Ionospheric whistler propagation

In a previous paper (Thomson Dowden 1977) it was found that down-going whistlers exit from ducts a few hundred km above the ISIS II altitude of 1400 km, a result confirmed here. Using a set of models chosen to fit the full range of variability of density versus latitude profiles measured at ISIS II altitude we now show by ray-tracing that transmission from duct to ground and from ground to duct is most likely for duct entry/exit in the altitude range 1200–1500 km. Thus as far as whistler trapping is concerned, ducts terminate at altitudes well above 1000 km, considerably higher than has been generally thought.Several consequences of this are discussed. Direction finding measurements on the ground will underestimate the L-value of ducts by 0.2 to 0.3. Of the whistler energy exiting from a given duct and reflected from the base of the ionosphere, only a small fraction will re-enter that duct, implying that 10–20 dB of magnetosphere amplification per hop is needed to sustain multi-hop whistlers and mid-latitude hiss. On the other hand some of the reflected energy may enter adjacent ducts, thus supplying a ready explanation for mixed-path whistlers.     

Ionospheric tomography: an algorithm enhancement

The Applied Research Laboratories, The University of Texas at Austin (ARL: UT), conducted a computerized ionospheric tomography (CIT) experiment called the “Mid-America CIT Experiment”, or MACE'93, from June through December, 1993. Portions of the data from this experiment have been input into various CIT algorithms to reconstruct a two-dimensional electron density image of the ionosphere. One of the CIT algorithms relies on model ionospheres to incorporate a priori information into basis functions that are used to represent the unknown ionospheric electron density structure. When global ionospheric models are used to generate the basis model ionospheres for this algorithm, the reconstructed ionospheres often underestimate the F₂ peak density and some reconstructions display incorrect latitudinal electron density variation. These erroneous reconstructions are mainly caused by the global ionospheric models incorrectly modeling the ionosphere in terms of the F₂ region thickness and latitudinal variations. This paper identifies an alternate set of basis ionospheres that improves the accuracy of the reconstructed ionospheres. In general, this enhancement has reduced the average per cent difference between actual and CIT reconstructed ionograms to 6% and the average absolute error between measured and CIT reconstructed f₀F₂ to 2.6%.     

Ionospheric reflection coefficient for television signals

It has been shown that the ionospherically reflected field strength for 50 MHz signals varies (statistically) in proportion to about the 18th power of f_oE_s at the midpoint between transmitter and receiver. This note is to point out that this observation provides support for the theory that the horizontal structure of E_s is due to a horizontal variation in the wind shear rather than horizontal convergence of metallic ions or a horizontal variation in the metallic column conterrt.     

Ionospheric modification experiments in northern Scandinavia

The heating facility at Ramfjordmoen near Tromsø, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D-region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F-region cross modulation.     

Ionospheric scintillation observations with radio interferometry

Radio astronomical interferometric observations are affected by atmospheric refraction, being particularly sensitive to inhomogeneities in the atmosphere. At frequencies below 2 GHz the influences of the ionosphere are significant in radio astronomy, especially for single dish observations and for connected element interferometry.Analytical expressions for the manifestations of weak ionospheric scintillation in radio interferometric observations, are derived. We indicate which ionospheric scintillation parameters can be derived from radio interferometric measurements. It is shown that the baseline dependence of the observed amplitude scintillation index implies a direct determination of the height of the region of random irregular electron distribution. Furthermore, the linear scale of the irregularities causing scintillation can be determined directly from the baseline dependence of the scintillation index S₄. From the mean square phase fluctuations as a function of interferometer baseline, the spatial scale of the irregularities responsible for this effect can also be determined. From a comparison with observational mid-latitude data we find indications that scintillation irregularities occur in the lower parts of the F2-layer. The spatial scale of irregularities causing amplitude scintillation is of the order of about 25 to about 500 metres. Phase scintillations are caused by irregularities with dimensions which are an order of magnitude larger.     

Ionospheric sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

Ionospheric plasma convection in the southern hemisphere

The first ionospheric plasma convection maps ordered by the y- and z-components of the IMF using only data from the southern hemisphere are presented. These patterns are determined from line-of-sight velocity measurements of the Polar Anglo-American Conjugate Experiment (PACE) located at Halley, Antarctica, with the majority of the observations coming from 65°–75° magnetic latitude. For IMF Bz positive and negative conditions, the observed plasma motions are consistent with a standard two cell pattern. For the periods from dusk through midnight to dawn, flow speeds are at least twice as large for Bz negative component compared with Bz positive. The observations about noon are significantly different from each other. For Bz positive, little ordered plasma motion is observed. For Bz negative, there are large anti-sunward flows the orientation of which is ordered by IMF By. These By orientated flows are consistent with theoretical predictions, and are anti-symmetric to those reported from the northern hemisphere. The two most significant differences from previous observations are that the convection reversal in the late morning sector for By negative conditions occurs at about a 4° lower latitude than the Heppner and Maynard (1987) model. This may be due to a seasonal bias in the PACE dataset. Also, the separatrix between eastward and westward flow near midnight has a very different shape dependent upon the orientation of IMF By. For positive By conditions, the separatrix is observed at progressively lower latitudes at later local times, but for By negative conditions, the separatrix appears at increasingly higher latitudes at later times.     

Ionospheric modification by high power radio waves

Powerful, high frequency (HF) radio waves can be used to temporarily modify the ionosphere. These controlled, active experiments have proven useful both for studies of the natural upper atmosphere through observations of ionospheric response to HF induced perturbations, and for basic physics investigations exploiting the ionosphere as a large, natural plasma laboratory-without-walls. This experimental diversity has attracted the attention and participation of physicists from a wide range of disciplines. As a result, HF ionospheric modification research continues to be strongly motivated by its many applications in the fields of aeronomy, space physics, plasma physics, and telecommunications science.     

Ionospheric wind measurements and activities in Swansea

Some of Sir Granville Beynon's contributions as a person and as an eminent physicist during 1956/57 at the University College of Swansea are considered. In particular, the spaced-receiver measurements of winds in the lower ionosphere are discussed; events and activities in that era are recalled.     

Ionospheric changes in response to IMF variations

This paper attempts to summarize the results of investigations of IMF effects on the ionosphere, published mostly in Russian, and to place them in context in up-to-date knowledge of IMF/magnetosphere/ionosphere relationships. Effects of the IMF sector structure and of the IMF B_z component turnings on the ionospheric F-layer are considered, including variations of position of the main ionospheric trough (MIT). The paper includes results of both theoretical calculations and observational data obtained mostly by the Cosmos-900, Intercosmos-19 and Cosmos-1809 satellites at subauroral, middle and low latitudes. The MIT position dependence on longitude has been derived as a background for further study. It has been shown that the nightside winter trough position at the storm growth phase correlates best with K_P index taken with a time delay τ, which is proportional to a disturbance growth rate ΔK_p/Δt. The MIT position dependence on D_st, B_z and B_y is also shown. Two troughs have been found to be formed usually in the storm recovery phase at postmidnight hours: these are the MIT (main ionospheric trough) and RIT (ring ionospheric trough) associated with the DR-current. In general the MIT position's response to B_z southward turnings corresponds well to changes of the amoral diffuse precipitation equatorial edge. For B_z southward turnings the height of the equatorial night-time F-layer lowers, and at equatorial latitudes f_oF2 decreases sharply, the latter effect being most pronounced at 03 LT. Large-scale internal gravity waves arriving at equatorial latitudes from the auroral oval cause intensification of the equatorial anomaly, both in daytime and night-time. A schematic pattern of a global ionospheric response to a magnetic disturbance is constructed using as an example the strong storm on 3–4 April 1979.     

Ionospheric control of polarization of low-latitude geomagnetic micropulsations at sunrise

Continuous observations of low-latitude Pc3 and Pc4 geomagnetic micropulsations were carried out at ASO (22.0°N, 198.0° geomagnetic coordinates) from November 1979 to July 1980 to confirm the ionospheric control of polarization characteristics of low-latitude pulsations presented by Saka etal. (1980). The present study confirms the previous result that D-component amplitude starts to increase with sunrise. From the present study, the following results are obtained : (1) the D-component amplitude, which is much smaller than the H-component amplitude before sunrise, increases as much as that of the H-component after sunrise, and this brings about the tilting of the major axis of the polarization ellipse from north to northwest; (2) the onset-time of the D-component increment (or tilting of the major axis) coincides with the appearance of the E-layer in the ionosphere within an hour, and the time of the coincidence shifts from season to season, in parallel with the change of sunrise ; and (3) the ellipticity of the polarization in the horizontal plane is not affected appreciably by sunrise.It is suggested that the Hall conductivity increment associated with the E-layer sunrise enhancement affects the characteristics of the D-component on the ground.     

Ionospheric electrodynamic model with an eccentric dipole geomagnetic field

A computer model of ionospheric electrodynamic processes using an eccentric dipole (ED) for the geomagnetic field has been developed. This is a development from existing models which are based on the centred dipole (CD) coaxial with the geographic axis. The ED dynamo model introduces or modifies the effects of hemispherical asymmetry and longitudinal variation in the dynamo processes through two explicit parameters—the geomagnetic field intensity and the length of the field lines. These parameters of the ED field have been quantified and displayed. An additional contribution to the above effects comes implicitly from the ionospheric parameters—plasma density and atmospheric tidal winds—which become asymmetric relative to the ED dip equator. The integrated effect of the geomagnetic and ionospheric parameters produces significant variation in the field line integrated ionospheric conductivity. The ED dynamo model shows that the peak height of the equatorial electrojet (EEJ) moves by over 2 km and height profiles of the EEJ display strong hemispherical asymmetry.     

Ionospheric modelling in support of single station location of long range transmitters

Position estimates derived from a large data base of bearing and elevation angles of signals from distant HF transmitters have been analysed, with a view to comparing the validity of available ionospheric models and to examining ionospheric limitations to the accuracy of single station location of such transmitters. In general, the accuracy of the position estimates is almost entirely controlled by a limited ability to model in sufficiently accurate detail the ionospheric effects on the signal propagation. Median miss distances for those cases with a reliable identification of the propagation mode were about 7% for both E and F2 propagation for all models considered. Difficulties were encountered with the International Reference Ionosphere, which failed to support the observed propagation in half the F2 propagation cases. Standard deviations of the bearing errors were about 0.5° for E modes and 0.7° for F2 modes and were largely attributable to the effects of the ionosphere and not to instrumental errors     

Ionospheric absorption at the magnetic equator using the A1 technique

A₁ absorption data on four operating frequencies, 1.8, 2.0, 2.2 and 2.5 MHz over a period of two years (1972–74) at Trivandrum (dip 0.6°S, geograph. lat. 8.6°N, long. 76.9°E) has been analysed to study its diurnal, day to day, long term and frequency variations. An empirical relation has been established between noon absorption and solar X-ray and 10.7 cm fluxes. Using this relation, the seasonal cosχ index has been evaluated. The deviation of the estimated absorption using this empirical relation from the observed absorption is found to be less than 10%.     

Ionospheric absorption and Pc1-type micropulsations following enhanced geomagnetic activity

Data of Pc 1-type micropulsations and ionospheric absorption derived from measurements at mid-latitudes were analysed during and after geomagnetically disturbed periods. In comparison to the normal Pc 1 activity, a rather reduced one has been found during disturbed conditions. A distinct increase of Pc 1 activity occurred, however, after selected key days in the investigated interval between days ‘+2’ and ‘+7’. A clear after-effect could also be shown for ionospheric absorption, measured at three frequencies in Kühlungsborn (φ∼55°N) and a quite moderate one at somewhat lower geographic latitudes (φ ∼ 48°–49°N). In Kühlungsborn the ionospheric after-effect is particularly prominent at all investigated frequencies in case of a simultaneous after-effect in Pc 1 pulsations. It is restrained in the LF-range and missing in the MF-range when the after-effect is lacking in Pc 1 pulsations.     

Ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect

Using long-term ionosonde measurements in mid-latitudes (Juliusruh: 54.6°N, 13.4°E; 1957–1990), the first experimental hints of a decrease of the peak height of the ionospheric F2-layer were found. In contrast to that the long-term variations of the peak electron densities in the F2-layer, as well as the E-layer, are small. These results qualitatively agree with the predictions of Rishbeth [(1990) Planet. Space Sci.38, 945] who expected a lowering of the E- and F2-layer caused by a global cooling of the strato, meso- and thermosphere due to the increasing greenhouse effect.     

Ionospheric conductivity dependence of the cross-polar cap potential difference and global Joule heating rate

We examine the extent to which the cross-polar cap potential difference ϕ and the global Joule heating rate, U, both determined by the magnetogram-inversion method (Kamideet al., 1981, J. geophys. Res. 86, 801), depend upon the assumed conductance models. For this purpose two statistically-determined conductance models developed by Siroet al. (1982, J. geophys. Res. 87, 8215) and ahn et al. (1983b, Planet. Space Sci. 31, 641), and a realistic conductance distribution estimated from bremsstrahlung X-ray image data (Ahnet al., 1989, J. geophys. Res. 94, 2565) have been used. As expected from earlier studies, U is less affected by the choice of conductance models than is ϕ. This is because U is a globally integrated quantity, and thus the local structures of the electric potential pattern do not affect it appreciably, whereas they are crucial in determining ϕ, which is defined as the difference between the maximum and minimum potential values usually found in the dawn and dusk sectors, respectively. A comparison between Uand ϕ based on the statistical conductance models and U and ϕ based on a realistic conductance distribution shows that there are considerable similarities, thus enabling us to use statistical conductance models as a first approximation in deriving such global quantities as the cross-polar cap potential difference and the global Joule heating rate in the study of solar wind-magnetosphere coupling. Several suggestions are made for improving the present available conductance models and some limitations (possibly intrinsic ones) are also discussed.     

Ionospheric electron content observations during the total solar eclipse of February 16, 1980

Observations on the Faraday rotation of a transionospheric VHF signal obtained from a network of four stations near the path of totality during the total solar eclipse of 16 February 1980 are reported. A small decrease of 3–4% in the total ionization has been obtained around the time of totality. Absence of any periodic structure following the eclipse indicates that the TIDs are not of significant amplitude in the present case to be detected by the Faraday rotation technique.