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.     

A modified technique to locate the sources of ELF transient events

An algorithm is proposed for the location of the sources of ELF transient events which occur in the Schumann resonance range. The procedure yields a uniformly rotating vector in the frequency domain. The vector is formed out of the recorded vertical electric and horizontal magnetic field components. The changes of this vector with frequency provide a pair of equations for the evaluation of two of the three unknown parameters: these are the distance from the source, phase and attenuation constants of the waveguide.     

Temporal variations of the global lightning activity deduced from the Schumann resonance data

Temporal variations of the global lightning activity were deduced from long-term Schumann resonance (SR) continuous records. The intensities of the horizontal magnetic field component in the vicinity of the first, second, and third SR modes were monitored at Tottori observatory (35.5°N, 134.33°E) from 1968. Variations of the effective source-observer distance were estimated using the ratios of the intensities of individual modes. This allowed us to obtain average diurnal variations of the global lightning activity for each month over a one-year period. The results show that the distances estimated between the field-site and the effective source are very stable, while the temporal changes of the fields and the global lightning intensity derived demonstrate substantial variability.     

Time domain polarization analysis of Schumann resonance waveforms

Schumann resonance waveforms in the lower ELF band are produced by distant lightning discharges. The multiplicity of world-wide thunderstorm centres provides a background spectrum which is generally incoherent. Superimposed on this background are larger amplitude events from individual thunderstorm centres. Source direction finding requires the determination of the polarization properties of these complex and superimposed waveforms. A time domain polarization analysis technique is described and applied to a variety of examples of Schumann resonance waveforms. The background waveforms display the anticipated level of incoherent polarization properties. It is demonstrated that the technique can isolate individual waveforms that are linearly polarized and which represent the arrival of short path signals from single source locations.     

Detection of elliptical polarization and mode splitting in discrete Schumann resonance excitations

Elliptical polarization and mode splitting have been detected in the magnetic component of discrete, well defined Schumann resonance excitations. These ELF excitations, which are large electromagnetic transients of approximately l s duration, are called Q-bursts and typically occur every few minutes. They are believed to be the signature of the impulsive excitation of the Earth-ionosphere cavity by ultra-large lightning currents. In this paper the magnetic polarization and spectral characteristics of four large Q-bursts are examined in detail using a new analysis technique. Two events display right-hand polarization and two display left-hand polarization. The theoretical polarization properties of the central and side multiplets of the Schumann resonances are used to define a local orthogonal coordinate system in the measurement frame in which these components may be separated. Maximum entropy spectrums computed separately for what are identified to be the central and side multiplets in this coordinate system show distinctly different eigenfrequencies for the lowest mode near 7.5 Hz. For the limited number of cases examined the magnitude of the line splitting detected using this technique is roughly 1.4–1.8 Hz, larger by nearly a factor of two than theoretical or observed values of the splitting previously reported. The frequencies of the side multiplets may lie either above or below the frequency of the central multiplet.     

Schumann resonance effects of electrical conductivity perturbations in an exponential atmospheric/ionospheric profile

Eigenmode solutions are computed for the n = 1 … 3 Schumann resonances in a perturbed, unmagnetized vertical atmospheric conductivity profile σ = 10⁻¹⁶ exp (z/3.1) mho m⁻¹ for z ⩽ 100 km and σ = 10⁻² mho m⁻¹ for z > 100 km. For the unperturbed exponential profile the radial electric field E_r is nearly constant z ≲ 40 km, and decreases rapidly above 50 km. The tangential field E_ϑ > E_r for z ≳ 65 km. The Joule dissipation profile in this case has an absolute maximum at about 50 km and a smaller relative maximum at 90 km with a deep relative minimum at 65 km. The maximum dissipation thus occurs in the middle atmosphere, making the Schumann resonances particularly susceptible to conductivity perturbations in this region. The perturbations of this study comprise Gaussian-shaped enhancements or depressions of FWHM ≈ 10 km impressed on the unperturbed profile. Eigenfrequencies and Q-values are computed for the full range of perturbation amplitudes 10⁻³−10³ and altitudes 30–90 km. The perturbations induce overall eigenfrequency variations of ± 1.0, ±1.5, and ±2.5 Hz in the n = 1, 2, and 3 modes, respectively, and Q-values spanning the range 3.5–11.0. The results of this calculation extend those of previous works investigating the Schumann resonance response to atmospheric conductivity perturbations, and may be useful for interpreting experimental observations in terms of external ionization source intensities of GCR, Lyman-α, or solar cosmic or X-rays, or variations in middle atmospheric chemical constituents.     

The spatial coherence of Schumann activity in the polar cap

The spatial coherence of the first two Schumann resonant modes has been studied at two locations in the polar cap separated by 1100 km. Measurements were made at Assistance Bay and Mould Bay, which have geomagnetic latitudes of 83° and 79°, respectively, and satellite time-keeping was employed to accurately synchronise the field stations. The coherence was found to be high, typically 95% for the first Schumann mode, and was unaffected by changes in K_p, a storm sudden commencement or a solar flare event. Polarization rotations were observed between the two stations, which could most likely be attributed to the coastline effect. The results are consistent with a stable propagation of Schumann activity from mid to high latitudes that is relatively unaffected by changes in the polar cap ionosphere.     

Microtremor HVSR Study of Site Effects in the Ilirska Bistrica Town Area (S. Slovenia)

The region of Ilirska Bistrica is one of the most seismically active areas of Slovenia, where 15 damaging earthquakes with maximum intensity equal or greater than V EMS-98 have occurred in the last 100 years. These earthquakes have shown that strong site effects are characteristic of the parts of the town that are built on soft Pliocene clay and sand overlain by Quaternary alluvium. Since there is a lack of boreholes and geophysical and earthquake data, the microtremor horizontal-to-vertical spectral ratio (HVSR) method was applied to a 250 m dense grid of free-field measurements over an extended area and to a 200 m dense grid in the town area in order to assess the fundamental frequency of the sediments. Measurements were additionally performed in ten characteristic houses to assess the main building frequencies. The effects of wind and artificial noise on the reliability of the results were analyzed. The map of the fundamental frequencies of sediments shows a distribution in a range of 1–20 Hz. The lower frequency range (below 10 Hz) corresponds to the extent of Pliocene clays and sand overlain by alluvium, which form a small basin, and the higher frequencies to flysch rocks, but variations within short distances are considerable. The measurements inside the buildings of various heights (2–6 stories) showed main longitudinal and transverse frequencies in the range 3.8–8.8 Hz. Since this range overlaps with the fundamental frequency range for Pliocene and Quaternary sediments (2–10 Hz), the danger of soil-structure resonance is considerable, especially in the northern part of the town. Soil-structure resonance is less probable in the central and southern part of the town, where higher free-field frequencies prevail. These observations are in agreement with the distribution of damage caused by the 1995 earthquake (M_L?=?4.7, I_max?=?VI EMS-98), for which a detailed damage survey data is available.     

Monitoring Schumann resonances—I. Methodology

The complex demodulation as a spectral technique has been used for the quasi-continuous determination of the actual frequencies of Schumann resonances. Applying this method, the first three modes of the vertical electric component have been measured regularly in the Nagycenk Observatory (47.6°N, 16.7°E) since May 1993.     

Steepening of reflectivity structures detected in high-resolution Doppler spectra of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar

Polar mesosphere summer echoes observed with the EISCAT 224 MHz radar frequently exhibit significant discontinuous offsets or jumps in the Doppler frequency. We can explain these frequency jumps as a result of a lifting of partially reflecting or scattering layers, which are distorted by bumps. These bumps can be caused by steepened refractivity variations, i.e. reflectivity structures. These suggestions are supported by model computations. We also notice that a relation exists between these structure shapes and gravity waves, which are steepened, but which do not necessarily break into enhanced turbulent velocity fluctuations.     

Acceleration of electrons in the ionosphere under the action of intense radio-waves near electron cyclotron harmonics

The effect of electron acceleration by intense ionospheric plasma turbulence induced by a high-power radio wave is studied theoretically, under conditions when the turbulence frequency is nearly equal to harmonics of the electron-cyclotron frequency. The turbulence located in a thin layer at the region of the pump-wave upper-hybrid resonance gives rise to the formation of an intense electron distribution function tail at high energies. Given the resonantly absorbed fraction of the pump-wave energy and the typical plasma turbulence scale, one can estimate the turbulence energy density and calculate the modified electron distribution function.     

Possible radioindications of anthropogenic influences on the mesosphere and lower thermosphere

From an analysis of the variations of various ionospheric characteristics influenced by global anthropogenic effects, it is shown that the collision frequency v_en is the parameter that changes most when the ratio between the carbon dioxide plus methane and the other components is changed. Since this collision frequency is directly involved in the formation of ionospheric absorption, the latter is recommended to be regarded as the most sensitive ground-based indicator of the global ‘cooling’ of the near-Earth space. Expressions are obtained for the estimation of man-made influences by absorption measurements. For medium latitudes, we recommend the frequency range from 400 to 800 kHz as a sensitive range for these measurements.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

Gyro line observations with the EISCAT VHF radar

This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.     

The Role of Ancient Fishing on the Desert Coast of Patagonia,Argentina

ABSTRACT

This article explores latitudinal and chronological variations in the exploitation of marine fish by the hunter-gatherers who occupied the Atlantic coast of continental Patagonia, Argentina, since the mid-Holocene. Results indicate a spatial gradient in the importance of fish. A significant reduction in the frequency of fish remains in the archaeological sites occurs at higher latitudes. The chronological trends identified also suggest differences between the assemblages of the northern and southern coasts of this long littoral area. The results obtained are further discussed in relation to identified environmental and cultural variation.     

LHR whistlers and lhr spherics in the outer ionosphere

Special types of VLF signals, which follow whistlers and spherics and have an anomalous dispersion near the lower hybrid resonance (LHR) frequency, have been observed on the low-altitude Intercosmos satellites. These signals have been named LHR whistlers and LHR spherics, respectively. A mechanism is suggested for the formation of their spectra, based on the peculiarities of quasi-resonance wave propagation at frequencies near the LHR frequencies. It is shown that the large dispersion observed may be accounted for by a significant increase in the propagation time of the wave as its frequency approaches the maximum in the LHR frequency profile.     

ASSESSMENT OF THE NATURAL FREQUENCY OF THE SEDIMENTARY COVER IN THE COLOGNE AREA (GERMANY) USING NOISE MEASUREMENTS

Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique.     

BOOKS IN SUMMARY

Book reviewed in this issue. Artifice and Design : Art and Technology in Human Experience . by Barry Allen . Between Mass Death and Individual Loss : The Place of the Dead in Twentieth ‐Century Germany . Edited by Alon Confino, Paul Betts, and Dirk Schumann. Reading Primary Sources : The Interpretation of Texts from Nineteenth‐and Twentieth ‐Century History . Edited by Miriam Dobson and Benjamin Ziemann. In the Light of Medieval Spain : Islam, the West, and the Relevance of the Past . Edited by Simon R. Doubleday and David Coleman. Diasporas . by Stéphane Dufoix . Was Ist Geschichte ? Aktuelle Entwicklungstendenzen von Geschichtsphilosophie und Geschichtswissenschaft . Edited by Wolfgang Eichhorn and Wolfgang Küttler. Past in the Making : Historical Revisionism in Central Europe after 1989. Edited by Michal Kope?ek. Time and History : The Variety of Cultures . Edited by Jörn Rüsen.     

The global distribution of ionospheric small-scale irregularities from topside sounding data

This paper examines the global distribution of electron density irregularities with scales of the order of several tens to hundreds of meters in the ionosphere by using topside sounder data from the COSMOS-1809 satellite obtained in May–June and December 1987. The diffuse traces of Z-waves on topside ionograms in a frequency band just below the upper hybrid resonance are used for diagnostics. These traces are attributed to the scattering of sounder-generated ordinary and slow extraordinary mode waves.