Storm-time variation of foE at a low latitude station

foE changes during geomagnetic storms are studied for Ahmedabad. Individual storms show erratic effects. The average curves show a possible 5–10% decrease in the post-Main Phase Onset period and a somewhat larger decrease after about 40 storm hours.     

Pulsing hiss,pulsating aurora and micropulsations

Simultaneous conjugate observations between the GEOS 2 geostationary satellite, an all-sky TV camera operated at Andenes, Norway and a magnetometer at Tromsø, are presented. A close correlation was found between the occurrence of pulsing ELF hiss on GEOS 2, pulsating aurora observed with the TV camera and the ground detected micropulsations. However, a one-to-one correlation between the individual pulses of all three phenomena was generally not found. Ten events were chosen for a case study where the optical pulsations could be classed into two types (either classical pulsating patches or propagating forms). Classical pulsations were seen to be associated with Pi(b) micropulsations while propagating forms showed a one-to-one correlation with Pi(c) micropulsations. Type matching between the optical pulsations and two different types of ELF hiss, agreed well on a statistical basis, but the exact relationship for individual events was not so clear. On occasions, the micropulsations possessed major frequency components whose periods were longer than the periods of the other two phenomena. This is shown to imply that the observed micropulsations were of ionospheric origin in these cases.     

Interplanetary magnetic field structure and the variations of the ELF and VLF emissions in the topside ionosphere

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters [the polarity of the sector structure, the signs and magnitudes of the B_y, and B_z, components of the IMF as defined in the Solar Ecliptic coordinate system (Nishida, 1978)]. The comparison shows that:

• 1.(i) the positive polarity of the IMF sector structure (when IMF vector is directed toward the Earth) involves an enhanced probability of the detection of larger emission field intensities (>25–30dB);
• 2.(ii) the emission median intensity is ~20dB higher at B_y > 0 compared with B_y < 0;
• 3.(iii) the 0.72 kHz emission median intensity in the polar caps and at night-side auroral latitudes is lower when B_z > 0 as compared with B_z < 0;
• 4.(iv) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.

     

Error sources and travel time residuals in plasmaspheric whistler interpretation

In the interpretation of observed whistlers by curve fitting, systematic travel time residuals appeared which were studied by extensive simulations using ray-tracing, numerical integration and curve fitting. The residuals were found to originate from the commonly used approximations in the refractive index and ray path of whistler mode waves, which result in travel time increments or decrements, not accounted for in whistler interpretation. These approximations and the assumed form of the electron density distribution also lead to systematic errors in the diagnostics of plasmaspheric electron density by whistlers. In addition, the effects of other error sources, including random measurement errors, are also reviewed briefly.It is shown that the fine structure of residual curves is connected to propagation conditions. Thus, their study may yield a new research tool for studying whistler trapping, ducting structures and other features of whistler propagation. The application of residual analysis in conjunction with digital matched filtering of whistlers seems to be especially promising for further whistler studies.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Recent findings on VLF/ELF sferics

Our recent activity on VLF/ELF sferics is reviewed, and this paper is composed of two parts. The first is a new method of estimating the propagation distance and ionospheric reflection height by using sophisticated signal processing for the dispersive tails of tweek sferics near the cutoff frequencies. In the second part, we have carried out the first attempt to apply our field-analysis direction finding (developed for whistlers) to tweek sferics; the corresponding experimental results and their interpretation are presented.     

On ELF transmission in the Earth-ionosphere waveguide

The propagation constant for ELF (extremely low frequency) propagation in the Earth ionosphere waveguide is determined analytically. The derivation carried out for a planar model, with the Earth's surface impedance Z_g> 0, confirms the important result obtained earlier by Greifinger and Greifinger [(1978), Radio Sci. 13, 831] in the limitZ_g = 0. The present method avoids the use of auxiliary potentials.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

The computation of ELF radio wave fields in the Earth-ionosphere duct

Evaluation of ELF radio wave fields using the mode theory requires the computation of Legendre functions. Four different representations of the Legendre functions in terms of hypergeometric series have been programmed for computation on a microcomputer. By an appropriate choice of series for given values of the propagation constant and distance from the source, only a few (typically eight) terms are needed to calculate the Legendre functions with a precision of six decimal digits.     

ELF fields for an inhomogeneous ionosphere with diffusive lower boundary

An ELF field equation is derived for an inhomogeneous ionosphere with diffusive lower boundary. Numerical results are presented to show that consistent ELF fields independent of the stratification of the ionosphere can be obtained by using this field equation.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

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.     

ELF emission in the outer ionosphere stimulated by short fractional hop whistlers

Results of a detailed study by digital methods of the energy spectra and the frequency-time characteristics of electromagnetic radiation stimulated by short fractional hop whistlers (S.W.) observed on Intercosmos 14 are presented. The phenomenon was observed at frequencies below I kHz in the ionosphere at altitudes from 360 to 450 km during summer night. It has been shown that the intensity of the stimulated emission is greater by an order of magnitude or more than that of the natural noise background and of the same order of magnitude or a little lower than the intensity of the triggering S.W. The duration of the stimulated emission varies from 0.27 to 0.64 s. The energy spectra of this emission have two maxima at frequencies of 730+ 35 Hz and 940+ 35 Hz, with a bandwidth ΔF ≃ 150 to 300 Hz.     

The relation between ionospheric profiles and ELF propagation in the Earth-ionosphere transmission line

Based on an approximate wave solution, it is shown that, for reflexion of ELF waves from a given ionosphere described by a simple profile of ionization density, the phase-integral method may be used above a certain level, and the ionosphere may be abolished below this level. The height of ELF reflexion thus determined is independent of angle incidence, but not of frequency. The level is the one where the ionosphere passes from rapidly varying to slowly varying behavior, judged in relation to the local wavelength.An approximate solution is obtained to the mode problem in the Earth-ionosphere transmission line in terms of four crossing plane waves, one pair having O wave polarization and the other X wave polarization. The velocity of phase propagation is calculated, and also the rate of attenuation due to leakage of energy into the region above the level of reflexion. The attenuation rate due to collisional absorption below the level of reflexion is also calculated using a method similar to that employed for dielectric loss in an engineering transmission line.As the frequency descends through the ELF band, penetration of the D-region occurs in succession for the O and X waves, leading to reflexion from the E-region at the Schumann resonant frequency and penetration of the ionosphere at micropulsation frequencies. Under quiet day-time ionospheric conditions the penetration frequency-band for the D-region is around 20–60 Hz in middle and high latitudes, but around 75–100 Hz at the equator. At a frequency low enough to be reflected primarily from the E-region under quiet ionospheric conditions, an increase in D-region ionization that is just sufficient to transfer primary reflexion from the E-region to the D-region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D-region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E-region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 Hz.     

Application of a simplified theory of ELF propagation to a simplified worldwide model of the ionosphere

The approximate theory of ELF propagation in the Earth-ionosphere transmission line described by Booker (1980) is applied to a simplified worldwide model of the D- and E-regions, and of the Earth's magnetic field. At 1000 Hz by day, reflection is primarily from the gradient on the underside of the D-region. At 300 Hz by day, reflection is primarily from the D-region at low latitudes, but it is from the E-region at high latitudes. Below 100 Hz by day, reflection is primarily from the gradient on the underside of the E-region at all latitudes. By night, reflection from the gradient on the topside of the E-region is important. There is then a resonant frequency (~300 Hz) at which the optical thickness of the E-region for the whistler mode is half a wavelength. At the Schumann resonant frequency in the Earth-ionosphere cavity (~8Hz) the nocturnal E-region is almost completely transparent for the whistler mode and is semi-transparent for the Alfvén mode. Reflection then takes place from the F-region. ELF propagation in the Earth-ionosphere transmission line by night is quite dependent on the magnitude of the drop in ionization density between the E- and F-regions. Nocturnal propagation at ELF therefore depends significantly on an ionospheric feature whose magnitude and variability are not well understood. A comparison is made with results based on the computer program of the United States Naval Ocean Systems Center.     

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromsø, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.     

On the polarity and continuing currents in unusually large lightning flashes deduced from ELF events

An analysis of ELF events has enabled the magnitude and duration of the continuing currents in 274 unusually large lightning flashes to be estimated. An exponential decay model of the variation of the current moment of the source lightning stroke with time was fitted to the measured spectra using an optimisation algorithm. Of the events analysed, 212 were of positive polarity (i.e. the initial polarity of the electric field was positive) and 62 were negative. The average magnitude of the peak current moment for positive events was 5.1 × 10⁷ Am with a standard deviation of 3.2 × 10⁷ Am while that for negative events was 2.9 × 10⁷ Am with a standard deviation of 0.6 × 10⁷ Am. The average time constant T of our data set is 32 ms with a standard deviation of 6 ms for positive events and 26 ms with a standard deviation of 5 ms for negative events. Positive events are therefore associated with somewhat larger and more variable current moments and slightly longer continuing currents. We believe that these positive events are most likely to be produced by positive cloud to ground lightning.