Showing papers by "Arthur D. Richmond published in 2007"

Connections between deep tropical clouds and the Earth's ionosphere

Abstract: [1] We report on a series of simulations with the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) which were designed to replicate and facilitate the interpretation of the longitudinal structure discovered in IMAGE satellite airglow observations of the equatorial ionization anomaly (EIA) at the far-ultraviolet (FUV) 135.6-nm wavelength during March–April 2002 equinox. Our TIME-GCM results indicate that the four-peaked longitudinal variation in the EIA observed by IMAGE-FUV near 20:00 local solar time can be explained by the effects of an eastward propagating zonal wavenumber-3 diurnal tide (DE3) that is excited by latent heat release associated with raindrop formation in the tropical troposphere.

An analysis of the momentum forcing in the high-latitude lower thermosphere

Abstract: [1] We analyze the dynamics of the high-latitude thermsopheric wind system below 170 km for negative IMF Bz by using a fully nonlinear model with a realistic distribution of the forcing. A transition of the forcing patterns and their relative contribution to the high-latitude lower thermospheric wind system occurs around 123 km under various conditions, weak or strong IMF, summer or winter. Winds around and above 123 km are sustained by the gradient-wind balance among divergent/convergent pressure gradient, Coriolis, and horizontal momentum advection (mainly centrifugal) accelerations. Below 123 km winds are maintained by the approximate balance of divergent/convergent pressure gradient, Coriolis, and Hall ion drag accelerations through modified geostrophy. The dominant contribution to the wind tendency (time rate of change) is the rotational component of the ion drag acceleration. The wind tendency above 123 km tends to resemble rotational Pedersen ion drag acceleration well, which reflects a rotated pattern of the E × B velocity. Near and below 123 km the wind tendency is also affected by the rotational component of the Hall ion drag acceleration whose pattern no longer closely resembles the pattern of the E × B velocity, and the wind pattern can differ significantly from that well above 123 km. Simulations for different strengths of the IMF and different seasons indicate that largely divergent/convergent Coriolis and horizontal momentum advection accelerations tend approximately to balance with the horizontal pressure gradient (as well as with divergent/convergent ion drag at lower altitudes) under various conditions. As the forcing increases the radius of curvature of the strong winds also tends to increase, so that the centrifugal acceleration does not increase quadratically with the maximum wind speed, and the tendency for a rough balance between the Coriolis and horizontal momentum advection accelerations in the duskside vortex above 123 km is maintained.

Simulation of equatorial electrojet magnetic effects with the thermosphere‐ionosphere‐electrodynamics general circulation model

Abstract: [1] In this work, the magnetic variations simulated by the NCAR thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) in the vicinity of the magnetic equator are examined to evaluate the ability of this model to reproduce the major features of the equatorial electrojet (EEJ) as observed on the ground as well as on board low-altitude orbiting satellites The TIE-GCM simulates electric currents of various origins and reproduces their associated magnetic perturbations We analyze the diurnal and latitudinal variations of the EEJ magnetic effects calculated on the ground in West Africa under approximately the same solar activity condition as in 1993 for the March equinox and June and December solstices The latitudinal and local time structures of these simulated results correspond well to those that are observed We also compare longitudinal variations of the midday EEJ magnetic perturbations observed by the CHAMP satellite with the model predictions Although the simulations and observations both show multiple maxima and minima in longitude, the locations of these extrema often disagree In the model most of the longitudinal variation of the magnetic variations is associated with nondipolar structure of the geomagnetic field We find that the modeled contributions of the thermospheric migrating diurnal and semidiurnal tides to the magnetic perturbations have large longitudinal variations, and we suggest that an increase in the amplitude of these tides in the TIE-GCM may cause them to play a major role in explaining the morphology of the EEJ longitudinal variation

Dependence of the high‐latitude lower thermospheric momentum forcing on the interplanetary magnetic field

Abstract: [1] We analyze the forces acting on the high-latitude lower thermospheric wind system below 170 km for Southern Hemisphere summer conditions, as a function of the interplanetary magnetic field (IMF) direction, on the basis of numerical simulations. The pattern and magnitude of the forces and their relative contributions to the wind system vary strongly with respect to the direction of the IMF. At higher altitudes, above 130 km, for negative By, strong anticyclonic winds are accelerated primarily by rotational Pedersen ion drag and are maintained by an approximate balance among the divergent/convergent Coriolis, horizontal advection, and relatively weak pressure-gradient accelerations. For positive By, the pressure-gradient acceleration is increased, while the inertial forces are reduced. For negative B z , in comparison with negative and positive By, the winds and forces extend to lower latitudes. The patterns of the accelerations for positive B z are similar to those for negative B z , but the magnitudes tend to be significantly smaller. At lower altitudes, below 120 km, the horizontal advection acceleration is less important but still contributes significantly to the maintenance of the neutral circulation in the polar cap region for positive B y . The difference of winds and forces above 130 km for negative and positive B y , with respect to winds and forces for zero IMF, show a simple structure with a strong anticyclonic or cyclonic vortex near the pole, respectively, centered differently for the two By directions. The difference of winds and forces for negative and positive B z are more complex than those for negative and positive By and extend to lower latitudes. Below 120 km, the difference of winds and forces for negative and positive By are much stronger near the pole than for negative and positive B z , indicating that the IMF By component tends to dominate effects on the neutral winds in the polar cap at low thermospheric altitudes. For all IMF conditions, at higher altitudes, the rotational ion-drag acceleration makes the dominant contribution to the neutral velocity tendency. This feature is most pronounced when the IMF B z is negative.

Modeling seasonal and diurnal effects on ionospheric conductances, region-2 currents, and plasma convection in the inner magnetosphere

Abstract: [1] The present study focuses on a modeling of the seasonal and diurnal effects on the dynamics of the coupled magnetosphere-ionosphere system under different solar illumination conditions, to try to reproduce some of the observations concerning the region 2 (R2) field-aligned currents (FAC). This is performed by introducing in the Ionosphere Magnetosphere Model (IMM) the Earth's rotation axis tilt, the dipole axis tilt and an eccentric dipole. The simulated patterns of the R2 FAC agree rather well with the observations. In particular the dayside FAC density is two times greater in the summer hemisphere than in the winter one. The results show that seasonal tilt of the dipole axis affects the distributions of the Pedersen conductances and FAC by 28.5–52.8% and do not have much influence on the distribution of the magnetospheric plasma, which is modified by only 0.9–3.9%. The diurnal variations induced by the tilt of the dipole axis are of the same order of magnitude, 5.5–29% for the Pedersen conductances and FAC and 0.4–7.1% for the magnetospheric plasma. They modulate the interhemispheric asymmetry of the FAC by 26–59%. The eccentric dipole induces an increase (decrease) of the daily variations of the conductances and the FAC by a factor 1.2–2.3 in the Southern (Northern) Hemisphere, irrespective of the season, which contributes to increase the asymmetry between the two hemispheres. This results in an increase of the daily variations of the ion maximum pressure and of the electron maximum energy flux at the December solstice and at the March and September equinoxes, but a decrease at the June solstice. The maximal variation of the magnetospheric plasma distribution amounts to 6.7% at December solstice. These results underline the importance of considering the three different effects at the same time.

Emulating the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM)

Abstract: We analyse the response of the TIE-GCM computer model of the upper atmosphere to atmospheric tides at its lower boundary and to variations of night-time ionization rates. TIE-GCM is an expensive code, and we choose to use our thirty evaluations of the model to construct an emulator, and then use the emulator for the analysis. TIE-GCM has some non-standard features, namely a periodic input, and modeloutput which is a periodic function of time, and our emulator must account for these. With only a limited number of evaluations our prior choices are important: we model the interaction between two of the inputs explicitly, as well as making informed choices for the emulator’s prior R2 and predictive variance. We validate our choices with detailed predictive diagnostics. Using the updated emulator we are able to quantify TIE-GCM’s response to atmospheric tides and night-time ionization rates.

Thermospheric/Ionospheric Extension of the Whole Atmosphere Community Climate Model (WACCM-X)

Abstract: : A key question in space weather and space environment study is to understand and to quantify the contribution to thermospheric/ionospheric variability by processes from both the lower atmosphere and from the magnetosphere and solar input. The primary goal of this project is address this question by developing a model that encompasses the entire atmosphere, from the ground/ocean to the upper thermosphere. We will achieve this goal by extending the NCAR Whole Atmosphere Community Climate Model into the thermosphere and ionosphere (WACCM-X), making use of the physics and many of the algorithms of the National Center for Atmospheric Research Thermosphere-IonosphereMesosphere-Electrodynamics General-Circulation Model (NCAR TIME-GCM). Such a seamless whole atmosphere model will enable us to self-consistently study how elements in the coupled upper atmosphere/ionosphere system interact with one another and to determine how this coupled system responds to the variable energy input from the sun and the complex interactions between the lower atmosphere/ocean and the middle and upper atmosphere. The information from this research will be useful for ONR to develop a seamless operational model that simulates the present day structure and dynamics of the thermosphere-ionosphere-mesosphere-lower atmosphere-system including its response to solar variability and global change.