Sharon L. Vadas

Generation of large-scale gravity waves and neutral winds in the thermosphere from the dissipation of convectively generated gravity waves

phase speeds of cH � 480–510 m/s, density perturbations as large as jr 0 /r j� 3.6–5% at z = 400 km, relative [O] perturbations as large as � 2–2.5% atz = 300 km, and total electron content perturbations as large as � 8%. This transfer of momentum from local, relatively slow, small scales at the tropopause to global, fast, large scales in the thermosphere is independent of geomagnetic conditions. The various characteristics of these large-scale waves may explain observations of LSTIDs at magnetically quiet times. We also find that this body force creates a localized ‘‘mean’’ horizontal wind in the direction of the body force. For the plume at 2120 UT, the wind is southward with an estimated maximum of vmax �� 400 m s � 1 that is dissipated after � 4h . We also find that the induced body force direction varies throughout the day, depending on the winds in the lower thermosphere.

Mechanism for the Generation of Secondary Waves in Wave Breaking Regions

The authors propose that the body force that accompanies wave breaking is potentially an important linear mechanism for generating secondary waves that propagate into the mesosphere and lower thermosphere. While the focus of this paper is on 3D forcings, it is shown that this generating mechanism can explain some of the mean wind and secondary wave features generated from wave breaking in a 2D nonlinear model study. Deep 3D body forces, which generate secondary waves very efficiently, create high-frequency waves with large vertical wavelengths that possess large momentum fluxes. The efficiency of this forcing is independent of latitude. However, the spatial and temporal variability/intermittency of a body force is important in determining the properties and associated momentum fluxes of the secondary waves. High spatial and temporal variability accompanying a wave breaking process leads to large secondary wave momentum fluxes. If a body force varies slowly with time, negligible secondary wave fluxes result. Spatial variability is important because distributing ‘‘averaged’’ body forces over larger regions horizontally (as is often necessary in GCM models) results in waves with smaller frequencies, larger horizontal wavelengths, and smaller associated momentum fluxes than would otherwise result. Because some of the secondary waves emitted from localized body force regions have large vertical wavelengths and large intrinsic phase speeds, the authors anticipate that secondary wave radiation from wave breaking in the mesosphere may play a significant role in the momentum budget well into the thermosphere.

Gravity Wave Radiation and Mean Responses to Local Body Forces in the Atmosphere

The authors determine the spectral linear solutions that arise in response to local 3D body forces and heatings in an idealized environment that turn on and off smoothly but not necessarily slowly over a finite interval in time. The solutions include impulsive through slowly varying body forcings. The forcings result in both a mean response, which is typically significantly broadened spatially in one direction, and a gravity wave response, which allows the fluid to reach this state. The gravity wave field depends on both the spatial attributes of the source and the forcing duration. The frequency of the wave response is the ‘‘characteristic’’ source frequency (formed from the source dimensions) if the forcing frequency is greater than the characteristic frequency and is the forcing frequency otherwise. The radiated gravity waves from zonal forcings have vertical wavelengths, which are approximately twice the vertical extent of the forcing, and horizontal wavelengths, which are at least twice the horizontal extent of the forcing. Wave excitation is increasingly inefficient when the forcing frequency is smaller than the characteristic source frequency. In addition, the mean responses are not confined to the source region; in general, significant spatial broadening of the mean responses occurs. If the source’s frequency is high and low, the responses are broadened horizontally and vertically, respectively, with the amount depending on the characteristic scales of the source. If the body forcing is in the eastward direction, then much or all of the ensuing zonal mean wind is eastward. However, for many realistic forcing scenarios, a large percentage of the ensuing zonal wind flows westward. These countersigned jets are displaced meridionally about the source. Thus, spatially confined body forcings create both gravity wave and mean responses if the forcings are fast enough; very slowly varying forcings create only mean responses.

A model study of the effects of winds on concentric rings of gravity waves from a convective plume near Fort Collins on 11 May 2004

[1] Using a convective plume model and a ray trace model, we investigate the effects of winds on concentric rings of gravity waves (GWs) excited from a convective plume on 11 May 2004, near Fort Collins, Colorado. We find that winds can shift the apparent center of the concentric rings at z = 87 km from the plume location. We also find that critical level filtering (for GWs with small phase speeds propagating in the same direction as the wind) and wave reflection (for high-frequency GWs with small horizontal wavelengths propagating in the opposite direction to the wind) prevent many GWs from reaching the OH airglow layer. Additionally, we find that strong winds disrupt the concentric ring patterns, causing distorted ‘‘squashed’’ ring and arc-like patterns instead. Using a zero wind profile and a representative April mean zonal wind profile, we compare our model results with observations of concentric rings at the Yucca Ridge Field Station (40.7N, 104.9W). We find that the model horizontal wavelengths and periods agree reasonably well with the observed data. We also compare the model temperature perturbations with the temperature perturbations calculated from the intensity perturbations. Because the observations show less critical level filtering than from the April wind profile and more critical level filtering than from the zero wind profile, we conclude that the winds on 11 May were likely somewhat smaller than the April zonal wind profile assumed here.

Periodic spacing between consecutive equatorial plasma bubbles

[1] We analyze three-years of data collected by a field-aligned airglow imaging system located at the Cerro Tololo Inter-American Observatory near La Serena, Chile to determine the occurrence of equatorial plasma bubbles (EPBs). On 317 of the 552 predominately clear nights of observations, structure indicative of EPBs is present. On 123 of these nights, multiple EPBs with periodic spacings were recorded with 88 nights showing 3 or more consecutive bubbles. We suggest that the periodic spacing of EPBs could be related to the properties of an underlying seed mechanism, namely gravity waves (GWs). The distribution of spacings compares favorably to the spectrum of GW induced traveling ionospheric disturbances (TIDs) measured by Vadas and Crowley (2010) from a similar geographic latitude in the northern hemisphere. Furthermore, the distribution of spacings decreases from 2006 through 2009, tracking the corresponding decrease in the thermospheric neutral temperature, T n As T n decreases, GWs with larger horizontal wavelengths have smaller initial amplitudes and cannot propagate as easily to EPB seeding altitudes. Thus, our observations are consistent with GW theory.

Numerical modeling of the global changes to the thermosphere and ionosphere from the dissipation of gravity waves from deep convection

During the minimum of solar cycles 23–24, the Sun was extremely quiet; however, tropospheric deep convection was strong and active. In this paper, we model the gravity waves (GWs) excited by deep convective plumes globally during 15–27 June in 2009 and in 2000 (previous solar maximum). We ray trace the GWs into the thermosphere and calculate the body force/heatings which result where they dissipate. We input these force/heatings into a global dynamical model and study the neutral and plasma changes that result. The body forces induce horizontal wind (uH′) and temperature (T′) perturbations, while the heatings primarily induce T′. We find that the forces create much larger T′ than the heatings. uH′ consists of clockwise and counterclockwise circulations and “jet”-like winds that are highly correlated with deep convection, with |uH′|∼50–200m/s. uH′ and T′ are much larger during 2009 than 2000. uH′ decreases slightly (significantly) with altitude from z∼150 to 400 km during 2009 (2000). T′ perturbations at z=350km primarily propagate westward at ∼460 m/s, consistent with migrating tides. It was found that planetary-scale diurnal and semidiurnal tides are generated in situ in the thermosphere, with amplitudes ∼10–40m/s at z=250 km. The largest-amplitude in situ tides are DW1, D0, DW2, SW2, SW3, and SW5. Smaller-amplitude in situ tides are S0, SE2, and SW3. Total electron content (TEC′) perturbations of 1–2.5 (2–3.5) total electron content units (TECU, where 1 TECU = 1016 el m−2) during 2009 (2000) are created in the upper atmosphere above nearby regions of deep tropical convection. For a given local time (LT), there are 2 to 3 TEC′ peaks in longitude around the Earth.

Satellite‐based measurements of gravity wave‐induced midlatitude plasma density perturbations

[1] Large amplitude anticorrelated wave structures appear at midlatitudes in the nighttime ionospheric plasma and neutral density measurements made at altitudes between 250 and 300 km by the Dynamics Explorer-2 satellite. The wavelengths along the satellite orbit track are generally longer than a hundred kilometers, and the vertical perturbation velocities are about 20 m/s. These characteristics are consistent with plasma motions driven by gravity waves in the neutral atmosphere as they propagate upward from lower atmospheric source regions. The altitude and the horizontal wavelengths observed provide in situ empirical validation of a recently developed gravity wave propagation model that includes the effects of both kinematic viscosity and thermal diffusivity at high altitudes.

Horizontal parameters of daytime thermospheric gravity waves and E region neutral winds over Puerto Rico

We report on the electron density perturbation amplitudes, periods (up to 60 min), horizontal and vertical wavelengths, phase speeds, and propagation directions of daytime traveling ionospheric disturbances (TIDs) from 115 to 300 km altitude using dual-beam experiments at the Arecibo Observatory (AO), Puerto Rico. As in previous studies, we find a near continuum of waves above the AO. While the TIDs propagate in nearly all directions except purely westward, we find that most propagate southward southeastward. We find that TID amplitudes increase nearly exponentially with increasing period, although with a much smaller slope for periods >30 min. TID amplitudes peak on the bottomside of the F region. Typical vertical wavelengths increase from less than 50 km at low altitudes to ∼100–300 km. Horizontal wavelengths increase from ∼70–100 km to ∼150–500 km over the same altitude range. Median vertical wavelengths, horizontal wavelengths, and periods increase with altitude up to z∼ 100–150 m/s. We also measure the E region horizontal neutral winds and find that they peak at ∼150 m/s near z∼105 km in the middle of the day. Wave phase speeds are in general greater than these ambient winds. In addition, by tracing individual wave packets vertically in altitude, we find that a packets vertical wavelength generally peaks near the altitude where its inferred ion velocity amplitude is maximum. The vertical wavelength generally decreases above this altitude, an observation that is consistent with gravity wave packet theory.

The spread F Experiment (SpreadFEx): Program overview and first results

We performed an extensive experimental campaign (the spread F Experiment, or SpreadFEx) from September to November 2005 to attempt to define the role of neutral atmosphere dynamics, specifically wave motions propagating upward from the lower atmosphere, in seeding equatorial spread F and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12 and the GUVI instrument aboard the TIMED satellite. Initial results of our analyses of SpreadFEx and related data indicate 1) extensive gravity wave (GW) activity apparently linked to deep convection predominantly to the west of our measurement sites, 2) the presence of small-scale GWactivity confined to lower altitudes, 3) larger-scaleGWactivity apparently penetrating to much higher altitudes suggested by electron density and TEC fluctuations in the E and F regions, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent direct links of these perturbations in the lower F region to spread F and plasma bubbles extending to much higher altitudes. Related efforts with correlative data are defining 6) the occurrence and locations of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, the 8) 2D (height-resolved) structures of plasma bubbles, and 9) the expected propagation of GWs and tides from the lower atmosphere into the thermosphere and ionosphere.

Traveling ionospheric disturbances over the United States induced by gravity waves from the 2011 Tohoku tsunami and comparison with gravity wave dissipative theory

The 11 March 2011 Tohoku earthquake generated a massive tsunami off the Pacific coast of Japan, which launched intense atmospheric gravity waves (AGWs) in the atmosphere. Within the context of this study, the Tohoku tsunami event was unique in the sense that it enabled a rare, controlled experiment for investigating how AGWs are launched, propagate, and dissipate in relation to tsunamis. This tsunami was a long-lived, rapidly traveling source of a rich spectra of AGWs excited just above the ocean-atmosphere interface. In this paper we use GPS total electron content (TEC) data from the United States (U.S.) to look for these AGWs in the ionosphere via their signatures as traveling ionospheric disturbances (TIDs). We find a spectrum of TIDs in the TEC data propagating in the direction of the tsunami that last for several hours over the West Coast of the U.S. and as far inland as western Colorado. The observed TIDs have periods that range from 14 to 30min, horizontal wavelengths that range from 150 and 400 km, and horizontal phase speeds that range from 180 to 260m/s. We use reverse ray tracing to show that the Tohoku tsunami was likely the source of these TIDs. Using the networks of GPS receivers in the U.S., we map the tsunami-launched TIDs over the western U.S. and investigate the spectrum of gravity waves enabling an enhanced understanding/verification of the properties of AGWs as a function of the launch angle.