A scintillation theory of fading in long distance HF ionospheric communications

Scintillation theory is used to study the fading of radio waves returned from the ionospheric F-region in the HF band at oblique incidence. For both low and high penetration frequencies calculations are made as a function of wave frequency, but emphasis is given to behaviour near the maximum usable frequency for one-hop F-transmission over distances up to 4000 km. Estimates are made of

• 1.(i) the fluctuations of phase both for long-term (~ 1h) and for short-term (~ the fading correlation time),
• 2.(ii) the fading correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the twinkling correlation distance,
• 5.(v) the quasi-period of twinkling,
• 6.(vi) the angular departure of the arrival azimuth from the mean and
• 7.(vii) the correlation bandwidth.

The slow fluctuations often experienced in long distance HF radio communications are a manifestation of twinkling, rather than fading. Under normal conditions in the F-region the correlation bandwidth is so large that the bandwidth of transmission at HF is controlled by the dispersive properties of the ionosphere, rather than by the scattering properties. The reverse is true in the presence of spread-F. For sufficiently strong spread-F the correlation bandwidth for long distance HF radio communications is of the order of only 10 Hz, thereby creating the phenomenon known as flutter fading. Theoretical estimates of the scintillation parameters are compared with experience in long distance HF ionospheric communications over the last 60 years. Agreement is promising, but future experiments should be designed in the light of theory; existing observational data do not provide some of the information needed.     

A scintillation theory of the fading of HF waves returned from the F-region: receiver near transmitter

Scintillation theory is used to study the fading of HF radio waves returned from the ionospheric F-region to a receiver close to the transmitter. Estimates are made of

• 1.(i) the fluctuations of phase both for long term (∼ an hour) and for short term (∼ a fading correlation time),
• 2.(ii) the correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the angular divergence in the direction of arrival around the zenith and
• 5.(v) the correlation bandwidth.

The calculations are made as a function of wave frequency for two ionospheric penetration frequencies representative of high day-time values and low pre-sunrise values. Results are compared with observations of fading made with ionosondes over the past 40 years. Precise comparison is rendered impossible by omissions in the experimental data caused by lack of guidance from scintillation theory. Nevertheless, agreement is promising. When fading is deep but spread-F-region is not well-developed there is a slow modulation of the fading. This is what, for optical propagation in the troposphere, is called twinkling. The slow fluctuations observed by Bramley and Ross in the HF band constitute ionospheric twinkling. Calculated quasi-periods of twinkling range from about an hour down to about a couple of minutes, while calculated quasi-periods of fading range from about a couple of minutes down to about a tenth of a second.     

Possible effects of solar variability on the middle atmosphere

This paper discusses the possible effects on the middle atmosphere of

• 1.(i) solar variability in emission in the u.v. and far u.v.,
• 2.(ii) solar proton events,
• 3.(iii) relativistic electron precipitation events,
• 4.(iv) corpuscular heating in auroras,
• 5.(v) joule heating by the auroral electrojet,
• 6.(vi) auroral NO production and
• 7.(vii) gravity wave emission by the auroral electrojet. It is important to establish the maximum depth in the atmosphere to which these effects go, in order to be able to determine whether the reflection of planetary scale waves from the troposphere is substantially altered by them.

     

The effect of the large ion temperature differences on the solutions of diffusion and continuity equations

Diffusion and continuity equations for O⁺ and H⁺ ions were integrated along a magnetic field line between 500 and 3000 km. Three sets of equations were used:

• 1.(1) the standard ones;
• 2.(2) modified according to the formulae given by St-Maurice and Schunk;
• 3.(3) those derived by Conrad and Schunk.

The differences between solutions obtained for each set of equations and the importance of the modifications introduced in the diffusion equations are discussed.     

Midday recovery of the polar cap absorption of 19–21 March 1990: a case study

The effect of the midday recovery of absorption (MDR) during the polar cap absorption (PCA) of 19–21 March 1990 is investigated using data from 25 riometer stations in both hemispheres. The measured variations of absorption are compared with those calculated from a model. Three main aspects are considered:

• 1.(1) The solar and geophysical conditions under which the effect appears,
• 2.(2) The essential morphological features of the phenomenon,
• 3.(3) The relative contributions of (a) diurnal variations in the geomagnetic cut-off energy and (b) an anisotropic pitch-angle distribution of the solar protons to the development of the MDR.

The principal morphological features of the MDR effect are found to be as follows:

• 1.(a) the width of the area affected by MDR is about 10° of invariant latitude,
• 2.(b) there are two regions, respectively below and above 65° latitude, in which the MDR properties are different,
• 3.(c) the maximum duration of the MDR effect is about 12 h,
• 4.(d) there are distinct geomagnetic conjugucy effects in MDR.

There are solid reasons to suggest that the MDR at latitude 65–70° is due to the combined influence of a diurnal variation of geomagnetic rigidity, and the pitch-angle anisotropy of the solar protons. The MDR at lower latitudes (Λ = 60–65°) seems to be produced primarily by diurnal variations of the cut-off rigidity.     

Nocturnal stratification of the ionospheric F-layer over Wuchang,China

Examples are given in the present paper of nocturnal ionospheric stratifications in lower midlatitudes, with the following characteristics:

• 1.(1) clear night-time stratification or low frequency F retardation, implying stratification
• 2.(2) a marked decrease in foF2 and a marked increase in h'F2 and hpF2
• 3.(3) a marked increase in electron concentration under the peak
• 4.(4) a close correlation with polar substorms, as deduced from the local magnetograms. It is suggested that the stratifications are due to particle precipitation and to modification of the low-latitude electric field by polar substorms.

     

Auroral radar observations at Siple Station,Antarctica

A dual coherent 50 MHz auroral radar installed at Siple Station, Antarctica, continuously monitored echo intensity and mean-Doppler velocity from 29 December 1976 until late March 1978. We describe here the experimental technique and present some statistical results including yearly averaged echooccurrence patterns and irregularity drift velocity characteristics. Our results show that:

• 1.(1) the irregularity drifts are approximately westward (poleward electric fields) in the evening, and eastward (equatorward electric fields) in the morning following the electric field reversal in the region of the Harang discontinuity;
• 2.(2) the Harang discontinuity under disturbed conditions (average K_p = 5₀) as seen by both radars is located around 2100–2300 MLT;
• 3.(3) the relationship between the irregularity drift velocity and the actual electron drift velocity is strongly dependent on the angle between the radar beam and the earth's magnetic field, as predicted by linear theory.

     

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.

     

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of

• 1.(i) a southward neutral wind
• 2.(ii) a vertical wind driven by Joule heating
• 3.(iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

     

Parameterization of the radiative flux divergence in the 15 μm CO2 band in the 30–75 km layer

A parameterization of the cooling rate of the earth's middle atmosphere due to radiation transfer in the 15 μm CO₂ band is proposed. The parameterization has been performed for such an atmospheric layer with local thermodynamic equilibrium (below approximately 75 km), for which in the major part of the frequency interval occupied by the 15 μm band the approximation of non-overlapping lines (above about 30 km) is justified. The overlap of lines has only been taken account of for superposed strong Q-branches at the centre of the 15 μm band. The parameterization takes into account the peculiar features of radiation transfer:

• 1.(1) the height step increases with the growing distance from the level, for which cooling is calculated;
• 2.(2) the dependence of line intensity on temperature has partially been taken account of in an explicit form. The procedure proposed is multiparametric: 42 parameters are prescribed for calculating cooling at a given level. With an error less than 0.5 K day⁻¹ the procedure can be recommended for calculating radiative cooling in the case of any temperature profile undisturbed by small- and meso-scale motions and, in particular, for use in the modelling of the middle atmospheric circulation.

     

Excessive absorption by atmospheric water vapor in the infrared and 5–17 cm−1 regions

Measurements of ‘excessive’ absorption by atmospheric water vapor, i.e. absorption in excess of water monomer absorption and droplet scattering predictions, are analyzed for the infrared and 5–17 cm⁻¹ regions. Both fair weather and fog data are considered. It is found that observations in both spectral regions can be explained by the clustering (hydrogen bonding) of ~ 10⁻³ of the vapor molecules. The development leads to several tentative conclusions:

• 1.(1) equilibrium cluster populations (and maximum absorption) are only attained forvaporin contact with liquid water;
• 2.(2) the clusters are formed by evaporation, are large, and cannot be modeled by Boltzmann statistics;
• 3.(3) cluster modes can account for unexplained spectral features at 5–17 cm⁻¹;
• 4.(4) excessive absorption both in the infrared and 5–17 cm⁻¹ regions probably can be explained by the same cluster species.

     

Synoptic data for solar-terrestrial physics: the U.K. contribution to long-term monitoring

In 1989 the Council of the Royal Society of London established a Study Group to investigate the current status and future requirements of solar-terrestrial monitoring in the United Kingdom. This paper summarizes the conclusions and recommendations of the U.K. Study Group. The rationale for regular synoptic monitoring of the solar-terrestrial environment is reviewed briefly. Instruments used for solar-terrestrial monitoring in U.K.-supported research programmes are listed, with special emphasis on research instruments that produce monitoring data. Some important applications of solar-terrestrial monitoring are outlined and criteria are presented for determining the priorities of various monitoring operations. High priority is attached to monitoring operations that are important for:

• 1.(i) deriving geophysical models and reference systems;
• 2.(ii) calculating the principal solar-geophysical indices;
• 3.(iii) producing long time series of good data, especially at sites of particular geophysical interest; (iv) detecting global changes;
• 4.(v) contributing to real-time forecasts of solar-terrestrial conditions, or to longer-term predictions and planning; and
• 5.(vi) providing background data for other observations and experiments. The existing U.K. programmes of solar-terrestrial monitoring make a highly prestigious contribution to national and international scientific activity. Synoptic measurements of the solar-terrestrial environment are crucial for underpinning present and future programmes of basic, strategic and applied research. Therefore, a core programme of synoptic monitoring must be maintained in the U.K. It is recommended that this core programme should be financed centrally through the Research Councils and reviewed quinquennially.

     

Equatorial plasma bubble eastward velocity characteristics from scanning airglow photometer measurements over Cachoeira Paulista

Regular observations were carried out of airglow depletion patch motion, associated with equatorial plasma bubble events, using east-west and meridional scan OI 6300 Å airglow photometers over Cachoeira Paulista (22°42′S, 45°W, dip −26°), Brazil. Results from the east-west scan photometer for 31 nights obtained during the period January 1980–February 1981 that are analysed in the present work show that eastward motion of airglow depletion patches is a regular phenomenon in the post-sunset period, extending often to early morning hours (~02 LT), during the summer-equinoctial months. Such moving airglow structures were absent during the winter months (June, July and August). The main results from this first extensive study of the airglow depletion patch zonal movements over Cachoeira Paulista are the following:

• 1.(1) the night-time mean eastward drift velocities for the months January–May 1980 are higher than those for the period September 1980–February 1981;
• 2.(2) plots of eastward velocity versus local time for the second period mentioned in (a) show a decrease of the drift velocity with local time, while for the first group the velocity oscillates around an apparently local time-independent mean value;
• 3.(3)the drift velocities vary in varying degrees with geomagnetic activity index K_p. These results are discussed in the light of other reported measurements of the zonal F-region plasma motions.

     

Balloon observations of stratospheric electricity above the South Pole: vertical electric field,conductivity, and conduction current

This paper summarizes the results of measurements of the electrical conductivity σ and vertical component of the vector electric field E_z acquired from eight stratospheric balloon flights launched from Amundsen-Scott Station, South Pole, in the austral summer of 1985–1986. The major findings of this research are as follows

• 1.(1) The data contribute to the set of global atmospheric electricity measurements and extend the work of COBB [(1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.] to determine the electrical environment of the south polar region
• 2.(2) The average vertical profile of the conductivity at the South Pole, when compared with profiles obtained at other Antarctic locations, suggests that the conductivity scale height may increase with increasing geomagnetic latitude across the polar cap.
• 3.(3) The conductivity profiles measured at the South Pole and other Antarctic locations differ significantly from polar cap model profiles. On the basis of these measurements, the model profiles appear to require modification
• 4.(4) The magnitudes of the E_z profiles were observed to vary from day-to-day by a factor of > 2
• 5.(5) In all of the flights the air-Earth conduction current J_z, calculated as the product of E_z and σ, decreased with altitude in agreement with previous direct measurements of the air-Earth current by Cobb [( 1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.]
• 6.(6) The magnitude of J_z was 2–3 times larger than the global average, which can be attributed to the lower columnar resistance of the atmosphere above the high-elevation Antarctic plateau. The magnitude of J_z agrees with that observed by Cobb, if the Cobb measurements are multiplied by the Few and Weinheimer [(1986), Factor of 2 error in balloon-borne atmospheric conduction current measurements. J. geophys. Res.91, 10937] correction factor of 2
• 7.(7) E_z from all of the flights during times of balloon float demonstrates characteristics of the classical ‘Carnegie’ diurnal variation, which is indicative of global influences on the ionospheric potential
• 8.(8) The influence of geomagnetic activity was observed as a decrease in the amplitude of the diurnal variation of E_z with increasing geomagnetic activity index K_p, which is the predicted effect at the South Pole of the magnetospheric polar-cap potential superimposed on the ‘Carnegie’ potential variation.

     

Pseudosaturation of gravity waves in the middle atmosphere: an interpretation of certain lidar observations

Irregular winds of the middle atmosphere, commonly attributed to gravity waves, often exhibit a vertical-wavenumber (m) spectral form approximating to Km⁻³ at sufficiently large m, with K a constant found to be relatively unvarying with time, location and even height. This behavior is widely believed to result from some saturation process, but the physical mechanism remains a matter for debate. There now exist three theories:

• 1.(a) linear instability,
• 2.(b) nonlinear wave-induced diffusion, and
• 3.(c) nonlinear waveinduced Doppler spreading. Each has produced the Km⁻³ form (but only as an approximation in the case of Doppler spreading) and values of K within a factor of three of one another and of observed values.

New data have revealed circumstances in which an approximation to the form Km⁻³ is again found, suggestive of saturation, but with values of K that increase by a factor of 5 or 10 on moving from the stratopause to the mesopause region. This height variation is incompatible with theories (a) and (b) if the m⁻³ form is taken to be induced by the corresponding saturation process (rather than by source spectra), but is shown here to be compatible with the Doppler-spread theory. Because of the continued growth of K with height, which must ultimately cease, the observations and corresponding theory are taken to represent pseudosaturation rather than fully developed saturation per se.     

Interplanetary magnetic field structure and the variations of the ELF and VLF emissions in the topside ionosphere

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters [the polarity of the sector structure, the signs and magnitudes of the B_y, and B_z, components of the IMF as defined in the Solar Ecliptic coordinate system (Nishida, 1978)]. The comparison shows that:

• 1.(i) the positive polarity of the IMF sector structure (when IMF vector is directed toward the Earth) involves an enhanced probability of the detection of larger emission field intensities (>25–30dB);
• 2.(ii) the emission median intensity is ~20dB higher at B_y > 0 compared with B_y < 0;
• 3.(iii) the 0.72 kHz emission median intensity in the polar caps and at night-side auroral latitudes is lower when B_z > 0 as compared with B_z < 0;
• 4.(iv) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.

     

The equatorial electrojet and associated currents as seen in Magsat data

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are:

• 1.(1) extremely weak,
• 2.(2) extremely variable with longitude,
• 3.(3) inconsistent with the pattern expected from a line or narrow sheet current.

It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data.Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February.Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m² for the horizontal current at 110km and about 10–20 × 10⁻⁹ A/m² for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda.Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.     

Studies of middle atmosphere dynamics: the research projects Middle Atmosphere Co-operation/Summer in Northern Europe (MAC/SINE), and MAC/EPSILON

The Earth's middle atmosphere covers the height region 10–100 km and has been extensively studied in recent years. The international programmes Middle Atmosphere Program (MAP) and Middle Atmosphere Co-operation (MAC) have made significant contributions to our understanding of this part of the atmosphere. This special issue reports on the results from two campaigns, MAC/SINE and MAC/EPSILON, carried out within the framework of MAP and MAC. The two projects both studied the dynamical state of the middle atmosphere at high latitudes and had different, but complementary, aims:

• 1.1. Middle Atmosphere Co-operation/Summer in Northern Europe (MAC/SINE) studied the general circulation and temperature structure as well as waves, turbulence and winds in high latitudes during summer solstice conditions. The experiments comprised a series of regular meteorological rocket firings and ground-based observations as well as four launches of sounding rockets. The campaign was carried out in July-August 1987.
• 2.2. MAC/EPSILON was a case study of middle atmosphere turbulence by means of instrumented sounding rockets, meteorological rockets and ground-based observations. The campaign comprised four salvoes in which all rocket and ground-based techniques were concentrated in time and space to make detailed measurements during events with strongly developed turbulence. During the campaign period the ground-based techniques were exploited to map the general behaviour of the middle atmosphere during autumn/early winter conditions. The campaign was carried out in October–November 1987.

Both campaigns were very successful and yielded a number of significant and interesting results, as demonstrated in the 21 articles in this special issue.     

Small-scale fluctuations of magnetic and electric components of the ELF and VLF wave fields in the sub-auroral topside ionosphere : stochastic characteristics of the wave field

When the Interkosmos-14 and Interkosmos-19 satellites crossed the region of spatially varying electron concentration in the topside ionosphere adjacent to the high-latitude boundary of the main ionospheric trough, it was discovered that there were simultaneous fluctuations of plasma density, temperature and the amplitudes (H_x and E_y) of the ELF and VLF radio/plasma emissions. The probability characteristics of the naturally perpendicular H_x and E_y fluctuations are analysed. The correlation coefficient R(_H, E_y) turned out to be less than 0.6 at frequencies of F ⩽ 4.65 kHz, while at higher frequencies R increases, up to 0.9 at 15 kHz. The following interpretations are proposed:

1. 1.1. While measuring noise emissions, as a rule a mixture of numerous elementary waves is recorded.
2. 2.2. At frequencies exceeding the local lower hybrid resonance frequency (in our case f_LHR ≈ 5 kHz), a mixture of electromagnetic waves experiencing the influence of the inhomogeneous electron concentration N_e is registered.
3. 3.3. At frequencies which are lower than the local value f_LHR the mixture mainly consists of ELF waves. The wave field has a complicated structure, and the dynamical coherence between electric and magnetic field components is not as simple as at VLF frequencies (f ≈ 15 kHz).
4. 4.4. It is shown that the wave components for a mixture of electromagnetic and electrostatic waves (for instance a mixture of VLF and lower hybrid frequency waves) have a lower correlation coefficient because the electrostatic waves are unrelated to the electromagnetic waves.
5. 5.5. The correlation analysis offers an opportunity to detect the presence of waves of various types in the wave mixture.