Doppler shift pulsations on whistler mode signals from a VLF transmitter

Whistler mode signals from the NAA transmitter (24 kHz) received at Faraday, Antarctica are processed to obtain the Doppler shift at a much higher time resolution than has previously been possible. This has allowed the observation of pulsations of about 13 mHz frequency which are believed to be associated with hydromagnetic waves in the magnetosphere. The pulsations are observed separately on signals with a number of discrete group delay features that can be interpreted as individual whistler ducts. Using the measured pulsation phase over the array of ducts the phase velocity and wave normal direction of the hydromagnetic wave in the equatorial plane are estimated. The direction of propagation is consistent with a source on the dayside magnetopause.The association between whistler mode Doppler shifts and hydromagnetic waves has been reported before but not, as far as we are aware, using an experimental technique that allows measurements on individual ducts in order to determine the direction of propagation of the hydromagnetic wave.     

In-situ observations of magnetospheric electron scattering by a VLF transmitter

High resolution pitch angle measurements of outer zone electrons in the energy range 12 keV−1.6 MeV were obtained at high altitude in the region of the high power VLF transmitter UMS [300 kW radiated at 17.1 kHz (Watt A. D., 1967, VLF Radio Engineering, Pergamon Press, Oxford)] while a resonant wave-particle interaction was in progress. Additional complementary electron measurements in the range of 36–316 keV were obtained in the drift loss cone by another satellite at low altitude along the drift path 75° east of the interaction region. The data from the low-altitude satellite confirm that UMS was precipitating particles in the inner zone, in the slot, and in the outer zone at the time that the high-altitude satellite was obtaining its data. The high-altitude pitch angle distributions indicate that, for this event, two types of scattering interactions were in progress. Particles with small pitch angles, up to 17.2° at the Equator, were being removed, resulting in an enhanced loss cone. Particles which were mirroring between 6500 km and the altitude of the spacecraft (7200) km were also being strongly scattered, resulting in a relative minimum in the pitch angle distribution around 90°. The data are interpreted as indicating that a cyclotron mode interaction with UMS waves was precipitating electrons with equatorial pitch angles up to 17.2° and that another process, perhaps electrostatic (ES) waves arising from the UMS radiations through a mode-conversion process, was present in the region above 6500 km and was efficiently scattering those particles which mirrored in that region     

Temperatures in the plasmasphere determined from VLF observations

Satellite and ground-based VLF recordings were made at SANAE, Antarctica from 1976 to 1979. In this paper we combine ground and satellite observations to determine temperatures in the plasmasphere. Scale heights in the plasmasphere are determined at high altitudes using a diffusive equilibrium model and measurements of equatorial electron densities and densities at about 3000 km. The temperatures corresponding to these scale heights show a gradual increase with increasing L-value and sharp increases of about 2000 K just inside the plasmapause.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.     

The Siple VLF transmitter as a multi-frequency probe of the Earth-ionosphere waveguide

1983 receptions of subionospheric signals radiated from Siple, Antarctica (L = 4.3) to neighboring stations Palmer (L = 2.3), Halley (L = 4.3), and South Pole (Λ = 74°), each ~ 1500 km from the horizontal (magnetically east-west) VLF transmitting antenna at Siple, were found to be strongly dependent upon azimuth and upon signal frequency. At Palmer, located equatorward in the broadside direction with respect to the antenna, signals near 2.5 kHz were often well defined, while the third harmonic of the transmitted signal, near 7.5 kHz, was not detected. Meanwhile, at Halley, the third harmonic was regularly observed and directionally stable, while the fundamental was often weak or undetectable. The field strength of the third harmonic component at Halley exceeded by ~ 40dB the level of the fundamental, when both were normalized to the same antenna input power. The large size of these effects is attributed in part to antenna properties that favor the endfire direction (toward Halley) at the 3d harmonic of the antenna half wave resonance frequency, and in general provide greater efficiency at higher frequencies. Other factors are high waveguide attenuation in the 2–4 kHz range and azimuth dependent differences in the propagating modes. The observed effects represent a way of extending the effective frequency range of the narrowband Siple antenna system, and also, by using the new crossed dipole configuration at Siple, of selectively probing certain regions of the Earth-ionosphere waveguide.     

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.     

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.     

Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations.Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.     

Multiple beam observations of mid-latitude ionospheric disturbances by the MU radar

The pulse-to-pulse beam steerability of the M U radar of Kyoto University enables us to observe multiple beam positions simultaneously. Based on 560 h of this type of data, we present two typical patterns of mid-latitude ionospheric disturbances and their horizontal traveling characteristics. Wavy structures have not been found in large-scale disturbances. Isolated disturbances travel primarily southward (equatorward) in disturbed conditions, while no preferred direction is observed in quiet conditions.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.     

VLF scattering from a column of ionization in the Earth-ionosphere waveguide

Choosing a highly idealized model, we analyze the scattering of VLF radio waves from a thin vertical column of ionization within the Earth-ionosphere waveguide. It is shown that mode conversion is produced if the column is limited in height and/or if the ionization is a function of height. However, the relative higher-mode content would be small in some situations such as in the case of the ionization produced by a cloud-to-ionosphere lightning discharge.     

Some differences in diurnal phase and amplitude variations for VLF signals

The daily change in illumination over a VLF propagation path produces a regular variation in the phase and amplitude of a received signal. Significant differences have been reported for such variations when measured at the same frequency over propagation paths of similar length but different orientation. Past attempts have been made to explain these observations in terms of stellar X-ray sources and a major variation in VLF propagation parameters at the magnetic equator. This paper shows that a minor azimuthal variation in the relative phase velocities of interfering waveguide modes propagating at night can account for the observations in some detail.     

A comparison of low- and mid-latitude D region electron densities derived from ground-based radio data

Electron density profiles, 70–110 km, for almost identical solar zenith angles over Central Europe and over Ahmedabad (India) are compared, avoiding seasonal ‘anomalies’. Low- and mid-latitude profiles agree very well in their structural features, the maximum deviation in magnitude being by a factor of 2 (around 90 km).     

Experimental estimates of electron density variations at the reflection height of VLF signals

To study the behaviour of the electron concentration at the reflection level of very low frequency (VLF) waves, two years of phase and amplitude records of the 12.9 kHz signals emitted from Omega-Argentina (43.20°S; 294.60°E) and received at Tucumán (26.90°S; 294.70°E) have been used. The experimental results are compared with values derived from the International Reference Ionosphere model (IRI-79). The experimental data show a seasonal variation not predicted by the model. Differences are explained in terms of changes of night-time atomic oxygen concentration, which control the electron density profile at the base of the night-time D-region, not taken into account in the IRI model. Values of atomic oxygen necessary to explain VLF data are comparable with published data.     

Atomic oxygen concentrations from rocket airglow observations in the equatorial region

A rocket payload designed to measure mesospheric sodium, hydroxyl and oxygen nightglow emissions, in addition to electron density and temperature, was launched from the Alcantara Launch Center (2°S, 44°W), Brazil, at 23:52 LST on 31 May 1992. The height profiles of the atomic oxygen OI557.7 nm and molecular oxygen Atmospheric (0-0) band emissions showed maxima at 100±3 km and 98±3 km, respectively. The emission data are used to calculate the atomic oxygen concentration profiles. The results show the validity for the equatorial region of the empirical parameters proposed by McDade et al. (1986).     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.