wind. Upward vertical wind can shift the convergence regions upward, while downward vertical wind shifts them downward, unlike the cases of the presence of only meridional and zonal winds. It also changes the ion convergence rate compared to the one with just a horizontal wind. It is shown for the first time that the upward constant wind also causes the convergence of ions with the maximum rate in the region where the ion-neutral collision frequency is equal to their cyclotron frequency. While demonstrating the presented theory by numerical simulations, HWM14 data is used for the meridional and zonal wind velocity profiles and the presence of its vertical component is assumed. In this case, in addition to the estimated ion drift velocity and its vertical changes, their initial distribution and ambipolar diffusion also determine the development of ion convergence/divergence processes. For a small magnitude of vertical wind velocity, its significant influence on the ions/electrons behavior is demonstrated, which shows the importance of both tidal wind and wind changes caused by the propagation of atmospheric gravity waves on the formation of the Es layer. In this theoretical research, it is shown that the realistic profile of the wind velocity, which takes into account the vertical component along with its zonal and meridional ones, is important for the prediction of the Es layers formation, as well as regions of ion depletion.

IONOSPHERIC TOTAL ELECTRON CONTENT VARIATIONS CAUSED BY THE TONGA VOLCANO EXPLOSION ON JANUARY 15, 2022

The capability of volcanoes to generate powerful explosive eruptions influencing the state of the ionosphere became known back in the 1980th. The Hunga-Tonga-Hunga-Ha’apai (Tonga for short) volcano explosion on January 15, 2022, has shown a surge of renewed interest in investigating effects in the Earth — atmosphere — ionosphere — magnetosphere system since this volcano can be rightfully classified as unique. A number of papers have already dealt with the ionospheric effects generated by the Tonga volcano. The temporal variations in the total electron content (TEC) were used to determine the number of volcano explosions to be five. The second and third explosions were the strongest, with the second being the most intense. The response of the ionosphere to the Tonga volcano explosion has been studied on local and global scales by making use of the Global Positioning System satellite constellation and measurements onboard the Swarm satellite network. In the vicinity of the volcano explosion, disturbances in TEC attained 5—10 TECU. In addition to the local effect, traveling ionospheric disturbances were observed to propagate, which were due to the generation and propagation of atmospheric gravity waves with speeds of 180 m/s to 1,050 m/s. Of particular importance to global-scale perturbations is the Lamb wave, which propagated with a speed of 315 m/s. At nighttime, plasma depletions of the equatorial ionosphere were revealed over the tropical Pacific Ocean when the electron density at 400—500 km altitude showed a decrease by 2-3 orders of magnitude. The length of these formations in longitude exceeded ~10 Mm, and they were observed for more than 4—5 h. The scientific objective of this study is further analysis of aperiodic and quasi-periodic perturbations in the ionosphere, which were caused by the Tonga volcano explosion, in a wide range of distances from the source of disturbance (from ~0.1 Mm to 5 Mm). To reveal the ionospheric response to the Tonga volcano explosion, the records of signals from Global Positioning System satellites have been analyzed. The intercomparison of temporal variations in TEC observed on the reference days and on the day when the volcano explosion occurred has resulted in the determination of basic principles of the generation of ionospheric perturbations and the estimation of numerical magnitudes of the parameters of the perturbations. Four groups of disturbances have been detected, each of which arrived at different time delays with respect to the moment of the volcano explosion. It is important to note that the time delay increases with increasing distance from the volcano to the observational instruments. The first group of speeds included the disturbances traveling with a speed close to 1,000 m/s and having an N-shaped profile. This perturbation was generated by a blast wave whose speed depended on the excess pressure and a priori exceeded the speed of sound. In the second group, the speed varied in the 336 m/s to 500 m/s range, within which the speeds of atmospheric gravity waves are found. The speeds in the third group exhibited variability within the 260—318 m/s limits, within which the Lamb wave propagates. The speed in the fourth group was estimated to be 190—220 m/s, which is a characteristic speed of the tsunami that was caused directly by the volcano explosion. The period of quasi-periodic perturbations varied from ~10 min to 20 min, while their amplitudes were from 0.5 TECU to 1 TECU. The observed ionospheric «hole» was proved to be produced by the volcano explosion directly, with the modules of the absolute and relative magnitudes of disturbances showing a tendency for decreasing with increasing distance from the explosion epicenter, from ~10 TECU to 2 TECU and from 37 % to 7 %, respectively. Contrary to the amplitude, the «hole» time delay and its duration exhibited an increase with distance from the volcano to the observational sensors, from 35 min to 100 min and from ~ 30—40 min to 120— 150 min, respectively. A mechanism for generating the ionospheric «hole» has been advanced, which is based on both the electric and non-electric processes (cracking, the friction of particles, condensation of water vapor, coagulation of water droplets, attachment of electrons, gravity segregation, etc.). The ionospheric «hole» is formed as a result of perturbing the global electric circuit, arising external electric currents, an increase in the electric field strengths by orders of magnitude in the atmosphere and the ionosphere, diffusion of the ionospheric plasma down to lower altitudes where the recombination processes become fast. The basic numerical characteristics have been established of the disturbances, whose fluctuations account for local time, the dusk terminator, sensor geographic locations, the location of subionospheric points on the satellite to receiver ray paths with respect to the equatorial anomaly, etc.

Three‐Dimensional Fourier Analysis of Atmospheric Gravity Waves and Medium‐Scale Traveling Ionospheric Disturbances Observed in Airglow Images in Hawaii Over Three Years

AbstractThree‐dimensional Fourier spectral analysis of airglow images is a useful technique to provide propagation characteristics of atmospheric gravity waves (AGWs) in the mesopause region and medium‐scale traveling ionospheric disturbances (MSTIDs) in the thermosphere observed at emission wavelengths of 557.7 nm (altitudes: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs). However, a statistical study of spectral analysis of airglow images over years has not been done yet at Hawaii. Here, we report the first spectral analysis of AGWs and MSTIDs using airglow images obtained at Haleakala (20.71°N, 203.74°E), Hawaii, during the three years from 2013 to 2016. The power spectral density (PSD) of AGWs tends to be larger toward north to northeast in summer and from north to west and southward in winter. The eastward (summer) and westward (winter) preferences are consistent with a wind‐filtering mechanism of AGWs by a mesospheric jet. Some correspondence is found between the locations of tropospheric convection activities and AGW propagation directions, suggesting that these mesospheric AGWs are originated from the troposphere. The PSDs of MSTIDs are larger in solstice than in equinox. This PSD enhancement in solstice is consistent with a sporadic‐E (Es) layer occurrence in either northern or southern hemisphere, suggesting that the MSTIDs over Hawaii could be caused by a coupling process between the Perkins and Es layer instabilities. A small correlation was found between the PSDs of AGWs in the mesopause region and the PSDs of MSTIDs, suggesting that the MSTIDs are partly caused by AGWs from the mesopause region.

Northward propagation of medium scale traveling ionospheric disturbances over Srinagar, J and K India

We report some interesting medium scale travelling ionospheric disturbances (MSTIDs) observed during the night of September 20, 2020 through an all sky CCD imager installed in Kashmir University (KU), Srinagar, India (34.08° N geographic latitude, 74.79° E and 25.91° N geomagnetic latitude). Three different MSTID events including a post-sunset MSTID, a mid-night MSTID and a pre-dawn MSTID are recorded in the airglow images during this night. The post-sunset and pre-dawn MSTIDs are seen to propagate northward while the MSTID observed near midnight time propagates towards west. As the MSTIDs during the night times over the northern hemisphere generally propagate southwestward, the northward propagation of nighttime MSTIDs over Srinagar makes this result an interesting one. More importantly, three different MSTIDs are observed at different times during the single night whose directions of propagation are not same either. We provide the detailed characteristics and evolution processes of these MSTIDs in this paper. The unusual northward motion of the MSTIDs over the Srinagar region is also addressed. The upward propagation of atmospheric gravity waves during this night is believed to be the responsible candidates for such unusual propagation direction of nighttime MSTIDs over the northern hemisphere.

Variations in the geomagnetic field that accompanied the 10 June 2021 solar eclipse

Urgency. At present, the existence of the geomagnetic effect of solar eclipses (SEs) is in question. The data presented in the literature are contradictory. Some researchers assert that the amplitude of the north-south component of the main geomagnetic field increases, while others that it decreases. The third group of researchers notes that this amplitude does not change at all, but instead the amplitude of the west-east component shows variations. In some cases, observations confirm the mechanism for the geomagnetic effect caused by disturbances in the Sq current system, while in other cases observations contradict with the mechanism. The difficulties that are encountered in observing the SE geomagnetic effect are caused by the fact that the magnetic field is subjected to the influence of many energy sources. The magnitude of the geomagnetic effect depends not only on the magnitude (phase) of the solar eclipse but also on the state of space weather, geographic coordinates of data acquisition, local time, season, etc. Therefore, the study of the geomagnetic effect from each new solar eclipse remains an urgent problem. The main feature of the 10 June 2021 Solar eclipse is its annularity. The maximum magnitude did not exceed 0.943, and the eclipse obscuration 89%. The aim of this work is to present the results of analysis of variations in the geomagnetic field that were recorded by the INTERMAGNET during the 10 June 2021 SE. Methods and Methodology. To analyze the effects in the main Earth’s magnetic field, the INTERMAGNET data have been utilized. The data have been analyzed from 15 magnetic observatories located between 77.47°-N and 48.17°-N latitude where the maximum phase varied from 0.943 to 0.124. The analysis was performed with 1-min temporal resolution providing a 0.1-nT resolution. To determine spectral content of the quasi-periodic variations, the systems spectral analysis has been used, which combines mutually complementary the short-time Fourier transform, the wavelet transform employing the Morlet wavelet as a basis function, and the Fourier transform in a sliding window with a width adjusted to be equal to a fixed number of harmonic periods. Results. An aperiodic geomagnetic effect of a solar eclipse has been detected and explained; it consists in a decrease by not greater than 30 nT in the level of the north-south component. The effect is explained by a variation in the ionospheric current density in the west-east direction as a result of a decrease in the electron density during the eclipse. A quasi-periodic geomagnetic effect of a solar eclipse has been revealed; it is explained by the generation of atmospheric gravity waves. The wave acts to modulate the ionospheric electric current, as well as to drag the electrons inducing additional quasi-periodic ionospheric current with a period equal to the wave period. The amplitude of the quasi-periodic variations was observed to be a few nanoteslas. The systems spectral analysis provided more precise values of periods of quasi-periodic variations in the geomagnetic field accompanying the solar eclipse, approximately 20 min and 35 min. Conclusions. The aperiodic and quasi-periodic geomagnetic effects are caused by the disturbance (generation) of the ionospheric current.

Multi-instrument study of longitudinal wave structures for plasma bubble seeding in the equatorial ionosphere

Large Scale Wave Structures (LSWS) in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content (dTEC) data obtained by ground-based GNSS receivers over the South American continent. By using dTEC-maps, we have been able to produce, for the first-time, two-dimensional representations of LSWS. During the period from September to December, the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble (EPB) development. From 17 events of LSWS observed in 2014 and 2015, wave characteristics were obtained: the observed wavelengths, periods, and the phase speeds are respectively, ~900 km, ~41 min and ~399 m/s; the waves propagated from the northeast to southeast. In some cases the front of the oscillation was meridionally aligned, extending to more than 1600 km, the first time such large extension of the wavefront has been reported. From F-layer bottom height oscillation data, measured by ionosonde, LSWS exhibit two different vertical phase propagation modes, in-phase and downward phase. The former mode indicates the presence of a polarization electric field in the F-layer bottom side; the latter suggests propagation of atmospheric gravity waves. The presence of LSWS near the solar terminator, followed by the development of EPBs, suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.

Formation of Multilayered Sporadic E under an Influence of Atmospheric Gravity Waves (AGWs)

The formation of multilayered sporadic E by atmospheric gravity waves (AGWs), propagating in the mid-latitude lower thermosphere, is shown theoretically and numerically. AGWs with a vertical wavelength smaller than the width of the lower thermosphere lead to the appearance of vertical drift velocity nodes (regions where the ions’ vertical drift velocity, caused by these waves, is zero) of heavy metallic ions (Fe+). The distance between the nearest nodes is close to the AGWs’ vertical wavelength. When the divergence of the ion vertical drift velocity at its nodes has a minimal negative value, then these charged particles can accumulate into Es-type thin layers and the formation of multilayered sporadic E is possible. We showed the importance of the ions’ ambipolar diffusion in the formation of Es layers and control of their densities. Oblique downward or upward propagation of AGWs causes downward or upward motion of the ion vertical drift velocity nodes by the vertical propagation phase velocity of these waves. In this case, the formed Es layers also descend or move upward with the same phase velocity. The condition, when the horizontal component of AGWs’ intrinsic phase velocity (phase velocity relative to the wind) and background wind velocity have same magnitudes but opposite directions, is favorable for the formation of the multilayered sporadic E at fixed heights of the sublayers. When the AGWs are absent, then horizontal homogeneous wind causes the formation of sporadic E but with a single peak. In the framework of the suggested theory, it is shown that, in the lower thermosphere, the wind direction, magnitude, and shear determine the development of the processes of ion/electron convergence into the Es-type layer, as well as their density divergence. Consideration of arbitrary height profiles of the meridional and zonal components of the horizontal wind velocity, in case of AGW propagation, should be important for the investigation of the distribution and behavior of heavy metallic ions on regional and global scales.

Three‐Dimensional Fourier Analysis of the Phase Velocity Distributions of Mesospheric and Ionospheric Waves Based on Airglow Images Collected Over 10 Years: Comparison of Magadan, Russia, and Athabasca, Canada

AbstractWe studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) using a three‐dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), over 10–13 years. The AGW propagation direction in summer was from northward to northeastward in ATH and northeastward in MGD with phase speeds of 20–60 m/s. In winter at ATH, they are more omnidirectional with weak preference from northward to southwestward with a speed less than 40 m/s, while another weaker power exists from northeastward to southeastward from 70 to 120 m/s. In winter at MGD, there was no dominant direction in the phase‐velocity spectra with spectral power an order smaller than ATH. We suggest that these AGW characteristics were caused by wind filtering and intensity and locations of tropospheric sources. The MSTIDs at ATH propagated westward in spring and winter and eastward in summer and fall. The MSTIDs at MGD propagated northeastward, eastward, and westward in spring, fall, and winter, respectively, with weaker power than that at ATH. The phase speeds are mostly less than 50 m/s except for fall. The propagation direction tends to change from southwest‐westward in the evening to northeast‐eastward after the midnight at ATH and from south‐southwestward in the evening to north‐northeastward after the midnight at MGD. We discuss possible reasons for these MSTID characteristics at high latitudes based on Perkins and E‐F coupling instabilities, high‐latitude plasma convection, and thermospheric neutral winds.

Analysis of the global ionospheric disturbances of the March 2015 great storm

AbstractThe global ionospheric storm in March 2015 was investigated by using data from over 3000 GPS stations worldwide. In this study, total electron content (TEC), rate of TEC (ROT), and ROT's standard deviation rate of the TEC index, as well as the second‐order difference operator TECT, were considered as main characteristic methods to distinguish ionosphereic disturbances. The results show that (1) based on the multiple methods above, we all observed that for the first time, there were three equatorward traveling ionospheric disturbances (TIDs) in the main phase of this storm. In North America, the disturbance zone expanded to ~40°N; the disturbance periods and AE peak stages were roughly synchronous. We suggest that these three TIDs were induced by the propagation of atmospheric gravity waves to low latitudes under the action of AE. (2) The most intense positive storm occurred over South America and the South Atlantic (over 300% enhancement; 00:00–05:00 UT on 18 March), whereas a negative storm was observed in the corresponding region of the Northern Hemisphere. Such inverse hemispheric asymmetry in intensity and structure can be explained by the variations of the thermospheric composition, the IMF By component, and the geomagnetic intensity. (3) On 18 March, a negative storm dominated globally (except at certain low latitudes), and tended to propagate equatorward and decay with time, which could be largely attributed to the storm circulation theory. And the evolution of the negative storm was further characterized by the foF2 variations of ionosondes.

Temperature changes over storms from measurements of spacecraft TIMED

In the present work we have studied changes of mesospheric temperature over the powerful storms Wilma, Haitang, and Katrina using measurements of the space vehicle TIMED. We have found the temperature increasing at the altitude range 80-100 km. We have found the explanations for the obtained results by the dissipation of the gravity waves. Propagation of atmospheric gravity waves in a non-isothermal, windless atmosphere, with taking into account the viscosity and the thermal conductivity, has also been modelled in this work. We have determined that the maximum of amplitude of the atmospheric-gravity waves at the considered characteristics corresponds to altitudes of near 90 km (mesopause). It was found that the main factor influencing propagation and dissipation of the wave in such cases is the vertical temperature gradient. Viscosity and thermal conductivity have less influence on the wave amplitude.

An analysis of atmospheric gravity waves observed in synthetic aperture radar images acquired over the Northern Caspian Sea

The generation and propagation of atmospheric gravity waves (AGWs) in the troposphere over the land and the ocean are common phenomena. Because AGWs have the properties of waves, they create on the sea surface and, in turn, on synthetic aperture radar (SAR) images of the sea surface, specific surface manifestations – a series of groups of quasi-parallel strips (or wave-like patterns) of different intensities. Data collected from remote-sensing satellites during recent years have shown that these waves are very often generated and extend over the Caspian Sea. The SAR images acquired by the European Environmental Satellite (ENVISAT), the Canadian SAR-equipped satellites Radarsat-1, and Radarsat-2 satellites over the Northern Caspian Sea during monitoring in 2009–2012 have allowed a detailed study of these AGWs. In this article, the analysis of a number of SAR images containing AGW manifestations is performed using image analysis and calculations of characteristics of the lower atmosphere and the parameters of the AGWs are estimated. The model results confirm that AGWs can be generated by specific atmospheric disturbances and are able to cross the entire northern part of the Caspian Sea. A probable generation mechanism explaining the occurrence of waves over the northern part of the sea is proposed and discussed.

Widespread tsunami-like waves of 23-27 June in the Mediterranean and Black Seas generated by high-altitude atmospheric forcing.

A series of tsunami-like waves of non-seismic origin struck several southern European countries during the period of 23 to 27 June 2014. The event caused considerable damage from Spain to Ukraine. Here, we show that these waves were long-period ocean oscillations known as meteorological tsunamis which are generated by intense small-scale air pressure disturbances. An unique atmospheric synoptic pattern was tracked propagating eastward over the Mediterranean and the Black seas in synchrony with onset times of observed tsunami waves. This pattern favoured generation and propagation of atmospheric gravity waves that induced pronounced tsunami-like waves through the Proudman resonance mechanism. This is the first documented case of a chain of destructive meteorological tsunamis occurring over a distance of thousands of kilometres. Our findings further demonstrate that these events represent potentially dangerous regional phenomena and should be included in tsunami warning systems.

Ranges of AGW propagation in the Earth’s atmosphere

The propagation of atmospheric gravity waves (AGWs) is studied in the context of geometrical optics in the nonisothermal, viscous, and thermal-conductive atmosphere of Earth in the presence of wind shifts. Parametric diagrams are plotted, determining the regions of allowed frequencies and horizontal phase velocities of AGWs depending on the altitude. It is shown that a part of the spectrum of AGWs propagates in stationary air in an altitude range from the Earth’s surface through the ionospheric F1 layer. AGW from nearearth sources attenuate below 250 km, while waves generated at altitudes of about 300 km and higher do not reach the Earth’s surface because of the inner reflection from the thermosphere base. The pattern changes under strong thermospheric winds. AGW dissipation decreases with an adverse wind shift and, hence, a part of the wave spectrum penetrated from the lower atmosphere to the altitudes of F2 layer.

Experimental evidence of a stratospheric circulation influence on mesospheric temperatures and ice-particles during the 2010–2011 austral summer at 69°S

A significant inter-annual decrease in polar mesosphere ice-particles, i.e., PMSE and PMC, during 2010–2011 is compared with earlier austral summers, in particular with 2009–2010. The first IAP iron lidar temperature measurement at Davis (68.6°S), Antarctica from 14 December 2010 are used to assess thermal effects of atmospheric processes on the mesopause region. We report low average temperatures of ∼125K measured by Fe-lidar near 90km when the PMSE season commenced, whereas temperatures were warmer in 2010–2011 compared to 2009–2010 at altitudes where PMSE normally occur (around 86km). Summer mesopause region temperature anomalies are derived using Aura MLS records. We reveal that the late break-down of the Antarctic stratospheric polar vortex on 5 January 2010, coupled with enhanced early summer mesospheric zonal wind field, provide a barrier to upward propagation of atmospheric gravity waves to be the main mechanism for the observed warm early summer season below the mesopause. The mesopause in 2010–2011 was unusually high and cold. We conclude that the timing of the annual break-down of the southern polar stratospheric vortex as manifest in zonal winds at 30hPa impacts mesosphere temperature and ice-particle formation early in the austral summer.

Characteristics of traveling ionospheric disturbances observed by the TIDDBIT sounder

HF Doppler sounders represent a low‐cost and low‐maintenance solution for monitoring wave activity in theF region ionosphere. HF Doppler sounders together with modern data analysis techniques can provide comprehensive traveling ionospheric disturbance (TID) characteristics, including both horizontal and vertical TID velocities and wavelengths across the entire spectrum from periods of 1 min to over an hour. Atmospheric and Space Technology Research Associates LLC has developed a new system called “TIDDBIT” (TID Detector Built in Texas), and data will be presented from a TIDDBIT system deployed in Virginia. These results reinforce the relationship between atmospheric gravity waves (AGWs) and TIDs. The TID propagation azimuths rotate through 360° in 24 h, mimicking the rotation of the thermospheric winds but with approximately a 90° offset. The rotation of TID azimuths and thermospheric winds in Virginia is similar to that observed previously by other Northern Hemisphere systems and opposite from the direction observed in Antarctica. These results illustrate the filtering effects that thermospheric neutral winds can have on the propagation of AGW. The completeness of the wave information obtained from the TIDDBIT system makes it possible to reconstruct the vertical displacement of isoionic contours over the ∼200 km horizontal dimension of the sounder array. Such information will be relevant for understanding the seeding of irregularities, as well as for several operational needs involving navigation, communication, and surveillance systems.

Comparative studies of methods of obtaining AGW's propagation properties

Abstract. Three among the existing methods of obtaining the properties (intrinsic period, wavelength, propagation direction) of atmospheric gravity waves (AGWs) were compared and studied by numerical method to simulate radar data. Three-dimensional fluctuation velocity satisfying dispersion equation and polarization relation of atmospheric gravity wave were generated, then the numerical data were analysed by these methods to obtain the properties of waves. We found that, hodograph analysis was accurate for a monochromatic wave in obtaining its wave period and propagation direction, but the analysis became erratic for the case of multiple waves' superposition. The error was especially large when data consisted of both upward propagating waves and downward propagating waves. The hodograph method became meaningful again if all the component waves propagated in the same direction and the resulting period was dominantly decided by the lowest frequency wave. Stokes parameters method would obtain statistically meaningful values of wave period and azimuth if the spreading of the azimuths among the component waves did not exceed 90° and the resulting period and azimuth were dominated by the lowest frequency wave component as well, irrespective of the vertical sense of propagation. Another method called phase and group velocity tracing technique was reconfirmed to be meaningful in measuring the characteristic wave period and vertical group and phase velocities of a wave packet: the characteristic wave period and vertical wavelength was dominated by the wave with the highest frequency among the component waves in the wave packet. Based on these numerical results, a composite procedure of data analysis for wave propagation was proposed and an example of real data analysis was presented.

Generation of Atmospheric Gravity Waves in the Polar Thermosphere in Response to Auroral Activity

Atmospheric gravity wave (AGW) is a typical phenomenon in the upper atmosphere. At mid/low latitudes, climatological sources such as unstable barometric activity in the troposphere play an important role to generate AGWs in the thermosphere. While these sources are also important at high latitudes, energy input from the magnetosphere has additional large contributions to AGW generation. This paper reviews previous studies of AGWs associated with auroral activity at high latitudes. Theoretical studies have indicated that Joule/particle heating and the Lorentz force are major processes for generating AGWs in the thermosphere. Many observations show that AGWs can propagate horizontally for thousands of km from the source region. The paper summarizes equations regarding AGW generation by Joule/particle heating and the Lorentz force, and discusses the relative importance of these two processes.

On the Discrimination of Radar Signatures of Atmospheric Gravity Waves and Oceanic Internal Waves on Synthetic Aperture Radar Images of the Sea Surface

Synthetic aperture radar (SAR) images acquired over the ocean frequently show sea wave-like patterns that have wavelengths well above those of ocean surface waves and that are sea surface signatures of oceanic internal waves (OIWs) or of atmospheric gravity waves (AGWs). However, it is often difficult to decide whether they result from the first or the second kind of waves, which has led many investigators to misinterpret SAR images of the sea surface. Based on solitary wave and radar imaging theories of AGWs and OIWs, we present criteria that help distinguish between them. However, there are cases where these criteria, which are based solely on the shape and structure of the features visible on the SAR images, yield ambiguous results. In these cases, one must resort to additional information on the generation of AGWs and OIWs, which are listed in this paper.

A class of coupled nonlinear Schrödinger equations: Painlevé property, exact solutions, and application to atmospheric gravity waves

The Painleve integrability and exact solutions to a coupled nonlinear Schrodinger (CNLS) equation applied in atmospheric dynamics are discussed. Some parametric restrictions of the CNLS equation are given to pass the Painleve test. Twenty periodic cnoidal wave solutions are obtained by applying the rational expansions of fundamental Jacobi elliptic functions. The exact solutions to the CNLS equation are used to explain the generation and propagation of atmospheric gravity waves.

Heating of the auroral ionosphere by traveling ionospheric disturbances initiated by atmospheric gravity waves

In the presence of perturbations of the thermospheric auroral region produced by traveling ionospheric disturbances during the propagation of atmospheric gravity waves, an analytical expression of the velocity of the thermospheric plasma is derived through magnetohydrodynamic formalism. The expressions of the Joule heating and the viscous heating are derived, and their rates of variation are presented. A threshold height for their transition has been determined from their ratio, which is in agreement with the experimental data. The analysis indicates that the time taken by the thermospheric plasma to reach a steady-state corresponds to the nature of the traveling ionospheric disturbances in the medium.