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).     

Solar cycle and seasonal variations of the low latitude OI 630 nm nightglow

Regular zenith measurements of the OI 630 nm nightglow emission have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9°S), Brazil, since 1975. The long series of observations during the period 1975–1982, including the ascending phase of the last solar cycle, permitted studies of solar cycle effects and seasonal variations. A large intensity increase, about seven times, from low solar activity to high solar activity has been observed. Also, the seasonal-nocturnal intensity variations show large changes between years of low and high solar activity. The characteristics of the variations observed are closely related to the equatorial electric field variations, since the observation site is under the southern equatorial ionospheric anomaly crest.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

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.     

Day-to-day ionospheric variations in a period of high solar activity

In November and December 1979 the solar 10.7 cm radio flux density, sunspot number, X-ray flux and EUV flux were high and very variable. The day-to-day variations of noon F2-layer height and Elayer electron density at three ionosonde stations (Slough, Port Stanley and Huancayo) are found to be well correlated with the day-to-day variations of solar activity. The short-term E- and F-layer variations are consistent with those derived from longer-term studies.     

Influence of solar flares and disturbances of the interplanetary medium on the atmospheric circulation

The atmospheric circulation response to solar flares and geomagnetic disturbances is studied. It is shown that solar flares cause an increase of the zonal circulation intensity in the latitudinal belt ϑ = 45–65°, while geomagnetic disturbances are associated with a decrease of the circulation intensity.The energy necessary to produce the observed atmospheric circulation changes is estimated. The variation of the atmospheric transparency is suggested to be a possible source of that energy.     

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.     

Simulation of ionospheric currents and geomagnetic field variations of Sq for different solar activity

Variations of ionospheric Sq electric currents and fields caused by changes in electric conductivity due to changes in solar activity are studied using the International Reference Ionosphere (IRI) model. Calculations are made for R (sunspot number) = 35 and 200 on the assumption of constant (1, −2)mode tidal winds. It is shown that electric fields grow when solar activity is high, because the ratio of the conductivity in the F-region to that in the E-region increases. Currents in the F-region become stronger than those in the E-region, and nocturnal currents are not negligible when solar activity becomes high. F-region currents also play an important role in the westward currents on the high latitude side of the current vortex. The calculated geomagneticH component at the equator has a depression around 1600 LT for R = 35, while it decreases smoothly from 1100 LT to 1900 LT for R = 200. This difference is consistent with the observed geomagnetic field variation. The ratio of total Sq currents obtained by our simulation is about 3.5, which is a little larger than is found in the observed results.     

A multi-station satellite radio beacon study of ionospheric variations during total solar eclipses

Faraday rotation data obtained at Delhi, Kurukshetra, Hyderabad, Bangalore, Waltair, Nagpur and Calcutta during the total solar eclipse of 16 February 1980 and at Delhi during the total solar eclipse of 31 July 1981 have been analysed to detect the gravity waves generated by a total solar eclipse as hypothesized by Chimonas and Hines (1970, J. geophys. Res. 75, 875). It has been found that gravity waves can be generated by a total solar eclipse but their detection at ionospheric heights is critically dependent on the location of the observing station in relation to the eclipse path geometry. The distance of the observing station from the eclipse path should be more than 500 km in order to detect such gravity waves.     

Long-period variations of wind parameters in the mesopause region and the solar cycle dependence

A solar dependence of wind parameters below 100 km was found by Sprenger and Schminder on the basis of long-term continuous ionospheric drift measurements (D1) in the l.f. range. For winter they obtained for the prevailing wind a positive correlation with solar activity and for the amplitude of the semi-diurnal tidal wind a negative correlation. Later on this result was confirmed by radar meteor wind measurements (D2) at Obninsk and further D1 measurements at KÜhlungsborn and Collm.However, after the years 1973–1974 a change in the behaviour of the zonal prevailing wind was observed. At this time we found a significant negative correlation with solar activity with an indication of a new change after 1983. This was obtained from D1 results in Collm and D2 results in Kühlungsborn not only for winter, but also for summer and even for annual averages. We conclude that this long-term behaviour points rather to a climatic variation with an internal atmospheric cause than to a direct solar control. The negative correlation with solar activity of the semi-diurnal tidal wind in winter remained unchanged (up to 1984) and also proved to be the same in summer and for annual averages. Recent satellite data of the solar u.v. radiation and the upper stratospheric ozone have shown that the possible variation of the thermal tidal excitation during the solar cycle amounts to only a few per cent. This is, therefore, insufficient to account for the 40–70% variation of the tidal amplitudes. Some other possibilities of explaining this result are discussed.     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

Temporal variations of ionospheric waves in the D- and F-regions

It is shown that mesoscale ionospheric wave disturbances in the D- and F-regions regularly occur at all times of day, night and season (characteristic periods ~100, 24, 12.6 min) and are a characteristic property of dynamic processes in the ionosphere.     

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.     

Numerical modeling of the energy storage and release in solar flares

This paper reports on investigation of the photospheric magnetic fieldline footpoint motion (usually referred to as shear motion) and magnetic flux emerging from below the surface in relation to energy storage in a solar flare. These causality relationships are demonstrated by using numerical magnetohydrodynamic simulations. From these results, one may conclude that the energy stored in solar flares is in the form of currents. The dynamic process through which these currents reach a critical value is discussed as well as how these currents lead to energy release, such as the explosive events of solar flares.     

The seasonal variation of the diurnal thermospheric winds over Millstone Hill during solar cycle maximum

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density and vertical plasma drift are used to determine the diurnal variation of neutral winds at F-layer heights over Millstone Hill (42.6°N, 71.5°W) for different seasons. The technique to derive the thermospheric winds from incoherent scatter radar data and the results for equinox conditions have been discussed by Roble et al. (1974). Data for three summer and three winter days in 1969 and 1970 are examined for seasonal characteristics during geomagnetic quiet time at solar cycle maximum conditions. The derived diurnal variation of the neutral winds for summer months shows a later morning transition from equatorward to poleward winds and an earlier evening transition from poleward to equatorward winds than during winter months. These times of transition have a pronounced influence on the ionospheric structure and are partly responsible for the characteristic ‘summer’ and ‘winter’ type behavior of the ionosphere observed over Millstone Hill. The diurnally averaged values of the thermospheric winds at 300 km also have a seasonal variation. During summer, the zonal winds are westward at about 15 m⁻¹ and are eastward at about the same velocity during the winter. The diurnally averaged meridional winds show a strong equatorward flow during summer of about 50 m s⁻¹ whereas during winter months the winds are considerably weaker and irregular, with an average equatorward flow on some days and poleward flow on others. The zonally averaged values are consistent with a mean meridional circulation from the summer hemisphere to the winter hemisphere at F-layer heights. However the irregular meridional winds in the winter indicate a weakening of this circulation near Millstone Hill.     

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.     

Predictability of ionospheric variations for quiet and disturbed conditions

Empirical models can predict the average monthly behaviour of the ionosphere and allow for day-to-day variability. Significant departures from these predicted conditions can be forecast using a range of solar, magnetic and ionospheric observations. The main improvements needed in the prediction models is that they should be made more precise, by introducing variable prediction limits, and should be improved by harnessing the impressive power of the current physical models so that ionospheric data can be assimilated in near real time. This strategic objective will lead to ionospheric weather forecasting.     

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.     

Diurnal and seasonal variability of the southern-hemisphere main ionospheric trough from differential-phase measurements

A differential-phase technique utilizing the radio transmissions of NNSS satellites was used to make measurements of the latitudinal variations of ionospheric vertical total electron content (TEC) in the southern mid-latitude trough region from the location of Macquarie Island (a cis-auroral site; geographic coordinates 54.5°S, 154.95°E, geomagnetic coordinates 64.5 S, 177.67 E, L = 5.38) for a period of 14 months during 1987–1989. The differential-phase method provided a means of observing a relatively large expanse of ionosphere whilst centered on the cis-auroral region which was well suited for trough studies. By monitoring the two transmitted radio signals at 150 and 400 MHz from the Navy Navigation Satellite System (NNSS) polar orbiting satellites it was possible to deduce the latitudinal variation of ionospheric vertical TEC for the duration of the satellite pass. The absolute TEC was derived from Faraday-rotation and ionosonde data obtained during the same period.The main findings of this work have been the high incidence of daytime troughs for all seasons and the relative low incidence of night-time troughs. Both summer and vernal equinox seasons display a greater occurrence frequency of daytime troughs than the winter and autumnal equinox seasons. Winter-time troughs at any time are less frequent than for any other season. Comparisons with the northern-hemisphere trough display a marked difference in occurrence frequency and distribution of troughs. An attempt to explain some of these features in the light of the high-latitude convection theory is offered. Case studies are given for all seasons to highlight these findings.