High-latitude D- and E-region investigations using imaging riometer observations

The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B_Y component of the interplanetary magnetic field. A typical example is examined.     

On the prediction of auroral radio absorption on the equatorial side of the absorption zone

Observations of auroral radio absorption made in Finland over the years 1972–1983 have been compiled for use in absorption predictions. The data are presented as values of Q(1), the probability of at least 1 dB absorption being recorded by a 30 MHz riometer, and the results are compared with the predictions of an existing model regarding the latitudinal variation, the influence of solar and magnetic activity indices, and the pattern of daily variation. New formulae for basing absorption predictions on the A_p index are proposed.     

The ionosphere: morphology,development and coupling

The morphology of the MAP/WINE winter is examined, principally from ground-based and satellite observations. Winter anomaly is evident, occurring in bursts with a west to east shift in time. Auroral activity, particularly with reference to the times of major rocket salvoes, is generally low, with Andøya to the south of the auroral boundary in most cases. Minor stratospheric warmings, of which 4 occurred, are found to correlate with minima in radio wave absorption. Salvo R1 was launched during one of the minor warmings.Using data from a broad sector of Europe, coupling between the lower thermosphere and mesosphere is seen over large areas. Westerly winds are associated with high absorption (winter anomaly) and reversal to easterly winds with stratospheric warmings and low absorption. It is found possible to select cases, from amongst the MT series of rocket launchings, corresponding to quiet conditions, stratospheric warming, winter anomaly and particle precipitation in the general absence of other effects. Examining D- and lower E-region ionisation profiles for these caes it is found that, compared with a quiet night, the stratwarm night shows the lower E-region to have reduced ionisation. The ionisation ledge is of similar shape in all cases, but occurs over different height ranges. The observed effects all point to transport being a major factor and the need to measure vertical transport over the range of geophysical conditions examined is highlighted.     

Morphology of the D-region in northern Scandinavia during the campaigns MAC/SINE and MAC/EPSILON in 1987

The morphology of the lower ionosphere was examined by the riometer and ionosonde networks in Scandinavia during the campaigns MAC/SINE and MAC/EPSILON. The campaigns were carried out during 1987 when the solar activity was low. The minimum sunspol number 14 was observed in 1986. The sunspot number was already increasing during the year 1987, while the Ap-value and riometer absorption were still decreasing. During the campaign MAC/SINE the rockets were launched during quiet periods, and during the campaign MAC/EPSILON during more disturbed periods. The variations in ionospheric parameters, ionospheric absorption and foF2 are presented for the campaign periods. Some interesting events are pointed out.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Measuring ionospheric movements using totally reflected radio waves

It is shown that for radio waves of a particular frequency reflected totally from the ionosphere the effect of refraction as well as reflection can be simulated by an effective reflecting surface. This mirrorlike surface will give the correct angle of arrival and Doppler shift for all radars operating at this frequency. It is theoretically possible for the effective reflecting surface to be folded back on itself, but this is unlikely except for F-region echoes refracted by sporadic E-clouds. If the surface is not folded and exists everywhere, it is always possible to describe its motion and change in terms of wave undulations. Experimental data for F-region echoes show that these wave undulations are very dispersive. However, the matching between the best fitting model and the experimental data is worse than expected for reasons we do not understand.     

An inversion procedure for obtaining collision frequency profiles from swept-frequency absorption measurements

A method is presented which inverts swept-frequency Al absorption data to obtain collision frequency profiles in the E- and F-regions of the ionosphere. The method gives consistent results from successive sets of measurements and the profiles obtained are consistent with other measurements of collision frequency. Accounting for D-region absorption is a difficulty affecting the accuracy of the collision frequencies obtained at the lowest heights, but model simulations show that values at higher heights are not affected seriously. The technique can be used to obtain results for the F1-region for which there are very few previous measurements.Comparison with theoretical calculations of collision frequency show agreement in the form of the altitude variation. That is, there is a rapid decrease with altitude through the E-region which becomes much less in the F-region so that the collision frequency becomes almost constant with height. This change is caused by electron-ion collisions becoming more dominant than electron—neutral collisions. However, consistent with other observers, we find a major discrepancy between the magnitude of the experimental and theoretical values. If the electron and ion temperatures are assumed equal, the experimental values are approximately five times greater. The discrepancy increases if Te >Ti in the theoretical calculations.     

The winter anomaly as seen in the propagation of low frequency 40 kHz radio waves

An analysis of propagation data for LF 40 kHz radio waves shows that the field strength of the sky wave is enhanced during day-time on winter anomaly days (WAD), in striking contrast to the severe attenuation of HF radio waves. This peculiar enhancement of the field strength may be ascribed to an increase in the reflection coefficient. The analysis also demonstrates that the reflection height is lower on WAD, which seems to be associated with enhancements of ionization in the D-region. Moreover, it was found that WAD are characterized by an earlier occurrence in the morning and a delayed occurrence in the evening of pronounced interference maxima and minima, respectively.     

A scintillation theory of the fading of HF waves returned from the F-region: receiver near transmitter

Scintillation theory is used to study the fading of HF radio waves returned from the ionospheric F-region to a receiver close to the transmitter. Estimates are made of

• 1.(i) the fluctuations of phase both for long term (∼ an hour) and for short term (∼ a fading correlation time),
• 2.(ii) the correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the angular divergence in the direction of arrival around the zenith and
• 5.(v) the correlation bandwidth.

The calculations are made as a function of wave frequency for two ionospheric penetration frequencies representative of high day-time values and low pre-sunrise values. Results are compared with observations of fading made with ionosondes over the past 40 years. Precise comparison is rendered impossible by omissions in the experimental data caused by lack of guidance from scintillation theory. Nevertheless, agreement is promising. When fading is deep but spread-F-region is not well-developed there is a slow modulation of the fading. This is what, for optical propagation in the troposphere, is called twinkling. The slow fluctuations observed by Bramley and Ross in the HF band constitute ionospheric twinkling. Calculated quasi-periods of twinkling range from about an hour down to about a couple of minutes, while calculated quasi-periods of fading range from about a couple of minutes down to about a tenth of a second.     

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.     

Mid-latitude frequency spread and its association with small scale ionospheric stratifications

A variety of ground based radio techniques have provided new information relating to the nature of mid-latitude F-region irregularities responsible for frequency spreading on ionograms. Firstly, an analysis of ionograms covering a restricted frequency band indicates that frequency spreading is primarily caused by duplicate traces which are often unresolved in group path on standard ionograms. Furthermore, where angle of arrival information is available, the duplicate traces are shown to represent reflections from markedly different directions and the spread in critical frequencies is therefore indicative of a horizontal gradient in the peak electron density over a scale size of the order of many tens of kilometres. Secondly, the individual duplicate traces themselves are shown to comprise quasi-horizontal trace (QHT) segments which are unresolved on conventional ionograms and contribute to the diffuse appearance of spread-F traces on those ionograms. Difficulties in attributing these observations to the widely held view that scattering from small scale structures is the causative mechanism are discussed.     

Interference of tidal and gravity waves in the ionosphere and an associated sporadic E-layer

Observations made on 10 July 1987 with the EISCAT UHF radar are presented. The F-region measurements of both electron density and field-aligned ion velocity show that an upward propagating gravity wave with a period of about 1 h is present. The origin of the gravity wave is probably auroral. The E-region ion velocities show a tidal wave and both upward and downward propagating gravity waves. The gravity waves have three dominant periods with a possible harmonic relationship and similar vertical wavelengths. These waves are either reflected at a single reflection level, ducted between two levels, or they are generated in a non-linear interaction between gravity and tidal waves. The E-region electron density is dominated by particle precipitation. After a short burst of more intense precipitation, a sporadic E-layer forms at 105km and then disappears 40min later. Within this time, the layer rises and falls by a few kilometres, following closely the motion of a convergent null in the velocity profile. We suggest that the formation and destruction of this layer is controlled by both the precipitation, which indirectly provides a source of metal ions through charge exchange, and the superposition of gravity waves and the tidal wave.     

Experimental studies of daytime LF radio propagation and their implications for D-region electron densities

Measurements are presented of interference phenomena in amplitude and phase of VLF and LF signals along propagation paths from central England to the Norwegian Sea. The data are interpreted by means of the ‘wave-hop’ propagation theory, incorporating full wave evaluation of ionospheric reflection coefficients with realistic D-region models. No published electron density profiles are found which completely satisfy the experimental data, but modified profiles are presented which provide a better fit to the observations.     

The seasonal variations in the nighttime 6300 Å emission at midlatitudes from ISIS-II spacecraft

Optical limb observations at F-region heights from the ISIS-II satellite have been used to study the seasonal variations in the 6300 Å limb emission for nighttime conditions and the aeronomic implications. The observations were carried out over the American zone at northern midlatitudes, and refer mainly to the period 1973–1975 of low solar activity.The observed seasonal variations in the emission seem to be mainly controlled by the electron density at F-region heights for nighttime and quiet geomagnetic conditions. The winter minimum is found to be deeper than the summer minimum. The obervations give clear evidence of semiannual variation in the emission. The phase variations agree closely with that of the semiannual variations in electron density and neutral atmospheric density at F-region heights. However, the amplitude variations of the semiannual variations are found to be larger than suggested by the observed F-region electron density. The observations during highly disturbed conditions possibly show the presence of gravity waves with wavelengths around 500 km, which could transport auroral energy to lower latitudes. The midlatitude enhancements observed during disturbed conditions seem to be related to the inward movement of the plasmapause.     

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.     

Incoherent scatter observations of thin ionization layers at Sondrestrom

High resolution incoherent-scatter observations of E-region thin (1–3 km) metallic ion layers are presented. Data were collected during three different periods from August 1990 to August 1991, in three different experimental modes. First, the antenna was directed vertically and the entire duty cycle was devoted to Barker coded multi-pulse [Zamlutti (1980) J. atmos. terr. Phys.42, 975–982] measurements to determine the densities and temperatures in the E-region with 300 m resolution. The second experiment measured the F-region electric field as well as the high resolution E-region densities. For the third experiment the antenna was scanned magnetic north-south while only the E-region densities were measured. The experiments were carried out on 16 different nights for a period of 4 h each night at a time near magnetic midnight. Thin ionization layers were observed on 12 of the 16 nights. The first experiment demonstrated that the thin layers are composed of a significant fraction of heavy metallic ions; assuming the layers are composed of a mixture of Fe⁺ and Mg⁺ a composition estimate of 63% Fe⁺ was obtained in one example. The second experiment investigated the relationship between the direction of the electric field and the presence of the thin layers. In these observations thin layers were only present when the electric field was pointed in the magnetic north-west or south-west quadrants, most frequently when the field was near magnetic west. Correlation between layer altitude and field direction was also observed, layers occurring at higher altitudes for fields directed in the north-west, and lower altitudes for fields directed to the south-west. The observations are compatible with the electric field mechanism for thin ionization layer formation. The scanning experiment showed that the layers were of a limited latitudinal extent, typically about 100 km up to a maximum of about 200 km.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Direct radar observations of TIDs in the D- and E-regions

We have observed Traveling Ionospheric Disturbances (TIDs) in the night-time D- and E-regions using a 2.66 MHz imaging Doppler interferometer radar. TIDs were observed in two distinct ways. In the first, the TID was manifested as discrete traveling surges, with average spacings of 54 min. The D-region surges were so well defined that they could be tracked as they passed close to overhead by using the phase differences across the antenna arrays. A velocity of 135 m s⁻¹ to the south was measured, giving a horizontal wavelength of 440 km typical of medium scale TIDs. The direction of phase travel relative to the horizontal was −6° (i.e. downwards). These observations were made during a night of extraordinary OH infrared mesopause structure activity made visible by the presence of a total lunar eclipse. In the second type of TID observation, we show the Doppler interferometer analysis of ripples on the under surface of sporadic-E layers taken on two nights of significant OH infrared and OI 5577 Å wave activity. The TIDs were observed to propagate at speeds of 120–300 m s⁻¹, with directions predominately toward the southwest, again typical of medium scale TIDs. These results show definite wave effects on MF radar returns and thus suggest that the measurement of mesospheric bulk winds with MF radars should be approached with some caution. Comparison of the TID characteristics with the OH structure characteristics show that the TIDs travel faster than the OH structures, have longer apparent horizontal wavelengths and generally travel in the opposite direction.     

A scintillation theory of fading in long distance HF ionospheric communications

Scintillation theory is used to study the fading of radio waves returned from the ionospheric F-region in the HF band at oblique incidence. For both low and high penetration frequencies calculations are made as a function of wave frequency, but emphasis is given to behaviour near the maximum usable frequency for one-hop F-transmission over distances up to 4000 km. Estimates are made of

• 1.(i) the fluctuations of phase both for long-term (~ 1h) and for short-term (~ the fading correlation time),
• 2.(ii) the fading correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the twinkling correlation distance,
• 5.(v) the quasi-period of twinkling,
• 6.(vi) the angular departure of the arrival azimuth from the mean and
• 7.(vii) the correlation bandwidth.

The slow fluctuations often experienced in long distance HF radio communications are a manifestation of twinkling, rather than fading. Under normal conditions in the F-region the correlation bandwidth is so large that the bandwidth of transmission at HF is controlled by the dispersive properties of the ionosphere, rather than by the scattering properties. The reverse is true in the presence of spread-F. For sufficiently strong spread-F the correlation bandwidth for long distance HF radio communications is of the order of only 10 Hz, thereby creating the phenomenon known as flutter fading. Theoretical estimates of the scintillation parameters are compared with experience in long distance HF ionospheric communications over the last 60 years. Agreement is promising, but future experiments should be designed in the light of theory; existing observational data do not provide some of the information needed.     

Meridional electric currents in the equatorial F-region

A study of the boundary conditions for the equatorial thermospheric transport equations by the authors has led to the theoretical prediction of the vertical electric field at the base of the F-region. Earlier, this result was applied to the calculation of the zonal wind field in the equatorial F-region. In this work, the aforementioned model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat.