F-layer irregularities' formation at auroral latitudes: radio wave scintillation and EISCAT observations

A coordinated experiment involving scintillation observations using orbital satellite beacons and CP-3-F program measurements by means of the EISCAT ionospheric radar facility is described. The results reveal the location of patches, containing kilometre-scale irregularities, in the vicinity of a region of an electron density minimum and an electron temperature increase. In the daytime under quiet geomagnetic conditions, the region of scintillations coincided closely with the southwards gradient in electron density, while a plasma drift velocity was mainly westwards V_E-W ≲ 0.3 km/s. In the evening, the region of the most intense irregularities was transformed to the northwards sense of the electron density gradient simultaneously with the plasma drift velocity reverse and the arrival of a significant southwards component V_N-S ≲ (1.5−1.0) km/s. EISCAT data demonstrated the patches' location in regions of an electron temperature increase. Processes operating to create kilometer-scale irregularities were analysed and estimated according to the data obtained. The assessments suggest that irregularities with a cross-field scale, equal to or greater than 1 km, and a field-aligned scale, equal to or greater than 30 km, were the result of growth of the thermomagnetic instability.     

Effects of composition and fountain effect on the annual variation of the day-time F-layer at low latitudes

The annual variation of the daytime F2-layer peak electron density (NmF2) is studied at two low latitude stations, Okinawa and Tahiti (geomagnetic latitudes ± 15°) for the sunspot maximum years 1979–1981. Observed values are compared with those calculated using the MSIS model and a simplified version of the continuity equation for day-time equilibrium conditions. Summer-winter differences imply an intensification of the fountain effect on the winter side of the equator at the expense of the summer side. This could be explained by a summer to winter neutral wind. Semi-annual variations, however, appear to be mainly due to changes in neutral composition.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

Non-ducted whistler-mode signals at low latitudes

In an earlier paper (Andrews, 1974) it was shown that a component of VLF signal from station NLK, near Seattle, received on a trans-equatorial path at Wellington appeared to have left the Earth-ionosphere waveguide at low latitudes and crossed the equator in the whistler mode. Results from a VLF recorder at Rarotonga (lat. 21°S) show that similar signals are indeed common at low latitudes. Recent calculations by Singh (1976) have established that a non-ducted VLF mode could exist at a latitude of about 20° in the presence of horizontal gradients of ionisation. It is pointed out that a second non-ducted path, first implied by Scarabucci (1970), appears possible at still lower latitudes when the O⁺ to H⁺ transition height in the ionosphere is high. While firm conclusions cannot be drawn, the low Doppler shift on the observed signals and the terrdency for their occurrence to maximise before dawn and at solar maximum are features which tend to favour the very low latitude path.     

Day-time Pi pulsations at equatorial latitudes

Low latitude Pi2 pulsations are considered to be the best indicators of the onset of magnetospheric substornis (Rostoker and Olson, 1978; Saito, 1979) and are hitherto believed to be mainly night-time phenomena. It is seen from this study utilising the pulsation records from Choutuppal (geomagnetic: 7°.5, 149°.3 E)and Etaiyapuram (geomagnetic: –0°.6.147°.5 E)and the “Common Scale Magnetograms” from the Auroral Electrojet (AE) stations during January–April 1976, that Pi2s do appear even during day-time on many occasions at equatorial latitudes in simultaneity with the onset of magnetospheric substorms at AE stations located in the night hemisphere. It is also found that the day-time Pis, unlike the night-time Pi2s, show enhancement in their amplitudes of Hx component at Etaiyapuram, situated at the dipequalor as compared to those at Choutuppal, well away from it. The results thus not only show the appearance of Pi pulsations during daytime in the equatorial zone, but also bring out the possible influence of the equatorial electrojet on their amplitudes at the dip equator.     

Quasi-periodic scintillation events at southern auroral latitudes

Quasi-periodic (QP) radio scintillations were observed during (1987) on 244 MHz and 1.5 GHz geostationary satellite transmissions in the southern auroral zone from Davis station (68.6°S, 78.0°E geographic, 74.6°S Aλ) in Antarctica. Three distinct types of OP events were identified, with occurrence times mainly restricted to the period 18-00 MLT. The substantial loss of signal associated with these events appears to be an important factor in determining the reliability of satellite links on 1.5 GHz in auroral regions. Previous observations at mid-latitudes of QP scintillations have noted a preference for large zenith angles and equatorward azimuths. It is demonstrated that a height transition in a densely ionized layer can produce QP scintillations in a manner analogous to a dense column of ionization but at lower ionization densities, as well as demonstrating a zenith angle and azimuthal dependence that is more consistent with observations than a column of ionization. At the occurrence times noted, the raypath may be intersecting the poleward edge of the trough where sporadic-E is a regular feature. QP scintillation events may result when the E_s-layer is height modulated by the passage of acoustic-gravity waves originating in the auroral zone.     

Scintillation at high latitudes during winter magnetic storms

Scintillation observations are described which were made at Kiruna in northern Sweden during three magnetic storm periods in the winter of 1984–1985. The results were obtained using transmissions from the multisatellite NNSS system, so that it has been possible to chart the development of scintillation activity over some 20° of geomagnetic latitude as a function of time for several days throughout each storm. A region of strong scintillation at the highest latitudes near magnetic noon is a common feature on all but the quietest days. This feature, probably associated with soft particle precipitation into the cusp, shows an abrupt boundary which moves equatorwards as the disturbance develops. In the magnetic midnight sector two latitudinally separate zones of scintillation are found, patchy at high latitudes although more sustained in the auroral zone. An absence of auroral scintillations around midnight UT can be followed by prolonged intense scintillation activity at auroral latitudes during the early morning hours on some disturbed days.     

An observational study of the D-region winter anomaly in lower latitudes during sudden stratopsheric warmings

An observational study of the D-region winter anomaly of HF radio wave absorption in lower latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. By means of large-scale isopleth analysis of the absorption index, ƒ_min, and of meridional winds near 70 km height along 60°N, it is found that there exists a winter anomaly in lower latitudes which is comparable in order to that in middle latitudes, resulting from a nitric oxide (NO) increase due to southward transport from higher latitudes by well-developed planetary wave winds. From the daily changes of absorption in the equatorial region, it is found that the enhanced absorption reveals an oscillation with a period of about 2 weeks and has its maximum in the region south of 20°N. The period is similar to that of planetary wave amplitudes in the winter stratosphere and mesosphere, suggesting that an effect of planetary waves could contribute to the equatorial anomaly of the absorption in the D-region.     

Magnetic variations at other latitudes during reverse equatorial electrojet

Several reverse equatorial electrojet events are examined. Magnetic variations from a sample of worldwide stations on the same days are compared with the quiet daily average for the same month. In most cases departures can be found elsewhere at the same time as the reverse jet. Various wind systems in the dynamo region might produce the observed results but different systems are required on different occasions.     

Geomagnetic bays and Pc5 pulsation substorms at high latitudes

Morphological features of Pc5 pulsation substorms and of geomagnetic bays are examined in the light of current theories using data from Fort Churchill—a high latitude Canadian station in the auroral zone. The bays are classified using actual magnetograms into four categories:

• 1.(1) sleeping bays,
• 2.(2) pulsating bays,
• 3.(3) loaded bays and
• 4.(4) transition bays. Characteristics of all these bays are detailed and special circumstances are described in which they tend to occur on the magnetograms. A comprehensive model of the magnetosphere which largely fits the observations has been detailed.

     

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Structure in the seasonal variations of Es at southern latitudes

The diurnal variations of the seasonal characteristics of sporadic-E occurrence have been studied by analyzing a large data set of ionosonde parameters for two southern hemisphere stations. The seasonal patterns are found to display anomalous short-term variations apparently not associated with solar control or the effects of dynamic meteorology.     

Mechanisms for observed total electron content pulsations at mid latitudes

Total electron content variations in the Pc3–Pc4 range of frequencies of the order of 4 parts in 10⁴ have been reported in apparent correlation with simultaneous ground based magnetic pulsation observations. By means of a term-by-term analysis of the continuity equation for electrons, the plausibility of various mechanisms is investigated. The most likely explanation is in terms of localized increases in the electron density at F-region heights caused by the field-aligned (compressional) component of the pulsation magnetic field. The analysis predicts a tendency for the amplitude of the TEC pulsations to vary in antiphase with ground-based measurements of the north-south component of the pulsation field.     

GOES satellite timekeeping for field stations at high Arctic latitudes

The feasibility of using the GOES satellite time signal is discussed for field stations at high Arctic latitudes. Results are presented for three ground stations on islands in the Canadian Archipelago. The stations range in latitude from 74 to 81° 30' N. At all locations, time code reception was found to be satisfactory and capable of providing accurate time reference for remote experiments. A simple design for a high gain helical antenna, used successfully at these latitudes for time signal reception, is also presented. The antenna, primarily intended for a small research field station, is portable, inexpensive and readily constructed.     

Solar quiet geomagnetic variations and E-region neutral winds at equatorial latitudes

A model of the ionospheric current system valid at zones close to the geomagnetic equator, taking into account the contribution of neutral winds, is proposed. From this, the external magnetic field at ground is calculated. Also, ground records of the geomagnetic field variations at the Peruvian equatorial zone were separated into their external and internal contributions. Using an iterative process a local particular fitting was found by comparing the separated external field to the one calculated with the proposed model.     

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.     

Solar and lunar variations in TEC at low latitudes in India

Total electron content data at Ootacamund (dip 6°N) during the second phase of the ATS-6 program are analysed for solar and lunar variations. Both the lunar semi-monthly and lunar semi-diurnal tides at Ootacamund are smaller in magnitude than at stations near the Appleton anomaly crest. The phases of the lunar oscillations however are almost the same as that at tropical latitudes. Thus the lunar tides in total electron content and in maximum F2-region electron density at the equatorial station are not in phase and present problems for the simple electrodynamic theory.     

Rocket altitude atmospheric X-rays from magnetospheric electrons at low and middle latitudes

Like auroral electrons, quasitrapped magnetospheric electrons mirroring in the upper atmosphere at low and middle latitudes will generate X-rays by the bremsstrahlung and K_α line excitation processes. These atmospheric X-rays may contribute a diffuse background to rocket-borne astronomy experiments launched from White Sands, New Mexico and Kauai, Hawaii. Calculations of the atmospheric X-ray spectrum at these launch sites based on observations of quasitrapped electrons indicate that this locally generated flux is comparable to the reported soft X-ray flux below a few keV.     

Observations of the reflection coefficient of the sporadic E-layer at high latitudes

A method to measure the reflection coefficient of the sporadic E-layer using a conventional ionosonde is described, and some results obtained at Sodankylä, Finland (Φ=63.8°, φ= 120.0°), are presented. The observations are often found to be in agreement with the theoretical frequency dependence of the reflection coefficient of a thin horizontal layer in the presence of mode coupling. Some of the results can be interpreted as indicative of scattering from small-scale irregularities or reflections from larger inhomogeneities in the E_s layer.