Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

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.     

Geomagnetic storms in the Antarctic F-region. I. Diurnal and seasonal patterns for main phase effects

New analysis procedures are used to show that the main phase mid-latitude storm effects conform to consistent patterns in local time when suitable selection rules are applied, with averaging over several years. Changes in $\text{ƒoF2}$, with respect to estimated quiet-time values, are analysed in terms of a_p(τ), a new geomagnetic index derived to take account of integrated disturbance. Reduction of $\text{ƒoF2}$ is greatest during the early morning hours, in summer, at higher geomagnetic latitudes, near solar minimum and through the more active periods. The various dependencies are quantitatively determined for the first time by creating an average ‘steady state’ disturbance, rather than following specific storm events. This approach will permit tests of competing theories using available modelling programs.     

The variability of ionospheric dynamo currents

The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents.     

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.

     

The ionospheric trough dynamics in the northern and southern hemispheres: the longitudinal and IMF effect

A study was made of the dynamics of the main ionospheric trough in the northern and southern hemispheres using data of ion density winter measurements on the Kosmos-900 satellite from 1977 to 1979. Significant longitudinal variations of the trough position have been found which prove to be different in the different hemispheres: in the northern hemisphere they have the shape of a double wave (period 180° longitude) with an amplitude of 4–6° of latitude by both day and night whereas, in the southern hemisphere, they exhibit a simple wave (period 360° longitude) with amplitude of about 6° in the night hours and 10–12° of latitude in the day hours.The analysis of the IMF influence on the trough position by day and night has shown both B_Z and B_Y to affect the shift of the ionospheric trough. It has been found that in the northern hemisphere the vertical and azimuthal IMF components act in opposite phase while in the southern hemisphere the effects of the two components are added. Analytic relationships between the trough shift magnitude and the values of B_Z and B_Y are discussed.     

The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

Riometer measurements of ionospheric radio wave absorption

Absorption was calculated at two height levels in the ionosphere, from different electron density profiles. The correlation between absorption and foF2 was studied. This study bears out theoretical results that riometer absorption occurs mainly in the D-layer and less in the upper parts of the ionosphere.     

Comparison of ionospheric electrical conductances inferred from coincident radar and spacecraft measurements and photoionization models

Height-integrated electrical conductivities (conductances) inferred from coincident Sondrestrom incoherent scatter radar and DMSP-F7 observations in the high-latitude ionosphere during solar minimum are compared with results from photoionization models. We use radar and spacecraft measurements in combination with atmospheric and ionospheric models to distinguish between the contributions of the two main sources of ionization of the thermosphere, namely, solar UV/EUV radiation and auroral electron precipitation. The model of Robinsonet al. (1987, J. geophys. Res.89, 3951) of Pedersen and Hall conductances resulting from electron precipitation appears to be in accordance with radar measurements. Published models of the conductances resulting from photoionization that use the solar zenith angle and the solar 10.7-cm radio flux as scaling parameters are, however, in discrepancy with radar observations. At solar zenith angles of less than 90°, the solar radiation components of the Pedersen and Hall conductances are systematically overestimated by most of these models. Geophysical conditions that have some bearing on the state of the high-latitude thermosphere (e.g. geomagnetic and substorm activity and a seasonal variation of the neutral gas distribution) seem to influence the conductivity distribution but are to our knowledge not yet sufficiently well modelled.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.     

Structure in the seasonal variations of ionospheric Es occurrence in the South Pacific

A survey is presented of the occurrence of strong ionospheric sporadic-E for South Pacific ionosonde stations covering a period of many years. The seasonal characteristics indicate the presence of strong non-solar effects with, for example, subtropical data showing strong afternoon enhancements of E_s activity in the autumn.     

Electron content measurements in the auroral zone using GPS: preliminary observations of the main trough and a survey of the degree of irregularity in summer

Ionospheric electron content was monitored from sites in or near Tromsø, Norway, for six months of 1993, using the transmissions from the satellites of the Global Positioning System (GPS). The data have been used for preliminary studies of two important phenomena of the high-latitude ionosphere: the main trough and the incidence of large irregularities. The latitude and motion of the trough were determined on several occasions during the spring period, and the results compared with previous data. Best agreement is with the formula of Collis and Haggstrom (1988). The incidence of large irregularities was surveyed during a four-month period, approximately from the summer solstice to the autumnal equinox, and the variation with time of day and magnetic activity has been determined. It was found that irregularities are considerably larger by night than by day, but that they are enhanced during both periods by increased magnetic activity. Statistical results are presented. It is suggested that these irregularities are the same as the “auroral blobs” previously studied by incoherent-scatter radar.     

Midlatitude measurements of the ionospheric electric field during the ALADDIN programme

Three measurements of ionospheric electric field were made during the 24 h ALADDIN rocket programme at Wallops Island (37°50′N, 75°29′W) on June 29–30, 1974. The first of these used a double probe instrument, flown at 1500 Local Solar Time, and the second and third measurements were made by barium cloud releases at evening and morning twilight. These three electric field vectors have been compared with the predictions of a number of models of electric field due to the dynamo effects of various atmospheric tides, and also of a possible magnetospheric origin. On the assumption that the measurements were made at a location equatorward of the afternoon convergence and poleward of the morning divergence in the electric field patterns related to the Sq current cystem, Stening's model of the diurnal variation of the electric field induced by the (1, −2) tidal model at the time of the Summer solstice correctly predicts the directions of the observed electric field. Forbes and Lindzen's model, incorporating the three major propagating tidal modes as well as the evanescent (1, −2) mode, also bears an acceptable relationship to the ALADDIN electric field directions. The ALADDIN E-field magnitudes are comparable with those obtained by ground-based observations (incoherent scatter) from Millstone Hill and from Saint Santin but are about half of Stening's model values, and three times those of Forbes and Lindzen.While the Millstone Hill E-field directions are compatible with the ALADDIN observations, Saint Santin E-field directions, at the same latitude but 75° difference in longitude, are distinctly different from ALADDIN, implying that longitudinal differences are significant.     

Simultaneous measurements of the ionospheric electric field by probes and radar methods

Ionospheric electric field values are presented, obtained simultaneously by the double probe technique on board a rocket and by two incoherent backscatter radar installations. The measurements were performed during auroral activity over northern Scandinavia. The spatial distribution of the field reveals pronounced local variations.     

Solar cycle and seasonal variations in the efficiency of solar flares in producing sudden ionospheric disturbances

The seasonal variation of the efficiency of solar flares in producing sudden field anomalies (SFAs) on 164 kHz, reported by Letfus and Nestorov (1977), has been investigated for the complete 20th solar cycle. This variation persists with one exception of the solar cycle minimum, when the efficiency considerably decreases. The results of Mitraet al. (1964) giving threshold solar radio flux for an SID occurrence have been found to be incorrect.     

GPS satellite measurements: ionospheric slab thickness and total electron content

A preliminary analysis was made of ionospheric slab thickness, τ, and total electron content, TEC, for southern Australia using GPS satellite measurements. It was found that at mid-latitudes τ has similar overall diurnal, seasonal and latitudinal variations in the southern hemisphere as in the northern hemisphere. However, there are appreciable differences between τ in the two hemispheres which would justify appropriate modifications to ionospheric models based on northern hemisphere data before being applied confidently to the southern hemisphere. The usefulness of GPS satellites together with ionosondes over a spread of latitudes was demonstrated in determining long-term variations of TEC and τ over a large area. It was concluded that as few as four GPS receivers could provide TEC for the whole of Australia in real-time, though approximately six receivers in convenient locations would be required in practice.     

The use of sunrise and sunset terminators to calibrate ionospheric movement measurements

The velocity of the terminators may be used to calibrate the measurements of ionospheric movements and this has been successfully carried out for an H.F. radar system.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

Comparisons of EISCAT and dynasonde ionospheric measurements: simple to moderately structured plasma densities

Plasma densities obtained from EISCAT's UHF incoherent scatter system are compared with profiles inverted from the digital ionograms of a co-located dynasonde. Excellent agreement is found for the bottomside ionosphere when conditions of horizontal stratification and classical photochemical equilibrium prevail. However, departures from such conditions are frequent and intense at Tromsø. Compensating errors of EISCAT calibration and long pulse convolution are resolved by analysis of power profile data. Good agreement is recovered for tilted and more complex ionospheric structure, provided that accurate echo location data are used to confirm a common volume. Monotonic inversion of the ionograms is inadequate. Dynasonde recordings are analysed to show characteristic structure in vertical and horizontal planes as a context for EISCAT measurements along a fixed (magnetic field) direction. Incoherent scatter and modern total reflection sounding, used together and coordinated in one consistent data reduction system, could produce a far more powerful ionospheric diagnostic program than either technique seems capable of providing alone.