Seasonal variations of day-time ionisation flows inferred from a comparison of calculated and observed NmF2

This paper reports on a comparison of calculated and observed monthly mean day-time ionospheric F2-peak density (NmF2) at a chain of stations from Japan to Australia for both solar minimum (1976) and solar maximum (1980). Nm values are calculated using the MSIS model for the observed peak heights (hmF2) and a simplified version of the continuity equation for day-time equilibrium conditions. The observed NmF2 values are always higher than the calculated ones in winter. This implies that a substantial downward flow of ionisation from above into the winter ionosphere is induced by the strongly poleward winter neutral wind which drives the ionisation down the field lines, lowering the peak height hmF2. In summer, winds are smaller, and the fluxes are more upward in comparison to winter. The seasonal variation of the ionisation fluxes and neutral winds are estimated for solar minimum, and compared with results of detailed calculations.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

The dynamic F2-layer over Arecibo

The idealized ‘servo’ model of the ionospheric F2-layer, developed by Rishbeth, ganguly and Walker (1978), is used to simulate the observed behaviour of the daytime F2-peak at Arecibo for sunspot minimum. Taking the east-west electric field to be given by the observed plasma drift velocity perpendicular to the magnetic field, the theoretical equations are integrated using a trial-and-error approach to match the observed values of field-parallel plasma velocity, and the height and electron density of the F2-peak. From the calculation is determined empirically the meridional pressure-gradient force associated with the meridional neutral-air wind. The local time variation during the day is found to be consistent with the semidiurnal variation given by the MSIS atmospheric model of Hedinet al. (1977a, b), though with a phase shift that varies with season; on some days in the fall the pressure-gradient force displays a strong equatorward ‘surge’ in the evening. The values of F2-layer loss and diffusion coefficients needed to match the data are broadly consistent with the MSIS model. The analysis thus validates the MSIS model by way of ionospheric parameters quite independent of the data from which MSIS was originally derived.     

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.     

The isobaric F2-layer

Cyclic diagrams, obtained by plotting the daily variation of the ionospheric electron density NmF2 against the height hmF2, are drawn for typical conditions at Slough (52°N) and Watheroo (30°S). Using the MSIS86 thermospheric model to relate the heights hmF2 to values of atmospheric pressure, the F2-peak is found to lie at nearly the same pressure-level at any given local time, over a wide range of geophysical conditions (season, solar cycle, magnetic disturbance). As local time varies, the pressure level corresponding to hmF2 varies in a way that is mainly determined by the local time variation of the thermospheric winds. This is verified for noon and midnight, using the MSIS86 model to compute the winds. The noon values of peak electron density (NmF2) are fairly consistent with theory (using values of solar ionizing flux as quoted in the literature), but with some discrepancies—particularly at sunspot maximum—that are probably due to uncertainties in chemical composition, or to the effects of vibrational excitation of molecular nitrogen. Overall, the analysis shows a remarkable consistency between ionospheric theory, the data and the MSIS model.     

Winds in the ionosphere—A review

Winds in the upper atmosphere, and their effect on the ionosphere, are reviewed with an emphasis on information useful to ionospheric studies. The winds are driven by pressure gradients from solar and auroral heating, with some forcing by tidal energy from below. Simple calculations which balance the pressure gradient by ion drag and Coriolis forces are generally unreliable, so large-scale numerical models of the coupled atmosphere and ionosphere are required. The accuracy of these global models is limited by uncertainties in the energy inputs at high latitudes and at the lower boundary (about 90 km). The best current wind data come from incoherent scatter radar or airglow installations, at a few sites and for only a few nights per month. Satellite data are also available for several years, and results to 1989 are incorporated in the global HWM90 model. This seems acceptable for determining mean winds at night, less good during the day, and least good in the southern hemisphere where few data were available. Plots are given to show the mean winds at different latitudes and longitudes, for use in ionospheric calculations.Meridional winds alter the height of the mid-latitude F layer, causing large changes in the effective loss rate. This is the major cause of observed seasonal changes, of differences between the hemispheres, and of changes at different longitudes. An increased knowledge of the winds is essential for further progress in F region studies. Ionospheric data provide the most promising route, using routinely scaled parameters. The simplest calculations compare observed peak heights, obtained from M (3000)F2, with the value h_o predicted by simplified “servo” equations. Errors occurring for some hours after sunrise can be overcome using model results to define h_o this allows rapid and accurate wind calculations at dip latitudes of 23–62°. Winds can also be obtained from full model calculations, designed to match observed values of peak height or density.     

The influence of neutral temperatures and winds on the F-Iayer height

Observations of neutral winds and temperatures obtained using a FabryPerot interferometer at Beveridge (37°28′S, 145°6′E) have been combined with h'F measurements from ionosondes at Canberra (35°21′S, 149°10′E) and Hobart (42°54′S, 147° 12′E). Data from 16 nights have been used to study the response (height change) of the F2-layer to changes in neutral wind and temperature. The observations have been compared with the ‘servo’ model of Rishbeth. It is found that the ‘night stationary level’ of the F2-layer depends on temperature, with the height changing by (13 ± 6) km per 100K. This agrees well with the prediction of the ‘servo’ model. There is reasonable overall agreement between the observations and the model predictions for the change in height produced by a given meridional wind. However, there is considerable scatter in the individual comparisons due to the approximations used to apply the theory to the observations. In particular, the effect of electric fields on the F2-layer height has been ignored.     

A comparison of the height of the maximum electron density of the F2-layer from real height analysis and estimates based on M(3000)F2

Estimates of the height of the F-layer peak based on formulations using the ionospheric transmission factor M(3000)F2 are compared with hmF2 derived from the real height analysis of digital ionograms acquired at a mid-latitude station. Based on the analysis of 27 hours of quiet data, our result shows that the M(3000)F2 methods are highly accurate and that the formulation developed by Bradley P. A. and Dudeney J. R., (1973, J. atmos. terr. Phys. 35, 2131) is most accurate.     

Ionospheric conductivity dependence of the cross-polar cap potential difference and global Joule heating rate

We examine the extent to which the cross-polar cap potential difference ϕ and the global Joule heating rate, U, both determined by the magnetogram-inversion method (Kamideet al., 1981, J. geophys. Res. 86, 801), depend upon the assumed conductance models. For this purpose two statistically-determined conductance models developed by Siroet al. (1982, J. geophys. Res. 87, 8215) and ahn et al. (1983b, Planet. Space Sci. 31, 641), and a realistic conductance distribution estimated from bremsstrahlung X-ray image data (Ahnet al., 1989, J. geophys. Res. 94, 2565) have been used. As expected from earlier studies, U is less affected by the choice of conductance models than is ϕ. This is because U is a globally integrated quantity, and thus the local structures of the electric potential pattern do not affect it appreciably, whereas they are crucial in determining ϕ, which is defined as the difference between the maximum and minimum potential values usually found in the dawn and dusk sectors, respectively. A comparison between Uand ϕ based on the statistical conductance models and U and ϕ based on a realistic conductance distribution shows that there are considerable similarities, thus enabling us to use statistical conductance models as a first approximation in deriving such global quantities as the cross-polar cap potential difference and the global Joule heating rate in the study of solar wind-magnetosphere coupling. Several suggestions are made for improving the present available conductance models and some limitations (possibly intrinsic ones) are also discussed.     

Variations of mean winds and tides in the upper middle atmosphere over a solar cycle,Saskatoon, Canada, 52°N, 107°W

Saskatoon (52 N, 107 W) medium frequency (MF) radar data from 1979 to 1990 have been analyzed to investigate the solar activity effects on upper middle atmospheric winds and tidal amplitudes. The period of study covers two solar maxima and a solar minimum; the continuous data allow a systematic analysis of solar cycle dependence on mean winds and tides. The height region of 79–97 km sampled in the study shows an apparent but very weak dependence of mean winds and tidal amplitudes on solar activity variation. The observed features are fairly consistent with the early results reported by Sprenger and Schmindkr [(1969) J. atmos. terr. Phy. 31, 217). The mean zonal wind and the semidiurnal tidal amplitudes appear to exhibit positive and negative correlations with the solar activity, respectively; the statistical significances of these correlations are generally low. There is a biennial periodicity evident in the zonal wind oscillations but this docs not have a consistent phase relationship with the equatorial stratospheric wind oscillations (QBO). The meridional winds and the tidal amplitudes are characterized with different and quite irregular periods of oscillations (2–5 yr). The diurnal tidal variations over the solar cycle are small and irregular, although amplitudes are slightly larger during the solar minimum years.     

Observations of winds in the Antarctic summer mesosphere using the spaced antenna technique

Observations of winds in the 60–100 km height range were made at Mawson (68°S, 63°E) during December 1981 and January 1982 with the MF spaced antenna technique. The prevailing winds are in accord with other recent observations made at high latitudes and show a peak in the zonal wind near 80 km with westward winds of 30 m s ⁻¹. The meridional winds maximize near 90 km with an equatorward flow of 10 m s⁻¹. The diurnal tidal components are in reasonable agreement with recent model predictions, especially in phase. The amplitudes tend to be larger than the model values. The semidiurnal tide is not as stable as the diurnal tide and shows evidence for interference effects between different modes.     

Subsidiary maxima in late evening NmF2 at a low latitude station at solar maximum

At solar maximum during the late evening hours (2100–2400 LT), N_mF2 at Tahiti frequently does not decrease monotonically but exhibits temporary subsidiary maxima. Thus, in 1980, of 66 evening periods for which good data were available, 20 showed monotonie decreases but the remainder exhibited such subsidiary maxima. In summer the subsidiary maxima correspond to hmF2 significantly lower than the monotonie decreases. This lower hmF2 during subsidiary maxima corresponds to a weakening or reversal of the equatorward neutral wind, accompanied by an increase in the flux from the equatorial fountain. In winter the subsidiary maxima are fully accounted for by increases in the flux from the fountain effect, probably due to increases in the strength of the equatorial electrojet.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

Observation and theoretical modelling of a region of downward field-aligned flow of O+ in the winter dayside polar cap

Combined optical and radar measurements of ion drift at high latitudes near the terminator show that large downward field-aligned ion flows occur below the F-peak. At an invariant latitude of 72° and in the local time period from 1100 to 1500, downward velocities of 400 m s ⁻¹ have been observed. At the same time, the poleward component of field-perpendicular ion velocity was only 100 m s ⁻¹. The high latitude ionospheric model of Queganet al. (1982), as modified by Allenet al. (1984), predicts downward field-aligned velocities with the same sign morphology as the observations, but with only one fifth of the magnitude. However, the existence of downward neutral winds might lead to non-linear amplification of the downward ion motion. Using the vertical wind measurements of Reeset al. (1984), a possible explanation of the fast ion flow is suggested.     

Do beats affect sun-weather correlations?

Kinget al. (1977) have presented evidence to suggest that variations in the height of the 500 mbar level may be solar induced. Using a superposed epoch analysis they show a high correlation between the 27.5 day variation (frequency 0.0364 day⁻¹) in the sunspot number and the tropospheric pressure using a ten year run of data. It is suggested here that using such an analysis, side bands in the isobaric height variations spaced equally either side of the 0.0364 day⁻¹ component may add to enhance the variation at 0.0364 day⁻¹. That such a situation may occur is confirmed by an analysis of isobaric height data at three Australian locations for the winter of 1973 which shows that the isobaric surfaces do not respond to the solar component, but that frequency components spaced equally either side of the solar components do occur.     

Shadow bands during the total solar eclipse of 11 July 1991

We recorded shadow bands just before and just after the total phase of the solar eclipse of 11 July 1991. The recordings were made using two broad band silicon photodiodes separated horizontally by 100 mm. They faced the zenith, near to where the eclipsed Sun lay as seen from our observing site close to San José del Cabo in Baja California. The irradiance fluctuations associated with the shadow bands were around 0.04 W m⁻² peak to peak on a background of 1–3 W m⁻². The cross-correlation function indicates that the shadow bands were moving at about 1.8 m s⁻¹ perpendicular to their extent. The power spectral density functions are in accord with the shadow band theory of Codona [(1986), Astron. Astrophys. 164, 415–427].     

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.     

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.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.