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.     

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.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985–1989 period. Annual as well as semi-annual waves are observed in the 50–70 km altitude region. In the latitude range of ±30 the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than ±15). There is a hemispheric asymmetry in the ozone annual wave in the 20 30 region, with northern hemispheric ozone having a larger amplitude than southern hemispheric ozone. In this region temperature is nearly in phase with ozone in both hemispheres and is reduced in amplitude in the northern hemisphere. The equatorial region is characterized by a strong semi-annual wave in addition to the annual variation, and temperature is nearly out of phase with ozone. At all latitudes there is a larger ozone concentration at sunrise than at sunset. The sunrise sunset difference increases with increasing altitude     

Investigation by ray-tracing of the effect of a summer-winter asymmetry on whistler ducting

A ray-tracing model of the inner magnetosphere (L < 6) is constructed for a plasma distribution asymmetric about the equatorial plane, thus representing summer and winter conditions in the two hemispheres. At the reference height of 900 km, the oxygen ion concentration and electron density are taken to vary by factors of ten and two respectively between the hemispheres. The concentrations of hydrogen and helium ions at the reference level are chosen to ensure electron density continuity across the equatorial plane. The altitude at which ducts terminate is modelled to differ between the two hemispheres in accordance with the numerical simulations of Bernhardt and Park (1977).It is shown that the different plasma distributions in the two hemispheres affect the paths of ducted rays and consequently the likelihood of the reception of one-hop whistlers in the conjugate hemisphere. The difference in final latitude between propagation in the symmetric and asymmetric models for the same initial latitude is largest when ducts extend down to 300 km altitude in the conjugate hemisphere. When ducts terminate at greater altitude, the effect of a difference in termination heights between the two hemispheres generally has a larger effect than that of the plasma asymmetry. Both these effects may play a role in determining the seasonal variation of whistler occurrence.     

A comparative study of H+ and He+ at sunspot minimum and sunspot maximum

The time-dependent equations of continuity and momentum for O⁺, H⁺ and He⁺ are solved for the section of a mid-latitude flux tube from the lower F2-region to the equator. H⁺ and He⁺ behaviour is compared for both sunspot minimum and sunspot maximum conditions at equinox concentrating on light ion replenishment following a magnetic storm and light ion fluxes.It is found that the He⁺ fluxes vary little from day to day throughout the replenishment period following a magnetic storm. In contrast, the H⁺ fluxes can vary considerably throughout the replenishment period, particularly at sunspot minimum. In addition, our results show that temperatures above 1000 km have an important influence on the variation and magnitude of H⁺ fluxes and consequently also influence the replenishment of H⁺. At sunspot minimum the He⁺ content reaches its maximum value after eight days of replenishment and then starts to decrease whilst the H⁺ content continues increasing. This is explained in terms of night-time H⁺ return flows dragging the He⁺ to regions where it can readily recombine. Consequently lightion depletion following a magnetic storm provides a loss process for neutral helium at sunspot maximum but not necessarily at sunspot minimum.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Theoretical calculations on the changes in the ionospheric composition and temperature profiles during storms

The changes in the ionospheric composition and temperature profiles, in the altitude range of 120–1000 km, due to different mechanisms currently considered important during storms, are estimated quantitatively for a low latitude station, Delhi, for moderate solar activity conditions using the computer method of Stubbe. The theoretical results reported here are discussed in the light of the available ion composition and temperature variations observed at low latitudes during storms with satellite data in the topside ionosphere. The results are presented for the three atomic ions O⁺, H⁺ and He⁺ which are important in the F-region and topside ionsophere. It is found that all the three atomic ions increase or decrease in phase with the change in the concentration of n(O) when there is no change in total neutral density. When the change in the exospheric temperature T_∞ with its consequent change in neutral composition and an additional storm time increase in N₂ by a factor of 2 is considered, O⁺ is found to increase in the topside and decrease in the bottomside ionosphere, whereas H⁺ and He⁺ decrease all throughout except for a small increase in He⁺ above 800 km during day. The effect of eastward electric field or southward (equatorward) winds during the day is to increase all three ions in the topside ionosphere and to decrease them in the bottomside ionosphere and vice versa for westward fields or northward (poleward) winds. At night, O⁺ shows the same type of behaviour as for day, while He⁺ shows an increase above 900 km and a decrease below that height for eastward fields or soutward winds and H⁺ shows an oscillating behaviour.Electron and ion temperature (T_e and T_i) during the day shows anticorrelation with the change in the electron concentration N_e (equal to total ion concentration), whereas at night it does not show any significant change except for the case of change in T_∞ and N₂.     

Can the high latitude ionosphere support large field-aligned ion drifts?

Ion velocities perpendicular and parallel to the geomagnetic field have recently been deduced by Smith et al. from bistatic measurements at 71° geomagnetic latitude in the afternoon sector. The results of this experiment include large (>400 m s⁻¹) downward ion velocities parallel to the magnetic field that persist for hours, small (100 m s⁻¹) ion velocities perpendicular to the magnetic field and electron density profiles with extremely narrow full-width at half-maximum. The explanation of these results was that the ionospheric flux tubes observed were near the terminator, and thus, sunlit at the top and in darkness at the bottom. The difference in production between the top and bottom of the flux tube creates an excess of ions at the top, which rapidly diffuse downwards. A three-dimensional, time-dependent model of the ionosphere has been used to test this explanation. Numerical experiments were performed to determine upper limits for the downward ion velocity. Assuming reasonable vertically-induced ion drifts due to either neutral winds or plasma convection, these upper limits were substantially smaller than the measurements. The location of the terminator was found to contribute a maximum of about 60 m s⁻¹ to the vertical ion velocity due to diffusion in a partially illuminated flux tube. In an attempt to explain the narrow density profiles without invoking an additional ionization source, the downward force in the model was arbitrarily increased, as would occur due to parallel electric fields in the ionosphere. Since the interpretation of these measurements as large field-aligned flows seems untenable by a model thought to be consistent with the currently accepted physics of the atmosphere, an alternate hypothesis is presented. If the common volume measurement is made in a region of O⁺ precipitation, then the line profile would not be Doppler shifted when viewed off-zenith. Therefore, the field-aligned velocities would be small, and the narrow width of the profiles would be due to enhanced electron densities in an O⁺ arc.     

The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.     

A comparison of northern and southern hemisphere TEC storm behaviour

Changes in total electron content during magnetic storms are compared at stations with similar geographic and geomagnetic latitudes and eastward declinations in the northern and southern hemispheres.Mean patterns are obtained from 58 storms at ±35° and 28 storms at ± 20° latitude. The positive storm phase is generally larger (and earlier) in the southern hemisphere, while negative storm effects are larger in the north. These changes reduce the normal asymmetry in TEC between the two hemispheres. Composition changes calculated from the MSIS86 atmospheric model agree well with the maximum decreases in TEC in both seasons (when changes in the F-layer height are ignored). Recovery occurs with a time constant of about 35 h; this is 50% longer than in the MSIS86 model. There is a marked diurnal variation at 35°S, with a rapid overnight decay and enhanced values of TEC in the afternoon. This pattern is inverted (and weaker) at 35°N, where night-time decay is consistently slower than on undisturbed nights. These results require a diurnal change in composition of opposite sign in the two hemispheres, or enhanced westward winds at night changing to eastward near sunrise. There is some evidence for both these mechanisms. Following a night-time sudden commencement there is a large annual effect with daytime TEC increasing for storms near the June solstice and decreasing near December. Storms occurring between November and April tend to give large, irregular increases in TEC for several days, particularly at low latitudes. In summer and winter at both stations, the mean size of the negative phase does not increase for storms with K_p> 6. The size of the positive phase is proportional to the size of the change in a_p in winter, while in summer a positive phase is seen only for the larger storms.     

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical dynamo action in the ionosphere

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical ionospheric dynamo are calculated self-consistently, using the assumption of infinite parallel conductivity. Tidal winds of (1, −2) mode, which are generally accepted as a main cause of Sq fields, are adopted as a wind model. Variation in universal time (UT) is examined by considering the discordance between conductivity and wind distribution, which are assumed to follow the geographic coordinate system, and geomagnetic dipole field. Observed UT variation of Sq current system is partly reproduced by our calculation. Calculation for solstice condition is performed by shifting conductivity distribution by 23.5° in latitude. Height integrated westward currents are much smaller in the winter hemisphere than in the summer hemisphere, though eastward currents are not so different in both hemispheres. This unbalance is compensated by the field aligned currents mainly from summer to winter hemisphere in the morning and vice versa in the afternoon. In both above asymmetric cases, structure of the equatorial electrojet is almost symmetric with respect to the equator. Total field aligned currents are rather large and comparable to currents in the ionosphere.     

D-region observations of polar cap absorption events during the EISCAT operation in 1981–1989

During the years 1981–1989, 71 solar proton events altogether were observed. Dividing the events into strong, p.f.u. > 1000 (p.f.u.—proton flux measured at geosynchronous satellite orbit in units of (cm² s sr)⁻¹), medium, 100 < p.f.u. < 1000 and weak events, p.f.u. < 100, only the strong and medium events have a considerable effect on the lower ionosphere. The mean daily absorption at 30 MHz (A), measured in the auroral zone, is >2 dB during strong events, <2 dB during medium events and < l dB during weak events. The most active year during the EISCAT operation was 1989 when 23 solar proton events were observed including six strong events. Diurnal variation of the electron density in the D-region during PCA is a function of the solar zenith angle. However, south of L = 5 a minimum in absorption is observed during the noon hours. During sunrise the absorption increases simultaneously with solar elevation angle, but during sunset there is about 2 h delay between the decrease of absorption and the solar elevation angle.     

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 effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

The morphology of dayside Birkeland currents

Intense (10⁵ A) electric currents flow into and from the Earth's two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west (Y) component of the IMF. The presence of an IMF Y-component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.     

The suppressing effect of field-aligned currents on VLF emissions in the magnetosphere

The growth rate of whistler-mode waves is calculated analytically for a bi-Maxwellian plasma in the presence of a beam of cool electrons. This beam is moving in the same direction as the gyroresonant electrons and in the opposite direction to the waves which are considered to propagate parallel (or anti-parallel) to the imposed geomagnetic field. A somewhat surprising result is found. This is that even if the anisotropy is greater than a critical value, which is strongly frequency dependent, the beam reduces the growth of the waves near half the electron gyrofrequency. For a field-aligned current density ~ 1 μA m⁻², this mechanism can explain the lack of signals near 1.4 kHz on auroral (return current) flux tubes. It can also explain the observed absorption of signals at half the electron gyrofrequency, around 7 kHz on L = 4 flux tubes, near the equatorial plane and just outside the plasmapause.     

Comparison of incoherent scatter observations of electron density,and electron and ion temperature at Millstone Hill with the International Reference Ionosphere

Millstone Hill incoherent scatter (IS) observations of electron density (N_e, electron temperature (T_e) and ion temperature (T_i) are compared with the International Reference Ionosphere (IRI-86) for both noon and midnight, for summer, equinox and winter, at both solar maximum (1979–1980) and solar minimum (1985–1986). The largest difference inN_e is found in the topside, where values of N_e given by IRI-86 are generally larger than those obtained from IS measurements, by a factor which increases with increasing height, and which has a mean value near two at 600 km. Apart from the bottom of the profile, which is tied to the CIRA neutral temperature, the IRI-86 T_e model has no solar cycle variation. However, the IS measurements during the summer reveal larger T_e at solar maximum than at solar minimum. At other seasons higher T_e at solar maximum occurs only during the daytime at the greater heights. Nighttime T_e is shown by the IS radar to be generally larger in winter than in summer, an effect not included in the IRI. This is apparently due to photoelectron heating during winter from the sunlit ionosphere conjugate to Millstone Hill. The day-night difference in T_i given by IRI-86 above 600km is not as large in the IS measurements.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.