H. G. Mayr

Magnetic storm effects in the neutral composition

The thermospheric wind circulation, excited during magnetic storms presumably by joule heating within the auroral zone, is shown to be an effective mechanism for removing atomic oxygen at high latitudes. Wind induced variations in O exceed the temperature effects up to 250 km. The calculated depletion is most pronounced at around 180km where the density can decrease by as much as a factor of two, consistent with the observed storm time variations in the ionosphere. At higher altitudes, this effect is canceled by the thermal expansion in atomic oxygen thus explaining the negligible response in the concentration of this atmospheric constituent under disturbed conditions when N2 increased by as much as a factor of ten.

Ionospheric storm effects at subauroral latitudes: A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large-scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

An equatorial temperature and wind anomaly (ETWA)

Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE-2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl-shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion-neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

Thermospheric gravity waves: observations and interpretation using the transfer function model (TFM)

Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.

Equatorial oscillations in the middle atmosphere generated by small scale gravity waves

A realistic parameterization scheme for the deposition of gravity-wave momentum in the middle atmosphere has been incorporated into the 2D version of a global-scale Numerical Spectral Model of the Earths middle atmosphere. Here we present early results, obtained with only the simplest assumptions for the incident gravity-wave spectrum—that it is azimuthally isotropic (i. e., identical flux in the four cardinal directions), globally uniform, and unchanging with season—and with essentially “untuned” values of tunable parameters. This model reproduces reasonably well the observed anomalous latitudinal temperature distribution and the zonal circulation of the upper mesosphere during solstice, just as other models do. It also produces relatively large oscillations in the mean zonal circulation of the middle atmosphere at low latitudes, descending in altitude with time. In the mesosphere and upper stratosphere, the dominant period is semi-annual and the maximum amplitude is about 20 m/s near a height of 50 km. At lower levels, the dominant period is about 20 months and the maximum amplitude is about 8 m/s near 25 km. These values resemble those associated with the observed Semi-Annual Oscillation and Quasi-Biennial Oscillation, respectively, leading us to conclude that small scale gravity waves may contribute significantly to both.

A theoretical model of the ionosphere dynamics with interhemispheric coupling

Abstract The ionospheric plasma is described by means of the momentum and continuity equations for O + , He + , H + and the energy equations for T e and T i . The ion production rate profiles and the photoelectron heating rates to the plasma are computed separately and serve as source functions in the continuity and energy equations respectively. The neutral atmosphere (composition, winds, temperature) as well as the incident photon spectrum are part of the inputs. Charge transfer, collisional energy loss processes, ion-neutral drag, diffusion and electron heat conduction are among the physical processes included in the calculations. Ion heat conduction is neglected. Assuming steady state, the equations are transformed into integral equations and then solved iteratively along geomagnetic field lines from the region of chemical equilibrium up to the equatorial plane. In this scheme non-local heating is included in a self-consistent way. The lower boundary conditions are obtained by satisfying photochemical and local energy equilibrium. Under conditions of asymmetry with respect to the equatorial plane the solutions are carried out for both hemispheres, and the upper boundary conditions are determined by requiring continuity of the physical parameters across the equatorial plane. In this way inter hemispheric plasma transport is introduced in a natural way. For symmetric conditions the transport fluxes are assumed zero at the Equator. An extension of this model to include time-dependent phenomena is presented.

The gravity wave Doppler spread theory applied in a numerical spectral model of the middle atmosphere .2. Equatorial oscillations

Mayr et al. [this issue] discussed a two-dimensional version of the numerical spectral model (NSM) of Chan et al. [1994a, b] that incorporates the Doppler spread parameterization (DSP) for momentum deposition by small-scale gravity waves (GW) developed by Hines [1997a, b] and presented numerical results describing the global scale seasonal variations in the temperature and wind fields of the middle atmosphere. Even with the simplest assumptions for the GW flux emanating from the troposphere, to be isotropic and independent of latitude and season, this model also produces significant oscillations in the equatorial zonal circulation which are discussed here. Our model results lead to the following conclusions: (1) At altitudes above 40 km, a periodicity of 6 month dominates, resembling the observed semiannual oscillation (SAO). The peak amplitude of this oscillation is close to 18 m/s near 50 km (20-30 m/s observed). A secondary maximum is excited near 80 km with an amplitude of about 11 m/s (15-25 m/s observed), whose phase is opposite to that at 50 km. In this altitude range, the downward phase progression is about 9 km/month, in agreement with observations. The computed SAO is confined to equatorial latitudes, as observed. (2) At altitudes below 40 km, the period of the computed oscillation is almost 21 months, approaching that of the observed quasi-biennial oscillation (QBO). The maximum wind amplitudes are close to 8 m/s (20 m/s observed), and the downward phase progression is about 1.6 km/month (1.3 km/month observed). The model also produces a QBO in the upper mesosphere, in qualitative agreement with recent UARS measurements [Burrage et al., 1996]. (3) When the eddy diffusivity is reduced by a factor of two, the QBO period increases to 30 months and the maximum wind amplitude approaches 13 m/s. Computer experiments are discussed for constant, equinoctial solar heating to elucidate the GW excitation mechanism for the equatorial oscillations in the zonal circulation.

A three-dimensional model of thermosphere dynamics—I. Heat input and eigenfunctions

Abstract A three-dimensional model of thermosphere dynamics is developed in terms of the eigenfunctions of the atmospheric system. Formulae for the external heat inputs like solar XUV-radiation and corpuscular heating during geomagnetic storms are derived in terms of these eigenfunctions. Those series contain tidal components (depending on local time) and planetary components (depending on seasonal time) which are the generators of tidal and planetary neutral atmospheric waves. The relative importance of corpuscular heating when compared with XUV-heating is estimated. Approximate analytic solutions for the generation and propagation of the atmospheric waves within the dissipative thermosphere excited by the solar heat input are given. It is shown that the eigenfunctions which are the Hough-functions within the nondissipative lower atmosphere change into the spherical surface functions within the dissipative thermosphere. Moreover, the density amplitudes of the wave modes decrease proportional to 1 n 2 where n is the zonal wave domain number of the spherical harmonics. Therefore, only the wave modes with low wave domain numbers n are significant at thermospheric heights.

Thermosphere and F-region plasma dynamics in the equatorial region

Abstract The dynamics of the equatorial thermosphere and the F-region ionospheric plasma are reviewed highlighting several features observed with in-situ satellite and ground-based experiments. Attention is given to the midnight temperature maximum (MTM) and related phenomena and to recent results on zonal neutral and plasma flows at F-region heights. The midnight temperature maximum and its midnight pressure bulge above 250 km altitude lead to neutral wind variations which significantly affect the F-region equilibrium height and its airglow emissions. During magnetically active periods, enhanced meridional winds from the poles lead to strong meridional intensity gradients (MIGs) in the atomic oxygen emission at 6300A; MIGs have been used to estimate the magnitude of meridional wind gradients during active periods, and these estimates are consistent with measurements using incoherent scatter radar and optical Fabry-Perot interferometry. The pressure gradients which drive the thermospheric wind have been estimated using averaged density and temperature data, and the results have been used to check the consistency of the current data base in terms of the momentum equation. New analyses of the AE-E data are presented as further evidence of the effect of the MTM on the latitude-local time distribution of the meridional wind reversal. The tidal decomposition studies of the neutral temperature and of both ion and neutral flows are reviewed. The zonal plasma flow is found to be closely coupled to the zonal neutral wind as a consequence of the F-region dynamo, and more recently, the F-region dynamo has been found to play an important role in an anomaly in the latitudinal distribution of the equatorial zonal plasma flow.

Magnetic storm dynamics of the thermosphere

Abstract A theoretical study of the Dst component of magnetic storms is presented. The dynamic characteristics are found significantly different for Joule dissipation and electron precipitation, leading to the conclusion that the former is probably the predominant heat source for the upper thermosphere. Composition measurements on OGO-6, which reveal markedly different characteristics in N 2 , O and He, can be explained on the basis of energy adveetion and diffusive mass transport by thermospheric winds. Essential features in the F 2-region response are explicable in terms of these dynamic processes. Electric field induced motions are estimated and it is concluded that resultant adiabatic heating could be significant.