Electric field sources in the quiet plasmasphere from whistler observations

Existing evidence for the ionospheric dynamo being the source of quiet time electric fields in the plasmasphere is reviewed. Part of a 24 h set of whistler data recorded continuously at Sanae, Antarctica (L = 4), during quiet magnetic (average K_p = 1) is analysed to obtain westward electric fields in the equatorial plane. These electric fields are examined as a function of L-value in order to infer their source. It is found that for periods of outward flow of plasma during the noon-midnight local time period, the electric fields are consistent with the dominant source being the ionospheric dynamo. There is some evidence that during the evening period of inward flow the electric fields are magnetospheric in origin, although this could also be consistent with a refined dynamo model. The observed whistler duct convection patterns do not fit either of two theoretical models, which invoke a magnetospheric field but not a dynamo field.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

Early Trimpi events from lightning-induced electric fields in the ionosphere: An alternative explanation

Two classes of ‘Trimpi’ modulation of VLF signals in the Earth-ionosphere waveguide have been identified in the literature. The more common type occurs l s or more after causative lightning strokes, the second in less than 100 ms. We explore the possibility that these early Trimpi events result from lighting-generated, electric field impulses lowering the mirror altitudes of trapped electrons. To overcome the mirror force on energetic electrons, upward-directed electric fields with strengths of a few tens of mV/m are required. This is well within the range of electric fields observed on sounding rockets above thunderstorms.     

Quasi-periodic particle precipitation and Trimpi activity at Halley,Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interaction.     

An asymmetry in the direction of arrival of whistlers at Sanae,Antarctica

The directions of arrival of over 1300 whistlers have been measured at Sanae, Antarctica and it is shown that whistlers arriving from lower latitudes than the station predominantly come from the west. In this paper we report on the observations and discuss the possible reasons for this asymmetric distribution of arrival bearings.     

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.     

Electron density profiles over the Southern Ocean from oblique incidence ionograms

This paper presents a first attempt to use oblique incidence ionograms over the 4500 km path from Sanae, Antarctica, to Grahamstown, South Africa, to deduce information about the ionosphere in the intervening regions. It is shown that existing methods for the reduction of oblique incidence ionograms to N(h) profiles give reasonable results even over the two-hop path involved. By comparison with vertical incidence ionograms made from a research ship below the reflection regions it is shown that the maximum observed frequency is normally limited by conditions at the southernmost reflection point, though this may be modified by ionospheric tilts, sunrise and sunset.     

Very-low frequency signals (2–10 kHz) received at Palmer Station,Antarctica, from the 21.4 km dipole antenna at Siple Station, 1400 km distant

Radio signals transmitted from the unique experimental VLF transmitter at Siple Station (76°S, 84°W), Antarctica, as well as VLF signals from communication and navigation systems and waves that propagate in the ionosphere and magnetosphere in the whistler mode, are regularly received and analysed at Palmer Station (65°S, 64°W), Antarctica. The amplitude and polarization properties of the Siple signals are predicted using a ray optics analysis. The amplitude of the signal received from Siple varies with frequency; observed nulls in the signal spectrum, where thesignal amplitude/alls 5–10 dB below what might be expected, are explained by the ray analysis. The amplitude spectrum is observed to be very sensitive to ionospheric conditions. Whereas the arrival bearings of signals from VLF transmitters other than Siple are found to be within 5° of their expected values, which is consistent with their expected vertical polarization and the operation of the DF system, an approximately 90° anomaly in the apparent arrival bearing of the signals from Siple is attributed to the essentially horizontal polarization of the received signal. The anomaly is found to be consistent with the theory of operation of the DF system. Occasional anomalies greater than 90° are explained in terms of a combination of polarization error and a smaller multi-path error. Siple two-hop signals and whistlers propagating on a common magnetospheric path showed arrival bearings and other properties consistent with a path end point within 200km of Siple. This suggests that these signals were received at Palmer with essentially vertical polarization.     

Size and location of lightning-induced ionisation enhancements from measurement of VLF phase and amplitude perturbations on multiple antennas

A 600-km array of five Trimpi receivers (“elements”) has been set up in New Zealand broadside to the VLF (22.3 kHz) transmitter, NWC, some 6000 km west, with element separations varying from 8 km to 550 km. Although such a five-element array is inadequate for imaging of lightning-induced ionisation enhancements (LIEs) by VLF holography, or inverse scattering, estimates of LIE size and location can be made if the shape and form of the LIE can be guessed or assumed, with even fewer elements. With five elements, tests of the assumed model can be made as well.Owing to its transform properties, the simplest model to use for scattering inversion is the Gaussian LIE distribution. For this model, and for single mode propagation, an inversion process is derived here for the full range of LIE and path dimensions, ranging from those for which the receiver is in the diffraction far field to those in which “geometric optics” dominate. This inversion process has some validity for small LIEs of other shapes of simple form. For more extreme models, the dominance of geometry or diffraction can usually be established in individual cases which then allows simple scaling procedures to be used in scattering inversion.Some 70 Trimpi events were observed on all five elements during a single night in July. 1991 (late winter). These were used to determine LIE location and size, and to test the applicability of various LIE models. It was found that most LIEs that night occurred over the Tasman Sea near the great circle from the VLF transmitter, NWC, to Wellington, generally some 500 to 2000 km from Wellington, and with north-south dimensions of 100–250 km. Much longer east-west dimensions (oriented towards NWC) are suggested to account for the very strong Trimpis observed. While about half of these LIEs that night could have had a smooth lateral spread (e.g., Gaussian), the remainder required varying degrees of fine structure, from “flat” or Butterworth LIEs to multiple LIEs as might be expected from multiduct whistlers, to explain the observed diffraction pattern exhibiting maxima and minima as well as the wide angular range over which simultaneous Trimpis were observed.     

Lightning-induced perturbations on VLF subionospheric transmissions

Phase and amplitude perturbations on VLF subionospheric transmissions from transmitter NWC to Dunedin have been studied on both MSK frequencies and at spaced receivers, 9 km apart. In any one event (a ‘Trimpi’) the phase and amplitude perturbation can be expressed in terms of a perturbation phasor. This is generally believed to be the result of lightning-induced electron precipitation (LEP) producing a localized increase in ionization near the normal reflection height for subionospheric (waveguide) VLF waves. Most of the Trimpis received on the NWC-Dunedin path can be best explained if the LEP ionization is sufficiently localized so that it acts as a scattering centre for the subionospheric VLF wave from the transmitter. It is then this scattered wave or echo at the receiver which makes the perturbation phasor. We call these ‘echo Trimpis’. The phase of the echo relative to the direct signal will differ on spaced antennae if the angle of arrival of the two signals differ. Similarly, this relative phase will vary with frequency if the group delay of the signals differ. Thus measurement of these differences allows location of the scattering centres, and so too the LEP. Locations made show a significant grouping in a region where the lightning intensity is high. This and other features strongly suggest that these echo Trimpis originate from local (southern hemisphere) lightning. This and other reasons are suggested to explain the high proportion of echo Trimpis on this path.     

On the influence of a plasma hot component on whistler propagation beyond the plasmapause

Theoretical spectrograms are computed for whistlers propagating beyond the plasmapause. The electron distribution function was modelled as consisting of a hot plus a cold component and an appropriate dispersion equation is used. A collisionless (CL) model is used for the cold electron concentration and for the hot electron component the derived model assumes a bi-maxwellian distribution function with a loss cone at the equator. The results indicate limits on the use of the cold plasma approximation (c.p.a.) in the study of magnetospheric whistler propagation beyond the plasmapause and show that whistler analysis with the c.p.a. may under or overestimate the L value of the path deduced from ground spectrograms, depending on the anisotropy of the hot component.     

Multi-station VLF direction-finding measurements in eastern Canada

The directions of propagation, in the earth-ionosphere waveguide, of multi-component two-hop whistlers recorded on 10 July 1972 by four VLF goniometer receivers in eastern Canada have been determined. Using the bearings of these great circle paths, triangulation of several whistler exit-points has been accomplished. The L-values of the whistler exit-points determined by this method are systematically lower than those expected from their nose frequencies, by ~ 0.6. Various explanations are discussed for this effect. The most satisfactory is that the whistler waves leave through the side of the ducts (in which they had propagated for most of their path through the magnetosphere) at an altitude of a few thousand kilometres, and then are refracted to lower L-values before exiting from the lower ionosphere. The results are consistent with both the duct termination altitude predicted by Bernhardt and Park (1977) for the appropriate conditions and also with the observed upper cut-off frequency of the whistlers.     

Whistler-mode propagation: results of model calculations for an inhomogeneous plasma

Waveforms computed using the new whistler model derived from Maxwell's equations were analysed. In the calculations, realistic model values for magnetospheric parameters were used. The results accurately described whistler-mode propagation in the magnetosphere and provide explanations for some features exhibited by real observed whistlers when they are analysed. In particular, solutions of the exact full-wave whistler model can explain the whistler fine structure, and may be used to develop a more accurate (matched filtering) fine-structure analysis method.     

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.     

Photometric observations of proton aurora at Sanae

Proton aurora was monitored at Sanae (invariant latitude 60°S) by recording Hβ emission with a tilting-filter photometer, and observed for K_p⩾ a 3_. Study of two nights of intense auroral activity during 1978 revealed that the times of onset of proton and electron auroral substorms and magnetic substorms agreed within 5 min. Further, electron aurora occurred poleward or equatorward of proton aurora for a location under the eastward or westward auroral electrojet respectively.     

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.     

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.     

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.