Elsayed Rasmy Talaat

Nonmigrating Diurnal Tides in Mesospheric and Lower-Thermospheric Winds and Temperatures

Abstract This paper presents analyses of nonmigrating diurnal tide signatures in High Resolution Doppler Imager mesospheric and lower-thermospheric winds and temperatures. A global comparison of both winds and temperature reveals equatorial features corresponding to nonmigrating tides. Structures interpreted as zonal mean and eastward nonmigrating diurnal tides display consistency between horizontal winds and temperatures. The second symmetric mode is prominent in the zonal mean and wavenumber 1. The gravest antisymmetric mode and the gravest symmetric or Kelvin mode are the main features in zonal wavenumbers 2 and 3. The amplitudes of the tides generally increase with altitude and maximize within 90–110 km.

Observations of the 6.5‐day wave in the mesosphere and lower thermosphere

Previous observations of atmospheric oscillations with zonal wave number 1 have consistently located waves of periods between 5 and 7 days. This study presents a global and seasonal analysis of the 6.5-day wave in High Resolution Doppler Imager daytime mesosphere and lower thermosphere horizontal winds, temperatures, and nighttime atomic oxygen at 95 km. The horizontal structures of all these atmospheric variables are similar to the gravest symmetric wave number 1 Rossby wave, i.e., the (1,1) mode. The seasonal and spatial analysis displays possible modification by the zonal mean wind. Finally, the observed vertical structures of the 6.5-day wave indicate that it is an internal Rossby wave, not an external or Lamb wave.

Thermosphere and ionosphere response to subauroral polarization streams (SAPS): Model simulations

An empirical model of subauroral polarization streams (SAPS) has been incorporated into the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). This SAPS driven TIEGCM is used to simulate the effect of SAPS on the global thermosphere and ionosphere during a moderate geomagnetically active period between day of year (DOY) 329 and 333 in 2008. Model results show: (1) SAPS caused an increase in global thermospheric temperature which became stronger with time. This neutral temperature increase was more significant in subauroral and auroral regions. Joule heating by the SAPS and the redistribution of this heat by dynamic processes were the primary mechanisms for the simulated global neutral temperature changes. (2) In the SAPS driven TIEGCM, the strong ion drag effect in the subauroral SAPS channel drove large changes in thermospheric winds. Zonal neutral winds had either an extra, separate channel of westward flow in the subauroral region in the afternoon-midnight sector or a broad westward wind jet that merged with the regular duskside auroral westward zonal neutral wind driven by the high latitude convection pattern. The exact latitudinal profile of the zonal winds depended on local time. (3) The response of neutral temperature and wind to SAPS was more significant at higher altitudes and exhibited seasonal/hemispheric asymmetry. (4) The heating to the thermosphere by SAPS also resulted in changes in thermospheric composition with upwelling of molecular rich air in subauroral and auroral regions and downwelling of atomic oxygen rich air at other latitudes. These changes in thermospheric composition contributed to the deeper and more extended ionospheric electron density depletions in subauroral middle latitude regions, as well as electron density increases along the equatorward edge of the SAPS channel in the afternoon sector.

An examination of inter-hemispheric conjugacy in a subauroral polarization stream

During geomagnetically disturbed conditions the midlatitude ionosphere is subject to intense poleward directed electric fields in the dusk-midnight sector. These electric fields lead to the generation of a latitudinally narrow westward directed flow channel in the subauroral region called a subauroral polarization stream (SAPS). If the magnetic field lines are treated as equipotentials, electrodynamic events such as SAPS are expected to occur simultaneously at magnetically conjugate locations with similar features. In this paper we present simultaneous observations of a SAPS event in both hemispheres made by midlatitude SuperDARN radars with conjugate fields-of-view. We analyze the relation between the geomagnetic conditions and the characteristics of the channels such as latitudinal location, electric field, total potential variations across the channels, and Pedersen current. The results suggest a strong correlation between the strength of the ring current and the latitudinal location of the channel. An inter-hemispheric comparison of the characteristics of the channel indicates that the potential variations across the channels are similar while the electric fields, Pedersen currents and latitudinal widths of the channel exhibit differences that are consistent with equal potential variations. We attribute these differences to seasonal differences in ionospheric conductivity between the hemispheres and magnetic distortion effects in the inner magnetosphere.

A Spectral Parameterization of Drag, Eddy Diffusion, and Wave Heating for a Three-Dimensional Flow Induced by Breaking Gravity Waves

Abstract There are three distinct processes by which upward-propagating gravity waves influence the large-scale dynamics and energetics of the middle atmosphere: (i) nonlocalized transport of momentum through wave propagation in three dimensions that remotely redistributes atmospheric momentum in both zonal and meridional directions from wave generation to wave dissipation regions; (ii) localized diffusive transport of momentum, heat, and tracers due to mixing induced by wave breaking; and (iii) localized transport of heat by perturbing wave structures due to dissipation that redistributes the thermal energy within a finite domain. These effects become most significant for breaking waves when momentum drag, eddy diffusion, and wave heating— the “breaking trinity”—are all imposed on the background state. This paper develops a 3D parameterization scheme that self-consistently includes the breaking trinity in large-scale numerical models. The 3D parameterization scheme is developed based on the general relat...

First Palmer and Millstone Hill midlatitude conjugate observation of thermospheric winds

The first midlatitude conjugate thermospheric wind observations in the American sector showed various degrees of conjugacy between Palmer (64°S, 64°W, magnetic latitude (MLAT) 50°S) and Millstone Hill (42.82°N, 71.5°W, MLAT 53°N) under three different geomagnetic conditions (recovery after a substorm, moderately active, and quiet). The agreement with the National Center for Atmospheric Researchs Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations also varies with the geomagnetic activity level. During substorm recovery, the observations at Palmer (PA) and Millstone Hill (MH) both showed strong westward zonal winds, which the standard TIEGCM greatly underestimated. Inadequate ion convection pattern size and lack of effect from Subauroral Polarization Streams (SAPS) may be the cause of the large discrepancy. The TIEGCM with a SAPS model produced stronger westward zonal winds near PA but did not change the zonal wind near MH. The empirical SAPS model needs further refinements. In general, there is better conjugacy with moderate geomagnetic activity levels. The TIEGCM also agrees better with the observations. Under geomagnetically quiet conditions, the meridional winds appear to be less conjugate. The agreement between the observations and model is reasonable. Optical conjugate observations are severely limited by the seasons and weather conditions in the two hemispheres. Yet they are necessary to understanding the thermospheric dynamics in the subauroral region and its relationship with geomagnetic activity levels. The comparisons with TIEGCM are necessary for future model improvements.

Diagnostic Analysis of Tidal Winds and the Eliassen–Palm Flux Divergence in the Mesosphere and Lower Thermosphere from TIMED/SABER Temperatures

Abstract For migrating tides or fast-moving planetary waves, polarization relations derived from the linear wave equations are required to accurately derive the wind components from the temperature field. A common problem in diagnosing winds from the measured temperature is the error amplification associated with apparent singularities in the wave polarization relations. The authors have developed a spectral module that accurately derives tidal winds from the measured tidal temperature field and effectively eliminates the error amplification near the apparent singularities. The algorithm is used to perform a diagnostic analysis of tidal winds and the Eliassen–Palm (EP) flux divergence in the mesosphere and lower thermosphere (MLT) based on the zonal mean and tidal temperature fields derived from 6 yr of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satelli...

Effect of short-term solar ultraviolet flux variability in a coupled model of photochemistry and dynamics

Abstract Variability in the solar ultraviolet radiative flux is known to cause changes in the chemistry and dynamics of the middle and upper atmosphere. Specifically, the 27-day solar rotation signal in irradiance has been correlated with responses in temperature and ozone. This study investigates the ozone and temperature responses in the upper stratosphere and mesosphere through analytic formulations and the Johns Hopkins University Applied Physics Laboratory (JHU/APL) 2D chemical–dynamical coupled model. From a simple ozone–temperature coupled analytical model, conditions are derived that would yield the greater sensitivities and negative phase lags in the ozone response as observed in the upper stratosphere. Using the JHU/APL photochemical model, both the diurnal and 27-day solar ultraviolet flux forcings are coupled to examine the effects of localized photochemistry on ozone response. A strong local-time dependence of the ozone response is then systematically explored. The JHU/APL 2D model is integra...

Diagnosis of Dynamics and Energy Balance in the Mesosphere and Lower Thermosphere

Abstract A diagnostic technique has been developed to consistently derive all the dynamical and chemical tracer fields based on one or a few well-measured fields such as temperature and ozone distributions. The technique is based on the new Johns Hopkins University/Applied Physics Laboratory (JHU/APL) globally balanced 2D diagnostic model that couples the dynamics with photochemistry. This model is especially useful for studying the mesosphere and lower thermosphere where dynamics, radiation, and photochemistry strongly interact. The novelty of the diagnostic model is to derive the wave drag and eddy diffusion coefficient directly from the better-defined thermal forcing with its major contributions derived from the zonal mean components. The latter is also affected by the advective and diffusive transports. The derived tracer distributions together with input field(s) provide the necessary radiative and chemical heating rates for the calculation of the thermal forcing. Two numerical experiments with diffe...

Gravity wave feedback effects on the diurnal migrating tide

Abstract Previous studies have suggested the interaction of gravity waves with the diurnal tide could account for some of the semiannual variation observed in the mesosphere and lower thermosphere. This study investigates these effects using the JHU/APL spectral tidal model and a new parameterization scheme for gravity wave induced heating. Further effects on the tides are also examined using diurnal chemical heating from the JHU/APL coupled 2D chemical-dynamical model. Of these processes, gravity wave driven eddy diffusion is found to produce the largest variations in the diurnal tides.