Effects of geomagnetic storms in the lower ionosphere,middle atmosphere and troposphere

Geomagnetic storm effects at heights of about 0–100 km are briefly (not comprehensively) reviewed, with emphasis being paid to middle latitudes, particularly to Europe. Effects of galactic cosmic rays, solar particle events, relativistic and highly relativistic electrons, and IMF sector boundary crossings are briefly mentioned as well. Geomagnetic storms disturb the lower ionosphere heavily at high latitudes and very significantly also at middle latitudes. The effect is almost simultaneous at high latitudes, while an after-effect dominates at middle latitudes. The lower thermosphere is disturbed significantly. In the mesosphere and stratosphere, the effects become weaker and eventually non-detectable. There is an effect in total ozone but only under special conditions. Surprisingly enough, correlations with geomagnetic storms seem to reappear in the troposphere, particularly in the Northern Hemisphere. Atmospheric electricity is affected by geomagnetic storms, as well. We essentially understand the effects of geomagnetic storms in the lower ionosphere, but there is a lack of mechanisms to explain correlations found deeper in the atmosphere, particularly in the troposphere. There seem to be two different groups of effects with possibly different mechanisms—those observed in the lower ionosphere, lower thermosphere and mesosphere, and those observed in the troposphere.     

Dynamical coupling of the auroral F-region ionosphere and thermosphere: case studies

During two 24 h periods of EISCAT observations in the summer of 1982, the F-region ion temperature and density responded differently before and after midnight to large ion convective flows. Such observations were recently reported at Chatanika (Alaska), however, the mechanism invoked to interpret these measurements (large day-to-night variation in electron density affecting the coupling between ions and neutrals) appears insufficient, for summer conditions, to account for the EISCAT observations. Hence, it is proposed, with the support of Fabry-Perot observations and numerical models, that in addition to the electron density asymmetry, the presence of a large southward neutral wind around midnight induces, through Coriolis coupling, a zonal neutral wind of an opposite direction to the convective flow. This enhances considerably the frictional energy and momentum transfer between ions and neutrals in the post-midnight sector.     

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

Lower ionosphere,lower atmosphere and IMF sector structure in winter

There are two basic types of IMF sector boundary crossing effect. One is observed in the IMF magnitude, geomagnetic activity, the nighttime lower ionosphere in high mid-latitudes and in an inverse form in cosmic rays and is reasonably well explained. The other is observed in the noon lower ionosphere in high mid-latitudes and in the troposphere; its interpretation remains open.     

Climatic trends of the mid-latitude upper atmosphere and ionosphere

Long-term variations of electron concentration in the mid-latitude ionosphere, independent of heliogeophysical conditions and the meteorological characteristics of the atmosphere, have been determined. It is suggested that a long-term decrease of atomic and molecular oxygen concentrations in the mid-latitude thermosphere is the most possible reason for the trends found in ionospheric parameters.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

Tides in the middle atmosphere

Some recent progress in the study of tides in the middle atmosphere are reviewed, with special emphasis placed on radar observations at high latitudes, as well as data analysis methods used in the calculation of tidal structures. Observations carried out outside the meteor zone with MST radars and satellites are also presented. Theoretical and numerical advances on the diurnal tide are extensively discussed. Finally, some outstanding problems, which we hope will be solved in the near future are raised: the existence of hemispheric asymmetries in tidal structure; the role played by non-migrating modes at meteor heights and short time scales variations of tides.     

The ionosphere: morphology,development and coupling

The morphology of the MAP/WINE winter is examined, principally from ground-based and satellite observations. Winter anomaly is evident, occurring in bursts with a west to east shift in time. Auroral activity, particularly with reference to the times of major rocket salvoes, is generally low, with Andøya to the south of the auroral boundary in most cases. Minor stratospheric warmings, of which 4 occurred, are found to correlate with minima in radio wave absorption. Salvo R1 was launched during one of the minor warmings.Using data from a broad sector of Europe, coupling between the lower thermosphere and mesosphere is seen over large areas. Westerly winds are associated with high absorption (winter anomaly) and reversal to easterly winds with stratospheric warmings and low absorption. It is found possible to select cases, from amongst the MT series of rocket launchings, corresponding to quiet conditions, stratospheric warming, winter anomaly and particle precipitation in the general absence of other effects. Examining D- and lower E-region ionisation profiles for these caes it is found that, compared with a quiet night, the stratwarm night shows the lower E-region to have reduced ionisation. The ionisation ledge is of similar shape in all cases, but occurs over different height ranges. The observed effects all point to transport being a major factor and the need to measure vertical transport over the range of geophysical conditions examined is highlighted.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

Vertical penetration of planetary waves into the lower ionosphere

Earlier work which provided evidence for coupling between pressure variations in the stratosphere and lower ionosphere in winter has been extended. Day-to-day changes in the height of fixed electron density isopleths in the E-region at a middle latitude often exhibit quasi-oscillations with amplitudes between 2 and 10km and periods between 5 and 30 days. It is found that the correlation between these oscillations and corresponding variations in the height of winter isobaric surfaces in the stratosphere, resulting from the presence of planetary-scale waves, is sometimes good and sometimes poor. Examination of the type of wave disturbance in the stratosphere and of the stratospheric zonal wind profiles suggests that the conditions for stratosphere-ionosphere coupling are met only when well-defined planetary waves of increasing amplitude with height are seen in the lower stratosphere and when the stratospheric zonal wind pattern is favourable to the vertical propagation of such waves.     

Net radiative heating in the middle atmosphere

The meridional distributions of both total solar and net radiative heating rates have been obtained between 30 and 110 km at both the solstice and equinox using Fomichev et al.'s total radiative long wave cooling data in the calculations of the net radiative heating. The contributions to the solar heating of O₃, O₂, CO₂ and H₂O have been investigated. For the ozone heating, the absorption of diffusive solar radiation from the ground and troposphere has been estimated. The 50–90 km layer is close to radiative equilibrium on a globally averaged basis. The importance of radiative cooling as an energy sink in the 90–110 km layer is apparently not less than that of the vertical eddy heat conduction. The ordered meridional circulation has been obtained under the assumption that the temperature variation, due to net radiative heating, is balanced by the adiabatic and temperature variations due to vertical air motion. The circulation model obtained is compared with other empirical models, which are reviewed. For the hemisphere and the 60–80 km layer, the two-cell circulation with the rising motion near the equator and pole from spring to autumn and above 80 km, the one-cell circulation with the sinking motion near the equator and equinox, seem to be most realistic. Also quite realistic for the period near the solstice is the same type of two-cell circulation in the 40–50 km layer and the sinking motion at low latitudes in the 50–60 km layer.     

Electrical structure in the high-latitude middle atmosphere

Middle atmosphere electrodynamics at high latitudes differs significantly from the normally assumed picture of a passive region through which electric fields of external origin couple. Large Vm ⁻¹ electric fields, both horizontal and vertical, have been observed within bounded regions of the upper stratosphere and lower mesosphere. They seem to occur only in regions where the electrical conductivity is a few times 10⁻¹⁰ S m⁻¹ or less and appear to be current limned. While low conductivity is necessary, it is not a sufficient condition for occurrence. The observed large horizontal electric fields were found to be anticorrelated with the local neutral wind. However, a generation mechanism of these electric fields is as yet unknown but must involve space charge separation rather than dynamo effects. Large variations in the conductivity were also observed to occur with fluctuations in magnetic activity, and these were found to be consistent with measured variations in energy deposition during auroral phenomena. Theoretical concepts of mapping of electric fields downward from the thermosphere along equipotential magnetic field lines were shown to hold qualitatively in the D-region at the mV m⁻¹ level. Perturbations affecting such models were determined to be small.     

Summer circulation in the Antarctic middle atmosphere

In November 1982 a partial reflection drifts system for the measurement of winds in the mesosphere and lower thermosphere was installed as part of the New Zealand Antarctic Research Programme at Scott Base (77.8 S, 166.7 E). Ross Island, Antarctica. The wind speed and direction are measured once an hour from echoes available at the time within a height range of 67–97 km. Initial observations made during December 1982, show westward winds between 70 and 90 km, reaching a broad maximum of about 25 m s⁻¹ around 85 km. There is a strong (10 m s⁻¹) meridional component away from the pole at heights of 85–95 km.     

Diurnal and semi-diurnal tides in the equatorial middle atmosphere

As part of the DYANA Programme, six rocket launchings (ship-borne) were conducted on three days in the equatorial region (Indian Ocean/Arabian Sea region). Using the temperature and wind data from these launchings, the diurnal and semi-diurnal tidal components in wind and temperature in the middle atmosphere are obtained and are compared with theoretical predictions. It is found that significant departures occur between the observed and theoretical values. The results are discussed in the light of current theoretical understanding of the tides.     

High resolution turbulence observations in the middle and lower atmosphere by the MU radar with fast beam steerability: preliminary results

Refractive index fluctuations or turbulence in the mesosphere, stratosphere and troposphere are observed with the aid of the fast beam steerability of the MU (middle and upper atmosphere) radar which operates at 46.5 MHz with 1 MW peak radiation power and 8330 m² antenna aperture. Morphology of the mesospheric and stratospheric turbulence is studied by making use of the high altitude and time resolutions. Sixteen beam observations based on the fast beam steerability reveal advection properties and spatial variability of echoing regions in the troposphere. These results demonstrate new possibilities for this system in the investigation of three dimensional structures of turbulence.     

Climatology of gravity waves in the middle atmosphere

An attempt is made at the statistical analysis of small-scale disturbances in the stratosphere and mesosphere with the aid of meteorological rocket observations at many stations from 77°N to 8°S for several years.By applying a high-pass filter to daily rocket data in the height range 20–65 km, wind and temperature fluctuations with characteristic vertical scales close to or less than 10 km are obtained, which are considered to be due to internal gravity waves. Results are expressed in terms of parameters which tend to emphasize smallscale vertical fluctuations and which should provide qualitative measures of gravity wave activity.It is found that the gravity wave activity shows a notable annual cycle in higher latitudes with the maximum in wintertime, while it shows a semiannual cycle in lower latitudes with the maxima around equinoxes. It is also found from the standard deviation around the monthly mean that the temporal variability of gravity waves is very large.     

Diurnal propagating tides in the low-latitude middle atmosphere

Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H₂O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO₂ parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km.     

Climatology of the middle atmosphere of the Southern hemisphere

Main features of spatial distribution and time variations of meteorological parameters in the Southern hemisphere at altitudes 25–80 km are reviewed on the basis of zonal empirical models revised in 1982. Meridional distribution and seasonal variations are analysed. For comparison purposes with the Northern hemisphere, a model developed by Cole and Kantor in 1978 is used. It is revealed that the compilation of new models of the Southern hemisphere atmosphere has not resulted in substantial revision of hemispheric-structure outlined earlier in studies conducted in the Central Aerological Observatory. Meridional distribution of the parameters in summer is characterized by higher values of temperature, pressure and density gradients in the stratosphere of the Southern hemisphere than in that of the Northern hemisphere. This resulted in greater advancement of the core of the summer-time easterly (low towards the equator in the Southern hemisphere than in its northern counterpart. In winter, meridional temperature gradients in the middle stratosphere are greater in the Southern hemisphere than those in the Northern hemisphere, however, rapid attenuation of the gradients with height is observed in the Southern hemisphere, and above 35–40 km they become negative near 50–60°S, in contrast to positive values typical for the Northern hemisphere stratosphere. In the wind field, specific features of the Southern hemisphere westerly flow are high intensity and relatively low altitude of the maximum speed (as compared to the Northern hemisphere).The phases of the annual temperature wave at low latitudes are similar south and north of the equator; south of 30°S a reversal of the phase is observed. The semi-annual oscillation of temperature and wind is less pronounced in middle and high latitudes of the Southern hemisphere than in the Northern hemisphere.The origin of the primary differences between the hemispheres is related mainly to lower intensity of large-scale stratospheric processes in the Southern hemisphere as compared to those in the Northern hemisphere, which is confirmed by values of the standard deviation of the parameters in the two hemispheres.In summer, temperature and pressure fields based on satellite data are symmetric relative to the poles in both hemispheres. In winter, the distortion of the mean pressure field in the mesosphere may be as great in the Southern as in the Northern hemisphere.