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.     

Solar cycle and long-period variations of mesospheric temperatures

Evidence for a temperature variation above about 55 km between years of high and low solar activity is found in rocket data of Volgograd (49°N, 44°E) 1969–1983, reaching a solar-cycle amplitude of 6K, whereas below 55 km no statistically significant solar cycle effect is detected. This mesospheric temperature variation is in qualitative agreement with a pressure variation at 80 km derived from lower ionosphere radio reflection heights near 51°N, 13°E, measured at Kühlungsborn/GDR, covering almost two solar cycles. When the solar cycle variation has been removed from these 80 km pressure data by means of a regression analysis, there remains a quasi-cycle of about 20 yr, which agrees well with observations of a general cooling of the northern mid-latitude stratosphere between 1965 and 1977, reported by other authors.     

A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

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.     

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.     

Modeling the low latitude ionospheric total electron content

The undisturbed ambient total electron content of the ionosphere in the equatorial region exhibits two characteristic features:

• 1.(i) a longitudinal behavior of the post-sunset variation of the ionization near the crests of the equatorial anomaly
• 2.(ii) an enhancement at lower latitudes following the post-sunset decay. During high solar activity periods the southern crest of the equatorial anomaly in the African longitude sector is characterized by a post-sunset maximum often exceeding the afternoon maximum. In the Indian and other longitude zones, the post-sunset peak is not so prominent. Instead, a ledge is obtained in the corresponding local time period. At lower magnetic latitudes, the ionization decays very rapidly around sunset, but an enhancement lasting 2–4 h is observed afterwards.

Numerical solution of the plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind, has been used to investigate the above two features. It is found that the pre-reversal peak of the E × B drift at the magnetic equator around sunset is the dominant mechanism responsible for the post-sunset behavior near the crests of the equatorial anomaly. The zonal wind causes an asymmetry of the total content in the northern and southern hemispheres. In African longitudes, where the magnetic declination is about 20°W, the southern crest is more developed at the expense of the northern counterpart. The north-south asymmetry is practically absent in the Asian sector, with its low (< 5°) declination angle. In the Pacific area, an easterly declination (about 9°E) results in a higher post-sunset ionization at the northern crest, although the asymmetry is less pronounced than that in the African zone. The night-time enhancement at lower latitudes has been found to be controlled by the post-sunset increase in the vertical drift, possibly also modulated by the neutral wind.     

Ray-tracing the paths of very low latitude whistler-mode signals

VLF whistler-mode signals with very low group delays (75–160 ms) received at night in Dunedin, N.Z., from the 23.4 kHz MSK transmissions of NPM, Hawaii (21.5°N, 158°W), are explained by ray-tracing along unducted paths. The typical vertical and horizontal electron density gradients of the night equatorial ionosphere are found to be sufficient to explain not only the typical group delays but also their decrease during the night and the typical frequency shifts observed on these signals. An important feature appears to be the decreasing starting and finishing latitudes (and the decreasing maximum height of the path) during the course of the night. The amplitude of the signals in relation to the expected collisional absorption in the ionosphere is discussed. A simple but quite accurate analytical expression suitable for ray-tracing is derived for the night electron density in the height range 170–1400 km, based on non-isothermal diffusive equilibrium and O⁺/O friction.     

Determination of the latitude of Sq focus and its relation to the electrojet variations

The latitude of the Sq focus is determined analytically using the method of natural orthogonal components. Geomagnetic data along the 75°E meridian have been used. The day-to-day variations of the latitude of the Sq focus thus determined are very highly correlated with the electrojet enhancements. The strength of the normal Sq field and the electrojet field are not correlated. It appears that as the Sq focus moves towards the equator the entire current pattern bodily moves equatorwards with an increased current concentration in the electrojet region.     

Oscillations of intensity and temperature of nightglow emissions: [01] 557.7 nm and 630.0 nm lines,and OH(6-2) band

Clear oscillations of intensity of the oxygen green and red lines and those of intensity and temperature of the OH(6-2) band, which were obtained during two total lunar eclipse nights, are presented. The wave periods found range mainly from 25 to 40 min. In connection with these clear oscillations of intensity and temperature, we have examined intensity variations of these airglow emissions on other nights and found two nights during which only the green line indicates clear oscillations in the post-midnight hour. Our results favor the suggestion of Peterson (1979).     

Experimental observations of very low latitude man-made whistler-mode signals

VLF signals at 23.4 kHz from NPM in Hawaii (lat 21.5°N) are commonly received at night in Dunedin, New Zealand with very low group delays of between about 75 ms and 160 ms and frequency shifts of several tens of milliHertz or more. The ratio of the frequency shift to the rate of change of group delay generally agrees with the ratio which would be expected from signals which have travelled through the equatorial ionosphere. Normal whistler-mode signals with delays of 0.3–0.6 s are quite frequently observed at the same time.     

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.     

Storm-time variation of foE at a low latitude station

foE changes during geomagnetic storms are studied for Ahmedabad. Individual storms show erratic effects. The average curves show a possible 5–10% decrease in the post-Main Phase Onset period and a somewhat larger decrease after about 40 storm hours.     

Monitoring schumann resonances-11. Daily and seasonal frequency variations

The time variations of the Schumann resonance peak frequencies for the first three modes are presented in the vertical electric component measured in the Nagycenk Observatory (47.6°N, 16.7°E) from May 1993 to August 1994. The average daily frequency patterns are different for the three modes, and each mode shows a distinct seasonal variation. The recurrence of this seasonal variation is also shown. The daily frequency range, in which the frequencies shift, is wider in winter than in summer in all three modes. The mean frequency level also shows a seasonal variation in the third mode. A spring-autumn asymmetry has been found in case of the first mode.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

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.     

Very unusual low latitude whistlers with additional traces of the Earth-ionosphere waveguide propagation effect

The present paper reports very unusual whistlers strongly influenced by the Earth-ionosphere waveguide propagation after emerging from the ionosphere, as observed simultaneosly at our two stations, Sakushima (geomag. lat. 24°) and Kagoshima (20°). These unsual whistlers are characterized by clearly exhibiting additional dispersion effects near the cut-off frequencies of the 1st and 2nd order modes of waveguide propagation and, to our knowledge, they are a new finding. All the subionospheric dispersion is deduced to occur between the ionospheric exit point and the receiver. Detailed spectral analysis, after extracting the small waveguide dispersion effect from the overall spectrum by taking the beat with the appropriate pseudo-whistler, has enabled us to determine the propagation distance of the ionospheric exit region from each station. These distances have then been used to locate the ionospheric exit region, which is found to be about 3000 km east of the stations and in the local sunrise time sector. The generation mechanism of such unusual whistlers is discussed in terms of the joint influences of the ionospheric transmission mechanism (longitudinal gradient of the ionosphere, wave scattering by density irreglarities) and magnetospheric propagation and characteristics of ducts.