Y. Z. Su

Comparison of satellite electron density and temperature measurements at low latitudes with a plasmasphere-ionosphere model

Observations made by the Hinotori satellite of the latitude and diurnal variations of electron density and temperature near 600 km altitude in the low-latitude region are studied by comparison with values from the Sheffield University plasmasphere-ionosphere model (SUPIM). The model results show that the observed features of higher electron density in the summer hemisphere and higher electron temperature in the winter hemisphere are caused principally by the difference in the summer and winter hemisphere values of the meridional neutral wind. Closer agreement between the modeled and observed values is obtained when the interhemisphere difference in the meridional wind, as given by the horizontal wind model (HWM) 90, is reduced and when the peak value of the daytime poleward wind is moved to the afternoon sector in the winter hemisphere and to the morning sector in the summer hemisphere. The model results also show that the altitude variation of the vertical E×B drift velocity plays an important role in the development of the ionospheric equatorial anomaly. The latitude and diurnal variations of the modeled electron density and temperature are in good agreement with the observations when the E×B drift velocity used by the model is in accord with the observations made by the AE-E satellite for magnetic field lines with apex altitude less than 400 km and at Arecibo for magnetic field lines with apex altitude greater than 2000 km; linear interpolation of the observed values is used for the intermediate magnetic field lines.

Season, local time, and longitude variations of electron temperature at the height of ∼600 km in the low latitude region

Abstract Electron temperature observed at the height of ∼600 km by the low inclination satellite Hinotori was studied in terms of local time, season, latitude, magnetic declination and solar flux intensity. The electron temperature shows a steep rise in the early morning (well known as “morning overshoot”), a decrease after that and again an increase at ∼18 hours (hereafter named “evening overshoot”). Generally the morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux. The evening overshoot becomes more pronounced in the higher latitudes in all seasons and more enhanced in the winter hemisphere as similar to the morning overshoot. These two overshoots show a slight difference in the 210° – 285° and 285° – 360° longitude sectors. This is most likely due to the difference in magnetic declination of these two zones and the resulting difference in the effect of the zonal neutral wind on the thermal structure in the low latitude ionosphere. Significant difference exists between IRI and the observation during daytime.

Longitudinal variations of the topside ionosphere at low latitudes: Satellite measurements and mathematical modelings

The longitudinal variations of the topside ionosphere at low latitudes observed by the Hinotori satellite during equinoctial periods at high solar activity are studied using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model values show that both the neutral wind and E × B drift velocities make important contributions to the observed longitudinal variations in the topside ionosphere. The displacement of the geographic and magnetic equators and the magnetic declination angle, which give rise to conjugate-hemisphere differences in the neutral wind in the magnetic meridian, are the principal causes of the observed north-south asymmetries in the electron density about the magnetic equator. A comparison of the modeled and observed electron densities shows that the modeled longitudinal variations are, in general, in qualitative agreement with the observations when the neutral winds are taken from the HWM90 thermospheric wind model. Improved agreement in the magnitudes is achieved if HWM90 is modified so that at low latitudes (1) the eastward component of the zonal wind is increased during the day and decreased at night and (2) the diurnal variations of the meridional wind in the northern hemisphere at eastern longitudes and the equatorward wind at around midnight at western longitudes are reduced. The model reproduces the observed longitudinal variations in the development of the equatorial peak electron density during the day and in the equatorial trough and associated crests during the afternoon and postsunset periods. The trough and crests are most prominent at around 2000 LT, where the crest-to-trough ratio varies from about 1.15 at eastern longitudes to about 2.0 at western longitudes. Model calculations show that the longitudinal differences of these features can arise from a longitudinal variation in the vertical E × B drift velocity.

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

Abstract 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 ( N max) 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 N max, 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 N max (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 N max and stronger daytime N max 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.

Variations of the ionosphere and related solar fluxes during solar cycles 21 and 22

Abstract The ionospheric electron content (IEC) and peak electron density (NmF2) data collected at low- and mid-latitudes during 1980–1991 and values of solar EUV (50–1050 A) fluxes obtained from the EUV91 solar EUV flux model are analysed to study the variations of the ionosphere during the intense solar cycles 21 and 22. The study shows that while the ionosphere responds linearly to the solar EUV fluxes its variation with the conventional solar activity index F10.7 is non-linear during both solar cycles. The behaviour of the ionosphere during the most intense solar cycle 19 has been shown to be similar to that during solar cycles 21 and 22 /1/. The ionospheric variations confirm that the relationship between the shorter (EUV and UV) and longer (10.7 cm) wavelength solar fluxes is non-linear during all solar cycles.

A modelling study of the longitudinal variations in the north-south asymmetries of the ionospheric equatorial anomaly

Abstract The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to study the effects of neutral winds of the north-south asymmetries in the ionospheric equatorial anomaly at longitudes 120 °E, 200 °E, 283 °E and 330 °E. The model results obtained for magnetically quiet equinoctial conditions at solar maximum produce north-south asymmetries in the equatorial anomaly in agreement with the observations. The study shows that the neutral wind causes the north-south asymmetries and that the asymmetries have longitudinal variation in accord with the longitudinal variations in the displacement of the geographic and geomagnetic equators and in the magnetic declination angle. At longitudes 120 °E and 283 °E, where the magnetic declination angle is small and the magnetic equators are located in opposite geographic hemispheres, the asymmetries in the anomaly are caused mainly by the asymmetries in the meridional wind. On the other hand, at longitudes 200 °E and 330 °E, where the geographic and geomagnetic equators are almost coincident and the magnetic declination angles are eastward and westward, respectively, the asymmetries in the anomaly arise mainly from the zonal wind. During daytime, in the hemisphere of stronger poleward wind, the crest values in TEC are weaker than in the conjugate hemisphere at all longitudes considered. The values of NmF2 can be stronger or weaker, depending on the competition between the effects of increased chemical loss rate and the downward flow of plasma from the plasmasphere caused by the stronger poleward wind. At night, after the magnetic meridional wind has changed direction and has been in that direction for some time, the stronger crests in both TEC and NmF2 occur in the hemispheres of stronger equatorward wind.

Night-time enhancements in TEC at equatorial anomaly latitudes

Abstract The seasonal and solar activity variations of night-time total electron content (TEC) enhancements and their latitudinal and longitudinal dependencies in the northern equatorial anomaly region (11–23° geomagnetic latitude) are studied by using data from three eastern longitude stations and one western longitude station. Two kinds of enhancement are observed: a postsunset enhancement which occurs most frequently in autumn and a postmidnight enhancement which occurs most frequently in summer. The enhancements, especially the postmidnight enhancements, are more frequent and stronger at the eastern longitude stations than at the western longitude station. The enhancements are weak and least frequent at 23° geomagnetic latitude; the seasonal dependence of the postsunset enhancements at this latitude is different from that at lower latitudes confirming that the effect of the prereversal enhancement of the equatorial fountain, which produces the postsunset enhancements in TEC in the equatorial anomaly region, rarely reaches this latitude even under very high levels of solar activity.

Altitude dependencies in the solar activity variations of the ionospheric electron density

In this study, 7 years (1986-1992) of measurements from the Japanese middle and upper atmosphere (MU) radar have been analyzed to investigate the solar activity variations of the ionosphere. The observations show strong altitude dependencies in the solar activity variations of the electron density. Below 300 km altitude, the electron density increases nonlinearly with F10.7, with the rate of increase being much lower when F10.7 is greater than 150. This nonlinear variation becomes weaker with increasing altitude. Above 450 km altitude, the electron density increases almost linearly with F10.7 when F10.7 lies in the range 100 to 250. For values of F10.7 less than 100, the electron density at low altitudes increases with increasing F10.7, while at higher altitudes (above about 400 km) the electron density remains almost constant. Mechanisms to explain the observed behavior have been investigated using the Sheffield University Plasmasphere Ionosphere Model. The model calculations show that while the variations with F10.7 of the solar EUV flux and neutral gas densities play important roles in the nonlinear variations of the electron density with F10.7, the correlation of the plasma loss rate with temperature being negative at low temperatures and positive at high temperatures is an important mechanism for the nonlinear variations at low altitudes. Model results also suggest that vibrationally excited N 2 strengthens the nonlinear variations of electron density with F10.7. The disappearance of the nonlinear variations at high altitudes, for values of F10.7 above 100, results from the altitude dependencies of the neutral gas variation on F10.7 and of the relative importance of plasma loss, production, and diffusion processes. At high altitudes, the plasma loss processes, which play an important role in the nonlinear variations at low altitudes, are unimportant when compared with the effects arising from plasma production and diffusion. The altitude dependencies of the electron density variations with F10.7, for values of F10.7 less than 100, are due mainly to the altitude dependencies of the neutral gas densities with F10.7.

Modeling studies of the middle and upper atmosphere radar observations of the ionospheric F layer

Observations of electron density and plasma drift velocity made during noon, June 25, 1990, to noon, June 27, 1990, by the middle and upper atmosphere (MU) radar at Shigaraki, Japan, are investigated by comparing them with values modeled by the Sheffield University plasmasphere-ionosphere model. The model calculations, with inputs for the E×B drift and neutral wind velocities derived from the observed plasma drift velocities, give diurnal variations for the electron density profile in reasonable accord with the observations and show that at the magnetic latitude of the MU radar (25°N) the effects of the west-east electric field on the electron density profile are comparable to the effects arising from the neutral wind. The observed feature that the F-region electron density is lower at around noon than during the morning and presunset periods results mainly from the effect of the daytime poleward neutral wind. At night the observations show an increase in the F2-layer peak electron density accompanied by a lowering of the F layer. This feature is shown to be caused by the postsunset eastward electric field which gives rise to a plasma flux from the plasmasphere to the ionosphere. It is found that the effects arising from the electric field are different from those arising from its replacement by an equivalent neutral wind derived from the E×B drift velocity.

Modelling studies of the longitudinal variations in TEC at equatorial-anomaly latitudes

Abstract The: longitudinal differences in total electron content (TEC) in summer in the northern equatorial-anomaly region are studied using observations from Wuhan (an east-Asian longitude station) and Palehua (a mid-Pacific longitude station), and values from the Sheffield University Plasmasphere-Ionosphere Model. The model results show that the differences in the observed values of TEC result from the longitudinal differences in the displacement of the magnetic and geographic equators and in the magnetic declination angle. During daytime, the TEC values are larger at Palehua mainly because the neutral air wind velocity in the magnetic meridian is equatorward at Palehua whereas it is poleward at Wuhan. Closer agreement between the modelled and observed daytime values of TEC is obtained when the phase of the daytime meridional neutral air wind velocity, as given by the HWM90 thermospheric wind model, is shifted towards the morning sector by 3 h. The observed longitudinal differences in nighttime TEC, particularly in the enhancements (which are most frequent and strongest at Wuhan), are caused principally by the longitudinal differences in the prereversal enhancement of the E × B drift velocity. The model calculations for Wuhan show that the time of peak enhancement in TEC, which occurs before midnight in the absence of neutral air winds, is shifted to after midnight by the neutral air wind.