J. Röttger

An updated review of polar mesosphere summer echoes: Observation, theory, and their relationship to noctilucent clouds and subvisible aerosols

Peculiar atmospheric radar echoes from the high-latitude summer mesosphere have spurred much research in recent years. The radar data (taken on frequency bands ranging from 2 to 1290 MHz) have been supplemented by measurements from an increasing arsenal of in situ (rocket borne) and remote sensing (satellites and lidars) instruments. Theories to explain these polar mesosphere summer echoes (PMSEs) have also proliferated. Although each theory is distinct and fundamentally different, they all share the feature of being dependent on the existence of electrically charged aerosols. It is therefore natural to assume that PMSEs are intimately linked to the other fascinating phenomenon of the cold summer mesopause, noctilucent clouds (NLCs), which are simply ice aerosols that are large enough to be seen by the naked eye. In this paper we critically examine both the data collected and the theories proposed, with a special focus on the relationship between PMSEs and NLCs.

Wave-like structures of large-scale equatorial spread-F irregularities

Abstract By means of non-great-circle propagation on the transequatorial path Lindau-Tsumeb, the equatorial spread- F occurrence in the African area is investigated. The observations yield the spatial and temporal distribution, the mean downward velocity, and the distribution of the eastward velocity of equatorial spread- F irregularities. Irregularity patches are found to be tilted to the east. Quasi-periodical structures in west-east extension are observed and can be explained by atmospheric gravity waves.

The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off-the-shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State-of-the-art exciter and receiver hardware has been developed in-house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron-induced spurious Doppler effects from the data. Intermediate-frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground-based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

Structure and dynamics of the stratosphere and mesosphere revealed by VHF radar investigations

Powerful VHF radars are capable of almost continuously monitoring the threedimensional velocity vector and the distribution of turbulence in the middle atmosphere, i.e. the stratosphere and mesosphere. Methods of radar investigations of the middle atmosphere are outlined and the basic parameters, mean and fluctuating velocities as well as reflectivity and persistency of atmospheric structures, are defined. Results of radar investigations are described which show that the tropopause level as well as a criterion on the stability of the lower stratosphere can be deduced. Besides mean wind velocities, VHF radars can measure instantaneous velocities due to acoustic gravity waves. The interaction of gravity waves with the background wind is discussed, and it is shown that cumulus convection is an effective source of gravity waves in the lower stratosphere. The vertical microstructure of the stratosphere, manifesting itself in thin stratified sheets in which temperature steps occur, is investigated by applying knowledge from investigations of the oceanic thermocline. Possible origins, like shear generation and lateral convection of the microstructure of the stratosphere, are discussed. Observations of gravity waves in the mesosphere are reviewed and their connection with turbulence structures is pointed out. Finally, some open questions which could be answered by further VHF radar investigations are summarized.

Travelling disturbances in the equatorial ionosphere and thier association with penetrative cumulus convection

Abstract By means of HF-ground-backscatter and HF-CW-Doppler experiments, characteristics of medium-scale travelling ionospheric disturbances (TIDs) in the equatorial zones of Africa and South America are investigated. An optimum fit of measured and calculated backscatter ionograms leads to the horizontal phase velocity, period and wavelength of medium-scale TIDs observed to travel meridionally out of the equatorial zone in Africa. The harmonic analysis of the TID occurrence throughout three years of observations yields seasonally consistent mean, diurnal and semi-diurnal components. A significant correlation of the TID occurrence and tropical rainfall activity is proved. Results from an HF-CW-Doppler experiment in Huancayo/Peru yielding the velocity, period, wavelength and azimuth of TIDs state the existence of medium-scale TIDs, travelling away from the tropical rainforest area. A summary of possible sources of atmospheric gravity waves in equatorial regions exciting the observed TIDs points to the penetrative cumulus convection in the intertropical convergence zone as the most likely generation mechanism.

Equatorial spread-F by electric fields and atmospheric gravity waves generated by thunderstorms

It is assumed that atmospheric gravity waves, resulting in travelling ionospheric disturbances (TIDs), and electric fields, generated by convective thunderstorms, have a reasonable influence on the large-scale structure of premidnight equatorial spread-F irregularities. The responsible mechanisms, viz the superposition of thunderstorm generated electric fields on the ionospheric electric fields being the determining factor for irregularity generation and the steepening of TID structures due to spatial resonance, are briefly outlined. It is recalled that convective activity is most pronounced in the intertropical convergence zone over the African and South American continents. A model based on the typical features of seasonal and geographical variation of tropical convection generating the TIDs is presented which can explain seasonal and geographical variations of premidnight equatorial spread-F occurrence.

Investigations of lower and middle atmosphere dynamics with spaced antenna drifts radars

Abstract Spaced antenna drifts (S.A.D.) experiments with the SOUSY-VHF-Radar are described. Wind profiles have been measured up to the middle stratosphere and at some heights in the mesosphere. Good agreement is found between the radar and radiosonde measurements of tropospheric and stratospheric winds. Similarities and differences between the S.A.D. method and the conventional Doppler beam swinging (D.B.S.) method are discussed. It is concluded that the S.A.D. method has some obvious advantages compared with the D.B.S. method. These essentially result from two fundamental features: (1) diffuse reflection from horizontally stratified turbulence structures is manifest at very high frequencies (VHP), (2) the S.A.D. method yields information on the spatial variation in addition to the temporal variation. Finally, the requirements for an operational radar system to investigate the lower and middle atmosphere with the S.A.D. method are summarized.

Remote sensing of the atmosphere by VHF radar experiments

Problems in atmospheric dynamics, in particular in micro- and mesoscale processes, which can be solved by radar investigations, are summarized. The method of atmospheric radar experiments is described. Some relevant results obtained with the new SOUSY-VHF-Radar are presented, including observations of a warm-front passage, of layered structures, and of vertical and horizontal velocities in the troposphere. A power profile of radar echoes from heights up to the stratosphere proves that the SOUSY-VHF-Radar in its final operating state will be able to investigate structures and dynamics in the mesosphere too.

Interpretation, reliability and accuracies of parameters deduced by the spaced antenna method in middle atmosphere applications

The spaced antenna method has proved to be an important and relatively inexpensive radar technique for making measurements of atmospheric wind velocities and other parameters. This discussion examines the reliability and accuracies of various parameters which can be measured with the technique.After a brief introduction, a series of comparisons of winds measured by the spaced antenna method and simultaneously by other techniques are presented. It is concluded that when using weak partial reflections in the height range 0–100km, the spaced antenna technique provides reliable estimates of the neutral air motion. Following this the assumptions made in applying the method are considered in more detail. The possibility of systematic errors and the likelihood of erroneous measurements are examined, and the accuracy of any particular measurement of wind speed is discussed. Previous objections to the technique are discussed, and in general shown to be invalid.Other parameters apart from wind speeds can be measured with the spaced antenna technique, such as pattern scale, the rate of natural fading, and angles of arrival. The meanings of these parameters are discussed in terms of physical quantities such as turbulent energy dissipation rates, small-scale gravity wave velocity fluctuations, and aspect sensitivities of scatterers, and it is indicated when and how these derived parameters can be applied to deduce meaningful physical quantities. The need for great caution in making these interpretations is discussed; for example it is not always possible to use the rate of natural fading to estimate the intensity of turbulence, although in some cases this is possible. Finally, interferometric applications of spaced antenna systems are discussed.

Drifting patches of equatorial spread-F irregularities -experimental support for the spatial resonance mechanism in the ionosphere

Abstract The spatial resonance mechanism which sets in when the plasma drift velocity matches the phase velocity of atmospheric gravity waves in the ionosphere is applied to explain large-scale structures of the equatorial spread- F . The resonance condition is examined for reasonable parameters of travelling ionospheric disturbances, plasma drift and neutral wind velocities. It is shown that the resonance condition can be fulfilled in equatorial regions during the post-sunset hours. The measured drift velocities of patches of range-type equatorial spread- F are similar to measured plasma drift velocities. Some spread- F structures observed with the RTI-technique at Jicamarca, range-type spread- F observed on ionograms of Huancayo as well as the occurrence of spread- F patches observed with transequatorial propagation experiments do occur after the vertical plasma drift reverses from upward to downward. The periods, wavelengths and velocities of the large-scale structure of the spread- F patches appear to be similar to those parameters for medium-scale TIDs. It is argued that a non-linear break-up of resonance-steepened TIDs gives rise to the quasi-periodic structures which are typical for the range-type equatorial spread- F .