Seasonal variations of tropical F-region nightglow emissions and correlative ionospheric studies

Simultaneous zenith measurements of the forbidden OI 630 nm and permitted 777.4 nm nightglow emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9° S), Brazil, during the period February 1983-May 1984, a period of medium solar activity. This first long series of simultaneous observations has been analysed to study the mean seasonal-nocturnal variations of these emissions in conjunction with simultaneous ionospheric data, obtained at the same location. Salient features of these observations are presented and discussed. The OI 630 nm emission mean seasonalnocturnal variations show the presence of pre- and post-midnight enhancements, with intensity levels slightly lower in the winter season. The OI 777.4 nm emission mean seasonal-nocturnal variations show a monotonie intensity decrease in time, with very low intensity levels during the winter season. A comparison has been made between the F-region peak electron densities, and heights determined from the optical and ionosonde remote sensing methods. In general, a good correlation is found between the measured and the nightglow inferred ionospheric parameters. The observed airglow intensity variations are also compared with those predicted by a semi-empirical low latitude ionospheric model.     

Correcting VLBI observations for ionospheric refraction

Radio astronomical observations with local interferometers and very long baseline interferometers (VLBI) are affected by atmospheric (i.e. tropospheric and ionospheric) refraction. A procedure to correct VLBI observations for ionospheric refraction using simultaneous ionospheric measurements is outlined. If networks of ionosonde and satellite observing stations operate during the astronomical VLBI observations, pathlength corrections ranging from several times the VLBI observing wavelengths to a fraction of this wavelength using the simultaneously collected ionospheric data are possible, provided that the geographical distribution of these ionosphere stations is similar to the VLBI network.     

Simultaneous nightglow and Na lidar observations at Arecibo during the AIDA-89 campaign

Simultaneous nightglow and Na lidar observations made at Arecibo during the AIDA-89 campaign are reported from between 0000 and 0530 LT (Local Time) on 6 April 1989 and on 9 April 1989. On 9 April the observations are consistent with the presence of a large amplitude 8.5 km vertical wavelength, 4 h period wave propagating through the 85–91 km region. A lower amplitude 80 min period wave is also observed. The results imply that the O₂ atmospheric band intensity peaked near 91 km while the OH Meinel (6,2) band intensity peaked near 85 km. The OH Meinel temperature and intensity are 160° out of phase which can be explained by low eddy diffusion and high ozone densities near the mesopause. The integrated Na abundance from 87 to 89 (90–92) km correlates well with the OH Meinel (O₂ atmospheric) band intensity. On 5–6 April the OH Meinel and O₂ atmospheric band intensities are not well correlated. The OH Meinel intensity is correlated with the integrated Na abundance from 86 to 88 km. Both the Na and OH measurements reveal the presence of an approximately 1 h period wave. The OH temperature data appear to be consistent with the OH Meinel band originating near 85 km. The O₂ atmospheric band data show the presence of a 2 h period wave. The integrated Na abundance data suggest that the O₂ atmospheric band peaks between 90 and 94 km. A large sporadic Na event which occurs near 6 UT appears related to the presence of a gravity wave near 95 km. In all of the observed waves there is good agreement between the wave parameters derived separately by the optical airglow and Na lidar techniques.     

The future of geomagnetic storm predictions: implications from recent solar and interplanetary observations

Within the last 7–8 years, there has been a substantial growth in our knowledge of the solar and interplanetary causes of geomagnetic storms at Earth. This review article will not attempt to cover all of the work done during this period. This can be found elsewhere. Our emphasis here will be on recent efforts that expose important, presently unanswered questions that must be addressed and solved before true predictability of storms can be possible. Hopefully, this article will encourage some readers to join this effort and perhaps make major contributions to the field.     

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.     

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.     

A climatology of quiet/disturbed ionospheric conditions derived from 22 years of Westerbork interferometer observations

Knowledge of the quiet and disturbed conditions in the propagation medium is essential for quality control of transatmospheric radio signals. This holds equally for the troposphere and the ionosphere. This paper describes a climatology of ionospheric irregularities obtained from observations of celestial radio sources by radio interferometry, i.e. by the Westerbork Synthesis Radio Telescope (WSRT) in The Netherlands. This instrument is located at geomagnetic mid-latitude. All WSRT calibrator observations in the 22-year period 26 June 1970–31 December 1991 have been checked for manifestations of ionosopheric effects. Although seasonal effects are clear, the occurrence and ‘strength’ of ionospheric irregularities show no dependence on solar activity. Assuming that the frequency of occurrence of ionospheric disturbances in spring and autumn are similar, it is found that ‘ionospheric’ winter starts on day 348 ± 3 and all seasons last for 3 months. Medium-scale travelling ionospheric disturbances (TIDs) occur most frequently during the daytime in winter periods. The occurrence of non-periodic irregularities is, however, not a function of time in the day. The daily variation in the amplitude and frequency of the occurrence of the TIDs suggests that the solar terminator and Joule heating near the electrojets do not contribute substantially to their generation. Generation of gravity waves may be caused by winds and tides in the lower thermosphere-mesosphere. This has to be investigated further.On the basis of the available data, a ‘disturbance measure’, indicating to what extent the ionosphere is ‘quiet’, is proposed. The output of this project may be of immediate use for different ionospheric investigations, such as ionospheric modelling and the study of excitation mechanisms for ionospheric irregularities.     

Pulse transit times and waveform observations for ionospheric propagation between England and Australia

Transit times based on the leading edge of pulses transmitted between England and Australia have been measured in the two opposite directions both on the ‘short’ and ‘long’ great circle paths, and on two frequencies. Synchronization was based on UTC time signals at the two ends, with time base stabilisation by the local TV line oscillator frequency. The delay times have been compared with calculated triangular paths for multi-hop, tilt-supported chordal hop as well as a ducted mode between the E- and F-layers. Broadening and rapid waveform changes of pulses make resolution into components very difficult. Observations are given of waveforms after computer averaging of some 2500–5000 pulses. The results are presented largely as experimental data which may be of use to other workers.     

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.     

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.     

Small scale ionospheric irregularities near regions of soft particle precipitation: scintillation and EISCAT observations

Results are presented from a coordinated experiment involving scintillation observations using transmissions from NNSS satellites and simultaneous measurements with the EISCAT ionospheric radar facility. The scintillation was used to indicate the presence of sub-kilometre scale irregularities while the radar yielded information on the larger structures in the background ionosphere. Two examples are discussed in which localised patches of scintillation were observed at L-shells near ‘blob’ like enhancements in F-region ionisation density. Elevated electron temperatures indicated that the enhancements may have had their origins in soft particle precipitation. While structuring of the precipitation on the 100 m scale cannot be completely ruled out as a source of the irregularities, in one case the blob gradient can be shown to be stable to the E λ B mechanism. The most likely cause of the irregularities appears to be shearing of the high velocity plasma flow in a region adjacent to the density enhancement. This region is characterised by a high ion temperature while the resulting scintillation has a shallow spectral slope.     

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.     

Solitons and ionospheric modification

The possibility of Langmuir soliton formation and collapse during ionospheric modification is investigated. Parameters characterizing former facilities, existing facilities, and planned facilities are considered, using a combination of analytical and numerical techniques. At a spatial location corresponding to the exact classical reflection point of the modifier wave, the Langmuir wave evolution is found to be dominated by modulational instability followed by soliton formation and three-dimensional collapse. The earth's magnetic field is found to affect the shape of the collapsing soliton. These results provide an alternative explanation for some recent observations.     

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.     

Reconstruction of horizontally-inhomogeneous ionospheric structure from oblique-incidence backscatter experiments

Fridman and Fridman [(1994) J. atmos. terr. Phys. 56, 115] suggested a method of reconstructing the horizontally-inhomogeneous ionospheric structure using vertical- and oblique-incidence backscatter sounding (OBS) ionograms measured at a single location. In the present paper this technique has been used to analyze experimental data and tested against independent vertical sounding (VS) measurements. By using the OBS and VS ionograms measured at Irkutsk as source data for the method we reconstructed ionization profiles over Tomsk (1050 km to the west of Irkutsk). We found that the reconstructed profiles are in reasonable agreement with the profiles obtained from VS measurements at Tomsk.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.