Neutral Air Winds Research Articles

Equatorial ionization anomaly development as studied by GPS TEC and foF2 over Brazil: A comparison of observations with model results from SUPIM and IRI-2012

The equatorial ionization anomaly (EIA) development is studied using the total electron content (TEC) observed by the Global Positioning System (GPS) satellites, the F2-layer critical frequency (foF2) as measured by digisondes operated in the Brazilian sector, and by model simulation using the SUPIM (Sheffield University Plasmasphere Ionosphere Model). We have used two indices based on foF2 and TEC to represent the strength of the EIA Southern Anomaly Crest (SAC), which are denoted, respectively, by SAC(foF2) and SAC(TEC). Significant differences in the local time variations of the EIA intensity, as represented by these two indices, are investigated. The observed SAC indices are compared with their values modeled by the SUPIM and also by the International Reference Ionosphere (IRI)—2012. The SUPIM simulations that use the standard E×B plasma drift and neutral air wind models are found to provide acceptable representations of the observed foF2 and TEC, and hence the indices SAC(foF2) and SAC(TEC) during daytime, whereas the IRI-2012 model is not, except during the post-midnight/sunrise hours. It is found that the differences in the local time variations between the SAC(foF2) and SAC(TEC) can be reduced by limiting the TEC integrations in height up to an altitude of 630km in the SUPIM calculations. It is also found that when the EIA intensity is calculated for an intermediate dip latitude (12°S) the difference between the local time variation patterns of the two corresponding indices in the experimental data and in the SUPIM results is reduced. For the IRI-2012 values, the subequatorial station modification does not appear to have any effect.

Morphology of large‐scale traveling atmospheric disturbances in the polar thermosphere

Using a fully coupled thermosphere‐ionosphere‐plasmasphere model, we investigate the morphology of modelled Large‐Scale Traveling Atmospheric Disturbances (LS‐TADs, hereinafter TADs) propagating both poleward and equatorward from auroral sources. We confirm observational evidence of daytime ion‐drag, reducing equatorward far‐field TAD activity. We further suggest an additional source‐dependent mechanism that associates TADs propagating preferentially poleward with an auroral electric field burst that contains an eastward rather than a westward component. These poleward TADs are superimposed on the high‐speed neutral air winds in the polar cap (originating from the day‐night plasma convection) and exhibit speed and spatial resonance effects with the neutral wind. The direction of propagation is affected by Coriolis forcing due to the Earth's rotation.

Mean thermospheric winds at middle latitudes of the northern hemisphere derived from longitudinal variations of the F2-layer peak height

Abstract Longitudinal variations of the vertical plasma drift W produced for any particular local time at middle latitudes of the northern hemisphere by the neutral winds from the horizontal wind model HWM-90, and longitudinal variations of the height of the F2-layer peak h m F 2 from the ionospheric model IRI-86, are approximated by the sum of two zonal harmonics (waves) with zonal wave numbers m = 1 and 2. Diurnal and seasonal variations of ( m = 1, m = 2) amplitudes and phases are investigated (by phase is meant the longitude λ where the crest of the wave is located). It is shown that m = 2 longitudinal variations in h m F 2 are produced mainly by the existence of m = 2 longitudinal variations of the vertical plasma drift, caused by the interaction of zonally-averaged thermospheric winds with the harmonic m = 2 in longitudinal variations of the geomagnetic field parameters. This suggests that zonally-averaged values of meridional and zonal winds can be derived from the Fourier-coefficients for cos 2λ and sin 2λ in longitudinal h m F 2 variations. Two sets of relations for wind calculations are obtained — simple and complex. The simple relations do not take into account longitudinal neutral-air wind variations themselves when producing longitudinal W variations. The complex relations are obtained taking into account m = 2 longitudinal variations of the meridional wind. The method neglects the effects of the electric fields and a possible error due to neglecting by the electric fields is discussed. Zonally-averaged winds calculated at latitude 53 °N from IRI-86 are in a reasonable agreement with winds predicted by the horizontal wind model HWM-90, although there are some differences in detail. To understand whether these differences are because of shortcomings in HWM-90 or in the suggested method, as well as to understand whether this method can be used for monitoring midlatitudinal thermospheric dynamics remotely, further investigations are needed.

The F-layer at sunrise

Abstract. It was noticed 60 years ago that at sunrise (i) the ionospheric critical frequency foF2 increases faster in winter than in summer and (ii) the increase begins at a greater solar zenith angle in winter. It was later suggested that this "seasonal sunrise anomaly" is due to a seasonal change of atomic/molecular ratio in the neutral air in the F2-layer. This paper uses the Sheffield University plasmasphere-ionosphere computational model (SUPIM) with the MSIS thermosphere model to examine the relative importance at sunrise of production, loss and diffusion processes, and the effect of neutral air winds. The results show that both (i) and (ii) can be explained in terms of neutral composition changes.

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

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.

Nighttime enhancements in ionospheric electron content in the northern and southern hemispheres

Nighttime enhancements in ionospheric electron content (IEC) observed at conjugate stations in the equatorial anomaly and mid-latitude regions under moderate to high solar activity conditions are studied. The observations at equatorial anomaly latitudes show that when an enhancement occurs in one hemisphere then an enhancement usually occurs in the conjugate hemisphere. The enhancement characteristics (frequency of occurrence, time of occurrence, amplitude, and duration) and their seasonal and solar activity variations are in agreement with the fact that the primary source of an enhancement is the post-sunset increase in the equatorial fountain. It is suggested that the north-south differences in the enhancement characteristics, e.g. the enhancement being more frequent and stronger in the southern hemisphere than in the northern hemisphere, are due to the north-south differences in the neutral air wind velocity. At mid-latitudes, on the other hand, when an enhancement occurs in one hemisphere then either no enhancement or only a weak enhancement occurs in the conjugate hemisphere; on no observed night does a strong enhancement occur in both hemispheres. The occurrence and other characteristics of the enhancements demonstrate that the primary source for the nighttime enhancements in IEC at mid-latitudes (i.e. the downward flow of plasma from the protonosphere to the ionosphere) is asymmetric; a strong downward flow occurs in only one hemisphere on any one night.

HF Doppler observations of vector plasma drifts in the evening F-region at the magnetic equator

Using HF Doppler radar in a spaced-receiver configuration, observations conducted on the vector plasma drifts in the equatorial F-region at Trivandrum during the afternoon-midnight period of March–April 1988, are presented. The three components of the F-region plasma drift at the magnetic equator are such that the North-South component corresponds to the meridional neutral air wind while the other two components represent the electrodynamic drift. The vertical component of the plasma drift is characterized by a prereversal upward enhancement which fits the interpretation based on the F-region dynamo operating at sunset. Equatorial spread-F, which is linked to the prereversal enhancement, appears to be inhibited during the main phase of magnetic storms. The East-West component of the plasma drift is westward during the afternoon and eastward during the night with mean peak velocities of 30 and 110 ms −1, respectively. The large night-time drift is believed to be due to a large reduction in the degree of electromagnetic coupling between the E- and F-regions during the night. The North-South component, representing the meridional neutral air wind, is found to be northward during the afternoon and southward during the night with respective mean peak velocities of about 50 and 70 m s −1. The neutral air wind shows a tendency to reverse from southward to northward at around midnight and is thought to be caused by the midnight bulge in the neutral air temperature. The observations which correspond to the lower F-region are found to be consistent with those reported earlier using other experimental techniques.

Observations of tides and gravity waves in the lower thermosphere at 70° N

Studies of the atmospheric tides and gravity waves at 70° N were made witt the EISCAT incoherent scatter radar on 3, 4 and 8 Aug 1988. This paper discusses measurements of the electron density and the field-aligned velocity, which is related to the horizontal meridional neutral-air wind. The phase of the 12 h oscillation in the neutral wind behaves smoothly with height on 3 Aug, but abrupt phase changes at 120 and 130 km are observed on 4 and 8 Aug, respectively. At lower heights the vertical wavelength of the 12 h oscillation is about 45 km, which corresponds to the (2,4) component of the semidiurnal tide. On 4 Aug the vertical wavelenght above 120 km altitude is about 200 km, which most probably indicates the dominance of the (2,2) mode. On the other hand, the phase above 130 km on 8 Aug and in the F-region on 4 Aug is nearly constant. The amplitude of the 12 h oscillation is quite variable from day to day, but it has a maximum at 100–110 km, which also speaks for the dominance of the (2,4) mode. In the F-region above 150 km, complex patterns of gravity waves are observed on all three days, which break up into discrete frequency ranges when subjected to filtering. The day-to-day variability of the 12 h oscillation may be due to interaction between the semidiurnal tide and the gravity waves.

Seasonal and longitudinal effects on plasmaspheric tube content

A mathematical model of the ionosphere and plasmasphere that includes an eccentric dipole geomagnetic field has been constructed to model, in the first instance, whistler results at L = 2.5. Observations show that there is a pronounced annual variation in plasmaspheric tube content at mid-latitudes and that this effect is modulated by longitude. The model results indicate that the geometry of the magnetic field (in terms of geographic coordinates) is a major factor in determining the magnitude of the annual variation, with neutral air winds and the degree of plasmaspheric refilling also important. A global variation in neutral atomic hydrogen abundance appears to have little influence.

Mid-latitude electron content modelling: The role of interhemispheric coupling

A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O +, H + continuity and momentum equations and O +, H + and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content ( N p ). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content ( N T ) and F2 region peak electron density ( N max) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on N T of the neutral air wind are not much different in winter from those in summer.

A modelling study of the effects of neutral air winds on electron content at mid-latitudes in winter

A modelling study of the effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in winter has been made. The theoretical models are based on solutions of time dependent momentum and continuity equations for oxygen and hydrogen ions. The computations are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It is found that the magnitude of the poleward neutral air wind velocity has a strong effect on the general magnitude of the electron content, but that the daily pattern of electron content variation is relatively insensitive to changes in the magnitude and phase of the wind pattern. These results are in contrast with the behaviour reported previously (Sethia et al., 1983) for summer conditions. However, the night-time electron content is increased by advancing the phase of the neutral air wind and decreased by retarding it. It appears that day-to-day variations in the electron content pattern in winter cannot be explained as effects of changing neutral air winds, which again contrasts with the findings for summer. As in summer, the wind has a major effect on the filling of the protonosphere, but in opposite sense. It is argued that the effect of the neutral air wind on the ionospheric and the protonospheric electron contents depends on the duration of the poleward wind in relation to daylight and on whether or not the wind reverses direction whilst the ionosphere is sunlit.

Ion transport in the mid-latitude F1-region

A study is made of electron concentration altitude profiles within the F1-layer. By using a mathematical model with a variety of atmospheric conditions and neutral air wind patterns we demonstrate the appearance of a daytime valley. Such valleys have been observed at St. Santin during summer for low-to-moderate levels of solar activity by Taieb et al. (1975,1978). Our calculations show that a valley occurs when there is a vertical shear in the meridional component of the neutral air wind velocity. This wind component must be directed poleward below the shear and equatorward above the shear. A valley will also occur when the meridional component of the neutral air wind velocity is directed poleward at all altitudes within the F1-region provided there is a rapid change in the velocity. We conclude from our results that the non-appearance of a valley at St. Santin during the winter and solar maximum observation periods of Taieb et al. (1975) is due to the neutral air wind pattern having an inappropriate form. Thus, it appears from observational evidence of the F1-layer that the pressure gradients existing in the neutral air during winter and at a solar maximum are unable to produce a wind pattern capable of producing a valley in the electron concentration profile.

The effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in summer

The effects of neutral air winds on the electron content ( N T ) and other parameters of the mid-latitude ionosphere have been modelled by means of mathematical solutions of the time-dependent continuity and momentum equations for oxygen and hydrogen ions. The geometry is chosen to represent a propagation path between a geosynchronous satellite and a ground station, and the computations are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It is demonstrated that the electron content responds markedly to the magnitude and phase of the neutral air winds and that the effect induced by the wind on the electron content shows a consistent quantitative relationship with the wind velocity, especially during daytime. Reasonable variations in the phase and magnitude of the wind produce a range of daily electron content patterns which encompass the range of daily variations observed. The computations show that the wind gives rise to enhanced filling of the protonosphere. This shows as a depressed value of the shape factor ( F), which by definition means that a greater fraction of the ionization is at higher altitudes. The depression of F is enhanced by a poleward wind and is suppressed or even superseded by an equatorward wind through changes of the electron density distribution with altitude.

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.

Directional currents in nocturnal E-region layers

In the mid-latitude E-region, the wind-shear mechanism produces thin ionized layers at levels where the vertical ion velocity is zero. We show that such layers conduct electric current only towards the magnetic equator, and not in the zonal direction. We surmise that this property may influence the electric field distribution in the nocturnal ionosphere, and possibly also the coupling between ion drifts and neutral air winds in the F-region. Detailed case studies of nocturnal layers located near the peak of ion Pedersen conductivity (around 130km) are needed to test this idea.

Night-time electron temperature troughs in the equatorial topside ionosphere revealed from RPA experiments on the ISS-b satellite

Night-time electron temperature ( T e ), electron density ( N e ) and the mean ion mass ( M i ) of the topside ionosphere at altitudes of about 1100 km have been obtained from the RPA experiments on ISS-b. Global maps of these quantities are derived by adopting the spherical surface harmonic functions fitted to the satellite data by means of a least-square method. The night-time map of T e shows a marked equatorial trough which is limited to a 100° longitudinal sector and a 20° latitudinal extent. The center of the trough is located a few degrees ‘summerward’ of the dip equator. The longitude of the trough is strongly controlled by the magnetic declination. This longitude changes rapidly around autumnal equinox from the region of maximum westward declination to that of eastward declination; the opposite change occurs around the time of vernal equinox. The maps of M i and N e in low latitudes have features apparently related to the T e trough. These features can be explained in terms of field-aligned interhemispheric plasma flow driven by the neutral air wind.

Ion temperature troughs induced by a meridional neutral air wind in the night-time equatorial topside ionosphere

A mathematical model has been developed to calculate consistent values for the O + and H + concentrations and field-aligned velocities and for the O +, H + and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered. It is shown that the O +-H + transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes.

The dynamic F2-layer over Arecibo

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

The topside equatorial ionosphere during daytime: Elevated plasma temperatures, the transequatorial O + breeze and the dynamics of minor ions

Steady-state calculations are performed for the daytime equatorial F2-region and topside ionosphere. Values are calculated of the electron and ion temperatures and the concentrations and field-aligned velocities of the ions O +, H + and He +. Account is taken of upward E × B drift, a summer-winter horizontal neutral air wind and heating of the electron gas by thermalization of fast photoelectrons. The calculated plasma temperatures are in accord with experiment: at the equator there is an isothermal region from about 400–550 km altitude, with temperatures of about 2400 K around 800 km altitude. The transequatorial O + breeze flux from summer to winter in the topside ionosphere is not greatly affected by the elevated plasma temperatures. The field-aligned velocities of H + and He + depend strongly on the O + field-aligned velocity and on the presence of large temperature gradients. For the minor ions, ion-ion drag with O + cannot be neglected for the topside ionosphere.

Nonlinear investigation of the spatial resonance effect in the nighttime equatorial F region

In the nighttime equatorial F region, zonal neutral air winds and gravity waves propagating across the magnetic field tend to drive the ionization as if there were no field. When the phase trace speed of a gravity wave equals the drift speed of the ionization, strong ionization perturbations may occur (Whitehead's spatial resonance effect). The resonance effect is described by a parametric differential equation. Without diffusion, it yields perturbations in the form of periodic δ functions. Therefore a coefficient of turbulent diffusion is introduced to limit the perturbation amplitude. Then the equation can be solved in terms of partial Fourier sums. Numerical calculations with reasonable F region parameters indicate that under spatial resonance conditions, gravity waves cause considerable secondary flows of ionization as a result of nonlinear effects. The contours of constant ionization density show steep or even breaking wave fronts similar to those observed by HF radar and radio propagation experiments in the large‐scale structure of the equatorial spread F. The spatial resonance mechanism produces field‐aligned tubes of decreased ionization density which may be the primary cause of ‘plasma bubbles’ detected by satellite and rocket probes.