HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed.A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.     

Source regions of medium scale travelling ionospheric disturbances observed at mid-latitudes

The characteristics of medium scale travelling ionospheric disturbances (MSTIDS) have been determined from observations carried out between 1972 and 1975 at Leicester U.K. (52°32′N, 1°8′W) using the HF Doppler technique. By reverse ray tracing through a model atmosphere an estimate of the source locations of these waves can be obtained. Auroral sources do not appear to represent an important generation mechanism for MSTIDs observed at mid-latitudes. The majority of waves originate at tropospheric altitudes at ground ranges of less than 1500 km from the observation point, and a moderate correlation is found between the occurrence frequency of MSTIDs and the intensity of the meteorological jet stream.     

The effects of travelling ionospheric disturbances on ionograms

The numerical synthesis of ionograms by ray-tracing in an analytic two-dimensional ionosphere is simplified by a technique which avoids the usual wastage of ray tracings. The technique is particularly suitable for obtaining ionogram sequences corresponding to a moving ionospheric disturbance. It is applied firstly to a moving tilted ionisation increase, which gives the familiar travelling-cusp records. Ionograms do not give a good measure of the vertical distribution of ionisation, and in the case considered lead to an overestimate of the horizontal size of the increase by a factor of about 2. For a travelling ionisation decrease or trough, the ionograms show an additional U-shaped trace overlapping the main trace. Calculated ionograms agree closely with some observations. It is difficult to obtain any measure of the size of the electron-density decrease; critical frequency scalings may give no indication of the passage of an intense disturbance. Similar results are obtained for a wavelike perturbation in an exponential topside ionosphere. Topside ionograms then show multiple ‘nose’ traces, following in general the curve for the unperturbed ionosphere. It is suggested that the multiple echoes frequently observed on topside ionograms may, in some cases, be due to refraction in large disturbances rather than the commonly-assumed ducting mechanism.     

Mid-latitude range spread and travelling ionospheric disturbances

This paper first discusses some early results (most hitherto unpublished) on off-vertical reflections which result from tilted isoionic contours associated with the passage of travelling ionospheric disturbances (TIDs). Azimuth of arrival and zenith angle information on these returning signals is discussed together with the role of these signals in producing both resolved and unresolved range spread on ionograms. Some ray tracing through a model ionosphere which incorporates wavelike structures is shown to predict fixed-frequency patterns (on recordings of virtual height vs. time) of converging and diverging satellite traces similar to those observed in practice. New experimental evidence is then presented to suggest (from the N(h) analyses performed) that TID wavetrains of several cycles are effective in producing spread traces on ionograms by specular reflections from the tilted surfaces of each cycle of these wavetrains. Ionograms from a modern ionosonde show that range spread consists primarily of discrete satellite traces even though at times these traces overlap and vary in intensity as a function of frequency thus creating a diffuse range spread ionogram.     

Reconstruction of travelling ionospheric disturbances parameters by a tomographic method

A tomographic method is proposed to reconstruct parameters of the inhomogeneous ionosphere on the basis of model representations using angles of arrival or total electron content. With such an approach, determining the function of the inhomogeneous ionosphere implies finding a set of parameters of TIDs. Suitable signals can be generated by satellites.     

Horizontal velocity dispersion of medium-scale travelling ionospheric disturbances in the F-region

Daytime observations of the horizontal velocity dispersion of medium-scale travelling ionospheric disturbances (TID) near the F2 peak height have been carried out using an array of HF Doppler sounders in central Japan. Cross-correlation analysis of sample records has shown that the horizontal trace velocity is a decreasing function of the period of fluctuations in the range 13.3–40 min. The theoretical dispersion of the atmospheric gravity waves is also calculated using Klostermeyer's (1974) method. Comparison between the observed and the calculated results suggests the possibility that the components of the lower period of the observed velocity dispersion may be a remnant of the quasi-evanescent mode pertinent to lower-height levels.     

Tropospheric gravity waves and their possible association with medium-scale travelling ionospheric disturbances

Analysis of pressure fluctuations observed over a period of several days using an array of microbarographs has shown the existence of long trains of gravity waves with two or more waves often present simultaneously. Meteorological data from radiosonde ascents indicates that many of the waves have a velocity which matches that of the background wind at some level within the troposphere. Generally this height corresponds to that of a frontal zone marking the transition between air masses and it is suggested that the waves may have been generated by shear flow instability within the frontal layer. Theoretical considerations, based on a three-layer model troposphere, show that some of the observed waves could have been ducted in or near the frontal zone. Some evidence is found to indicate that a non-linear wave-wave interaction between pairs of waves occurring simultaneously in the frontal zone could yield secondary waves with the characteristics of the gravity waves which had been observed in the thermosphere at appropriate times and whose group paths were traced to source regions in the troposphere in the general vicinity of the microbarograph array.     

Detecting travelling ionospheric disturbances in incoherent scatter data using the maximum entropy method

Two different methods of determining autoregressive coefficients in Maximum Entropy Method (MEM) were compared, investigating power spectra of synthetic signals consisting of three sinusoids and white noise of relatively high level. Spectral line shifts due to the initial phases changes were evaluated. Burg's method was chosen for determining power spectra of experimental signals. Simulating the incoherent scatter data obtained with EISCAT, it was shown that the increasing noise and wave attenuation effects are difficult to separate. Integrating the ME spectral density it was found that, in many cases, it is impossible to reproduce the true power of each harmonic of the signal.     

On medium scale travelling ionospheric disturbances as seen through total electron content power spectra

Modulation phase of 140 MHz with respect to 360 MHz signals from ATS 6 satellite recorded at Slough (51.5°N, 0.6°W geographic latitude and longitude; 54.3° geomagnetic latitude) at one-minute interval are power spectral analysed to derive dominant periodicities corresponding to Travelling Ionospheric Disturbances. From the significant peaks in the spectra, an occurrence peak at periods between 10 and 15 min and a secondary peak at 60–65 min are seen. From cross-spectral analysis of the same records from three stations separated by a few hundred kilometers for a short period, the speed and azimuth of the propagating disturbances are determined. During the day-time, most of the waves in the period range 30–100 min are seen to propagate at azimuths of 90–160°. At night-time they propagate poleward. Theoretical computations of the azimuth response of TEC to typical gravity waves, including the effect of neutral winds, show that the observed azimuths of propagation are in reasonably good agreement with theoretical predictions.     

spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite.At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.     

Refraction distortions of transionospheric radio signais caused by changes in a regular ionosphere and by travelling ionospheric disturbances

This paper generalizes experimental data on variations of the angles of arrival of transionospheric radio signals caused by changes in a regular ionosphere and by effects of medium-scale travelling ionospheric disturbances (TIDs). The data are based on radio astronomical observations of discrete sources and compact active features on the Sun as well as on angular measurements of signals from artificial Earth satellites with geostationary and circular orbits.The experimental data are interpreted through calculations of refraction corrections using a Gaussian model of a regular ionosphere disturbed by a three-dimensional travelling wave (the TID model) as well as an adaptive model of a regular ionosphere. Some possibilities of correcting refraction distortions with the use of appropriate models and ionospheric diagnostic tools are discussed.     

Simultaneous observations of travelling ionospheric disturbances by two-dimensional radio interferometry and the differential Doppler technique applied to satellite signals

The main object of the campaign reported here was to compare TID characteristics obtained from two essentially different observation techniques: (1) observation of the apparent angular position shifts of Virgo A by the Nançay radioheliograph (47.33°N, 2.15°E) gave azimuths and periods of travelling ionospheric disturbances (TIDs); (2) differential Doppler shifts of signals from NNSS-satellites recorded simultaneously at Tours (47.35°N, 0.70°E), Nançay and Besançon (47.32°N, 5.99°E) provided azimuths and latitudinal wavelengths. Observations were made during the period 10–30 November 1987, between 6 and 12 h UT. It is found that azimuths obtained from the two techniques are consistent if sufficient averaging over wave trains is performed: averaging over several hours for radio interferometry and averaging over the whole satellite trace for the differential Doppler technique. Averaging is necessary because of (1) the intrinsic dispersion in wave azimuth, (2) the broadness of observed wave spectra and the dispersive properties of gravity waves, and (3) the spatial separation of ionospheric points for the two techniques. Good agreement between the azimuths was achieved by setting the altitude of the TIDs, which is used in the differential Doppler analysis, to about 250 km, appreciably lower than the maximum in electron density (about 350 km). The mean azimuth of observed TIDs was 12° East from South with a standard deviation of about 30°. The dominant period and horizontal wavelength of the observed TIDs were 40 min and 450 km. The East-West coherence length of the TIDs was found to be only of the order of 200 km.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

Some aspects of mid-latitude daytime ionospheric disturbances

This paper is concerned with the ionospheric irregularities produced during daylight hours in mid-latitude regions by medium-scale travelling ionospheric disturbances (MS-TIDs) as these disturbances pass overhead at a recording station. These irregularities are detected as ionogram trace distortions as well as by spread-F conditions particularly related to the second-hop reflections of HF radio waves. The existence of a significant amount of spread on second-hop ionogram traces during the passage of two particular MS-TIDs is illustrated and discussed. Experimental evidence is presented to suggest that, similar to the results for night-time spread-F, the occurrence of these daytime events is inversely related to variations in the neutral-particle density of the upper atmosphere, for the annual and sunspot-cycle variations. However, the results do not show a similar inverse relationship for the diurnal variation.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

Schumann resonance frequency variations during sudden ionospheric disturbances

Fluctuations of the fundamental and harmonic frequencies of the Schumann (earth-ionosphere cavity) resonances measured near Tromsø, Norway, on 28 March 1979 are examined on a timescale of ≲ 20 minutes. Their relationships with two sudden ionospheric disturbance events and one other geophysical event is investigated. The experimentally observed frequency variations are, in some cases, at variance with the theoretical predictions of Madden and Thompson (1965).     

The ionospheric plasma flow and current patterns of travelling convection vortices: a case study

Following a short duration density enhancement in the solar wind, observed by the AMPTE/IRM spacecraft, transient disturbances appeared in the polar ionosphere in the prenoon local time sector which were identified as Travelling Convection Vortices (TCV). This event has been studied intensively by combining radar and magnetometer observations. EISCAT radar was operated in the special programme U.K.-POLAR which provides F-region plasma parameters from invariant latitudes around 72° at a rate of one sample per 15 s. The combined data set provides a detailed picture of the drift pattern of the plasma and the three-dimensional current distribution. There are two Hall current eddies drifting westward at a speed of 0.15° s⁻¹. The leading one circulating clockwise is associated with a downward field-aligned current and the oppositely circulating eddy with an upward current. The ionospheric conductivity seems to be enhanced in the leading vortex compared to the trailing, although the latter is connected to an upward field-aligned current. Still unexplained is the mechanism generating the electric field which drives the vortices. The direction of the electric field observed in the ionosphere is opposite to that expected if the source were a compression of the magnetosphere.     

The characteristics of VHF echoes from the summer mesopause region at mid-latitudes

Radar observations have been carried out at Aberystwyth (52.4°N, 4.1°W) during the period May–August 1990 to search for echoes analogous to the Polar Mesosphere Summer Echoes commonly observed at high latitudes. Signal strength measurements in the vertical and at 12° from the vertical are used to examine the aspect sensitivity of echoes, and Doppler measurements at 6° from the vertical and in the vertical to estimate wind velocities. The observations show the presence of two types of moderately strong echoes from heights above 80 km. On most days a spectrally broad echo is observed with characteristics consistent with isotropic turbulence scatter. On certain days between mid-June and mid-July, stronger spectrally narrow echoes are observed with characteristics similar to Polar Mesosphere Summer Echoes.     

Multiple beam observations of mid-latitude ionospheric disturbances by the MU radar

The pulse-to-pulse beam steerability of the M U radar of Kyoto University enables us to observe multiple beam positions simultaneously. Based on 560 h of this type of data, we present two typical patterns of mid-latitude ionospheric disturbances and their horizontal traveling characteristics. Wavy structures have not been found in large-scale disturbances. Isolated disturbances travel primarily southward (equatorward) in disturbed conditions, while no preferred direction is observed in quiet conditions.