ABSTRACTS

Abstract 1

Afraimovich E.L. et al., 1991, "Simultaneous measurements of the polarisation, angles of arrival, Doppler frequency, and amplitude of the VHF radio signal", Radio Science, 26, pp 1177-1198.

This paper gives a description of the transionospheric radiosonde (TIR) designed for simultaneous measurements of the main parameters of the ETS 2 radio signal at 136 MHz (polarization, angles pf arrival, frequency Doppler shift, and amplitude, with at time resolution of 30 s; amplitude scintillations using spaced antennas with a resolution of 0.2 s). These data, referenced to exact time, are stored on magnetic tape and diskettes for the purpose of making a subsequent digital processing on a PDP-11 computer and personal computers. Data from the TIR radio interferometer can be useful when studying the dynamics of a geostationary satellite in orbit; when making a complex study of the transionospheric propagation of radio waves; and when studying the structure, dynamics, interaction, and transformation of ionospheric irregularities simultaneously in a wide range of scales (from 100 m to 1000 km). The objective of this paper is also to attract the researcher's attention to the data obtained by these authors, for possible cooperation on the level of problem formulation or loint secondary treatment and interpretation of data.

Abstract 2

Bowman G.G. and Hajkowicz L.A., 1991, "Small scale ionospheric structures associated with mid-latitude spread-F", Journal of Atmospheric and Terrestrial Physics, 53, pp 447-457.

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from a modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuation in the scintillation recording on the occassion it seems likely (with the support of other evidence in the ionograms) that the small-scale structures present were field-aligned.

Abstract 3

Fridman S.V., 1990, "The formation of small -scale irregularities as a result of ionospheric plasma mixing by large-scale drifts", Planetary and Space Sciences, 38, pp 961-972.

This paper reports some results derived by studying statistical characteristics of irregularities which are produced as a result of plasma mixing by random drifts. It is found that as time progresses, the irregularity amplitude grows in proportion to Characteristics of the irregularity spectrum undergo more violent changes. For example, the spectrum width grows according to a law close to the exponential one. Thus, relatively large-scale random motions (of the size of ~ 10 km, for example) rapidly generate a broad spectrum of small-scale irregularities as small as meter scales. A broadening of the spectrum is stopped by a transverse diffusion or by the longitudinal diffusion and recombination. An assessment is made of the influence of these processes. Equations are obtained, which describe the evolution of the correlation function of plasma density fluctuations for given statistical properties of irregular mixing motions in the case of low-intensity mixing. For the Guassian spectrum of random drifts, numerical calculations of spectra of irregularities appearing from the initial state, which contain only a smooth large-scale gradient of plasma density, are made. In the intial stage the calculated irregularity spectrum is also close to the Guassian one and does not involve small scales. A gently-sloping short-wavelength part is then found, where the dependence of spectral power on the wave vector is close to a power law with index -1 (for a one-dimensional spectrum). Subsequently, the character of the spectrum remains unchanged. The formation mechanism for irregularities investigated in this paper can operate effectively under conditions of middle and high latitudes.

Abstract 4

Fukao S. et al., 1988, "First VHF observation of midlatitude F-region field-aligned irregularities", Geophysical Research Letters, 15, pp 768-771.

We describe the first midlatitude F-region VHF radar observations of small-scale Field-Aligned Irregularities (FAI). Looking northward at ~ 34° elevation the locus of perpendicularity with the geomagnetic field B remains inside the radar beam, such that FAI can be observed, at radar ranges from 350 to 500 km, approximately. The preliminary results presented here show the large scale (> 20 km) configuration of small-scale (322.6 cm, half the radar wavelength) F-region irregularities as well as typical spectral results. Our most important finding is that at least on some occassions there is a quasi-periodic variation in time and space, on scales of approximately 12 minutes and 50 km, of the observed plasma instabilities.

Abstract 5

Fukao S. et al., 1991, "Turbulent Upwelling of the mid-Latitude Ionosphere 1. Observational Results by the MU Radar", Journal of Geophysical Research, 96, pp 3725-3746.

In this paper we present the detailed results of a series of experiments designed to study the coherent backscatter of 50-MHz radar waves from the mid-latitude F region. Data were obtained with the active phased array MU radar in Japan and include some auxiliary E region coherent echoes as well. As in other turbulent ionospheric phenomena the intense nonthermal scatter comes from irregularities oriented parallel to B. The strongest echoes correspond to the irregularities at least 20 dB stronger than thermal backscatter at the same frequency from typical F region densities at the same range. Simultaneous observations with ionosondes show that these echoes occur during strong mid-latitude spread F. As defined by ionosondes, the latter phenomenon is certainly much more widespread than the turbulent upweling events described here, but we believe that in some sense these corespond to the most violent mid-latitude spread F. The strongest echoes occur in large patches which display away Doppler shifts corresponding to irregularity motion upward and northward from the radar. At the edges of these patches there is often a brief period of toward Doppler before the echoing region ceases. On rare occasions comparable patches of strong away and toward Doppler are detected, although in such cases the Doppler width of the toward echoes is much narrower that that of the away echoes. The away patches often are characterized by mean velocities well over 250 m/s and Doppler widths (full width at half maximum) of 50 m/s. the multiple beam capability at MU allowed us to track the patches in the zonal direction on two days. The patches moved east to west in both cases at velocities of 125 m/s and 185 m/s, respectively. There is a distinct tendency for the bottom contour of the scattering region to be modulated at the same frequency as the patch occurence frequency as wellas at higher frequencies. This higher-frequency component may correspond to substructures in the large patches and to the E region coherent scatter patches which were detected simultaneously in several multiple beam experiments. In the companion paper (Kelley and Fukao, this issue), we explore a number of possible explanations for this phenomenon in more detail.

Abstract 6

Hajkowicz L.A. and Dearden D.J., 1988, "Observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle", Journal of Atmospheric and Terrestrial Physics, 50, pp 511-517.

An extended period (1973-1985) of recording of random and Fresnel type quasi-periodic (QP) scintillations in southern mid-latitudes, using satellite beacon transmissions at a frequency of 150 MHz, has provided some new information on the morphology of scintillation producing irregularities.

It has become evident that a pronounced daytime increase of the random type of scintillations in the southern winter (at 1200-1600 LT) occurs throughout the solar cycle and becomes a distinct daytime maximum during the years of sunspot minimum. Scintillations are most intense in the pre-midnight period in the southern summer (2000-2400 LT). There is a gradual decline in scintillation activity by about 40% from the period of sunspot maximum to the period of sunspot minimum. It appears that a specific type of sporadic-E, so-called constant height Es (Esc), is responsible for daytime scintillation activity in winter. Night-time scintillations are strongly correlated with the presence of the range-spread type of spread-F, but not so with the frequency-spread type.

There are two peaks in the occurrence of QP scintillations, predominantly in the southern summer: in the late morning (0800-1000 LT) and in the pre-midnight period (2000-2400 LT). The daytime QP scintillations occur mainly polewards of the station, whereas the night-time scintillations are recorded predominantly equatorwards. There is a distinct increase in the occurrence number of QP scintillations with a decrease in the sunspot number.

Abstract 7

Hajkowicz L.A., 1989, "Simultaneous Recordings of VHF Scintillation Occurrences over a Wide Range of Southern Latitudes", Indian Journal of Radio & Space Physics, 18, pp 2-9.

Scintillation studies have been conducted simultaneously at low latitude and auroral zone stations using amplitude data from the VHF beacon orbiting satellite transmissions at a frequency of 150 MHz. It is evident that a general rapid decrease in random scintillation level, from nighttime to daytime (the dawn period) druing the years of sunspot maximum, is preserved over a wide range of sub-ionospheric latitudes (from about 20° to 70° south geom. lat.). The variations in the diurnal scintillation levels are similar over a wide range of latitudes. In general, nighttime maxima and daytime minima in scintillations are associated with a similar variation in the level of auroral geomagnetic disturbances. Auroral scintillation activity shows a distinct seasonal pattern, with equinoctial maxima and solsticial minima, coinciding with a similar variation in the corresponding level of auroral magnetic K-index. The mid-latitude scintillation activity departs from this seasonal pattern; the principal maximum is observed during the summer solstice and the minimum during the vernal equinox. On a few occassions it was possible to discern in the auroral ionosphere the presence of regular, quasi-periodic (QP) scintillations; all QP scintillation events occurred at relatively high zenith angles equatorwards of an auroral station. This is in agreement with preferential direction of occurrence of the regular fading at lower latitudes. It is evident that strong random scintillations, extending from sub-auroral latitudes to the polar cap, can be present even during quiet geomagnetic conditions (K-index ~ 1); this is relatively unknown feature of the auroral scintillation morphology. It appears that the enhanced auroral scintillations of this type are associated with the occurrence of a specific type of sporadic-E, the so called constant-height sporadic-E (Esc), consisting of small-scale irregularities.

Abstract 8

Hajkowicz L.A., 1994, "Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum", Journal of Atmospheric and Terrestrial Physics, 56, pp 391-399.

A 5-yr study (1987-1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20-55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found; so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorwards edge of the auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988-1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.

Abstract 9

Kumagai H. and Ogawa T., 1986, "Behaviour of mid-latitude F-region irregularities deduced from spaced-receiver VHF scintillation measurements", Journal of Atmospheric and Terrestrial Physics, 48, pp 221-230.

Using three closely spaced antennas, mid-latitude night-time scintillations at 136 MHz were observed under geomagnetically quiet conditions during June-July 1982. By means of correlation analysis, characteristics of ionospheric irregularities such as drift velocities, shapes and sizes were investigated for seven scintillation events. In the premidnight hours mostly southward or southwestward drifts were observed, while in the postmidnight hours the percentage of northward drifts increased. Mean northward and southward drift velocities were 25 ms^-1 and 23ms^-1, respectively. Large northward drifts (>80ms^-1) were occasionally observed in the postmidnight hours. Peculiar drift reversals from northward to southward occurred abruptly in the midst of strong and high-pitch scintillations. They were accompanied by sharp increases in TEC. The directions of the major axes of the diffraction pattern ellipses are coincident with those of the geomagnetic field lines, which suggests that the irregularities are well field-aligned. The mean axial ratio and mean minor radius are 6.2 and 183m, respectively. The axial ratio increases and the minor radius decreases with increasing S₄.

Abstract 10

MacDougall J.W., 1990, "Elongation of midlatitude scintillation irregularities", Journal of Atmospheric and Terrestrial Physics, 52, pp 151-160.

This paper reports a study of the length of the midlatitude F-region irregularities which cause scintillations. The length of scintillation irregularities is usually measured by the Full Correlation Analysis (FCA) method. If observations from a typical 3-receiver array, with small spacing, are used the length is always found to be only a few kilometers. Basic ionospheric theory indicates that the irregularities should be much longer than this. In this experiment scintillations of beacon transmissions from polar-orbiting satellites were observed on a 3-receiver array with 310 m spacing, and a fourth receiver 4.6 km away was used to check the lengths measured. Analysis of the scintillation observations from the 310 m array used the FCA method. In all cases the FCA lengths, from the small array, were much shorter than those measured by the distant reveiver. Measurements using the 4.6 km receiver gave an average ratio of 44.5 and a half-length (in ionosphere) of 9.4 km. It appears that random errors in the correlation functions cause the FCA to underestimate the length of the ground pattern of irregularities when the spacing of the receiver is much less than the pattern size.

Abstract 11

Rodger A.S. and Aarons J., 1988, "Studies of ionospheric F-region irregularities from geomagnetic conjugate regions", Journal of Atmospheric and Terrestrial Physics, 50, pp 63-72.

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antartica are used to investigate the diurnal and seasonal variations in the simultaneous occurrence of medium-scale (~1-10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occassions at night when data are considered on an hour by hour basis. During solstices, the relationship is dominated by occassions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are ocassionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.

Abstract 12

Zvezdin V.N. and Fridman S.V., 1992, "Regimes of ionospheric turbulance from fractal analysis of satellite radio signal scintillations", Journal of Atmospheric and Terrestrial Physics, 54, pp 957-962.

Samples of amplitude scintillations of the radio signal from a geostationary satellite obtained at a midlatitude station near Irkutsk were processed. For calculating the fractal dimensionalities the GRASSBERGER and PROCACCIA [(1983) Physica D 9, 189] algorithm was used. Results of the data processing tend to divide into two groups. One group includes those realizations for which it was possible to obtain reliable estimates of dimensionality. Three of the seven realizations considered were in this group, and the fractal dimensionalities were found to be low (3.12-4.5). The other data fall within the second group; a reliable estimate of dimensionality for them is unobtainable in terms of the method used. We suppose that this is attributable to the high dimensionality of the process. Power spectra of the signals of this group are close to those with an exponent of -2. The spectra of the signals in the first group are markedly steeper. On the basis of the data analyzed it is supposed that there exist two modes of ionospheric turbulence in midlatitudes, namely the mode with the low dimensionality typical of localized turbulent processes, and the mode with the high dimensionality typical of homogeneous turbulence that covers an extensive region of the ionosphere.

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Last updated 07/02/1997 by Mark Keir