On the formation of atmospheric inhomogeneities in the solar terminator region

The present work is a systematic review cum report on theoretical and experimental investigations of the Solar Terminator (ST) as a physical, permanent, and global source of waves in near-Earth space (NES). From the founding work by Beer (1973) Nature242, 34–35 and Somsikov and Troitskii (1975) Geomagn. and Aeron., 15, 856–860, up to the present, twenty years of theoretical and experimental investigations on the effectiveness of wave generation in various geophysical situations have been considered. Linear mechanisms of atmospheric inhomogeneity formation are considered for the case of a moving discontinuity in some of the atmospheric parameters, whereas nonlinear mechanisms are characterized by various instabilities caused by the gradient in solar radiation absorption. Numerical simulations of an ST model and the pressure wave field generated by it are discussed.     

Weak nonlinear theory of the ionospheric response to atmospheric gravity waves in the F-region

The nonlinear ionospheric response to atmospheric gravity waves is studied in an approximate fashion using a new approach. The concept of nonlinear travelling ionospheric disturbances (TIDs) is outlined, and the nonlinear behaviour of atmospheric gravity waves is calculated. A principal result is that harmonics are generated which cause the wave velocity perturbation to deform. The ionospheric response is investigated by solving the continuity equation for ionization in the F-region. The distortion of the TIDs waveform produced by the nonlinear interactions is depicted. The nonlinear TIDs depart seriously from a cosinusoidal wave described by previous linear TID theory. The distorted TIDs appear as ‘sharp peak’ and ‘sawtooth’ waveform shapes. The ‘peaks’ can be upward or downward, and the ‘sawteeth’ forward or backward, depending on the wave parameters. The nonlinearly distorted TIDs show a good agreement with various observed ionospheric irregularities produced by atmospheric gravity waves.     

The effects of non-linearity on thermal tides in a 3-D numerical model

Absorption of solar radiation is taken as the cause of atmospheric tides, which are simulated by a global, 3-D primitive equation model for the altitude region 0 km–120 km. To investigate non-linear effects, two model versions are used, one with the complete non-linear equation set, the other with the linear equation set. Tide simulations are then performed under the same conditions (background atmosphere and radiation for solstice) with both model versions and directly compared. The diurnal tide can be regarded as a linear phenomenon, whereas the semi-diurnal tide is modified in lower latitudes above 65 km by non-linear effects. Due to the interaction of the tidal components with the background wind, a strong westward zonal flow is generated in the non-linear model above 70 km.     

Interaction of solar wind with the magnetopause-boundary layer and generation of magnetic impulse events

The transport of mass, momentum, energy and waves from the solar wind to the Earth's magnetosphere takes place in the magnetopause-boundary layer region. Various plasma processes that may occur in this region have been proposed and studied. In this paper, we present a brief review of the plasma processes in the dayside magnetopause-boundary layer. These processes include

• 1.(1) flux transfer events at the dayside magnetopause,
• 2.(2) formation of plasma vortices in the low-latitude boundary layer by the Kelvin-Helmholtz instability and coupling to the polar ionosphere,
• 3.(3) the response of the magnetopause to the solar wind dynamic pressure pulses and
• 4.(4) the impulsive penetration of solar wind plasma filaments through the dayside magnetopause into the magnetospheric boundary layer. Through the coupling of the magnetopause-boundary layer to the polar ionosphere, those above processes may lead to occurrence of magnetic impulse events observed in the high-latitude stations.

     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 1. Theoretical model

A simplified full-wave method adapted to the propagation of very obliquely incident LF radio waves is developed. For a selected ionosphere model the wave-field structure is calculated inside a horizontally stratified ionosphere and the peculiarities of the reflected field are clearly described. The penetration of the investigated radio waves in the lower ionosphere at noon-time is found to be restricted to a layer several wavelengths thick. The reflected wave is created entirely by the mechanism of partial reflections and the region responsible for its formation is usually below 70 km. The influence of some typical parameters of the electron density profile, as well as the atmospheric pressure and temperature, on the attenuation of the investigated radio waves is demonstrated. It is also found that the reflection at very oblique incidence depends mainly on the height of the bottom of the ionosphere.     

The isobaric F2-layer

Cyclic diagrams, obtained by plotting the daily variation of the ionospheric electron density NmF2 against the height hmF2, are drawn for typical conditions at Slough (52°N) and Watheroo (30°S). Using the MSIS86 thermospheric model to relate the heights hmF2 to values of atmospheric pressure, the F2-peak is found to lie at nearly the same pressure-level at any given local time, over a wide range of geophysical conditions (season, solar cycle, magnetic disturbance). As local time varies, the pressure level corresponding to hmF2 varies in a way that is mainly determined by the local time variation of the thermospheric winds. This is verified for noon and midnight, using the MSIS86 model to compute the winds. The noon values of peak electron density (NmF2) are fairly consistent with theory (using values of solar ionizing flux as quoted in the literature), but with some discrepancies—particularly at sunspot maximum—that are probably due to uncertainties in chemical composition, or to the effects of vibrational excitation of molecular nitrogen. Overall, the analysis shows a remarkable consistency between ionospheric theory, the data and the MSIS model.     

Variations of mean winds and tides in the upper middle atmosphere over a solar cycle,Saskatoon, Canada, 52°N, 107°W

Saskatoon (52 N, 107 W) medium frequency (MF) radar data from 1979 to 1990 have been analyzed to investigate the solar activity effects on upper middle atmospheric winds and tidal amplitudes. The period of study covers two solar maxima and a solar minimum; the continuous data allow a systematic analysis of solar cycle dependence on mean winds and tides. The height region of 79–97 km sampled in the study shows an apparent but very weak dependence of mean winds and tidal amplitudes on solar activity variation. The observed features are fairly consistent with the early results reported by Sprenger and Schmindkr [(1969) J. atmos. terr. Phy. 31, 217). The mean zonal wind and the semidiurnal tidal amplitudes appear to exhibit positive and negative correlations with the solar activity, respectively; the statistical significances of these correlations are generally low. There is a biennial periodicity evident in the zonal wind oscillations but this docs not have a consistent phase relationship with the equatorial stratospheric wind oscillations (QBO). The meridional winds and the tidal amplitudes are characterized with different and quite irregular periods of oscillations (2–5 yr). The diurnal tidal variations over the solar cycle are small and irregular, although amplitudes are slightly larger during the solar minimum years.     

A multistation study of long period geomagnetic pulsations at cusp and boundary layer latitudes

The characteristics of 1–20 mHz (Pc5) geomagnetic pulsations recorded during the daytime on the ground at cusp and boundary layer latitudes have been examined. On quiet and moderately disturbed days the major spectral contributions are due to three different mechanisms. Sustained oscillations whose properties are consistent with the Kelvin-Helmholtz instability at the low latitude boundary layer are the dominant mechanism at −70 to −75 geomagnetic latitude. Transient irregular pulsations are frequently seen at single stations at the foot of polar cap and boundary layer field lines. Occasionally similar transients occur essentially simultaneously at widely spaced stations accompanied by absorption spikes on riometer records. The latter signals are most likely due to solar wind pressure pulses on the magnetopause. At cusp latitudes the major spectral contribution arises from sustained irregular pulsations centred on magnetic noon. Although their occurrence is related to the proximity of the cusp's particle signature, it may be more appropriate to discuss these signals in terms of fluctuations in boundary layer or mantle currents.     

Multiple molecular scattering and albedo action on the solar spectral irradiance in the region of the UVB ( ⩽ 320 nm) : a preliminary inventory

Solar UVB, a fundamental element in our environment, was measured with cadmium cells by Paul Götz in Arosa more than sixty years ago and described in his book entitled : Das Strahlungsklima von Arosa [Götz (1926). Springer, Berlin]. Afterwards, in order to ensure uniformity in field experiments, he introduced in his atmospheric measurements a chemical sensor, the Bioclimatic Ultraviolet Dosimeter. This dosimeter, by its cylindrical form, was adapted to an instantaneous measurement of the global UVB radiation at different sites. The global radiation embraces the whole of the group of direct solar irradiances with molecular scattering (sky radiation) and ground reflection (albedo) together with its scattered spectral component.Numerical results from detailed theoretical calculations aimed at evaluating the various absolute effects associated with height, solar zenith angle and surface albedo have been obtained for the standard atmosphere. The variations with solar zenith angles from 0 to 90 and albedos between 0 and 1 are presented for a spherical terrestrial atmosphere at selected wavelengths between 301 and 325 nm in the UVB region     

Influence of solar flares and disturbances of the interplanetary medium on the atmospheric circulation

The atmospheric circulation response to solar flares and geomagnetic disturbances is studied. It is shown that solar flares cause an increase of the zonal circulation intensity in the latitudinal belt ϑ = 45–65°, while geomagnetic disturbances are associated with a decrease of the circulation intensity.The energy necessary to produce the observed atmospheric circulation changes is estimated. The variation of the atmospheric transparency is suggested to be a possible source of that energy.     

Seismic Assessment of a Historical Masonry Building in Switzerland: The “Ancien Hôpital De Sion”

This article presents the results of the evaluation of the seismic safety of the Ancien Hôpital de Sion, an important Swiss architectural heritage building, situated in the Canton of Valais, the region with the highest seismic hazard in Switzerland. Three-dimensional Applied Element (AEM) modeling of the whole structure has been performed and validated. The adopted modeling strategy, together with nonlinear dynamic analysis, was able to represent the actual behavior and failure mechanisms typical of complex masonry structures, in addition to a good computational efficiency compared to other available numerical approaches. The local collapse mechanisms have been also studied through a kinematic limit analysis based on rigid block rotation. Both linear and nonlinear approaches have been followed together with the capacity spectrum method. The results provided by the different methodologies have been compared with the aim to provide possible insights concerning a general procedure for the assessment of the safety of such type of structures.     

Interannual fluctuations of the stratospheric temperature over the north polar region

The monthly means for the years 1964–1991 of 30 hPa temperatures over the North Pole and averaged over the 70–90°N region are analyzed. A multiple regression model is used to find long-term monthly trends and possible linear associations between these temperatures and the QBO, ENSO, and the 11-yr solar cycle. The model's residuals are examined for detection of other periodic interannual fluctuations in Arctic temperatures.It is found that the interannual variations of temperature at 30 hPa over the Arctic are a superposition of the oscillations due to the QBO, ENSO, the 11-yr solar cycle, and approximate 6-yr periodic fluctuations of unknown origin. The QBO, ENSO, and the solar cycle effects in the Arctic temperature explain about 35% of the total variance of the temperature monthly anomalies. In winter, the QBO, ENSO, and the 11-yr solar cycle signals in the temperature data depend on the phase of the equatorial QBO. The polar vortex seems to be warmer (colder) than normal when the West (East) phase of the equatorial QBO in a period of high solar activity. The monthly temperature trends over the Arctic show seasonal variations with positive trends in February and March. The year-round trends (sum of the monthly trends) are about −0.5 K per decade.     

The effects of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes

The influence of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes is investigated. A 3-dimensional neutral wind model is developed and used together with an atmospheric model in a gravity-wave ray-tracing analysis. It is demonstrated that the thermospheric wind can act as a filter for waves travelling at unfavorable angles to the mean flow, via the mechanisms of reflection and critical coupling. This wind filtering action rotates clockwise diurnally through 360° in the northern hemisphere. Observational evidence is presented which supports these predictions. Extensive modelling indicates that (a) faster and longer period waves are least affected by the neutral winds and (b) fixed-height (e.g. HF Doppler) observations of medium scale gravity waves is only likely to be possible for waves generated locally (within 500–1000 km). Waves generated at greater distances are probably dissipated before reaching the observation region.     

Global change in the mesosphere-lower thermosphere region: has it already arrived?

This tutorial review describes some possible future scenarios for changes in temperature and water vapor in the mesosphere-lower thermosphere (MLT) region (50–100 km). The structure and dynamics of this region are controlled by physical processes, some of which are very different than in the lower atmosphere, such as gravity-wave breaking, radiative transfer in non-local thermodynamic equilibrium and airglow cooling. The couplings between the various atmospheric properties are illustrated by the use of a 2D zonally-symmetric model ranging from 16 to 120 km. The importance of temperature and water vapor for the occurrence and scattered brightness of mesospheric clouds (at a height of about 83 km) is described in terms of their influence on nucleation, growth and sedimentation of ice particles. At the cold mesopause at high latitude, IR effects would warm the region without dynamical feedbacks, which in the 2D model to be described, cause a net cooling at all latitudes and seasons. The effects of a future doubling of carbon dioxide and methane (and a past halving) are examined by means of the same 2D model. All models predict a future lowering of temperature throughout much, if not all of the MLT region, as a result of enhanced IR cooling and dynamical feedbacks. The rise of methane will lead to an enhancement of water vapor concentrations throughout the upper atmosphere. The cloud existence region, defined in terms of water-ice saturation, is predicted to extend to lower-latitude, high population areas in the future. In a glacial-era scenario, the existence region is found to be confined to a small region near the summertime polar mesopause. Over the past century, with a doubling of methane and a 30% increase in carbon dioxide, the mesospheric cloud existence region may, have advanced from near the pole to its current location inside the 50°–90° latitude zone. The uncertainties in current models and need for further studies are discussed.     

A nonlinear simulation of the thermal diurnal tide

A simplified middle atmosphere general circulation model is used to investigate the nonlinear behavior of the thermal diurnal tidal waves. In the model, only a westward moving diurnal tide generated by heating with zonal wavenumber 1 is considered. The tidal wave propagation is simulated by a full nonlinear calculation with a convective adjustment scheme and a Richardson number dependent vertical eddy diffusion.The numerical results show that the growth of the diurnal tide due to the density stratification is effectively suppressed and a relatively constant amplitude distribution with height is realized by the convective adjustment in the lower thermosphere. It is also shown that mean zonal winds and mean meridional circulations are induced by the diurnal tidal waves in the region where the tidal waves are breaking by convective instability, in accordance with the wave-mean flow interaction theorem.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

Energy deposition by precipitating keV range protons and neutral hydrogen atoms into the equatorial atmosphere: dependence on the neutral atmosphere model

The problem of the precipitation of keV range protons and neutral hydrogen atoms into the equatorial atmosphere is studied by the Monte-Carlo technique, and the energy deposition is calculated for different atmospheric models corresponding to low, medium and high solar activity conditions. It is found that the total energy deposited is independent of the atmospheric model and is equal to 16% of the incident energy, but the energy deposition profile varies. The dependence of the profile parameters on the neutral atmospheric model is examined in detail. An empirical model of the Chapman type distribution, involving only the neutral atmospheric density, has been developed to represent the energy deposition profile for any solar activity condition.     

The influence of high speed plasma streams on the ionospheric plasma

As shown by statistical investigations, high speed plasma streams (HSPS) in the solar wind cause direct ionospheric effects in the D- and Es-layers at auroral and subauroral latitudes due to increasing precipitation of high energetic particles as well as indirect effects in the F2-region at high, middle and equatorial latitudes caused by auroral heating processes. The ionospheric effects increase with the strength of the HSPS and are most pronounced for HSPS during IMF pro sectors (sectors with negative B_z-component). Seasonal differences of the ionospheric response to solar velocity changes are caused by the IMF influence (maximum effect at equinoxes) as well as internal atmospheric reasons (enhanced variability during winter).     

Electric field generation at the magnetospheric boundary for northward IMF

We discuss three different processes which generate electric fields at the magnetopause during northward interplanetary magnetic field (IMF) conditions. These are (1) Petschek-type magnetic field reconnection, (2) magnetic field diffusion, and (3) viscous-like interaction resulting from the Kelvin-Helmholtz instability. For northward IMF all three processes lead to the formation of a boundary layer on closed magnetic field lines adjacent to the magnetospheric boundary. The thickness of the boundary layer depend on Petschek's parameter in the first case, the magnetic Reynolds number in the second case, and an effective Reynolds number in the third case. In each case coupling between the boundary layer and the ionosphere occurs via field-aligned currents. These field-aligned currents result from the penetration into the polar ionosphere of the electric field generated at the magnetospheric boundary. These currents are closed by a transverse current in the boundary layer and the associated Lorentz force causes a decrease of the kinetic energy of the solar wind plasma inside the boundary layer. As a result of this velocity decrease the thickness of the boundary layer increases on both flanks of the magnetosphere near the equatorial plane. The convergence of the boundary layer on the dawn and dusk sides leads to antisunward plasma flow in the magnetospheric tail.     

Recent results in auroral-zone scintillation studies

Nighttime data from the Defense Nuclear Agency's Wideband satellite have consistently shown a pronounced scintillation enhancement at the point where the propagation vector lies within the local L-shell. Simultaneous observations at two well-separated stations and spaced-receiver measurements have shown that this feature is caused by a latitudinally narrow, unstable F-region ionization enhancement that produces sheet-like intermediate-scale structures. A more detailed characterization of the source region has been derived from simultaneous measurements from the TRIAD satellite and the Chatanika radar.The source region is typically convecting southward, whereby the poleward gradient at its southern boundary is stable to the gradient-drift instability. Thus, Ossakow and Chaturvedi (1979) postulated the current-convective instability as a source mechanism. Birkeland currents, when sufficiently intense, can destabilize a region that is otherwise stable to the gradient-drift instability.The sheet-like anisotropy of the irregularities is perhaps the most intriguing feature of the instability. Two mechanisms have been postulated to explain it. This paper reviews the development and current status of our understanding of this recently discovered high-latitude instability.