The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a threedimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.     

The influence of thermospheric winds on exospheric hydrogen on Venus

Monte Carlo models of the distribution of atomic hydrogen in the exosphere of Venus were computed which simulate the effects of thermospheric winds and the production of a “hot” hydrogen component by charge exchange of H⁺ and H and O in the exosphere, as well as classic exospheric processes. A thermosphere wind system that is approximated by a retrograde rotating component with equatorial speed of 100 m/sec superimposed on a diurnal solar tide with cross-terminator day-to-night winds of 200 m/sec is shown to be compatible with the thermospheric hydrogen distribution deduced from Pioneer Venus orbiter measurements.     

Atmospheric scattering effects on ground-based measurements of thermospheric winds

Inherent in observations of thermospheric winds from the ground with the Fabry-Perot interferometer is the assumption that the measured Doppler shift is a property of the source medium viewed by the instrumental line of sight. However, ground based airglow observations in regions of weak airglow emission near large intensity gradients may be contaminated by scattered light. Light from areas where the emission is strong can be scattered by the lower atmosphere into the field of view of the observations. Thermospheric winds deduced from the observed Doppler shifts will then show apparent convergence or divergence with respect to the site of observation. Examples of this effect are found in observations by the Michigan Airglow Observatory station located near the auroral zone at Calgary, Alberta. Simulation calculations based upon an experimental model for a significant scattering atmosphere also showed results with either convergence or divergence in the apparent neutral wind field observed by the station.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—I. Observations of vertical winds

Observations of vertical and horizontal thermospheric winds, using the OI (3P-1D) 630 nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia and in Svalbard (Spitzbergen) have identified sources of strong vertical winds in the high latitude thermosphere. Observations from Svalbard (78.2N 15.6E) indicate a systematic diurnal pattern of strong downward winds in the period 06.00 U.T. to about 18.00 U.T., with strong upward winds between 20.00 U.T. and 05.00 U.T. Typical velocities of 30 m s^?1 downward and 50 m s^?1 upward occur, and there is day to day variability in the magnitude (30–80 m s^?1) and phase (+/- 3 h) in the basically diurnal variation. Strong and persistent downward winds may also occur for periods of several hours in the afternoon and evening parts of the auroral oval, associated with the eastward auroral electrojet (northward electric fields and westward ion drifts and winds), during periods of strong geomagnetic disturbances. Average downward values of 30–50 m s^?1 have been observed for periods of 4–6 h at times of large and long-lasting positive bay disturbances in this region. It would appear that the strong vertical winds of the polar cap and disturbed dusk auroral oval are not in the main associated with propagating wave-like features of the wind field. A further identified source is strongly time-dependent and generates very rapid upward vertical motions for periods of 15–30 min as a result of intense local heating in the magnetic midnight region of the auroral oval during the expansion phase of geomagnetic disturbances, and accompanying intense magnetic and auroral disturbances. In the last events, the height-integrated vertical wind (associated with a mean altitude of about 240 km) may exceed 100–150 m s^?1. These disturbances also invariably cause major time-dependent changes of the horizontal wind field with, for example, horizontal wind changes exceeding 500 m s^?1 within 30 min. The changes of vertical winds and the horizontal wind field are highly correlated, and respond directly to the local geomagnetic energy input. In contrast to the behaviour observed in the polar cap or in the disturbed afternoon auroral oval, the ‘expansion phase’ source, which corresponds to the classical ‘auroral substorm’, generates strong time-dependent wind features which may propagate globally. This source thus directly generates one class of thermospheric gravity waves. In this first paper we will consider the experimental evidence for vertical winds. In a second paper we will use a three-dimensional time-dependent model to identify the respective roles of geomagnetic energy and momentum in the creation of both classes of vertical wind sources, and consider their propagation and effects on global thermospheric dynamics.     

Spatial mapping of the thermospheric neutral wind field

There are many possible observing strategies available for mapping the thermospheric wind field by using observations of the Doppler shift of the O (¹D) airglow with a Fabry-Perot interferometer. The determination of the neutral wind field from observed line-of-sight velocities invariably involves some assumptions about the nature of the wind field. A standard method of observing employs the assumption that horizontal gradients in the wind field are linear. An analysis of measurements from Arecibo, Puerto Rico, that makes use of this assumption, is discussed. For work at high latitudes this assumption may be unrealistic. An alternative approach that requires that local time and longitude be interchangeable, but eschews the assumption of linear gradients has been developed and used at Ann Arbor, Michigan, and Calgary, Alberta. We examine these different techniques, and illustrate the discussion with some typical results.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—II. Theory and numerical modelling of vertical winds

Recent observations of strong vertical thermospheric winds and the associated horizontal wind structures, using the 01(3P-1D)nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia have been described in an accompanying paper (Paper I). The high latitude thermosphere at a height of 200–300 km displays strong vertical winds (30–50m ms^?1)of a persistent nature in the vicinity of the auroral oval even during relatively quiet geomagnetic conditions. During an auroral substorm, the vertical (upward) wind in the active region, including that invaded by a Westward Travelling Surge, may briefly(10–30 min)exceed 150 m s^?1. Very large and rapid changes of horizontal wind structure (up to 500 m^?1 in 30 min) usually accompany such large impulsive vertical winds. Magnetospheric energy and momentum sources generate large vertical winds of both a quasi-steady nature and of a strongly time-dependent nature. The thermospheric effects of these sources can be evaluated using the UCL three-dimensional, time-dependent thermospheric model. The auroral oval is, under average geomagnetic conditions, a stationary source of significant vertical winds (10–40 m s^?1). In large convective events (directly driven by a strong momentum coupling from the solar wind) the magnitude may increase considerably. Auroral substorms and Westward Travelling Surges appear to be associated with total energy disposition rates of several tens to more than 100 erg cm^?2s^?1, over regions of a few hours local time, and typically 2–5° of geomagnetic latitude (approximately centred on magnetic midnight). Such deposition rates are needed to drive observed time-dependent vertical (upward) winds of the order of 100–200m s^?1.The response of the vertical winds to significant energy inputs is very rapid, and initially the vertical lifting of the atmosphere absorbs a large fraction (30% or more) of the total substorm input. Regions of strong upward winds tend to be accompanied in space (and time) by regions of rather lower downward winds, and the equatorward propagation of thermospheric waves launched by auroral substorms is extremely complex.     

Atmospheric scattering effects on ground-based Fabry-Perot measurements of thermospheric winds: An inversion technique

The theoretical development of a technique to recover velocities measured with a Fabry-Perot interferometer in the presence of scattered light is presented. Simulations are carried out which show that the inversion is effective in recovering actual velocities, especially in instances when the observed brightness is dominated by scattered light.     

Ion and neutral composition changes in the thermospheric region during magnetic storms

The structural differences of the ion and neutral composition in the thermospheric region are studied by solving a system of basic ionospheric and atmospheric equations. The study shows that the compositional changes during a magnetic storm arise largely as a result of changes in the neutral composition at the turbopause. A decrease in [O]/[N₂] in the lower atmosphere triggers a complex chain of events which results in an increase of the neutral gas temperature, depletion of the O⁺ layer and enhancement of NO⁺. The relative changes in these layers occasionally produce a sequence of electron density profiles giving rise to the so-called G condition. It is shown that, compared to the neutral atmosphere, the ionosphere is much more sensitive to the changes in [O]/[N₂] in the lower thernaospheric region. Since the ionospheric parameters can be measured much more accurately than the atmospheric parameters, it is argued that they should form an integral part of the observational data required to construct the atmospheric models.     

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.     

The influence of neutral winds on He+ distributions in the equatorial ionosphere

The effects of F-region neutral winds on the distribution of He⁺ in the equatorial ionosphere have been examined using a theoretical model and an observational data set. It is shown by the model that components of neutral wind in the magnetic meridian up to only 50 m s^? can produce He⁺ gradients in the northern and southern sectors of a flux tube that differ by more than 80%. This is associated with interhemisphere transport velocities of He⁺ as large as 15 m s^?1 at 800 km. A substantial latitude gradient in the He⁺ distribution across the dip equator also results from the redistribution of He⁺ The changes in the He⁺ concentration at the dip equator and the latitude distribution of He⁺ in response to different neutral wind components is determined from the model and used to construct longitude distributions of He⁺ to compare with observations made at equinox. Good agreement between the calculations and observations is obtained both at the geographic and geomagnetic equators using the relationship between neutral winds, interhemispheric transport velocity and He⁺ concentration derived from the model. If these relationships can be extrapolated to accommodate the different conditions expected during solstice, we can also discuss the He⁺ distributions expected during this season.     

The seasonal effect of nitric oxide cooling on the thermospheric U.V. heat budget

It has recently been shown that radiative cooling of vibrationally excited NO at 5.3μm could result in a significant loss of heat from the lower thermosphere. In contrast to this effect, recent rocket measurements of atomic oxygen fine structure radiation at 63 μm suggests that radiation entrapment may greatly reduce the effectiveness of this process in cooling the lower thermosphere. In this paper we examine the effects of these processes on the u.v. heating efficiency ?. We point out that in the past the definition used for the heating efficiency runs counter to its logical application, and that strictly speaking, the above processes should be included as cooling processes in the energy equations. However, in order to compare out findings with past work we include the 0 and NO radiative cooling in the calculation of an effective heating efficiency, ?. We find that ? can vary from 45 to 60% depending on how the two radiative cooling mechanisms are included in the calculation. In addition it is found that the shape of the altitude profile of the heating efficiency varies significantly with season, while the peak value remains relatively invariant.     

The role of neutral winds and ionospheric electric field in forming stable sporadic E-layers

Using the combined measurements from a rocket flight through a stable intense sporadic E-layer, we examine the shortcomings of conventional wind shear theory of ion layer formation, principally in underestimating the role of the ambient ionospheric electric field. Our results imply that the ionospheric electric field may control the stability and precise location of such ionisation layers within a region of convergent ion flow.     

The effect of the equatorially symmetric zonal winds of Saturn on its gravitational field

The penetration depth of Saturn’s cloud-level winds into its interior is unknown.A possible way of estimating the depth is through measurement of the effect of the winds on the planet’s gravitational field.We use a self-consistent perturbation approach to study how the equatorially symmetric zonal winds of Saturn contribute to its gravitational field.An important advantage of this approach is that the variation of its gravitational field solely caused by the winds can be isolated and identified because the leading-order problem accounts exactly for rotational distortion,thereby determining the irregular shape and internal structure of the hydrostatic Saturn.We assume that(i)the zonal winds are maintained by thermal convection in the form of non-axisymmetric columnar rolls and(ii)the internal structure of the winds,because of the Taylor-Proundman theorem,can be uniquely determined by the observed cloud-level winds.We calculate both the variation △J_n,n=2,4,6...of the axisymmetric gravitational coefficients J_n caused by the zonal winds and the non-axisymmetric gravitational coefficients △J_(nm) produced by the columnar rolls,where m is the azimuthal wavenumber of the rolls.We consider three different cases characterized by the penetration depth 0.36 R_S,0.2 R_S and 0.1 R_S,where R_S is the equatorial radius of Saturn at the 1-bar pressure level.We find that the high-degree gravitational coefficient ( J_(12)+△J_(12)) is dominated,in all the three cases,by the effect of the zonal flow with |△J_(12)/J_(12)|100%and that the size of the non-axisymmetric coefficientsdirectly reflects the depth and scale of the flow taking place in the Saturnian interior.     

Fabry-Perot determinations of midlatitude F-region neutral winds and temperatures from 1975 to 1979

Fabry-Perot interferometer measurements of the Doppler shifts and widths of the nightglow 630.0 nm line at Laurel Ridge Observatory, Pennsylvania are presented for the period 1975 to 1979, covering both solar minimum and solar maximum conditions. The F-region neutral wind vectors v_n and temperatures T_n deduced from these measurements show both day-to-day changes and overall seasonal patterns in the nocturnal variations during geomagnetically quiet conditions. Divergence in both the meridional and zonal horizontal flow is noted on occasion. The v_n results are compared with models including only solar EUV heating and those with EUV plus a high latitude heat source. The aggregate v_n data for solar cycle minimum conditions agree best with model predictions for winter zonal and equinoctal meridional winds and worst for winter meridional and summer zonal winds. At solar cycle maximum the predicted, rapid transition at equinox from summer to winter wind patterns and vice-versa is observed. The T_n data are in reasonable agreement with the MSIS model predictions.     

A short wavelength instability in the neutral sheet of the earth's geomagnetic tail

In an earlier paper, Bowers (1973), ion plasma oscillations were found to be unstable in the steady state developed by Cowley (1972) for the neutral sheet in the Earth's geomagnetic tail. In this paper a similar stability analysis is carried out but for a different steady state, suggested by Dungey, with the result that unstable waves with frequencies near the electron plasma frequency are found. In the Dungey steady state the current necessary for magnetic field reversal is carried by plasma originating from both the magnetosheath and the lobes of the tail. This modifies the steady state proposed by Alfvén and subsequently developed by Cowley in which all the current is carried by plasma from the lobes of the tail thereby fixing the cross-tail potential Φ. With magnetosheath plasma present the value of Φ is no longer fixed solely by parameters in the lobes of the tail but the cross-tail electric field is still assumed localised in the dusk region of the sheet as in the Cowley model due to the balance of charge required in the neutral sheet. The value of Φ can be expected to increase as magnetic flux is transported to the tail which inflates and causes flux annihilation because the magneto-sheath plasma in the neutral sheet has insufficient pressure to keep the two lobes of the tail apart. The Vlasov-Maxwell set of equations is perturbed and linearised enabling a critical condition for instability to be found for modes propagating across the tail. Typically, this condition requireseΦ≳KT _m whereT _m is the temperature of magnetosheath electrons. The instability occurs in the presence of cold plasma which hasE×B drifted into the neutral sheet from the lobes of the tail. This contrasts with the usual two stream instability which is stabilised by the cold plasma. Once precipitated the instability may be explosive provided current disruption occurs, for then a further increase in Φ will result which drives a greater range of wave numbers unstable thereby causing even more turbulence and an even larger cross-tail electric field. Because of this behaviour the instability may be a trigger for a substorm.     

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.     

The effect of damping on geomagnetic pulsation amplitude and phase at ground observatories

We present some results from a model of forced oscillations of the magnetosphere. The purpose of this work is to examine the effects and consequences of damping on geomagnetic pulsations as observed on the ground. The aim of the current work is to quantify the amount of damping applicable to geomagnetic pulsation waveforms. Ionospheric conductivities vary with latitude and time of day and this variation will effect the damping of geomagnetic pulsations. The variations in ionospheric conductivities are taken into account to predict the changes in amplitude and phase of geomagnetic pulsations over an extended latitudinal array of ground observatories. Three situations are modelled where the damping factor γ/ω_n, which is related to the amplitude loss per cycle, is different: (i) γ/ω_n approximately equal to 0.01, this corresponds to the ionospheric Joule damping of Newton et al. (1978); (ii) λ/ω_n equal to 0.1, this value is consistent with the empirically determined day-time damping factors from the observed latitude-dependent transient decays of the pulsation single effect events discussed by Siebert (1964). The value of 0.1 as the damping factor is taken as typical of day-time conditions and its effect on amplitude and phase for continuous pulsations is considered; and (iii) λ/ω_n is latitude-dependent; three different levels of damping are used appropriate for the night-time conditions associated with the auroral electrojet, plasmatrough and plasmasphere.The results from the model suggest that observationally determined damping factors are greater than those computed from ionospheric Joule damping alone. The model also illustrates the broadening of the latitudinal resonance width with increasing damping and the reducing of the phase change across resonance to less than 180°. The model also successfully reproduces features of pulsation single effect events and Pi2 pulsations.     

Measurements of high latitude thermospheric winds by rocket and ground-based techniques and their interpretation using a three-dimensional,time-dependent dynamical model

A three-dimensional, time-dependent model of thermospheric dynamics has been used to interpret recent experimental measurements of high altitude winds by rocket-borne and ground-based techniques. The model is global and includes a self-consistent treatment of the non-linear, Coriolis and viscosity terms. The solar u.v. and e.u.v. energy input provides the major energy source for the thermosphere. Solar u.v. and e.u.v. heating appear to be inadequate to explain observed thermospheric temperatures if e.u.v. heating efficiency (ε) lies in the range 0.3 < ε < 0.35. If the recent solar e.u.v. data are correct, then a value of ε between 0.4 and 0.45 would bring fluxes and observed temperatures into agreement. The Heppner (1977) and Volland (1978) models of high-latitude electric field are used to provide sources of both momentum (via ion drag) and energy (via Joule heating). We find that the Heppner Model CO (equivalent to Volland Model 1) is most appropriate for very quiet geomagnetic conditions (K_p ? 2) while Model A (equivalent to Volland Model 2) provides the necessary enhancement at high latitudes for conditions of moderate activity (K_p ～ 4). Even with the addition of a polar electric field, there still appears to be a shortage of high-latitude energy input in that model winds tend to be 10 m s^?1 poleward of observed winds under quiet or average geomagnetic conditions. This extra energy cannot be provided by enhancing the polar electric fields since the extra momentum would cause disagreement with the observed high latitude winds. High latitude particulate sources of relatively low energies, ~100 eV, seem the most likely candidates depositing their energy above about 200km. Relatively modest amounts of energy are then required, < 10¹⁰W global, to bring the model into agreement with both high- and mid-latitude neutral wind results.     

Midlatitude F-region neutral winds and temperatures during the priority regular world day 14 August 1980

Midlatitude F-region neutral winds and temperatures determined from Fabry-Perot interferometer measurements of the doppler shifts and widths of nightglow 630.0 nm line profiles are presented for the priority regular world day 14 August 1980. They exhibit, in many respects, the observed behavior for other summer, geomagnetically quiet nights at solar maximum. The neutral temperature decreases from 1500°K after sunset (21 h LT) to a minimum of ˜ 1200°K before dawn (05 h LT), except to the north of the observatory. The zonal winds are eastward at sunset at 50 m/sec, decrease to zero at 02 h LT and are westward just before dawn. The meridional winds are zero just after sunset and reach a maximum equatorward value of 50–70 m/sec at local midnight but do not decrease as predicted; instead, they remain at roughly these values towards dawn. The NCAR thermospheric general circulation model (TGCM) is used to predict the global upper atmospheric temperature and circulation patterns for this world day. The model predictions agree with the measured neutral temperatures and exhibit qualitative similarities to the measured neutral winds. It is concluded that inclusion in the model of ion drift at midlatitudes should improve the agreement with observations.