Starting models for the real height analysis of ionograms

Procedures are described for use in the real height analysis of ionogram data using the ordinary ray only, to allow for the presence of underlying (low density) ionisation. A controlled extrapolation of the virtual heights, with upper and lower limits, gives some useful correction under most conditions. For more accurate and consistent results a synoptic model is used to give a mean starting height at a fixed frequency of 0.5 MHz. Constraints are placed on the profile shape between 0.5 MHz and the lowest observed frequency ₁, to minimise the variations with different methods of analysis and different values of ₁. Suitable model starting heights are described for day and night conditions, and presented in tabular and graphical form. Equations are also given from which the model starting height can be calculated directly as a function of the local time, the month and the station latitude.     

Formation of ionisation layers responsible for blanketing Es during daytime counter-electrojet over magnetic equator

A model using photochemistry and transport due to electric fields and gravity wave winds has been used to explain the formation of ionisation layers observed over an equatorial station Thumba (dip 0°47′S) with rocket-borne Langmuir probes during two daytime counter-electrojet periods. These layers were seen as blanketing E_s-layers with an ionosonde at Thumba. Convergence of the metallic ions due to three-dimensional gravity wave winds and a westward electric field appears to be mainly responsible for the observed ionisation layer over the equator.     

Seasonal variations of day-time ionisation flows inferred from a comparison of calculated and observed NmF2

This paper reports on a comparison of calculated and observed monthly mean day-time ionospheric F2-peak density (NmF2) at a chain of stations from Japan to Australia for both solar minimum (1976) and solar maximum (1980). Nm values are calculated using the MSIS model for the observed peak heights (hmF2) and a simplified version of the continuity equation for day-time equilibrium conditions. The observed NmF2 values are always higher than the calculated ones in winter. This implies that a substantial downward flow of ionisation from above into the winter ionosphere is induced by the strongly poleward winter neutral wind which drives the ionisation down the field lines, lowering the peak height hmF2. In summer, winds are smaller, and the fluxes are more upward in comparison to winter. The seasonal variation of the ionisation fluxes and neutral winds are estimated for solar minimum, and compared with results of detailed calculations.     

The ionosphere: morphology,development and coupling

The morphology of the MAP/WINE winter is examined, principally from ground-based and satellite observations. Winter anomaly is evident, occurring in bursts with a west to east shift in time. Auroral activity, particularly with reference to the times of major rocket salvoes, is generally low, with Andøya to the south of the auroral boundary in most cases. Minor stratospheric warmings, of which 4 occurred, are found to correlate with minima in radio wave absorption. Salvo R1 was launched during one of the minor warmings.Using data from a broad sector of Europe, coupling between the lower thermosphere and mesosphere is seen over large areas. Westerly winds are associated with high absorption (winter anomaly) and reversal to easterly winds with stratospheric warmings and low absorption. It is found possible to select cases, from amongst the MT series of rocket launchings, corresponding to quiet conditions, stratospheric warming, winter anomaly and particle precipitation in the general absence of other effects. Examining D- and lower E-region ionisation profiles for these caes it is found that, compared with a quiet night, the stratwarm night shows the lower E-region to have reduced ionisation. The ionisation ledge is of similar shape in all cases, but occurs over different height ranges. The observed effects all point to transport being a major factor and the need to measure vertical transport over the range of geophysical conditions examined is highlighted.     

Observations of the reflection coefficient of the sporadic E-layer at high latitudes

A method to measure the reflection coefficient of the sporadic E-layer using a conventional ionosonde is described, and some results obtained at Sodankylä, Finland (Φ=63.8°, φ= 120.0°), are presented. The observations are often found to be in agreement with the theoretical frequency dependence of the reflection coefficient of a thin horizontal layer in the presence of mode coupling. Some of the results can be interpreted as indicative of scattering from small-scale irregularities or reflections from larger inhomogeneities in the E_s layer.     

Short gravity wave periodicities during AIDA

During the AIDA campaign in Puerto Rico 1–12 April 1989, the intensities of four airglow emissions were monitored: 557.7 nm (OI), 589.3 nm (Na doublet), 630.0 nm (OI) and 732.5 nm (OH 8-3). Various average brightnesses are reported. We present a summary of the periodicities from one minute to one hour observed in the three emissions originating from the lower thermosphere, based on 36 h of observing time. The periods show a preference for shorter time scales and there is a noticeable midnight minimum in wave activity, although not in brightness level. Detailed data on 19 well-defined periodicities are presented. Evidence is also presented to indicate that when a disturbance was present simultaneously in more than one airglow layer, its period was usually somewhat longer in emission layers at higher altitudes.     

Temporal behaviour of lightning HF radiation at 3 MHz near the time of first return strokes

Simultaneous recordings of the electric field and high frequency emissions (HF) at 3 MHz at times close to the first return stroke are presented. It is shown that there is no time delay between the beginning of the return stroke electric field and the build up of the HF. The delay observed in previous studies may be due to propagation effects. The results in turn suggest that the first return stroke from the very beginning is a strong source of HF at 3 MHz.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

The Shaking Frequency Effect in the Dynamics of a Model Sandy Layer

Experimental results showing the frequency-dependent behavior of both dry and saturated sandy layers subjected to a horizontal excitation on a shaking table are presented. The largest settlements of a dry layer correspond to two specific frequencies. In the case of a saturated layer, there is a single peak frequency corresponding to the largest depth of sinking of a measuring plate in liquefied subsoil. The first peak of settlements coincides with the single peak of sinking in liquefied soil. The eigenfrequencies of the layer were estimated. A modification of the compaction law was proposed for low shaking frequencies.     

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.     

Expanding the utility of Uranium-series dating of speleothems for archaeological and palaeontological applications

Uranium-series (U-series) dating of speleothems is frequently used for palaeoclimate studies but its archaeological and palaeontological applications are limited to stratigraphically significant speleothem formations, such as flowstones, to provide maximum and minimum temporal points. This study targeted soda straw stalactites for U-series dating. In contrast to other speleothem formations, soda straw stalactites are fragile, have typically short life spans (usually only years but rarely up to a few hundred years) and are frequently incorporated into cave deposits. Results of thermal ionisation mass spectrometry (TIMS) U-series dating of soda straw stalactites from palaeontological excavations at Blanche Cave, South Australia, show that small, dense, clean straws tend to yield ages closest to the time of stratigraphic deposition. A correlation between soda straw U-series ages and radiocarbon ages of charcoal from the site is shown. Multiple soda straw age determinations for a single stratigraphic unit are required in order to verify age concordance. In conjunction with other dating methods, U-series dating of soda straw stalactites may help to significantly constrain the age of stratigraphic units and associated archaeological and palaeontological deposits in order to provide more robust chronologies. Here we advocate the collection of soda straw stalactites at future excavations of limestone cave sites.     

Small scale ionospheric irregularities near regions of soft particle precipitation: scintillation and EISCAT observations

Results are presented from a coordinated experiment involving scintillation observations using transmissions from NNSS satellites and simultaneous measurements with the EISCAT ionospheric radar facility. The scintillation was used to indicate the presence of sub-kilometre scale irregularities while the radar yielded information on the larger structures in the background ionosphere. Two examples are discussed in which localised patches of scintillation were observed at L-shells near ‘blob’ like enhancements in F-region ionisation density. Elevated electron temperatures indicated that the enhancements may have had their origins in soft particle precipitation. While structuring of the precipitation on the 100 m scale cannot be completely ruled out as a source of the irregularities, in one case the blob gradient can be shown to be stable to the E λ B mechanism. The most likely cause of the irregularities appears to be shearing of the high velocity plasma flow in a region adjacent to the density enhancement. This region is characterised by a high ion temperature while the resulting scintillation has a shallow spectral slope.     

Dispersion of chirp pulses by the ionosphere

Nonlinearity of the phase time delay vs frequency of the ionospheric channel results in frequency dispersion. This distorts wideband signals and leads to amplitude reduction and ‘elongation’ of narrow pulses. Its effect on frequency modulated continuous wave signals (FMCW or chirp) is to broaden the width of the compressed pulse and to produce a chirp signal at the output of the detector instead of the ideal sine function. Several workers have studied this distortion and derived expressions which related the width of the compressed pulse to the first order derivative of the group time delay vs frequency. These expressions are limited to the case when the bandwidth of the channel is greater than the bandwidth of the transmitted chirp signal. In this paper a generalized expression for the time-bandwidth product of the chirp signal at the output of the detector is derived. Numerical calculations for the width of the compressed pulse vs its time-bandwidth product for various window functions are presented. The results are then applied to experimental data obtained over a short skywave radio link.     

Rapid onset,rapid decay (RORD), phase and amplitude perturbations of VLF subionospheric transmissions

Rapid onset (few ms), rapid decay (~ls) perturbations or RORDs occur frequently on the west-to-east signal from NWC to Dunedin, more often than not with classic Trimpis. They do not appear on an NWC mimic signal directly injected into the antenna and so cannot be broadband bursts. There is no delay between the initiating sferic and RORD start, implying that they are produced not by whistler-induced electron precipitation but directly by lightning. Observations on a multi element array show that classic Trimpis and RORDs initiated by the same sferic usually come from measurably different directions, so the lightning-induced ionisation enhancements (LIEs) which cause them must be laterally displaced. They may also be vertically displaced to explain the differing decay rates (30s versus 1 s). We conclude that RORDs are VLF echoes from vertical columns of ionisation at around 40km altitude and having vertical dimensions of some tens of km and horizontal dimensions of 1–2km, since such a column would scatter sufficient signal to fit observed amplitudes. Cloud-to-ionosphere (CID) lightning discharges (also called “cloud-to-space” and “cloud-to-stratosphere” discharges) of these visible dimensions have been observed on mountain observatories and on board the Space Shuttle.     

Effect of SMA Braces in a Steel Frame Building

Pull-back and shaking table test results on a simple model of a three-storey structure that includes shape memory alloys (SMA) copper-based dampers are presented and discussed. The model corresponds to a rigid-framed steel structure and the dampers to austenite CuAlBe wires inserted as bracing at each story. The inclusion of the dissipators in the structure increases the percentage of critical damping from 0.59% for the bare case to 5.95% for the braced system. At the same time, the structural stiffness increases making the first fundamental frequency change from 2.5–3.7 Hz (0.4–0.27s). The net effect of these two factors is a 30–60% reduction of peak relative displacements compared to the ones obtained without dissipation devices when the structure is subjected to earthquake records. Depending on records frequency contents, a reduction of the peak accelerations to near 58% also can be obtained. Additionally, a crude nonlinear analytical model has been studied that can predict the earthquake responses reasonably well.     

Monitoring schumann resonances-11. Daily and seasonal frequency variations

The time variations of the Schumann resonance peak frequencies for the first three modes are presented in the vertical electric component measured in the Nagycenk Observatory (47.6°N, 16.7°E) from May 1993 to August 1994. The average daily frequency patterns are different for the three modes, and each mode shows a distinct seasonal variation. The recurrence of this seasonal variation is also shown. The daily frequency range, in which the frequencies shift, is wider in winter than in summer in all three modes. The mean frequency level also shows a seasonal variation in the third mode. A spring-autumn asymmetry has been found in case of the first mode.     

Use of VLF transmissions in the location and mapping of lightning-induced ionisation enhancements (LIEs)

Lightning-induced ionisation enhancements (LIEs) are usually produced by short (~ 1 s) bursts of energetic electrons precipitated from the radiation belts in the process of amplifying whistlers. During their short life (~ 30 s) LIEs diffract or otherwise modify stable transmissions of VLF waves propagating in the two-dimensional Earth-ionosphere waveguide. This causes perturbations (‘Trimpis’) on the same time scale in the phase and amplitude of these VLF waves. Unless the LIEs are large (> 100 km) and smoothly varying (e.g., Gaussian distribution of ionisation enhancement) in the horizontal directions, the LIEs need not be on the great circle path (GCP) from VLF transmitter to receiver to produce Trimpis. Large and smooth LIEs produce ‘GCP Trimpis’, while small or structured LIEs produce ‘echo Trimpis’. The two can usually be distinguished, if Trimpi phase and amplitude are monitored and if the Trimpis are observed at several frequencies or on two or more spaced receivers simultaneously.If only GCP Trimpis are considered, the causative LIEs can be located and mapped by geometric optics using a network of receivers of sufficient density (spacing ~ 100 km) and a few transmitters. Provided all Trimpis are identified as GCP, their mere detection is sufficient for location. This is equivalent to locating the LIE ‘shadows’ cast onto arrays of spaced receivers by two or more transmitters. If this GCP identification is not made, or is just assumed, location and mapping (size estimation) errors can be quite large. At VLF (λ ~ 15 km) this geometric optics approach cannot be used to study the horizontal fine structure of LIEs since LIEs producing GCP Trimpis have no fine structure.Small or structured LIEs cast a diffraction pattern onto an array of spaced receivers. If both the phase and amplitude perturbation of echo Trimpis are measured at each receiver of the array, holographic techniques can be used to reconstruct the two-dimensional map or image of the causative LIEs. It is shown that, for a single system of one transmitter and a receiver array, this allows high resolution (~ 10 km) in the azimuthal dimension only. Equally high resolution in both horizontal dimensions can be achieved with two orthogonal systems. This technique works equally well on GCP Trimpis to map the causative LIEs (which are large and structureless) without incurring location errors thereby.     

Three-component a.c. E-field observations during the rocket and scatter experiment in 1988–1989 under radar auroral conditions in northern Scandinavia

During the ROSE (ROcket and Scatter Experiments) campaigns of 1988–1989 in northern Scandinavia a newly designed spherical probe (short name: KUSO, KUgelSOnde) was used to determine, in the frequency range 120–3500 Hz, the three components of the a.c. electric field being present under radar auroral conditions in the auroral E-region. Different from the normally utilized large d.c. double probes observing potential differences, the small (d = 23 cm) metallized KUSO probe measured the a.c. currents flowing through the sphere. These are mainly determined by the a.c. electric field existing in the surrounding plasma, and by the resitivity of the (small) Debye layer around its surface. Thus, if the relevant parameters are measured at the same time on one rocket the required a.c. E-field vectors can be deduced. With the aid of a simultaneously recording star sensor the rocket attitude could further be traced back to a non-rotating ground-based coordinate system. Afterwards the directions of the oscillating E-fields were deduced, both in the plane perpendicular to the Earth's magnetic (H-)field and relative to the H-field vector at the same time. The frequency distributions of these directions for about every second of flight time, for example, of more than 8000 E-field vectors contained in one such distribution, are discussed. Moreover, using FFT analyses, the spectral energy distributions of the total a.c. E-fields are obtained for the different heights. Different smaller spectral ranges of the E-field could be synthesized within the total bandwidth. The results of all the analyses are compared with the STARE, the EISCAT and the d.c. E-field observations. The electron densities measured by the Digital Impedance Probe (DIP) are also presented.     

Energy deposition rates by charged particles

A summary of measurements of the precipitation of electrons and positive ions (in the keV-MeV range) detected aboard eight rockets launched within the Energy Budget Campaign from northern Scandinavia is given, together with corresponding satellite data. In some cases strong temporal variations of the downgoing integral fluxes were observed. The fluxes provide the background for the calculated ion production rates and altitude profiles of the energy deposition into the atmosphere at different levels of geomagnetic disturbance and cosmic noise absorption. The derived ion production rates by energetic particles are compared to other night-time ionisation sources.     

MODIFICATION OF SITE RESPONSE IN PRESENCE OF LOCALISED SOFT LAYER

Experimental and numerical simulations are performed to evaluate the modification of ground response resulting from either the presence of soft layers or occurrence of partial liquefaction. Results from two densely instrumented dynamic centrifuge tests are presented to show the ambiguous role played by the presence of a soft layer. It was found that the lateral extent of the soft layer has significant influence on the overall response of the layered strata and any structure founded on it. The experimental observations are supported by simplified numerical analysis. The amplification or deamplification of the input motion is found to be a function of the ratio of the width of soft layer to the wave length. Based on the numerical analysis, a general function describing the site amplification is presented which may be used as a guide in seismic design of foundations in such layered strata.