Abstract. A time-dependent, nonlinear, fully compressible, axisymmetric, f-plane, numerical model is used to simulate the generation of small-scale gravity waves in the upper mesosphere and lower thermosphere by intense deep convection in the troposphere. The simulations show that major convective storms in the tropics excite a broad spectrum of upper mesosphere-lower thermosphere gravity waves above the storm centers. The wave field includes a component that is guided in a thermal duct in the lower thermosphere and propagates horizontally outward from above the storm. Storms excite oscillations over the source which are initially confined to a stratospheric duct but leak into the thermospheric duct over time generating a long train of small-scale-ducted waves. This ringing phenomenon persists for several hours after the storm has ended. The ducted disturbances may propagate large distances from the source and explain observations of a strong summertime anisotropy favoring southward propagation of small-scale waves observed in the airglow over Adelaide more than 2000 km to the south of the storm events.
[1] A time-dependent, nonlinear, fully compressible, axisymmetric, f-plane, numerical model is used to simulate the propagation of acoustic waves in the mesosphere and thermosphere by intense deep convection in the troposphere. The simulations show that major convective storms in the tropics launch acoustic waves into the mesospherethermosphere directly above the storm centers. The principal feature of the overhead acoustic wave field in the period interval of $3 to 5 min is a trapped oscillation below about 80 km altitude with a period of $5 min and a nearly vertically propagating wave with about a 3-min period above this height. Acoustic oscillations in the troposphere-mesosphere duct have a standing wave character directly over the storm; the oscillations that propagate off vertical mainly reflect off the top and bottom of the duct resulting in a horizontally propagating acoustic field within the duct. The acoustic oscillations that are mainly confined to the troposphere-mesosphere duct persist for about an hour after the storm has ended. The vertically propagating thermospheric acoustic oscillations are waves propagating upward from the thunderstorm source through the stratosphere-mesosphere. These predominantly 3-min waves are strongly driven for $30 min after the storm event and weaken with time thereafter. The vertically propagating thermospheric acoustic waves may be the source of the F-region 3-min oscillations. Intense acoustic disturbances directly above thunderstorms may be responsible for localized heating of the thermosphere.
[1] We have used a model of thermospheric gyres with simplified geometry (azimuthally symmetric cylindrical coordinate) to study dynamical adjustment for high-latitude gyres spun-up into rapid motion by ion drag. In our simulations, winds are spun-up for an hour subject only to circumgyre ion drag forcing from strong radial electric fields with peak values of ±75 mV m À1 centered at a radial distance of 1500 km. The winds in the core of the jets approach 500 m s À1 . The major finding is that the imbalance between the inertial forces (centrifugal and Coriolis) and the pressure gradient force during the spin-up of high-latitude gyres drives a strong radial circulation, and this circulation is a significant contributor to the radial circulation forced by all sources. This agradient circulation attempts to establish a gradient-wind balance between the inertial and pressure gradient forces. A feature of general circulation model simulations of the high-latitude lower thermosphere is that, subtracting the warming of the whole polar cap due to Joule heating, the counterrotating gyres driven into motion by ion convection have opposite thermal polarities, with the cyclonic gyre developing a cold core (low density) and the anticyclonic gyre a warm core (high density). We suggest that this is accomplished by the radial circulation forced by the aforementioned imbalance between the inertial and pressure gradient forces. While diabatic heating over the polar cap acts to elevate the temperature (raise the density) over the polar cap as a whole, the changes induced by the dynamically induced circulation account for the fact that cyclonic gyres in the lower thermosphere are relatively colder and denser and the anticyclonic gyres are relatively warmer and less dense. We also examine the radial circulation forced by the initial stages of spin-down and show that the spin-down circulation is significant.
An established high‐resolution dynamical model is employed to understand the behavior of the thermosphere beneath the Earth's magnetic cusps, with emphasis on the factors contributing to the density structures observed by the CHAMP and Streak satellite missions. In contrast to previous modeling efforts, this approach combines first principles dynamical modeling with the high spatial resolution needed to describe accurately mesoscale features such as the cusp. The resulting density structure is shown to be consistent with observations, including regions of both enhanced and diminished neutral density along the satellite track. This agreement is shown to be the result of a straightforward application of input conditions commonly found in the cusp rather than exaggerated or extreme conditions. It is found that the magnitude of the density change is sensitive to the width of the cusp region and that models that can resolve widths on the order of 2° of latitude are required to predict density variations that are consistent with the observations.
Measured E region neutral winds from the Atmospheric Response in Aurora (ARIA 1) rocket campaign are compared with winds predicted by a high-resolution nonhydrostatic dynamical thermosphere model. The ARIA 1 rockets were launched into the postmidnight diffuse aurora during the recovery phase of a substorm. Simulations have shown that electrodynamical coupling between the auroral ionosphere and the thermosphere was expected to be strong during active diffuse auroral conditions (Walterscheid and Lyons, 1989). This is the first time that simulations using the time history of detailed specifications of the magnitude and latitudinal variation of the auroral forcing based on measurements have been compared to simultaneous wind measurements. Model inputs included electron densities derived from ground-based airglow measurements, precipitating electron fluxes measured by the rocket, electron densities measured on the rocket, electric fields derived from magnetometer and satellite ion drift measurements, and large-scale background winds from a thermospheric general circulation model. Our model predicted a strong jet of eastward winds at E region heights. A comparison between model predicted and observed winds showed modest agreement. Above 135 km the model predicted zonal winds with the correct sense, the correct profile shape, and the correct altitude of the peak wind. However, it overpredicted the magnitude of the eastward winds by more than a factor or 2. For the meridional winds the model predicted the general sense of the winds but was unable to predict the structure or strength of the winds seen in the observations. Uncertainties in the magnitude and latitudinal structure of the electric field and in the magnitude of the background winds are the most likely sources of error contributing to the differences between model and observed winds. Between 110 and 135 km the agreement between the model and observations was poor because of a large unmodeled jetlike feature in the observed winds (140 m s-1). Agreement between the present simulation and the earlier simulations of Walterscheid andLyons (1989) is favorable, although the winds in the present simulation are considerably weaker for particle precipitation of similar characteristic energy and flux. The reasons for the difference were the smaller latitudinal extent of the model diffuse aurora and the weaker electric fields in our simulation. We have shown that the enhanced electron densities and electric fields associated with the postmidnight diffuse aurora provide the potential for a rapid acceleration of the zonal winds as shown by Walterscheid and Lyons (1989). However, the modeled response to the large-scale electric field is too great. This suggests that the assimilated mapping of ionospheric electrodynamics (AMIE) electric field is also too large. The actual electric field is most likely reduced locally in regions of enhanced ionization and conductivity within the diffuse aurora. In addition, we have shown that the "exotic" jetlike wind feature between 110 and 13...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.