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

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.

     

Comparisons between EISCAT observations and model calculations of the high latitude ionosphere

Calculations using a numerical model of the convection dominated high latitude ionosphere are compared with observations made by EISCAT as part of the UK-POLAR Special Programme. The data used were for 24–25 October 1984, which was characterized by an unusually steady IMF, with Bz < 0 and By > 0; in the calculations it was assumed that a steady IMF implies steady convection conditions. Using the electric field models of Heppner and Maynard (1983) appropriate to By > 0 and precipitation data taken from Spiroet al. (1982), we calculated the velocities and electron densities appropriate to the EISCAT observations. Many of the general features of the velocity data were reproduced by the model. In particular, the phasing of the change from eastward to westward flow in the vicinity of the Harang discontinuity, flows near the dayside throat and a region of slow flow at higher latitudes near dusk were well reproduced. In the afternoon sector modelled velocity values were significantly less than those observed. Electron density calculations showed good agreement with EISCAT observations near the F-peak, but compared poorly with observations near 211 km. In both cases, the greatest disagreement occurred in the early part of the observations, where the convection pattern was poorly known and showed some evidence of long term temporal change. Possible causes for the disagreement between observations and calculations are discussed and shown to raise interesting and, as yet, unresolved questions concerning the interpretation of the data. For the data set used, the late afternoon dip in electron density observed near the F-peak and interpreted as the signature of the mid-latitude trough is well reproduced by the calculations. Calculations indicate that it does not arise from long residence times of plasma on the nightside, but is the signature of a gap between two major ionization sources, viz. photoionization and particle precipitation.     

Observations of high latitude ionospheric conductances

A brief historical review of the development of models of the ionospheric conductivities with special emphasis on high latitude regions and the auroral zone is presented. It is with great admiration that we must conclude that the physical understanding of the importance of the ionospheric conductances was well perceived by pioneers like Schuster and Birkeland a hundred years ago. Progress in the basic theoretical fundamentals was achieved in the late 1920s and 1930s. Realistic estimates were not derived until the first rocket probes measured the electron and ion content at different altitudes in the 1950s.Today we have a superior technique in resolving electron density profiles of high time and height resolution by incoherent scatter radars on the ground. The challenge that we are facing is to obtain global conductivity maps, especially at high latitudes, with a time and spatial resolution which match the details in auroral substorm phenomena. If that can be achieved, great progress in the understanding of detailed dynamical coupling in the ionosphere, magnetosphere, and thermosphere systems is expected. The imaging technique as demonstrated by the DE-satellite can be the tool which eventually materializes our desires for increased knowledge.     

Small scale irregularities associated with a high latitude electron density gradient: scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using NNSS satellites and special program measurements with the EISCAT ionospheric radar facility is described. The results reveal the presence of sub-kilometre scale irregularities in the vicinity of a long lived steep equatorwards gradient in electron density. Evidence is presented of a southwards plasma flow which would cause the gradient to be unstable to the E Λ B gradient-drift mechanism. An instability growth time of about 4 min has been estimated from the observations. Cooler electron temperatures associated with enhanced densities rules out soft particle precipitation as an irregularity source in this case.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

Observations of a day-time mid-latitude ionospheric trough

A summer, dayside, mid-latitude trough detected by a digital ionosonde located at Halley (76°S, 27°W, L = 4.2) is described. The trough is found to be present in the F2-region only and its movements are found to conform to known trough dynamics. The F1-layer shows a greater degree of development within the trough; slant type sporadic E reflections are present underneath the trough minimum. Satellite data from the northern hemisphere show a conjugate trough, with rapid ion flow occurring within it. Possible formation processes for the trough are examined. It is unlikely that depleted nightside plasma could have contributed to the trough. The trough is formed by the effect of enhanced F2 recombination rates combined with a differing solar production term for the plasma associated with the trough minimum and equatorial edge.     

Lidar sounding of the mesospheric sodium layer at high latitude

Long series of laser sounding of the sodium layer have been performed at Heyss Island (80.4°N) during the polar winters of 1977–1978 and 1978–1979. The measurements show large and rapid variations of the sodium total content (a factor of 2, about 1000s). Those variations and the correlated modification of the sodium layer could be interpreted as the response of the layer to internal gravity waves.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Voltage and current sources of high latitude thermospheric movements

In studies of the high latitude thermospheric movements caused by electromagnetic fields, the driving source in the magnetosphere is usually treated as a voltage source. There may be situations where the driving source can be treated as a current source. The transient behaviours of the thermospheric movement due to the two sources are quite different. We illustrate this.     

Penetration of hydromagnetic waves of cylindrical symmetry through the high latitude ionosphere

The problem is formulated, and boundary conditions are developed, in order to solve numerically the equations for the penetration of hydromagnetic waves of horizontal cylindrical symmetry through a stratified high latitude ionosphere and atmosphere due to sources above the ionosphere. There are two orthogonal polarization modes. The numerical results are shown for various cases. It is found that near the ground the horizontal components of both the electric and magnetic fields along the circumference direction are much smaller than the radial components.     

Incoherent scatter radar contributions to high latitude D-region aeronomy

Recent aeronomical work on the high latitude D-region is reviewed, restricting the discussion to observations of the D-region by the incoherent scatter technique. Emphasis is given to chemical aeronomy, which governs part of the coupling between the neutral atmosphere and the ionosphere, and forms the basis for the global role of the high latitude D-region. Details of the dynamics of the high latitude D-region, and thus the actual coupling with regions below and above, are, however, not discussed in this context. The aeronomical consequences of special high-latitude phenomena are discussed. These include the effects of the polar summer, precipitation of high energy electrons during auroral substorms and high ionization of the D-region during solar proton events. A detailed discussion is given on selected studies concerning the series of solar proton events that occurred in 1989. Problems of ion and neutral chemistry are readily accessible with incoherent scatter measurements through chemical modelling of the D-region. In this way the continuous nature of incoherent scatter measurements can be utilized to expand our knowledge of the D-region, which earlier was mainly based on momentarily sounding rocket experiments. However, it is pointed out how the interpretation of incoherent scatter data from the D-region strongly benefits from multi-instrument configurations. An outline is given of a possible new development based on the combined use of the Tromsø heating facility and the EISCAT incoherent scatter radars.     

Production and control of ion-cyclotron instabilities in the high latitude ionosphere by high power radio waves

The possible generation and suppression of ion-cyclotron waves in a collisional plasma by external high power electromagnetic (EM) waves with frequency close to the local upper-hybrid frequency is considered. It is shown that the ion cyclotron instability can be destabilized (stabilized) for ω₀ <ω_UH (ω₀ > ω_UH), where ω₀ is the pump frequency of the EM wave. The results are applied to naturally occurring ion-cyclotron instabilities in the high latitude ionosphere.     

Determination of the vertical neutral temperature and wind profiles using EISCAT and HF Doppler radar

Simultaneous observations by EISCAT and an HF Doppler system of a TID are presented. Crossspectral analysis of the data allows the vertical variation of the neutral temperature and horizontal wind in the thermosphere to be determined.     

A case study of a high latitude ionospheric electron density depletion

Incoherent scatter measurements inside and outside an ionospheric electron density depletion are described. The density depletion is probably caused by an enhancement of NO⁺-ions and subsequent dissociative recombination. The NO⁺-ions are increased because high electric fields present at the geographical location of the density depletion speed up the reaction O⁺ + N₂ → NO⁺ + N. The electron as well as the ion temperature within the density depletion are strongly enhanced, the latter due to Joule heating, also caused by the electric field.     

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.     

Ion-neutral dynamics in the high latitude ionosphere: first results from the INDI experiment

The INDI experiment consisted of a series of joint observations between EISCAT and a Fabry-Perot interferometer (FPI) situated at Kiruna. The FPI measured the 630 nm neutral oxygen emission at eight positions on a 30° elevation circle, giving a spatial average of the neutral wind field with a time resolution of about 15 min. The radar performed a seven-position, near-meridional scan in a region common to the optical measurements. Such simultaneous observations of the ionised and neutral components of the Earth's atmosphere allow a study of the ion energy balance and the coupling between species. The first stage of the analysis was to derive the neutral wind from the EISCAT data using the simplified ambipolar diffusion and ion energy equations. This was then compared with the direct measurements from the FPI. There was good agreement between derived and measured meridional winds, but the zonal wind values, although showing the same trends, differed in magnitude by a factor of 3–5. The reasons for this are discussed. The meridional wind data was used to derive the ion-neutral collision frequency. This was a factor of 2 or 3 less than recent model values. Preliminary comparisons of the measured electron densities with the 630 nm emission intensity gave clues to the chemistry of the emission process.     

VLF transmissions observed at anomalously high latitude above the ionosphere in the conjugate hemisphere

Ariel 3 and 4 satellite observations of the GBR 16 kHz and NAA 17.8 kHz transmissions above the ionosphere in the conjugate hemisphere show that their wave-fields generally show a rapid reduction in signal strength for geomagnetic latitudes greater than 55°–60°. Sometimes, however, the signal strength has been observed to be high in the invariant latitude range > 60°. At certain times during these observations, the signal showed clear evidence of amplification, whilst at other times the pattern of signal strength was displaced to higher latitude with the signal strength integrated over latitude being unchanged from that normally observed.It is shown that the plasmapause can guide both the NAA and GBR signals but that the efficiency of this guiding depends on the plasmapause position. The important condition is found that the plasmapause must be situated sufficiently equatorwards that half the equatorial electron gyrofrequency at the plasmapause position is greater than (or approximately equal to) the transmitter signal frequency. Ray-tracing calculations in a realistic magnetosphere model indicate that for the 16 kHz GBR signal, the efficiency of guiding falls off for L_pp, (the L-value of the plasmapause) > 3.0 and guiding effectively ceases for L_pp > 3.5.Guidance by the plasmapause results in a wave-field at higher latitude than for non-guided propagation. This will only occur when, following geomagnetic storms, the plasmapause position is at a sufficiently low L-value. This is in agreement with the experimental observations of anomalously high latitude signal reception following strong magnetic storms (K_p ≥ 4+).     

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.