P.J.S. Williams

Non-linear interactions between tides and planetary waves resulting in periodic tidal variability

Abstract Data obtained from a meteor radar at Sheffield, U.K. (53°27′N, 1°35′W) between 30 June 1989 and 6 October 1994 have been used to study the variability of tidal amplitudes and the possibility that this is caused by non-linear interactions with planetary waves. Instances of regular variations in tidal amplitude with periods corresponding to those of simultaneously-observed planetary waves are investigated. The observed tidal variability is analysed in the light of a current theory of non-linear interaction between tides and planetary waves, which predicts the presence of secondary waves having frequencies that are the sum and difference of the primary waves involved in the interaction. Three case studies are presented in which periodogram analysis reveals the presence of these secondary waves and a bispectral analysis of the same data is used to confirm these findings. The suitability and limitations of this latter technique as a detector of quadratic phase coupling in data sets of the type used in this study are discussed.

Regions of strongly enhanced perpendicular electric fields adjacent to auroral arcs

Abstract In a joint campaign involving EISCAT, the Cornell University Portable Radar Interferometer (CUPRI), and sounding rockets, we have observed short-lived elevations of E -region electron temperatures, indicating the presence of strong electric fields. The use of a new pulse-code technique has considerably improved our EISCAT data in regions of low ionospheric electron densities. It has been found that strong and apparently short-lived enhancements of electric fields and associated E - region electron temperatures occur more commonly than long-lived ones. However, earlier EISCAT data with simultaneous optical recordings (and also some CUPRI radar data from the ERRRIS campaign) indicate that many of these events are, in fact, not short-lived, but occur in localized regions and are associated with drifting auroral forms. We show that the observed elevations of electron temperatures are created by very intense electric fields which can be found within narrow regions adjacent to auroral arcs. We discuss our observations against the background of models for electric field suppression or enhancement in the vicinity of auroral precipitation.

The generation and propagation of atmospheric gravity waves observed during the Worldwide Atmospheric Gravity-wave Study (WAGS)

Abstract During the Worldwide Atmospheric Gravity-wave Study (WAGS) in October 1985, the EISCAT incoherent scatter radar was used to observe the generation of atmospheric gravity waves in the auroral zone in conjunction with a network of magnetometers and riometers. At the same time a chain of five ionosondes, an HF-Doppler system, a meteor radar and a radio telescope array were used to monitor any waves propagating southwards over the U.K. The EISCAT measurements indicated that in the evening sector both Joule heating and Lorentz forcing were sufficiently strong to generate waves, and both frequently showed an intrinsic periodicity caused by periodic variation in the magnetospheric electric field. Two occasions have been examined in detail where the onset of a source with intrinsic periodicity was followed by a propagating wave of the same period which was detected about an hour later, travelling southwards at speeds of over 300 m s−1, by the ionosondes and the HF-Doppler radar. In both cases the delay in arrival was consistent with the observed velocity, which suggests a direct relationship between a source in the auroral zone and a wave observed at mid-latitude.

Worldwide atmospheric gravity-wave study in the European sector 1985-1990

Four campaigns of the Worldwide Atmospheric Gravity-wave Study (WAGS) have taken place in the European sector. On many occasions the onset of auroral activity in the evening and midnight sector, as indicated by EISCAT measurements of the electric field, was associated after a suitable delay with the detection of periodic ionospheric disturbance travelling southward over the U.K. at speeds between 500 and 1000 m s−1. The velocity and wavelength of the TIDs corresponded to large-scale atmospheric gravity-waves. The characteristic periods of the travelling disturbances were similar to the intrinsic time scales of the auroral activity for periods of 40 min or more, but variations on a time scale of 20 min or less were strongly attenuated. The r.m.s. amplitude of the auroral electric field was proportional to the r.m.s. amplitude of the HF Doppler-shift associated with the gravity-wave. The time-lag between the onset of strong auroral activity and the arrival of the travelling disturbance over the U.K. was usually about an hour, suggesting a source region about 2000 km north. Similar levels of activity in the afternoon did not appear to produce strong waves in the far field. This is possibly due to ion-drag in the daytime ionosphere although the effects of the lower sensitivity of the HF Doppler-network during daytime must also be considered.

EISCAT observations of tidal modes in the lower thermosphere

Abstract EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E -region and at 279 km in the F -region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E -region, it is possible to estimate the neutral wind velocity in the E -region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.

The measured relationship between electric field strength and electron temperature in the auroral E-region

When strong electric fields are applied to the auroral E-region they cause a modified two-stream instability which heats the electron population. The theory of electron-plasmon collisions [Robinson (1986) J. atmos. terr. Phys. 48, 417] predicts the relationship between the strength of the applied field and the corresponding increase in electron temperature at different heights. This relationship is non-linear and to make a valid comparison of the predicted increases in Te with the values observed accurate measurements must be made with a time resolution which matches the rapid variations in field strength. The measurements must also be corrected for the effects of neutral velocity, and care must be taken to minimise statistical bias when the results are averaged, including the effect of small errors in the assumed direction of the magnetic field. Results from EISCAT using alternating codes show a very good agreement between theory and observation. However, the significance of this comparison is limited by uncertainties in the assumed values of the electron-neutral collision frequency and the electron cooling rate.

VHF coherent radar signals from the E region ionosphere and the relationship to electron drift velocity and ion acoustic velocity

The Scandinavian Twin Auroral Radar Experiment (STARE) coherent radar system measures the Doppler shifts caused by ∼1 m plasma waves in the high-latitude E region ionosphere. These Doppler velocities are here related to the electron drift velocity and ion acoustic velocity derived from measurements with the incoherent radar system European Incoherent Scatter (EISCAT). The Doppler velocity is limited in magnitude to near the ion acoustic velocity in the plasma. For large flow angles θ, i.e., the angle between the radar line of sight and the electron drift velocity, the Doppler shifts are equal to the component of the electron drift velocity on the line of sight. For θ ∼ 40° the Doppler velocity is equal to the ion acoustic velocity at 105-km altitude, and for decreasing flow angle the Doppler velocity increases. For 0° < θ < 60° the variation with flow angle can be described as cosαθ, where the α decreases from 0.8 to 0.2 with an increase in drift speed from ∼400 to 1600 ms−1. The ratio of the line-of-sight velocity for θ ∼ 0° to the ion acoustic velocity decreases from 1.2 at low velocities to 1.05 at large velocities. The systematic variations of the Doppler shifts with drift speed and flow angle make it possible, in principle, to recover the electron velocity from the coherent radar measurements. The observations are used to illustrate how well the recovery is possible in practice.

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Abstract Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of 1. (i) a southward neutral wind 2. (ii) a vertical wind driven by Joule heating 3. (iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

Large field-aligned velocities observed by EISCAT

Abstract Large upward ion fluxes are observed near the polar cap/auroral zone boundary in the midnightdawn sector. Results from a recent model show that large upward velocities will occur in response to strong frictional heating and these results are consistent with the observations presented in this paper. There is particularly good agreement between the spatial and temporal morphology of the heating and the upward flux of ions which favours this mechanism to explain the observations. However, the time and location are also those associated with other acceleration mechanisms which cause upward motion of energised ions and these may also contribute to the large velocities observed.

The amplitude of auroral backscatter—I. Model estimates of the dependence on electron density

A model of the auroral backscatter amplitude, in the form discussed by Uspensky and Oksman et al., has been derived for the radar geometry appropriate to joint observations by the PGI auroral radars at Karmaselga and Essoyla and the EISCAT incoherent scatter radar. The model shows how refraction effects cause a strongly non-linear dependence of backscatter amplitude on electron density in the E-region. It also explains why the macro aspect sensitivity for auroral radar operating at a frequency of about 45 MHz is only 1–2 dB per degree for aspect angles greater than 5°.