The suppressing effect of field-aligned currents on VLF emissions in the magnetosphere

The growth rate of whistler-mode waves is calculated analytically for a bi-Maxwellian plasma in the presence of a beam of cool electrons. This beam is moving in the same direction as the gyroresonant electrons and in the opposite direction to the waves which are considered to propagate parallel (or anti-parallel) to the imposed geomagnetic field. A somewhat surprising result is found. This is that even if the anisotropy is greater than a critical value, which is strongly frequency dependent, the beam reduces the growth of the waves near half the electron gyrofrequency. For a field-aligned current density ~ 1 μA m⁻², this mechanism can explain the lack of signals near 1.4 kHz on auroral (return current) flux tubes. It can also explain the observed absorption of signals at half the electron gyrofrequency, around 7 kHz on L = 4 flux tubes, near the equatorial plane and just outside the plasmapause.     

The morphology of dayside Birkeland currents

Intense (10⁵ A) electric currents flow into and from the Earth's two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west (Y) component of the IMF. The presence of an IMF Y-component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.     

On the frequency shift in modulated VLF emissions

The nature of the shift between the frequency of geomagnetic pulsations registered on the Earth and the modulation frequency of VLF emissions registered on board satellites is analyzed. The results obtained are compatible with the mechanism of VLF emissions modulations founded on the interaction between magnetosonic waves propagating perpendicular to the geomagnetic field and the energetic electrons exciting VLF emissions. It is shown that the frequency shift considered can be used for pulsations source localization in the magnetosphere.     

A review of observational,theoretical and numerical studies of VLF triggered emissions

We review theoretical and numerical studies of VLF triggered emissions generated by manmade signals propagating in the whistler mode in the magnetosphere. Instead of listing all the past studies on the subject we select some of those works which give important ideas and the theoretical basis for future studies. The main purpose of the paper is to clarify what problems remain unresolved and to make suggestions for future studies.We first describe the phenomenon of VLF triggered emissions and give a brief review of experiments and observations. We then summarize the basic physics of VLF wave-particle interactions, which is indispensable for an understanding of the triggering mechanisms. We review important ideas and theories, as well as numerical models so far employed, summarize the present understanding of the phenomenon and consider briefly those problems which need further study. Especially, we focus our attention on numerical simulations whose capabilities in nonlinear physics are increasing with the increasing power of supercomputers.     

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.

     

The effect of geomagnetic activity on north polar currents in winter

Average north polar currents for the winter season have been derived from geomagnetic hourly means at six levels of geomagnetic activity. The twin-vortex pattern, normally associated with more disturbed conditions, persisted even on the quietest (mean Ap ≈ 1) days. When Ap increased to 24, the Harang discontinuity appeared earlier by about 3.5 h, though the current system as a whole remained unchanged in orientation. The average quiet day seems to be associated with a weak IMF oriented northwards and towards the Sun, with a low solar wind velocity and low proton temperature.     

The effect of induced currents in the sea on magnetic bays observed at a coastal observatory

It is widely accepted that return currents from the auroral electrojet are responsible for the magnetic bays in the D component. The different directions of the return currents, at either end of the electrojet, means that the sign of the D bay should change as the station moves from east to west of the central meridian of the electrojet. However, there is confusion as to whether this change occurs, giving rise to different models for the substorm current system. The possible effect of induced currents on magnetic bays is shown to be significant and should be examined for each observatory before using their magnetic bays to infer substorm current systems.An analysis of magnetic bays at Halley Bay (75°S, 27°W), a coastal site in Antarctica, shows that a change in the sign of D bays does not occur. It is shown that induced currents flowing in the sea, parallel to the coast, would have an effect that is consistent with the observations.     

On the effect of a beam of energetic electrons on the whistler-mode growth rate due to gyroresonant interactions

The dispersion relation for whistler-mode waves in the presence of an idealised beam of energetic electrons that is moving either parallel or antiparallel to the gyroresonant electrons is analysed. The dependence of the real part of the dispersion relation on the velocity of the beam is shown to be responsible for an additional growth rate, for the beam antiparallel to the gyroresonant electrons, or for a reduced growth rate for the parallel condition. The effect could be important in explaining some features of VLF emissions such as hiss which propagate through the magnetosphere in the opposite direction to the gyroresonant electrons.     

The diffraction of VLF radio waves by the Antarctic ice cap

Measurements of the amplitude and phase of VLF radio signals from the Omega transmitters on La Reunion Island and in Argentina have been made on routine Antarctic re-supply nights from Christchurch, New Zealand. It has been found that when the propagation paths to the transmitters cross the Antarctic ice cap, the direct path signals are very rapidly attenuated below the receiver noise level, the dominant signal source then being provided by the radio waves diffracting around the edge of the ice cap. These results have been made possible by the simultaneous use of the phase and amplitude data in a synthetic aperture antenna type analysis.     

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.     

The effect of solar and lunar currents on simultaneous phase path,group path and amplitude measurements

The solar and lunar variations in the phase path, group path and amplitude of a fixed frequency transmission were obtained at the September equinox over a slightly oblique path. The phase of the lunar semidiurnal tide in the phase path and amplitude were similar, the maxima occurring near 0200 lunar time, whereas the group path had a maximum near 0800 lunar time. These results were compared with other results obtained near the same location. The results suggest a complex situation in the E-region, where the height of the lunar current depends on season, and also suggest that the location and distribution of the solar and lunar currents may be different.     

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.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

The variability of ionospheric dynamo currents

The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents.     

The effect of growth on stable nitrogen isotope ratios in subadult bone collagen

Stable nitrogen isotopes have been used to reconstruct infant feeding practices as nursing infants have elevated δ¹⁵N ratios compared with their mothers. However, infancy is also a time of rapid growth, which may alter nitrogen isotope diet‐to‐tissue spacing. Several studies have documented a decrease in δ¹⁵N during growth in tissues with relatively fast accretion rates. This study investigates the effect that the growth of long bones, via collagen accretion, has on δ¹⁵N ratios. Long bones from individuals aged seven to nineteen years were obtained from a protohistoric ossuary in Ontario, Canada. Analysis of juveniles and adolescents permitted the examination of growth in a group who were not also nursing. It is concluded that a nitrogen isotope growth effect is not detectable in bone collagen from juveniles and adolescents, because: (1) δ¹⁵N ratios are not significantly different among the epiphyses, metaphyses and diaphysis of a growing long bone; (2) δ¹⁵N ratios are not significantly different between faster‐growing versus slower‐growing metaphyses; and (3) δ¹⁵N ratios are not significantly different between bones (or areas of a bone) that are still undergoing growth, versus bones that have ceased growing. The relatively slow speed of collagen accretion may explain why a growth effect is not manifested. Ultimately this research lends support to the use of nitrogen isotopes from bone collagen for infant feeding reconstructions. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of congestion and agglomeration on multifactor productivity growth in Dutch regions

It is well known that labour productivity growth in Europe isslowing down, against an increasing growth rate in the US. TheNetherlands is one of the countries in Europe with the lowestgrowth rates of productivity. This article looks at this phenomenonfrom a regional perspective and presents the results of a growthaccounting exercise applied to regional industry data of TheNetherlands between 1995 and 2002. We find that slow productivitygrowth in The Netherlands is particularly situated in the economiccore regions and is caused by slow multifactor productivity(mfp) growth. A substantial part of this slow mfp-growth canbe explained by the fact that positive agglomeration advantagesare overruled by negative congestion effects caused by trafficjams.     

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.     

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.