Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae,Antarctica

‘Trimpi’ amplitude perturbations on VLF signals received at Sanae, Antarctica, have been identified using a new computerised technique. Our survey of 1982 data, taken during magnetically disturbed times, shows that events of short duration (<25 s) constitute 60% of all events detected and that all events found are amplitude attenuations with deviations from quiescent levels ranging up to 90%. It is unusual, at Sanae, to observe the causative whistler with a Trimpi event. This, together with further evidence from Trimpi occurrence statistics, may suggest that the gyroresonant interactions responsible for some of the events occur with non-ducted whistler mode waves. A method for estimating the extent of the precipitation region is presented.     

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.     

Auroral radar observations at Siple Station,Antarctica

A dual coherent 50 MHz auroral radar installed at Siple Station, Antarctica, continuously monitored echo intensity and mean-Doppler velocity from 29 December 1976 until late March 1978. We describe here the experimental technique and present some statistical results including yearly averaged echooccurrence patterns and irregularity drift velocity characteristics. Our results show that:

• 1.(1) the irregularity drifts are approximately westward (poleward electric fields) in the evening, and eastward (equatorward electric fields) in the morning following the electric field reversal in the region of the Harang discontinuity;
• 2.(2) the Harang discontinuity under disturbed conditions (average K_p = 5₀) as seen by both radars is located around 2100–2300 MLT;
• 3.(3) the relationship between the irregularity drift velocity and the actual electron drift velocity is strongly dependent on the angle between the radar beam and the earth's magnetic field, as predicted by linear theory.

     

The geoelectric field at Davis station,Antarctica

An electric field mill is used to measure the vertical component of the geoelectric field at Davis station, Antarctica (68.6°S, 78.0°S, geographic coordinates; 74.6°S magnetic latitude). Local influences on the measurements are determined. Approximately a year of data is subjectively examined to determine periods when the ‘fair-weather’ electric field is expected to be dominant. Using a ‘cumulation of consecutive differences’ method, small intervals of data are combined to determine winter, spring and autumn diurnal ‘fair-weather’ electric field curves. A paucity of intervals not locally influenced precludes determination of a summer diurnal curve.The seasonal-diurnal curves each show a peak between 19 UT and 22 UT that is similar in temporal location and relative magnitude to the global, fair-weather, seasonal diurnal curves (see Reiter, 1992, p. 130). A local influence persists between 03 UT and 10 UT and precludes determination of a magnetospheric influence on the geoelectric field for these data.     

Gravity-wave motions in the mesosphere and lower thermosphere observed at Mawson,Antarctica

We present the results of MF radar observations of mean winds and waves in the height range 78–108 km at Mawson (67°S, 63°E), Antarctica. The measurements were made in the period from 1984 to 1990. Climatologies of the prevailing zonal and meridional circulations made with a 12-day time resolution show that the mean circulation remained relatively stable over the 6 yr of observation. Climatologies of gravity-wave motions in the 1–24 h period range were also generated. These reveal that the r.m.s. amplitudes of horizontal wave motions near the mesopause (~90 km) are about 30 m s⁻¹, and that there is some anisotropy in the motions, especially at heights below 90 km. Meridional amplitudes are larger than zonal amplitudes, which suggests a preference for wave propagation in the north-south direction. Comparisons with MST radar wind observations made near the summer solstice at Poker Flat, Alaska (65°N) and at Andøya, Norway (69°N) show similarities with the Mawson observations, but the wave amplitudes and mean motions are larger in magnitude at the northern sites. This suggests hemispheric differences in wave activity that require further study.     

Subglacial Meltwater Channels at Cape Jones,Obruchev Hills,Antarctica

Subglacial curved and winding meltwater channels, and Sichelwannen are recorded from Cape Jones in the Obruchev Hills. Such channels, sometimes referred to as P-forms, may have a variety of origins. After briefly considering the origins, it is deduced that those at Cape Jones were formed subglacially while the ice surface of the Denman Glacier became lower and bedrock was exposed The presence of water-laid glacial deposits on the summit of Cape Jones, and lakes on and adjacent to the eastern margin of the Denman Glacier suggests that the sudden release of impounded meltwater was important     

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.     

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.     

First Dynasonde observations of F-region plasma flow at Halley,Antarctica

Plasma flow vectors have been derived from data recorded by the Advanced Ionospheric Sounder (operating as a Dynasonde) at Halley, Antarctica (76°, 27°W). Single bulk flow vectors derived from the motion of echo reflection surfaces in the overhead F-region ionosphere are consistent both with plasma flow vectors, poleward of Halley, observed simultaneously by the PACE HF radar and also, for various levels of geomagnetic activity, with published mean plasma flow at the same invariant geomagnetic latitude (62°). The results demonstrate application of the method and lend support to existing evidence that the velocity measured by this kind of technique, at least for moderate to active geomagnetic activity at high latitudes, represents ionospheric plasma flow.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

Propagation characteristics of night-time whistlers in the region of equatorial anomaly

In this paper results of simultaneous multi-station observations of night-time whistlers obtained at very low latitudes for the last three years are presented. The propagation mechanism of these whistlers is also discussed. Some records can be shown to have the characteristics of ducted propagation by calculating the dispersion of the whistlers along a ducted path and by noticing the feature of strong intensity, as well as the existence of three-hop echoes. Whistlers in some other records, however, can be shown to be propagating along a non-ducted path by numerical ray-tracing, in which a non-dipole field (IGRF) is used and a negative horizontal gradient of ionization in the equatorial ionosphere is taken into account. The agreement of the results of calculation with observed facts seems to confirm the existence of a definite whistler path at very low latitudes. Finally, based on analysis of typical events, the effects of magnetic storms on the parameters of low latitude whistlers due to variations in the equatorial ionosphere are also discussed.     

Simultaneous observations of the quasi 2-day wave at Mawson,Antarctica, and Adelaide,South Australia

Observations of the Austral quasi 2-day wave at Mawson, Antarctica (67°S, 63°E) are presented and compared with those from Adelaide (35°S, 138°E). The data were obtained from partial-reflection radars which have been measuring winds continuously since mid-1984, and the results presented here are the first to record the 2-day wave in middle atmosphere winds from Mawson. They show that 2-day period oscillations of 10–15 m s⁻¹ are a regular feature of the high latitude southern hemisphere summer. The wide longitude and latitude separation of the radar stations permits estimates of propagation velocity and latitude phase structure, and results are consistent with the passage of a westward travelling Rossby-gravity (3, 3) wave.     

Vertical motions in the thermosphere over Mawson,Antarctica

High time resolution measurements of Doppler shift and broadening of the (OI) >1630 nm emission in the night airglow and aurora have provided determinations of vertical velocities and temperatures in the neutral thermosphere over Mawson, Antarctica. The vertical wind exhibits a large, rapid and complex response to geomagnetic energy input. Upward winds greater than 50 m s⁻¹ are frequently associated with the expansion phase of auroral substorms. Following the disturbance, prolonged periods of downward winds produce temperature enhancements of 200K outside the source region, thus providing a mechanism for the redistribution of geomagnetic energy. Oscillatory behaviour consistent with thermospheric gravity waves is observed during both quiet and disturbed conditions.     

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.     

A neural network approach to the classification of electron and proton whistlers

Fractional whistlers, whistlers, and proton whistlers are automatically identified and characterized by means of a neural network. A feed-forward neural network with Time Delay Neural Network (TDNN) architecture is used. It has the ability to represent structures in frequency time diagrams; a set of 50 spectrogram elements (5 Fourier components × 10 time intervals) serves as input to the network. Applications to date have used ELF data recorded on board the low-altitude AUREOL-3 satellite. A first neural network was designed to identify and characterize fractional whistlers and whistlers. A set of 997 vector data is used for the training phase and 1088 other vector data are used for evaluating performance. It is observed that fractional whistlers and whistlers can be distinguished from noise with an accuracy of 90%. A second neural network, with the same architecture, was used for studying proton whistlers. Although the training database contains less examples, the accuracy of the classification is 89%. Neural networks of this type could be used in satellites for real-time classification and characterization of electron and proton whistlers.     

Changes in the Ice Boundary of the Vestfold Hills,East Antarctica, 1947 to 1990

The position of the ice boundary around the Vestfold Hills, East Antarctica (68°30'S, 78°00'E) was observed using a 43 year record of aerial photography from 1947 to 1990. The southern boundary, formed by Sørsdal Glacier, showed two main areas of ice retreat and no areas of advance. Stagnation and retreat of this boundary is interpreted to be contraction to former limits following the Chelnok Glaciation which occurred 7lt; 700 years BP. The eastern or continental ice sheet boundary showed five sites of rapid, localised incision and downwasting around supraglacial gullies. While no area of continental ice was noted to advance, continued accumulation and firnification of wind-drifted snow in the proglacial zone of the ice sheet in the southeastern corner of the hills may signify a locally positive ice mass balance. Until ice mass balance studies are carried out in the ablation zone surrounding the hills, evidence concerning whether the ice sheet boundary is retreating, stable or advancing will remain equivocal.     

The polarisation of whistlers received on the ground near L = 4

The observed polarisation of the horizontal magnetic components of whistler mode signals received at Halley, Antarctica (L≈ 4.3), is in many cases that expected from a simple model of the transionospheric and sub-ionospheric propagation in the southern hemisphere; i.e. right-hand elliptical (field vectors rotate clockwise, looking towards the source) for ionospheric exit points close to the receiver, tending towards linear for more distant exit points. This suggests it may be possible to use the observed polarisation to estimate the propagation distance. However, in other cases, in certain frequency ranges, left-hand elliptically polarised signals have been observed. More realistic models do predict polarisation reversals at certain frequencies and exit point to receiver distances, but not over such a wide frequency range as has sometimes been observed. Also, in some cases, signals with nearly right-hand circular polarisation have been observed for exit points at large distances where linear polarisation would be expected.     

Association of whistlers with lightning discharges on the Earth and on Jupiter

The association between whistlers and lightning discharges has been reviewed on the basis of terrestrial ionospheric satellite observations of VLF radio noise. Evidence indicating that the observed low-latitude radio noise is associated with thunderstorms includes (1) amplitude distribution and noise properties, (2) geographical location, (3) diurnal variation in activity, and (4) diurnal variation of frequency spectrum. Corresponding studies on the propagation of sferics in the ionosphere and the excitation of whistlers recently carried out for Jupiter are presented here and compared with the terrestrial studies.