The incoherent scatter technique has played an important role since 1970 in the study of the electrodynamics of the ionosphere, thanks to its ability to measure the altitude profile of the ionospheric currents and to give access simultaneously to all electrodynamic parameters of the ionosphere: electric fields, neutral winds, conductivities, and electric current densities. The various calculation schemes involved in the derivation of this set of parameters are presented. The main results obtained up to now in three different latitude regions, from the auroral electrojets to the equatorial electrojet, are reviewed. The most promising developments of these studies in the near future are tentatively indicated. They appear to be of prime importance for progress toward a satisfactory description of electric current flows in the ionosphere-thermosphere system.

View full abstract

Long-period variations of winds observed by the radio reflection technique at Saskatoon, from 65 to 110km, 1969-1975, show some significant differences from the CIRA 1972 model. The maximum amplitude of the annual wave below 80km is found to be only 22ms^-1, while CIRA shows 45ms^-1 at this latitude. The semiannual wave below 75km at Saskatoon has an amplitude only half that of CIRA. Its phase shows a consistant downward propagation from 118 to 70km at 8km month^-1. No appreciable terannual wave was found at Saskatoon, but a weak 3-month wave shows the same phase progression pattern as for the semiannual. A QBO is found at Saskatoon with amplitude of 10ms^-1 above 100km.

View full abstract

Recent theoretical models predict complicated tidal structures in the lower thermosphere which deviate from the classical Hough mode structures commonly used in E-region dynamo calculations. The present paper investigates the electrodynamic effects of such a theoretical wind field, and examines its consistency with measured magnetic effects on the ground. The model of tidal structure is constructed by synthesizing diurnal and semidiurnal contributions excited in-situ and propagating upwards from the mesosphere and below. The individual tidal structures, which are inseparable in their latitude and height dependence, are each determined by solving the linearized tidal equations for a spherical, rotating, viscous atmosphere with anisotropic ion drag. The amplitudes and phases of the individual tidal components are calibrated with incoherent scatter and satellite measurements. The dynamo computations are generally in good agreement in amplitude and phase with the diurnal and semidiurnal harmonics of the observed ground variations at minimum and maximum levels of solar activity. There are, however, real discrepancies on the order of 20% in amplitude and 1 to 2hr in phase which require explanation. In interpreting our theoretical simulations, we attempt to point out the structural features of the E-region tidal winds and conductivities which are most critical to establishing such a consistency between theory and experiment, and to evaluate the status of dynamo theory with particular regard to the structure and variability of the solar tidal winds.

View full abstract

Atmospheric acoustic-gravity waves associated with severe local thunderstorms, tornadoes, and hurricanes can be studied through the coupling between the thermosphere and the troposphere. Reverse group ray tracing computations of acoustic-gravity waves observed by an ionospheric Doppler sounder array, show that the wave sources are in the neighborhood of storm systems and the waves are excited prior to the storms. It is suggested that the overshooting and ensuing collapse of convective turrets may be responsible for generating the acoustic-gravity waves observed. The results of this study also show that the study of wave-wave resonant interactions may be a potential tool for investigating the dynamical behavior of severe storm systems using ionospheric observations of atmospheric acoustic-gravity waves associated with severè storms.

View full abstract

A statistical analysis of a vast global experimental material of irregularity ionization horizontal drift radio physical measurements at ionospheric levels up to 130km allows one to reveal the basic features of the dynamical regime of the lower thermosphere. Intercalibration of radiomethods by simultaneous rocket and radio meteor measurements and physical considerations allow the measurements to be interpreted as an information on the neutral wind in the radio wave reflection region. In the mid-latitude ionospheric E-region the prevailing wind has a clearly pronounced seasonal variation in contrast to equatorial and high-latitude zone. In the high-latitude E-region there is observed an independent circulation cell probably linked with the properties of the thermal regime in the polar lower thermosphere. The drift measurements for 85-95km by the method of spaced receivers in the long waveband in Eastern Siberia are reported for 1974-1976. The results are compared with simultaneous similar measurements at the same latitudes in Central Europe. The differences which are thought to be associated with climatic conditions at the observation sites, are emphasized.

View full abstract

A world-wide change of geomagnetic fluctuations which have a period of several days was investigated. Two current systems were newly found. One is MDP 1 (Modified DP 1) which is characterized by three current vortices in the polar and middle latitides with westward zonal currents in the lower and equatorial regions. The other is MDP 2 (Modified DP 2) which has two vortices (one vortex in MDP 1 seems to be dissipated) in the polar region, with westward zonal currents in the lower and equatorial regions.
A day-to-day change of the Sq focus location which sometimes shows a clear 7 day period is interpreted as a result of the westward zonal current accompanied by MDP 1 and MDP 2.
Since the several day period geomagnetic fluctuations form a world-wide current system similar to well known DP 1 and DP 2, it is concluded that this current system may not be of the planetary wave origin but it must be of the magnetospheric origin.

View full abstract

Effects of the interplanetary magnetic field (IMF), particularly its polarity, on the lower thermosphere are briefly reviewed. A fairly well-known indirect correlation is that both long-lasting southward IMF and the boundary between eastward and westward fields often cause magnetospheric disturbances which produce storm related thermospheric variations. A direct correlation is that the IMF westward (or eastward) component produces clockwise (or counterclockwise) circular electric currents in the lower thermosphere over the polar region. Computer simulation studies of a possible mechanism of this effect are presented. IMF effects on the change of the magnetospheric configurations without any appreciable thermospheric variation are also included, and suggestions for future investigations are provided.

View full abstract

Abnormal features of the regular daily variation S_R could be of great interest for a full understanding of the physical sources of this phenomenon, especially of the tides and electric conductivity conditions in the E layer. The subauroral part of the S_R (a positive deviation from the night level, increasing polewards and occuring during a few hours at any longitude when the sun is crossing the meridian of the magnetic pole) and the polar cap S_R (MAYAUD, 1965b) on the one hand, the concept of the invasion of PRICE and WILKINS (1963) on the other hand are reviewed and update. With respect to the former, one shows that they cannot be caused by the action of the azimuthal component B_Y of the interplanetary field, and one suggests that their source could be strong localized dynamo effects inside the polar caps, bringing about a special and single current vortex in these regions. Some equatorial counter-electrojet effects contradict the concept of the invasion and suggest the existence of vortices with reversed direction of rotation of the currents, as it would be the case for other counter-electrojet effects (MAYAUD, 1977).

View full abstract

Approximately four hundred neutral wind profiles of the lower thermosphere, obtained by rocket-borne techniques since 1957, have been analysed in terms of diurnal and latitudinal variability. Some 15 of these measurements have been carried out during daytime conditions, providing an insight into the dynamics of the daytime thermosphere.
The dominant tidal modes (1, 1), (1, -2), (2, 2) and (2, 4) have each been identified in the statistical analysis, and their amplitudes and phases determined as a function of altitude within the lower thermosphere. Agreement is found between the characteristics of the data and recent models of the tides in the lower thermosphere. The (1, -2) mode, for example, appears more strongly in the meridional than zonal components; however, its phase appears to be 3 to 4hr advanced compared with model calculations. This mode may be strongly enhanced after even moderate geomagnetic activity, indicating that the high latitude energy source responsible for the enhancement may also cause the phase advancement relative to models of the (1, -2) tide as generated only by in situ solar EUV heating.

View full abstract

The results of fifteen rocket measurements of neutral wind profiles, the ionospheric electric field and electron density at mid-latitudes are compared with the predictions of the dynamo theory of mid-latitude ionospheric currents (Sq) and the measurements of the ground-level magnetic perturbation. In daytime, the computed ionospheric currents agree well with both ground-level magnetic observations and Sq models, while the electric field structure is consistent with a dynamo source of such fields, although the field amplitude appears to be about 50% of that predicted by models such as those of STENING (1973). At twilight there is a generally poorer agreement between the calculated currents and those indicated by magnetometers and by Sq models, probably due to difficulties of determining a precise magnetic baseline corresponding to zero electric current. At night, the wind system is occasionally strong enough to drive currents of 5 to 10 amps km^-1 unless these are opposed by locally-induced, polarization electric fields of the order of 5 to 10mVm^-1. Such fields may be readily induced, at mid-latitudes, by the high-speed, night-time wind systems set up by high latitude energetic processes during geomagnetic substorms, without requiring a direct penetration of magnetospheric, convective fields to low magnetic latitudes.

View full abstract

The current state of ionospheric wind dynamo theory is reviewed. Observational and theoretical advances in recent years have permitted more accurate models of the dynamo mechanism to be presented than previously, which have lent further credence to the validity of dynamo theory as the main explanation for quiet-day ionospheric electric fields and currents at middle and low latitudes. The diurnal component of the wind in the upper E region and lower F region appears to be primarily responsible for average quiet-day currents, although other wind components give significant contributions. Observationally, there is a need for better spatial and temporal coverage of wind and electric field data. Theoretically, there is a need for further consideration of the mutual dynamic coupling among winds, conductivities, electric fields, and electric currents, and for better modeling of nighttime conditions.

View full abstract

Using ground magnetometer data from several stations near the dip equator, the magnetically quiet-time characteristics of the daytime normal electrojet and the morning and afternoon counter-electrojets are established and illustrated. In particular, the solar cycle, seasonal, longitudinal and day-to-day variations of counter-electrojets as well as their dependence on lunar phase are studied. In order to investigate the influence of solar tidal winds in the production of counter-electrojets, the equatorial electric fields generated by such winds through the dynamo mechanism are calculated, using a numerical dynamo simulation model developed earlier at UCLA. From these calculations it is found that westward electric fields responsible for the afternoon counter-electrojets can be produced in the local summer solstice (when these events are most common) by a combination of the solar semidiurnal (2, 2) tide and the diurnal (1, -2) tide. An example from January 1964 is discussed to illustrate that afternoon counter-electrojet events occurring at all longitudes on the same day are likely to be associated with abnormalities in the global S_q or S_R current system. It is suggested that the lunar semi-diurnal tide is a more important causative agent for the morning rather than the afternoon counter-electrojets.

View full abstract

A new method based on covariance analysis is used to map the current patterns associated with variations of the equatorial electrojet. These patterns show that the electrojet is the expected equatorial enhancement of a coherent, large scale current system. This current system is further broken down and found to be composed of a part which is identified as the standard lunar current system and a part which is probably due to localized dynamo region wind fluctuations near the equator. Neither of these parts can be identified as the Sq current system. The problem will be investigated further in the second part of this study.

View full abstract

The ‘separation’ procedure described in Part I has been used to divide the Sq current system into three subsystems. The first of these is associated with seasonal variations and has current strength of about 100kA. Its origin appears to be semidiurnal winds.
The second subsystem is called the ‘Fuquene’ system and has a strength about 150kA (±50%). It appears to be due to magnetospheric sources and, at least at the equator, may not flow at ionospheric heights.
The third subsystem is called the ‘constant’ and has a strength of about 220kA (±50%). It is possibly due to direct ionospheric heating.

View full abstract

Several rocket borne magnetometer experiments have been conducted from Thumba to measure the equatorial electrojet currents under different solar and geophysical conditions. The data from these experiments have been used to study the vertical structure of the electrojet, the relation between strength of the jet and the magnetic field variation at the ground and the causes of large day-to-day variability of the amplitude of H variation. The results show that the experimentally measured current density peaks at a significantly different altitude compared to the altitude of the peak of Cowling conductivity calculated using model values of ionospheric parameters. The analysis indicates that given the magnetogram from a station under the electrojet, it is possible to predict the vertical distribution of current density in the electrojet on a quiet day. The mechanism of stabilisation of two stream instability suggested by the theoretical workers ro explain the back scatter radar echoes from type I irregularities on high jet days does not become operative atleast upto jet strengths equivalent to ΔH≤140nT.

View full abstract

The height integrated current density in the electrojet over Thumba has been measured between the years 1966 and 1976 on nine different occasions by rocketborne magnetometers. Using this data and the H variation data from a spread of ground magnetic stations in the region of the electrojet, the depth of the non-conducting layer has been evaluated. The results are compared with the results obtained from POGO and Kosmos 321 satellite experiments. It is shown that the depth of non-conducting layer in the region around Thumba is 230±20km. Using the above data, the internal and the external contributions to the diurnal variation of geomagnetic field observed at the ground have been evaluated.

View full abstract

Investigations of the AC electric fields associated with plasma instabilities in the equatorial electrojet were carried out from Thumba using Langmuir double probes. AC electric fields, the electrojet current density and the electron density and its fluctuations were measured in near simultaneous launchings on two different occasions near local noon when the electrojet intensity was high. A third experiment was conducted in the early morning hours when the electrojet was not yet developed. The two noon time experiments detected AC electric fields associated with cross field instability, both during the ascent and the descent, between 85 and 105km altitude. In the early morning experiment AC electric fields in the spectral band 10-100Hz were observed between 115 and 130km altitude.

View full abstract

A new method is proposed for calculation of electric charge distribution in the ionosphere associated with the dynamo action for any given air motion. If the dynamo action is asymmetric in the northern and southern hemispheres, the electric potential difference of the order of 1kV will be produced (if the field-aligned current is prohibited) at the conjugate pair of stations in middle latitudes although the potential difference will be almost cancelled by the field-aligned currents in the magnetosphere flowing from the winter hemisphere to the summer hemisphere. It is also shown that the height-gradient of the ionospheric conductivity plays a very important role for charge separation in the ionosphere, the effect of which intensifies the eastward equatorial electrojet on the dayside.

View full abstract

The Institute of Experimental Meteorology carries out regular wind measurements by the meteor radar at three stations, Heiss Island (from 1964), Obninsk (from 1964) and Molodezhnaya, the Antarctica (from 1967). The seasonal course of wind regime parameters in the meteor zone in different years was found to be relatively stable in comparison with site-to-site variations of these parameters. It is shown that wind velocity variations with the periods from 2-5 to 10-20 days are typical of the meteor zone and are connected with corresponding variations in stratomesosphere. The analysis of dates for different years showed that in the meteor zone the spring reversal of circulation begins much earlier in stratosphere; more earlier (later) dates of the reversal in stratosphere correspond to more earlier (later) dates of the reversal beginning in the meteor zone.

View full abstract

A comparison between the winds near 97 kilometers altitude has been made from observations of the 17924K (5577Å) OI line emission at Fritz Peak Observatory (39.8N, 195.5W) and with a meteor radar facility at Atlanta (34N, 84W), from August 1974 to November 1975. Since the optical emission measurements are made only at night, the nighttime meteor radar measurements have been used, weighted by an airglow emission rate profile. The results show general agreement in both the zonal and meridional wind vectors, but with the variations in the amplitude of the meridional winds at the northernmost station (Fritz Peak Observatory) larger than those at Atlanta, a result of the smoothing inherent in producing the meteor winds.

View full abstract