Equatorial spread-F: a review of recent experimental results

A number of striking experimental verifications of the Rayleigh-Taylor instability, as applied to equatorial spread-F, have been made in the past few years. We review these observations and attempt to synthesize them with parallel theoretical developments. The results also pose some new questions concerning the turbulent cascade of energy and the evolution of an inner scale for the phenomenon. We review recent observations of bottomside altitude modulation and discuss the possible role of gravity waves in initiating the equatorial spread-F process.     

Equatorial spread-F and neutral atmospheric turbulence : a review and a comparative anatomy

A number of satellite and rocket plasma density spectra obtained during equatorial spread-F conditions are presented and discussed in the light of similar measurements in the neutral atmosphere. We discuss this comparison in some detail and find both distinct similarities and subtle differences. The horizontal spectral measurements show a peak at an outer scale quite similar to the scale of the undulations caused by gravity wave interactions with the ionosphere. This feature is similar to a buoyancy subrange but it is easy to show that the amplitudes of the plasma fluctuations are too large to be directly driven by the neutral atmosphere. At intermediate scales the plasma fluctuations have a one-dimensional horizontal spectrum with a power law well described by a ($\text{−5}\text{3}$) slope. Once again it can be shown that the neutral fluid cannot be similarly structured at 400 km altitude due to the high viscosity coefficient. A plasma cascade process seems to be operating but it is not at all clear how the spectrum is formed. Furthermore, vertical power spectra seem to run the gamut in spectral form with slopes (n) varying in the range from −1 to −3. So the horizontal spectra are near universal in form, while the vertical spectra are quite variable.     

Equatorial spread-F by electric fields and atmospheric gravity waves generated by thunderstorms

It is assumed that atmospheric gravity waves, resulting in travelling ionospheric disturbances (TIDs), and electric fields, generated by convective thunderstorms, have a reasonable influence on the large-scale structure of premidnight equatorial spread-F irregularities. The responsible mechanisms, viz the superposition of thunderstorm generated electric fields on the ionospheric electric fields being the determining factor for irregularity generation and the steepening of TID structures due to spatial resonance, are briefly outlined. It is recalled that convective activity is most pronounced in the intertropical convergence zone over the African and South American continents. A model based on the typical features of seasonal and geographical variation of tropical convection generating the TIDs is presented which can explain seasonal and geographical variations of premidnight equatorial spread-F occurrence.     

Equatorial night-time spread-F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread-F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread-F condition is particularly outstanding and persistent at night and the occurrence of spread-F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread-F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread-F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.     

Methods and results: A review of two recent publications

In reviewing two new books on archaeology's relationship with the Bible, Thompson points out a series of issues in which he is in agreement with William Dever, particularly in regard to the understanding of the Bible as a composite literary product, a library involving many different genres which was completed in the Hellenistic period, and having a wide range of surviving remnants of the past. These, however, are much more substantial than Dever allows and involve a much more complicated, secondary discourse than Dever recognizes. The central issue with which he disagrees with Dever relates to the origins of an "Israelite ethnicity," especially in regard to the reading of the Merneptah stele and the early settlements of Iron I Palestine. Finally, Thompson concludes that Dever's concentration on the analysis of realia , as the primary value of archaeology for history writing does not respond to the promise of an independence of Palestinian archaeology in history writing. In contrast, Thompson finds Neil Silberman and Israel Finkelstein's book to offer a much more coherent understanding of the problems of integrating the Bible with archaeological results and recognizes their new thesis as valuable. Their discussion of the Omride character of the legends of the United Monarchy brings scholarship much further along the way in the discussion of biblical origins and the Bible's relationship to the history of Palestine. He raises some problems with their dating of the deuteronomistic history to the time of Hezekiah and Josiah and questions their historical evaluation of the supersessionist quality of the biblical texts.     

Equatorial ionospheric electric fields during magnetospheric disturbances: local time/longitude dependences from recent EITS campaigns

Data sets collected during a few coordinated Equatorial Ionosphere-Thermosphere System (EITS) observational campaign periods, mainly from the Brazilian and Asian longitude sectors are analysed in this paper. Ionosonde magnetometer and Ionospheric Electron Content (IEC) data from the EITS-1 and -2 campaigns (during March and December 1991) are complemented by interplanetary magnetic field and some ground based data sets from other campaigns. The analysis focuses on the response of the equatorial ionospheric heights and ionization anomaly to disturbance electric fields, identified as a direct penetration electric field associated with IMF B_z changes and development of the ring current (especially the asymmetric component), and that produced by a disturbance zonal neutral wind. New evidence on the local time and longitudinal dependences of these electric fields constitute the main results of this paper. Especially, a large eastward electric field (associated with the asymmetric ring current) in the dusk-dawn sector causes significant expansion of the EIA in this sector, and amplification of the evening prereversal uplift of the F-layer over Brazil. Significant inhibition of the evening prereversal electric field enhancement seems to be produced by the disturbance zonal wind associated with the magnetic disturbances prevailing several hours earlier. Some tentative evidence on the Brazilian dusk sector disturbance field being larger than that of the Asian dusk sector support the existence of a longitude asymmetry in the intensity of the disturbance electric field.     

Negro problems in tropical Africa: Reviews of recent books

Changes in Scottish planning since the 1960s are discussed, especially the greater involvement of central government and increased consultation with the public. Steady pressure from the economic crisis and from technological change has limited the scope of the regions in planning though they retain a strategic role.     

A comparison of mid-latitude and equatorial-latitude spread-F characteristics

Experimental evidence using a fast-swept-gain technique on an ionosonde is presented to support the idea that mid-latitude spread-F irregularities are large-scale wave-like structures. Also,diurnal and annual distributions of spread-F occurrence at an equatorial station at times of low sunspot activity are shown to be similar to those found for mid-latitude stations. The sunspot-cycle variation of post-midnight spread-F occurrence is also found to be similar in the two latitude regions. The similarity of certain spread-F characteristics at both mid- and equatorial-latitude regions is discussed. An attempt is made to reconcile current spread-F models for these two latitude regions by proposing that the primary spread-F structures for equatorial regions are large-scale wave-like structures. It is further proposed that the small-scale plasma instabilities have a role of modifying the traces resulting from specular reflections from the large-scale structures.     

Satellite and rocket observations of equatorial spread-F irregularities: a two-dimensional model

Recent rocket and satellite measurements of equatorial F-region irregularities have been able to resolve wavelengths comparable to the meter-size sensitivities of the Jicamarca and Altair radar backscatter techniques. In a July 1979 rocket campaign at the Kwajalein Atoll, vertical profile measurements by ‘in situ’ plasma probes showed the F-region marked by a number of large scale plasma depletions, each having its own distribution of smaller scale irregularities and a trend toward a co-location of the more intense irregularities with positive gradients of larger scale features. Similar measurements on the S3-4 Ionospheric Irregularities Satellite have shown large scale depletions (1–3 orders of magnitude) with east-west asymmetries that point toward the western wall as the sight for the more intense plasma density fluctuations. The combined rocket and satellite measurements provide a two-dimensional model of macroscopic F-region depletions with small structures tending to develop more readily on the top and western boundaries. The model and associated power spectral analyses is in concert with a developing catalog of radar observations and the predictions of numerical simulations which employ the Rayleigh-Taylor instability as the primary mechanism for the generation of intermediate wavelength irregularities.     

Equatorial scintillations: advances since ISEA-6

Since the last equatorial aeronomy meeting in 1980, our understanding of the morphology of equatorial scintillations has advanced greatly due to more intensive observations at the equatorial anomaly locations in the different longitude zones. The unmistakable effect of the sunspot cycle in controlling irregularity belt width and electron concentration responsible for strong scintillation in the GHz range has been demonstrated. The fact that night-time F-region dynamics is an important factor in controlling the magnitude of scintillations has been recognized by interpreting scintillation observations in the light of realistic models of total electron content at various longitudes. A hypothesis based on the alignment of the solar terminator with the geomagnetic flux tubes as an indicator of enhanced scintillation occurrence and another based on the influence of a transequatorial thermospheric neutral wind have been postulated to describe the observed longitudinal variation.A distinct class of equatorial irregularities known as the bottomside sinusoidal (BSS) type has been identified. Unlike equatorial bubbles, these irregularities occur in very large patches, sometimes in excess of several thousand kilometers in the E-W direction and are associated with frequency spread on ionograms. Scintillations caused by such irregularities exist only in the VHF band, exhibit Fresnel oscillations in intensity spectra and are found to give rise to extremely long durations (~ several hours) of uninterrupted scintillations. These irregularities maximize during solstices, so that in the VHF range, scintillation morphology at an equatorial station is determined by considering occurrence characteristics of both bubble type and BSS type irregularities.The temporal structure of scintillations in relation to the in situ measurements of irregularity spatial structure within equatorial bubbles has been critically examined. A two-component irregularity spectrum with a shallow slope (p₁ ~ 1.5) at long scalelengths (> 1km) and steep slope (p₂ ~−3) at shorter scalelengths has been found in both vertical and horizontal spectra. Phase and intensity scintillation modelling was found to be consistent with this two-component irregularity spectrum.Finally, the information provided by the major experimental undertaking represented by Project Condor in the fields of night-time scintillations and zonal irregularity drifts with be briefly outlined.     

Equatorial electrojet and radio scintillations

It is shown that the day-time scintillations of VHF radio waves at the equatorial station, Huancayo, are very small, of the order of 1–2 dB peak, during the equatorial electrojet condition. If the event of complete or partial counter-electrojet occurring either on quiet or during disturbed conditions is followed by the occurrence of blanketing type of Es, then only strong day-time scintillations are observed. Counter-electrojet events followed by only the absence of Es are not found to produce any additional scintillations. Thus the day-time VHF scintillations near the magnetic equatorial regions are due to the sharp plasma gradient associated with blanketing type of sporadic E region.     

Recovery of DNA from archaeological insect remains: first results,problems and potential

We report the recovery of short fragments of PCR amplifiable ancient DNA from exoskeletal fragments of the grain weevil Sitophilus granarius (L.) extracted from Roman and medieval deposits in Northern England. If DNA preservation in archaeological insect remains is widespread then many applications in the spheres of evolutionary studies and archaeology can be conceived, some of which are outlined.     

Some effects of geomagnetic activity and ionospheric height rises on mid-latitude spread-F occurrence

The analysis investigates the base heights of the ionosphere (h′F) when spread-F is recorded at Brisbane, Australia, for 2 separate periods, namely July and August 1966 when spread-F occurs frequently and September and October 1966 when the activity is much lower. It is found that for July and August there is little tendency for spread-F to occur preferentially when the base height is above the average for the period. However, for the September–October period, spread-F occurs more often (by a factor of 2 or 3 depending on the geomagnetic activity) when the base height is above rather than below the average height. Also, this analysis shows that the overall spread-F occurrence (for both periods investigated) decreased to some extent following increased geomagnetic activity. This suppressed activity in the hours following geomagnetic activity is confirmed by superposed-epoch analyses using K indices (for Macquarie Island) as controls. It is suggested that the results of all the analyses might be explained by invoking a transition height in the ionosphere (controlled by the neutral-particle density of the upper atmosphere). Ionospheric off-vertical reflections from above this height would be recorded as spread-F traces in this model. This transition height would be low in July and August when the neutral-particle density is low and higher in September and October. It is further proposed that changes in the neutral-particle density could also be associated with reduced spread-F activity following increased geomagnetic activity, as well as influencing the diurnal, annual and sunspot-cycle variations of spread-F occurrence in mid-latitudes.     

VHF radio wave scattering due to range and frequency types of equatorial spread-F

Comparing vertical incidence ionograms during spread-F conditions at the equatorial station Huancayo and modified range time interrsity records of 50 MHz scatter echoes at Jicamarca, it has been shown that the range type of spread-F is very efficient for the back-scattering of VHF radio waves. On the otherhand, the frequency type of spread-F does not seem to produce strong echoes. It is suggested that the range type of spread-F ionogram is due to the reflection (ω = ω_p) of radio waves from large scale irregularities with structure as small as 3 m below or near the base of the F-region The frequency type spread-F ionogram is suggested due to scattering from large scale irregularities with no 3 m counterpart situated around the region of peak ionisation density.     

spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite.At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.     

The role of acoustic gravity waves in the generation of spread-F and ionospheric scintillation

In the aggregate, acoustic gravity waves in the F-region constitute a spectrum of geophysical noise extending from the frequencies involved in diurnal variations up to the Brunt-Väisälä buoyancy frequency. They drive a roughly uniform power spectrum of travelling ionospheric disturbances (TIDs) with vertical scales of the order of the atmospheric scale height H and with horizontal scales extending from the radius of the Earth down to H. It has been known since the 1950s that this permits multiple normals onto the F-region from an ionosonde, thereby creating the multiple-trace type of spread F on ionograms. At shorter scales the spectrum of TIDs decreases in strength and, below the mean free path of the neutral atmosphere, creates a spectrum of plasma turbulence aligned along the Earth's magnetic field. Progressively shorter scales are responsible for phase scintillation, for amplitude scintillation and for blur-type spread F on ionograms. A weak extension of the spectrum to scales less than the ion gyroradius is responsible for spread F and transequatorial propagation in the VHF band. Under evening conditions in equatorial regions a band of TIDs with wavelengths of the order of 600 km can, at times, have a phase velocity that matches the drift velocity of the plasma (Röttger 1978). This band of TIDs is then amplified until it breaks (Klostermeyer 1978). The associated explosive increase in plasma turbulence creates the plume phenomenon discovered by Woodmn and La Hoz (1976).     

Equatorial X-rays and their effect on the lower mesosphere

In May 1975, a sequence of rocket and balloon experiments was launched from Chilca Base, Peru (12.5°S, 76.8°W, Dip = −0.7°) as part of a program called Operation Antarqui. Detailed analysis and comparison of the data have yielded a significant and unexpected result, namely the first direct measurement of lower mesospheric response to a galactic X-ray source. This result could only have been determined at the equator, where cosmic ray background effects are minimal.Our objective was to seek out the equatorial energetic electron belt, sporadically reported to have fluxes approaching auroral levels, measure the X-ray bremsstrahlung radiation produced by this particle belt, and determine the influence of this radiation on the mesosphere and stratosphere. High altitude rockets were used to measure electron and X-ray fluxes, while near simultaneous samplings of O₃, conductivity and meteorological parameters were conducted in the lower atmosphere. In addition, monitoring of cosmic rays and energetic X-rays near 40 km was made with balloon-borne instrumentation.The results show no trace of unusual electron fluxes up to 335 km but do show a strong enhancement of X-rays between an early and late night comparison on 23–24 May. This X-ray enhancement has been found to be consistent with the observed doubling of the conductivity (and hence ionization) near 65 km. The conductivity increase cannot be explained by any other known source. Preliminary attempts to identify the X-ray source currently favor Sco X-1 as the primary source of X-ray enhancement. This conclusion is consistent with the known position of Sco X-1 for each of the two measurements, and with the energy spectrum of the observed radiation vs that of Sco X-1. A power spectrum analysis of the X-ray data has further shown a periodicity of intensity in phase with the payload coning period, which further verifies the point source origin of the X-rays.This analysis demonstrates that stellar X-ray sources can have measurable influence on lower mesospheric ionization. It also shows that the equatorial energetic electron belt is not a permanent feature of the region.