Sir Granville Beynon

During my active career as the head of the Max-Planck-Institute for Aeronomy, I had many occasions of a most fruitful cooperation with Sir Granville Beynon. I remember three of them especially well.     

Sir Granville Beynon and URSI

Sir Granville Beynon has contributed much to the activities of URSI. Some memorable moments are recalled here.     

Sir Granville Beynon and international science

Sir Granville Beynon is a towering figure on the world stage of science and engineering. He has personally assisted developing countries, of which India was a particularly fortunate one, to train their young workers in the methods of research and to introduce new experimental techniques. Special mention is made here of his support for radio science in India and for his influential contributions to URSI, to the IGY (AGI in French), the IQSY and the MAP.     

The EISCAT facility

A new incoherent scatter radar facility, EISCAT, is now in operation in northern Scandinavia. The facility itself and its capabilities for performing high latitude upper atmosphere research are described.     

Thermospheric wind measurements with EISCAT

Thermospheric wind measurements with the EISCAT UHF radar around the evening Harang discontinuity are presented both in the E- and F-layers. Within the E-layer auroral oval the Lorentz and Coriolis force are shown to be more or less in balance. The neutral velocity is a factor of the order of two smaller than the ion velocity and is on average advanced 90° in a clockwise direction compared to the ion velocity. In the low electron density region just before the Harang discontinuity and outside the auroral oval a large (~250 m s⁻¹), thermally dominated neutral wind is closely followed by the ion wind in the antisolar direction. There is also a large downward flow present just before the Harang discontinuity. In the F-layer the neutral wind approximately follows the ion convection pattern, except for a couple of hours after the sudden change in the ion convection just after the passage of the evening Harang discontinuity. The close resemblance between the equilibrium ion and neutral flow when the neutral-ion collision frequency is close to twice the Earth's angular velocity may be connected to back pressures created by Joule heating in the case of an appreciable ion-neutral velocity difference.     

Large field-aligned velocities observed by EISCAT

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.     

EISCAT observations of a sequential Es layer

An experiment was carried out during the summer of 1982, using the EISCAT incoherent scatter radar, to investigate sequential Es: layers of enhanced electron density in the ionospheric E-region. Results are presented for an event occurring on 30 July, and the observations compared in a simple way with those expected from the wind-shear theory. Comparisons are made with data available from ionograms from Tromsø, Kiruna and Sodankylä, at which sites Es layers are also seen, though with differences that may be due to differences of wind direction.     

A narrow auroral arc observed with EISCAT

On the evening of 13 January 1983 we made simultaneous observations of optical and radar aurora using low light television cameras together with the EISCAT radar system. At 19 h 16 m 06 s UT an extremely bright auroral arc moved rapidly (about 2 km s⁻¹) through the EISCAT radar beam. The associated rapid rise and fall in the E-region electron density indicates that there was an intense narrow electron beam associated with the optical arc. We estimate that the ionisation rate in the E-region increased at least 20-fold (from 1 × 10¹⁰ m⁻³ s⁻¹ to >2 x 10¹¹ m⁻³ s⁻¹) for 1 or 2 s as the arc passed by. In addition, there was a brief (<4 s) increase of 130% in the signal returned from 250 km altitude which coincided with the arc crossing the radar beam at that height. In view of this coincidence, we find that a possible explanation is that the increase arose from short-lived molecular ions, for example vibrationally excited N⁺₂ ions, produced in the F-region by soft precipitation associated with the arc.     

Methods of measuring plasma velocity with EISCAT

Using the EISCAT system it is possible to determine the total vector of plasma velocity—and hence the electric field—both in the “monostatic” and “tristatic” mode.Results are presented for the evening of 18 May 1982 during the reversal of plasma velocity from westward to eastward. A comparison demonstrates that the random errors in the measured velocity are smallest in the monostatic mode using data taken at Tromsø, where the signal-to-noise ratio is highest. There is, however, a systematic error in the monostatic measurements due to horizontal gradients in plasma velocity.     

The EISCAT mesospheric measurements during the CAMP campaign

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10¹⁰ m⁻³ in the upper D-region. The measured meridional winds were lower than 10 m s⁻¹. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.     

Gyro line observations with the EISCAT VHF radar

This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.     

Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.     

Errors in velocities determined from EISCAT data

It has been reported that unusually large errors are observed in velocities derived from EISCAT data in which the power in the returned signal falls to less than 2% of that in the background noise. This suggestion has been investigated using a specialised interactive analysis package, and possible causes, both in the data and in the analysis, were considered. Subsequently, an adaptation of the usual procedure for velocity determination was considered. This involved applying the normal method to the raw data without first subtracting the background noise. The advantages of this technique are discussed, in particular for data in which the returned signal is relatively weak.     

Heating rates measured by EISCAT: latitudinal variations

Data from EISCAT are used to determine Joule and particle heating rates. As CP3 programs include scans in the meridian plane of Tromsø, some latitudinal extents of these rates are deduced. They are shown for a period of 24 h on 18–19 January 1984.If a linear relationship is assumed between height-integrated Joule heating rates and variations of the horizontal component of the magnetic field at ground level, the corresponding scaling factor depends on a certain ratio of the conductivities. Variations of this ratio versus time and latitude are shown for the same period.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

Calibration of electron densities for the EISCAT UHF radar

The accuracy of electron densities in routinely analysed EISCAT data is discussed and comparisons are made between the EISCAT measurements and those determined by rockets and an ionosonde.     

Observations of the high latitude trough using EISCAT

Data taken by EISCAT are presented as contours of electron density, ion and electron temperature and plasma velocity versus invariant latitude and local magnetic time.Three nights near midsummer were studied and in each case a trough in electron density occurred north of invariant latitude 64° shortly after local midnight (MLT 0200) and remained a prominent feature for about 3 h before moving poleward. The minimum in electron density was associated with a marked increase in ion temperature, but the electron temperature showed litttle change. In this respect the high latitude trough is clearly different from the mid-latitude trough.Full velocity measurements were not available for all three nights, but it seems that the appearance of the trough followed the start of a strong eastward plasma velocity combined with a strong upward velocity along the magnetic field line. The sudden change in plasma velocity causes frictional heating, which explains the increase in ion temperature. Upward plasma velocity is a major factor in the formation of the trough, with enhanced recombination making a smaller contribution.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

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.