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.     

Topside and intelhemispheric ion flows in the mid-latitude plasmasphere

A modelling study has been carried out of field-aligned ion flows in the topside ionospheres of conjugate hemispheres under solstice conditions at mid to low latitudes. In the model calculations coupled time-dependent O⁺, H⁺ and electron continuity, momentum and heat balance equations are solved along dipole magnetic field lines at L = 1.5 and 3.0 Sunspot medium and sunspot minimum atmospheric conditions are considered.It has been found that thermal coupling between conjugate hemispheres gives rise to strong flows of O⁺ in the topside ionosphere of the summer hemisphere that are directed upwards at conjugate sunrise and directed downwards at conjugate sunset. At conjugate sunrise in the winter hemisphere there is a small upward-directed signature in the O⁺ field-aligned flux; there is no observable signature in the O⁺ field-aligned flux in the winter hemisphere at conjugate sunset. There are strong upward and downward flows of O⁺ at local sunrise and local sunset, respectively, in both the summer and winter hemispheres.At both L = 1.5 and 3.0 the 24 h time-integrated interhemispheric H⁺ flux is in the direction summer hemisphere to winter hemisphere. At L = 1.5 its magnitude is in good agreement with the magnitude of the 24 h time-integrated plasma (O⁺ + H⁺) field-aligned flux at 1000 km altitude; there are no such agreements at L = 3.0.A study of the roles played by the individual terms of the O⁺ momentum equation has demonstrated the complex structure of momentum balance. Certain of the terms may be orders of magnitude greater than the combined total of the individual terms, i.e. the O⁺ field-aligned flux.     

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.     

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.     

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 effect of the electric field and neutral winds on E-region ion densities and conductivities at low latitudes

A model to calculate electron densities and electrical conductivities in the ionospheric E-region at low latitudes has been developed. Calculations have been performed under photochemical equilibrium and including plasma transport due to the electric field and neutral winds. Results have been compared with observations at Arecibo (18.15°N, 66.20°W), Thumba (8°32′N, 76°51′E) and SHAR (14.0°N, 80.0° E). Good agreement is obtained for Arecibo. For Thumba and SHAR agreement is satisfactory for altitudes above 110 km. Below 100 km, model predictions are too low in comparison with the observed data. The effect of plasma transport on electron densities and Hall and Pedersen conductivities is investigated in detail. A combination of neutral winds and a downward (or westward) electric field can compress the plasma into a thin layer. An upward electric field along with the neutral winds gives rise to a broad, multilayered profile. The ratio of height-integrated Hall to Pedersen conductivities changes from 1.2 to 2 in some cases.     

Diurnal variations in F2 peak density and ionization flux above Tahiti

The effects of composition and ionization fluxes on the diurnal variation of NmF2 at an equatorial anomaly zone station (Tahiti) are separated. The calculated diurnal variation of the fluxes agrees well with what would be expected from published equatorial E × B drift observations and global neutral wind models. A correlation analysis shows that lower hmF2 is often accompanied by larger NmF2, in spite of a much larger recombination rate. This illustrates the dominance of the fountain effect and neutral wind induced interhemispheric transport at this station.     

Observations of neutral winds in the polar cap during northward IMF

Long term remote observations of neutral winds at F-region altitudes have been performed at Thule Air Base (lat. 76.5°N, long. 69.0°W), Greenland, and Søndre Strømfjord (lat. 67.0°N, long. 50.9°W), Greenland. The former site is very close to the geomagnetic pole, while the latter site is within the polar cap for several hours each night on either side of geomagnetic midnight. Wind data corresponding to clear sky conditions and Kp ⩽ 4 were sorted according to the sign of the IMF B_z component. The averaged maximum poleward flow near midnight LST was reduced by approximately one third during B_z northward conditions. If the magnitude of B_y was less than the magnitude of the northward B_z component, then the averaged poleward flow was further reduced by one half. In addition, if B_z > 5 nT, then sunward directed horizontal neutral winds were observed at the very highest latitudes near noon LST.     

North-south asymmetry in the ionospheric equatorial anomaly in the African and the West Asian regions produced by asymmetrical thermospheric winds

The effect of asymmetrical thermospheric winds on NmF2 at the dip I = 30° and its magnetic conjugate point have been computed for equinox conditions to study asymmetry in the ionospheric equatorial anomaly in the African and West Asian regions. The wind models of I11 et al. and Chan and Walker have been used in our computations. During the daytime, due to the winds NmF2 in the northern crest becomes greater than NmF2 in the southern crest; at night the reverse is true in both regions. It is shown that the observed asymmetry in NmF2 at the equatorial crest in the African sector can be well explained by considering the effects of asymmetrical winds with respect to those in the West Asian sector.     

Nocturnal high-latitude E-region in winter during extremely quiet conditions

The quiet night-time E-region at high latitudes has been studied using the EISCAT UHF radar. Data from three subsequent nights during a long period of low magnetic activity are shown and typical features of electron density are described. The background electron density is observed to be 5·10⁹ m⁻³ or smaller. Two types of enhancements above this level are observed ; one is due to charged particle precipitation associated with the F-region trough and the other is composed of sporadic-E layers due to waves in the neutral atmosphere. The sporadic-E is observed to exist almost continuously and to exhibit a regular diurnal behaviour. In addition to the typical afternoon and morning sequential layers, a third major descending layer is formed at night after the passage of the F-region trough The afternoon layer disappears simultaneously with the enhancement of the northward trough-associated electric field and the night-time layer appears at high altitudes after the field has again been reduced to a small value. It is suggested that metal ions from low altitudes are swept by the electric field to the upper E-region where they are again compressed to the night-time layer. A set of steeply descending weaker layers, merging to the main night-time layer are also observed. These layers are most probably caused by atmospheric gravity waves. Theoretical profiles for molecular ions indicate that the strongest layers are necessarily composed of metal ions but, during times when the layers are at their weakest, they may be mainly composed of molecular ions.     

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.     

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density.In concert with the increase in magnetic activity to K_p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms⁻¹ occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm⁻¹ westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f₀F2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.     

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.     

Neutral density cells in the high latitude thermosphere—2. Mechanisms

NCAR-TIGCM simulations predict mesoscale cellular structures in the high latitude neutral density at altitudes from 120–350 km. During magnetically active conditions, the density structure at 200 km consists of low-density cells near dawn and dusk and high-density cells near noon and midnight. Mechanisms causing the structured density cells are a result of thermosphere-ionosphere coupling and can be explained in terms of dynamic meteorology. For example, at high latitudes ion drag causes the neutral circulation to flow cyclonically in the dawn sector and anticyclonically in the dusk sector. Low densities are contained within the cyclonic circulation at all altitudes. Below about 170 km, the densities inside the anticyclonic flow are high, while above that altitude densities within the anticyclonic flow are low. While typical dynamic meteorology explains low densities in the centre of cyclonic circulation and high densities inside anticyclonic circulation, the dusk low-density cell in the centre of anticyclonic flow is unexpected. The anticyclonic dusk low-density cell is explained by anomalous antibaric flow due to high-speed winds. 120 km and 200 km altitudes are used to demonstrate the relationship between the high latitude densities and winds as well as the effect of joule heating and auroral particle precipitation on the density structures.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Measurements of gradients in ionospheric parameters with a new nine-position experiment at Millstone Hill

A new nine-position experiment is now routinely carried out with the Millstone Hill incoherent scatter radars which allows estimation of spatial gradients in the measured ionospheric scalar parameters N_e, T_e, and T_i, and in the components of the ion velocity vector v_i. Use of this technique results in improved estimates of basic and derived parameters from incoherent scatter data at times of significant gradients. We detail the data analysis method and present the first results from this new experiment. The gradients in N_e and in the components of vi are used to compute the motion term in the ionospheric F region continuity equation ▿ · (Nv), which is then combined with ∂N/∂t to estimate the O⁺ recombination rate β at night. Meridional neutral winds U_mer are computed from the field-aligned ion velocity v_∥ and a calculation of the O⁺ diffusion velocity v_d, and it is found that horizontal gradients in the ion velocity field at times significantly affect the calculation of the neutral winds.     

Dynamo action in sporadic-E formation

A theoretical study of the effects of background winds on wind shear-produced sporadic-E layers requires an account of the dynamo electric fields which result from the plasma motion produced by these winds. When a sporadic-E layer is carried downward by a descending wind shear the final height of the layer may vary by some 10 km depending on the background wind. Due to a loading effect on the dynamo, for a given background wind, the final height may vary by about 6 km depending on the degree of ionization in the E-layer in the magnetically conjugate hemisphere. The time scale for dispersal of a sporadic-E layer by drifts in wind-induced electric fields may be as little as 2 h.     

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.     

The influence of geomagnetic activity on the upper mesosphere/ lower thermosphere in the auroral zone. I. Vertical winds

A scanning Fabry-Perot spectrometer (FPS), located at Mawson station, Antarctica (672S, 63°E, invariant latitude 70°S) was used to obtain vertical wind, temperature, and emission intensity measurements from the λ558 nm emission of atomic oxygen. The measured temperature is used to assign an approximate emission height to the observations. A spaced-antenna partial-reflection radar was run concurrently with the FPS from which the presence of enhanced ionization in the D-region could be inferred from the return heights and strengths of the echoes. Large upwards winds of approximately 30 m s⁻¹, at altitudes less than 110 km, appear to be a direct response of the neutral atmosphere to intense auroral events. It is suggested that the observed upwelling is a result of particle heating at heights below the principal emission height. At higher altitudes, vertical winds of a similar magnitude are also measured during geomagnetically disturbed conditions, although here they do not appear to be associated with particular auroral events. In this case it is suggested that upwelling is produced by a combination of Joule and particle heating.     

Low latitude electrodynamic plasma drifts : a review

Radar and radio measurements have provided detailed information on the dependence of F-region electrodynamic drifts on height, season, solar cycle and magnetic activity. Recently, satellite ion drift and electric field probes have determined the variation of low latitude ionospheric drifts over a large range of altitudes and latitudes. The general characteristics of the quiet time plasma can be explained as resulting from E- and F-region dynamo and interhemispheric coupling processes. The low latitude and equatorial zonal and upward/poleward components of the plasma drift respond differently to geomagnetic activity. Disturbance dynamo effects are responsible for the drift perturbations following periods of enhanced magnetic activity. The prompt penetration of high latitude electric fields to lower latitudes produces large perturbations on the upward/poleward drifts, but has no significant effect on the low latitude and equatorial zonal drifts. A number of processes such as ‘overshielding’, ‘fossil wind’ and magnetic reconfiguration were suggested as being responsible for the direct penetration of high latitude electric fields to lower latitudes. Detailed low latitude and global numerical models were used to study the characteristics of low latitude and equatorial plasma drifts and their response to changes in the polar cap potential drop or in the high latitude field-aligned currents. These models can reproduce the latitudinal variation of the perturbation electric fields and their diurnal variations, but are still unable to account for several aspects of the experimental data as a result of the complexity of the high latitude and magnetospheric processes involved.