Mid-latitude frequency spread and its association with small scale ionospheric stratifications

A variety of ground based radio techniques have provided new information relating to the nature of mid-latitude F-region irregularities responsible for frequency spreading on ionograms. Firstly, an analysis of ionograms covering a restricted frequency band indicates that frequency spreading is primarily caused by duplicate traces which are often unresolved in group path on standard ionograms. Furthermore, where angle of arrival information is available, the duplicate traces are shown to represent reflections from markedly different directions and the spread in critical frequencies is therefore indicative of a horizontal gradient in the peak electron density over a scale size of the order of many tens of kilometres. Secondly, the individual duplicate traces themselves are shown to comprise quasi-horizontal trace (QHT) segments which are unresolved on conventional ionograms and contribute to the diffuse appearance of spread-F traces on those ionograms. Difficulties in attributing these observations to the widely held view that scattering from small scale structures is the causative mechanism are discussed.     

Spread-Es structure producing apparent small scale structure in the F-region

When transmitting on 5.8 MHz the Bribie Island HF radar array synthesizes a beam that is 2.5 wide. The beam can be steered rapidly across the sky or left to dwell in any direction to observe the fading rates of echoes within a small cone of angles. With the beam held stationary, the time scale associated with deep fading of F-region echoes is usually more than 5 min. This is consistent with the focusing and defocusing effects caused by the passage of ever-present medium-scale travelling ionospheric disturbances (TIDs). On occasion the time scale for deep fading is much shorter, of the order of tens of seconds or less, and this is thought to be due to the interference of many echoes from within the beam of the radar. It is shown that the echoes are not due to scatter from fine structure in the F-region, but rather due to the creation of multiple F-region paths with differing phase lengths by small, refracting irregularities in underlying, transparent spread sporadic-E, (Spread-E_s). The natural drift of the Spread-E_s causes the phase paths of the different echoes to change in different ways causing the interference.Two methods are used to investigate the rapidly fading F-region signals. Doppler sorting of the refracted F-region signal does not resolve echoes in angle of arrival suggesting that many echoes exist within a Fresnel zone [Whitehead and Monro (1975), J. atmos. terr. Phys. 37, 1427]. Statistical analysis of F-region amplitude data indicates that when the range spread in E_s is severe on ionograms, then a modified Rayleigh distribution caused by the combination of 10 or so echoes is most appropriate. Using knowledge of the refracting process the scale of E_s structure is deduced from these results. Both methods find a Spread-E_s irregularity size of the order of 1 km or less. It is proposed that the Rayleigh type F-region signals seen by Jacobsonet al. [(1991b), J. atmos. terr. Phys. 53, 63] are F-region signals refracted by spread-E_s.     

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread-F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.     

Small-scale ionospheric structures associated with mid-latitude spread-F

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 the 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 fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.     

Response of night-time equatorial F-region to magnetic disturbances

The response of the equatorial night-time F-region to magnetic stormtime disturbances has been examined using mainly ionograms recorded at Trivandrum and magnetograms recorded at high, middle and low latitudes during the magnetic storm of 23–26 November 1986. The analysis revealed a close coupling between the equatorial F-region and high latitude magnetic field disturbances originating in solar wind-magnetosphere interactions. The presence of spread-F on ionograms during this period is found to be consistent with the Rayleigh-Taylor instability mechanism for the growth of the irregularities.     

Post-sunset F-region vertical velocity variations at magnetic equator

The vertical drift velocity of the F-region in the post-sunset period at the magnetic equatorial station Trivandrum has been studied using a HF phase path sounder. The study revealed the presence of quasi-periodic fluctuations with periods in the range 4 30 min superposed on a steady vertical motion as a regular feature of the equatorial F-region in the post-sunset period. The fluctuations in the vertical velocity arc attributed to the east west electric field fluctuations generated by internal atmospheric gravity waves. The vertical velocity fluctuations can provide the necessary seed perturbations for the growth of equatorial spread-F irregularities.     

In-situ studies of equatorial spread-F over SHAR—steep gradients in the bottomside F-region and transitional wavelength results

RH-560 rockets instrumented with Langmuir probes were launched from SHAR, India (dip 11°N) for in-situ studies of electron density irregularities associated with equatorial spread-F (ESF) when the F-region plasma was drifting down and strong range spread-F was observed with an ionosonde at SHAR. A high variability was observed in the steepness of the base of the F-region. The bases were found to be steeper during the periods when the F-region plasma was drifting down. On one of the flights irregularities were observed in the region around 280 km where the gradients in electron density were downwards, indicating that the gradient drift instability is the main mechanism for their generation. Assuming a power law of the type P_k∝ kⁿ for irregularities of transitional scale (20–200 m), it was found that the spectral index n ranges between −1.5 and −4.6, when the mean integrated spectral power P_T of the irregularities in the above scale size range varied from −45 to −12 db. A relationship between n and P_T was observed and can be represented by a Gaussian function using the above expression; the altitude variation of n normalized for a P_T value of −10 db showed that the nature of spectral index remains the same between 230 km and the apogee of the rocket. This is at variance with the observations of Kelley et al. [(1982), J. geophys. Res. 87, 1575] that 280 km is the threshold altitude for the steep drift wave type of spectra to a shallower spectra.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of 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 occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the 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 occasionally 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.     

On the occurrence of equatorial spread-F in the evening hours

By comparing electron drift velocities at Jicamarca with corresponding ionograms and VHF radio scintillation records at Huancayo it has been shown that the day-to-day variability in the occurrence of equatorial spread-F irregularities in the post-sunset period depends critically on the time of reversal of the Sq electric field. The field reversal before sunset does not produce any spread-F in the evening hours, while the continuation of the day-time electric field for a couple of hours after sunset at normal strength is a favourable condition for generating spread-F.     

EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Small scale sub-auroral F-region irregularities were observed on 6–7 February 1984 by the two HF radars of the EDIA experiment while the EISCAT UHF system was scanning the ionosphere between 57° and 66° invariant latitude at a slightly different longitude. The bistatic EDIA system was mainly designed to detect the F-region irregularities at sub-auroral latitudes and to measure their perpendicular velocities. This paper is devoted to an examination of the morphology of the irregularity regions detected by the HF radars and of their production mechanisms, by comparison with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360–430 km height are not associated with any large scale horizontal density gradients in the F-region (350km). However, a strong north-south gradient observed at lower altitudes (150–200km), which is likely to indicate the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars. The EISCAT electron temperature measurements at 350km height also show horizontal gradients which are well correlated with the small scale F-region irregularities. We discuss implications of these observations on the mechanisms of production of irregularities in the sub-auroral F-region.     

Night-time equatorial F-region zonal plasma drifts from VHF backscatter radar observations

Night-time equatorial F-region plasma drifts are deduced from VHF backscatter radar observations of F-region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.     

Effective electron collision frequency measurements in the E- and F-regions

The collision frequency v in the ionosphere has often been determined by measuring differences in the amplitude and group path of two closely spaced signals reflected in the region of high group retardation. In this paper we describe a method of measuring v using a CW double-side-band modulated signal reflected obliquely in the ionosphere. This allows v to be determined on a continuous basis and it is found that the value of v obtained is 1–5 × 10⁴ s⁻¹ for the E-region and ~ 10³ s⁻¹ for the F-region. It is shown that measurements made just after sunset, when the E-region is still present, are more representative of E-region values than F-region.     

Angular characteristics of radio pulses reflected from the subpolar ionosphere

This paper presents the results derived by measuring angular spectra of HF-radio pulses reflected from the subpolar ionospheric F2-region (62°N) using vertical-incidence soundings and a phase direction finder with Doppler filtering. The results correspond to three main types. One is the classical mirror reflection from the undisturbed ionospheric F2-region, typical of mid-latitudes (deviations from zenith do not exceed 3°; the angular spectrum width is less than 1°). The second type includes oblique diffuse reflections with a deviation from zenith of from 10 to 45°. The azimuth of arrival of these reflections is distributed in the range from 0 to 360°, the angular spectrum width is from 5 to 10°, and the range varies from 400 to 600 km. The third type includes anomalous mirror reflections with small deviations from zenith (not greater than 3°) but with substantially larger detection ranges (for example, 500km) as compared with the main reflections (250–300 km).     

Measuring ionospheric movements using totally reflected radio waves

It is shown that for radio waves of a particular frequency reflected totally from the ionosphere the effect of refraction as well as reflection can be simulated by an effective reflecting surface. This mirrorlike surface will give the correct angle of arrival and Doppler shift for all radars operating at this frequency. It is theoretically possible for the effective reflecting surface to be folded back on itself, but this is unlikely except for F-region echoes refracted by sporadic E-clouds. If the surface is not folded and exists everywhere, it is always possible to describe its motion and change in terms of wave undulations. Experimental data for F-region echoes show that these wave undulations are very dispersive. However, the matching between the best fitting model and the experimental data is worse than expected for reasons we do not understand.     

HF observations of the vertical structure of mid-latitude spread-Es

The Bribie Island HF radar array (27°S, 153° E) can be set up to make angle of arrival and Doppler shift measurements throughout the range of spread-E_s, layers. Results of this experiment show that the range spread seen on ionograms is not due to multiple reflection with varying obliquity, but rather a genuine height spread exists. Where velocity measurements can be reliably made, reflector velocity appears to be a slowly varying function of height. Spread-E_s, can be blanketing or non-blanketing, sequential or non-sequential and at first impression it seems that the chief difference between spread-E_s, and normal E_s, is a small scale, partially transparent structure in lower regions that allows higher regions to be observed. It is suggested that on occasion spread-E_s, irregularities are further modulated by the passage of gravity waves.     

Mid-latitude spread-F structure

Spread-F has been observed at frequencies of 1.98, 3.84 and 5.80 MHz and multiple angles of arrival have been resolved using an HF radar near Brisbane (27°S, 153°E). The spreading of the ionogram trace has been shown to be due to a spread in angles of arrival of echoes, rather than any ‘vertical’ spreading. The reflection process appears to involve total specular reflection rather than scattering. The previously reported very strong bias for angles of arrival from the north-west at Brisbane is supported. The direction of movement of the reflection points is not radial and therefore, the structure cannot be purely frontal with purely linear movement, as is often supposed. The velocities are much less than for coexisting travelling ionospheric disturbances. The variations of angle of arrival, range and rate of change of range with frequency do not fit previously proposed ideas of the plasma distribution and an alternative is suggested in which the distortions of the isoionic surfaces resemble small, elongated, asymmetrical ‘hills’ or ‘dips’.     

Simultaneous observation of the quasi-two-day variations in the lower and upper ionosphere over Europe

The large scale character of the observed quasi-two-day fluctuations in the whole ionosphere (from D- uptoF-region maximum) over Europe is shown. The study is based on the lower and upper ionospheric data obtained in Sofia (42.9°, 23.4°E), Ebre Observatory (40.9°N, 0.5°E) and El Arenosillo (37.1°N, 6.7°W) during two summer intervals: June–August 1980 and 1983. The obtained prevailing periods for the F-region fluctuations are 52–55 h and the mean amplitude is higher than 1 MHz. It was found that the fluctuations propagate westward with a mean phase velocity between 4.6 and 6° /h. The quasi-two-day variations in the F-region maximum are probably generated by flucutations in the mesospheric, neutral wind. During the time when well developed quasi-two-day fluctuations exist in the mesospheric neutral wind, similar variations are observed in the lower ionosphere also. Possible mechanisms for generating the D- andF-region electron density fluctuations from these oscillations in the neutral wind are proposed.     

Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.     

The seasonal variations in the nighttime 6300 Å emission at midlatitudes from ISIS-II spacecraft

Optical limb observations at F-region heights from the ISIS-II satellite have been used to study the seasonal variations in the 6300 Å limb emission for nighttime conditions and the aeronomic implications. The observations were carried out over the American zone at northern midlatitudes, and refer mainly to the period 1973–1975 of low solar activity.The observed seasonal variations in the emission seem to be mainly controlled by the electron density at F-region heights for nighttime and quiet geomagnetic conditions. The winter minimum is found to be deeper than the summer minimum. The obervations give clear evidence of semiannual variation in the emission. The phase variations agree closely with that of the semiannual variations in electron density and neutral atmospheric density at F-region heights. However, the amplitude variations of the semiannual variations are found to be larger than suggested by the observed F-region electron density. The observations during highly disturbed conditions possibly show the presence of gravity waves with wavelengths around 500 km, which could transport auroral energy to lower latitudes. The midlatitude enhancements observed during disturbed conditions seem to be related to the inward movement of the plasmapause.