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.     

Vertical motions in the thermosphere over Mawson,Antarctica

High time resolution measurements of Doppler shift and broadening of the (OI) >1630 nm emission in the night airglow and aurora have provided determinations of vertical velocities and temperatures in the neutral thermosphere over Mawson, Antarctica. The vertical wind exhibits a large, rapid and complex response to geomagnetic energy input. Upward winds greater than 50 m s⁻¹ are frequently associated with the expansion phase of auroral substorms. Following the disturbance, prolonged periods of downward winds produce temperature enhancements of 200K outside the source region, thus providing a mechanism for the redistribution of geomagnetic energy. Oscillatory behaviour consistent with thermospheric gravity waves is observed during both quiet and disturbed conditions.     

Observations of thermospheric neutral winds by the UCL Doppler imaging system at Kiruna in northern Scandinavia

Since the 1982/1983 winter, the UCL group, in collaboration with the Swedish Institute for Space Physics (previously Kiruna Geophysical Institute), has operated a Doppler imaging system at the high latitude station of Kiruna (67°N, 22°E). The Doppler imaging system is an imaging Fabry-Perot interferometer of 13.2 cm aperture. This instrument has been operated on a ‘campaign’ basis for mapping thermospheric winds using the OI emission at 630 nm (240 km altitude) from a region up to about 400 km radius about Kiruna. In November 1986, the performance of this wide-field Doppler imaging system was augmented by improvements to the detector and all-sky optics. We present data from December 1986, obtained during periods with both clear skies and active auroral and geomagnetic conditions. Maps of the neutral wind flow within the auororal oval during disturbed conditions and near magnetic midnight show continuous and rapid changes of thermospheric winds. The typical scale sizes of eddies observed within the mean flow around magnetic midnight are 100–300 km, with fluctuations at all time scales resolved by the 10 min between successive Doppler images. The local and short period fluctuations appear to be a filtered response of the thermosphere to rapid local variations of the convection and precipitation patterns, within a background of global scale changes     

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.     

Variability of winds and temperatures in the lower thermosphere

High-resolution daytime incoherent scatter radar measurements of plasma temperatures and drifts in the ionospheric E-region above Millstone Hill (42.6°N, 71.5°W) have been used to derive horizontal neutral winds and temperatures in the lower thermosphere (105–130 km) during five multi-day campaigns in 1987–1991. The underlying semi-diurnal tidal component has been determined from the observations, with characteristic average amplitudes of 50 ± 15 m/s and 30 ± 10 K. Phase propagation with altitude follows the expected structure of semi-diurnal tidal modes, but reveals complex coupling of tidal modes, particularly above 115 km. Day-to-day variability in the winds and temperatures is large, and the deviations from the semi-diurnal harmonic can exceed 40 m/s and 50 K. No strong correlations have so far been found with geophysical parameters to explain the observed variability.     

Thermosphere dynamics

Some recent investigations of thermosphere dynamics, carried out in the U.S.S.R., are reviewed briefly. The global empirical models of thermospheric motions are obtained on the basis of ground-based HF and meteor radar measurements of ionospheric irregularities drifts. Numerical modelling of large scale thermospheric electrodynamics for the low and mid-latitudes for quiet geomagnetic conditions, is presented. Disturbances of thermospheric wind systems from high latitude heat sources are considered. The response of lower thermosphere dynamics due to variations of solar and geomagnetic activity are discussed.     

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled,three-dimensional,global model,including variations of the interplanetary magnetic field

The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thermospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal thermospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

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.     

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread-F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Long-period motions in the equatorial mesosphere

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3–10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2°N, 157°W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9°N) and Ascension Island (8°S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January–June) than in the second half-year (July–December). An annual cycle dominates the meridional winds with maximum velocities (5–10m s⁻¹) attained at about 90km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3–10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.     

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.     

A study of the thermospheric forces at a high latitude site on two days of differing geomagnetic activity

Data from the Fabry-Perot Interferometer and Dynasonde at Halley (75.5°S, 26.6°W, L ∼ 4.2), Antarctica, have been used to calculate the forces acting on the high latitude thermosphere. Two case studies of the forces have been undertaken to study why the thermospheric zonal wind speeds are typically so different on nights with different geomagnetic activity. One case study analyses the forces on a geomagnetically active night and the other analyses them on a geomagnetically quiet night. Even on the geomagnetically active night, it is found that the ion drag force is not necessarily the largest force at any one time. Simple comparison of the magnitudes of the forces does not make it very clear which ones dominate in controlling the motion of the thermosphere. This can be seen more clearly by rewriting the momentum equation so that the neutral velocity is expressed in terms of the ion velocity, and the other forces normalized by the ion density. It then becomes clear that, in the evening, the differences in the neutral velocity are due to increases in both ion density and ion velocity, while in the morning, only changes in ion density are important. Thus, although the ion drag force is often not the largest force, it appears that changes in it can account for the variations in neutral velocity between the two nights that were studied.It has also been shown as part of the analysis that whether or not the viscosity needs to be considered when calculating the ion drag force at an altitude of 240 km depends on the ion density profile. If the profile has a single peak then it is only necessary to consider the ion density at 240 km. It is, however, possible that just considering the ion density at this altitude may lead to an underestimate of the effective ion drag force if more than one peak is present.     

Variabilities in the thermospheric temperatures in the region of the crest of the equatorial ionization anomaly—a case study

Results on spectroscopic measurements of thermospheric temperatures made from a low latitude station, Abu (24.6°N, 72.7°E, geographic; 18°N dip latitude), India, situated in the crest region of the equatorial ionization anomaly (EIA), are presented. On many occasions, these measurements reveal large deviations frorn the predictions of the neutral atmospheric model, MSIS-86, bringing out its limitations as applied to the equatorial and low latitude thermosphere. The role played by large-scale geophysical processes like the EIA, equatorial spread F (ESF) and the midnight temperature maximum (MTM), all of which influence the thermal structure of the upper atmosphere, is examined in the context of explaining the differences between the measured temperatures and model predictions. It has been conclusively shown that Joule heating associated with ESF irregular electric fields is not solely responsible for the observed deviations, and the possibility of a significant role by the EIA related processes is indicated.     

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.     

Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002̄23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

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.