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.     

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.

     

Night-time equatorial F-region zonal plasma drifts from VHF backscatter radar observations

Night-time equatorial F-region plasma drifts are deduced from VHF backscatter radar observations of F-region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.     

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S_10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.     

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.     

Field aligned airglow observations of transequatorial bubbles in the tropical F-region

It is possible to form images of the tropical F-region ionization structures, variously labelled as ‘bubbles’, ‘plumes’, or ‘depletions’, in a plane perpendicular to the magnetic field by observing the airglow emissions associated with them in a field aligned direction. Structures which are present at altitudes from 250 km to more than 700 km above the dip equator map down to the 250–350 km region, where recombination and associated airglow emissions occur, ranging from the equator to dip latitudes of 15° or more. The structures can be viewed in a field aligned direction from sites in the range 17°–23° dip latitude. Measurements with high angular resolution (as small as 0.1° in the meridian) could show structures as small as 2 km. It is possible to make simultaneous measurements in both 6300 and 7774 Å recombination emissions, from which the height h_max of the peak plasma concentration n(e)_max on the field line can be estimated from a ratio of the emission rates. It is possible to make maps of n(e)_max and h_max either by raster scanning the sky in the two emissions or by imaging them onto an imaging detector. Useful data can be obtained from one site over a range of 20° in dip latitude and 10° in dip longitude. Observations in the same magnetic meridian as a backscatter radar system are desirable, as also are observations from near magnetic conjugate points. Imaging characteristics for the observation sites in the range of dip latitude 17°–23° have been calculated.     

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.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Ground-based optical observations of daytime auroral emissions from Antarctica

First results obtained from ground-based observations on daytime amoral emission intensities from the Indian station, Maitri, in Antarctica, are presented and discussed. These results show striking increases in intensities in all the emissions (viz. low- and high-energy electron-induced and proton-induced emissions), during noon hours in a latitudinal region near the plasmapause and far from the conventional cusp zone. Wave-like disturbances are seen on moderately disturbed days in proton-induced emissions. Plausible causes for such enhancements and disturbances are discussed.     

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.     

A comparison of plasma densities by EISCAT and the Dynasonde from auroral altitudes: evidence of intense structure

Under conditions of moderately-energetic particle precipitation typical of the equatorward side of the auroral oval, plasma densities obtained from routine analysis of EISCAT Common Program data are often a factor 2 to 5 smaller than those suggested by co-located digital ionograms. We consider the reasons for this disagreement, and in particular we reject the implications of diffractive and multiplyrefractive scatter as alternatives to the usual plasma-frequency interpretation of ionogram echoes. We examine the effects of the (5 min and shorter) temporal averaging applied to the EISCAT data and conclude that together with the evidently small size (perhaps as little as 20 km) and high velocity of these structures, this accounts for much, if not all, of the disagreement. We point out the significance of the higher plasma densities in the 100–150 km height range for estimates of Joule and particle heating.     

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     

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson,Antarctica

Zenith observations of the oxygen λ1630 nm auroral/airglow emission (produced at an altitude of ∼220 to ∼250 km) were obtained with the Mawson Fabry-Perot Spectrometer (FPS) during three ‘zenith direction only’ observing campaigns in 1993. The data show many instances of strong (50 to 100 m s⁻¹) upwellings in the vertical wind, when the auroral oval is located equatorward of the zenith. Our data appear consistent with the existence of a region of upwelling up to ∼ 4° poleward of the poleward boundary of the visible auroral oval, rather than short duration, explosive heating events. The upwellings are probably the vertical component of wind shear produced by reversal of the zonal thermospheric winds, which occurs near the poleward boundary of the visible auroral oval. Zenith temperature was also seen to increase when the oval was equatorward of Mawson, showing rises of up to 300 K or more. However, this increase is at times unrelated to the upwellings, and seems to be caused by the expansion of the warm polar cap over the observing site.On a number of nights the boundary between the polar cap and the auroral oval was observed to pass over our site several times, occasionally showing a quasi-periodic expansion and contraction. We speculate that this quasi-periodic movement may be related to periodic auroral activity that is known to generate large-scale gravity waves.     

First low-power VHF radar observations of tropospheric,stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree⁻¹. Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.     

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.     

Simultaneous observations of the enhanced plasma line and of the reflected HF wave at Arecibo

A suitably devised pulsing sequence of the powerful HF transmissions radiated towards the ionosphere below the penetration frequency made it possible to separate the attenuation of the reflected HF wave due to thermal and to ponderomotive type parametric instabilities. The separation was possible on account of the different growth times of the thermal and ponderomotive type parametric instabilities. At the same time the power in the 430 MHz enhanced plasma line was recorded.The results show that previously accepted explanations of the overshoot in the 430 MHz enhanced plasma line for 5.3 MHz HF transmissions are invalid. A strong overshoot was observed but the attenuation of the powerful HF wave was too small to be observed. For 3.175 MHz HF transmissions substantial attenuation was observed on both time scales.     

F-region neutral winds from ionosonde measurements of hmF2 at low latitude magnetic conjugate regions

The behavior of the F2-peak height difference Δh_mF2, between low latitude magnetic conjugate points, is known to be governed by thermospheric winds blowing along the magnetic meridian. Ground based ionosonde measurements of h_mF2, at two pairs of magnetic conjugate stations, have been analysed in conjunction with the results of a realistic dynamic computer model of the tropical ionospheric F-region, to determine thermospheric wind velocities. The behavior of monthly average values of the sum, at conjugate points, of the thermospheric horizontal wind velocity component in the magnetic meridian, at low latitudes, has been inferred for months of solstice and equinox, as well as for periods of low and high solar activity.     

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.