Neutral Thermosphere Research Articles

A global model of the neutral thermosphere in magnetic coordinates based on OGO 6 data

Data from the OGO 6 satellite have been analyzed in magnetic latitude and magnetic local time coordinates for various seasons and magnetic activity levels. These measurements show considerable detail, particularly in the auroral regions where the energy inputs are well organized in this coordinate system. This detail is not readily observed in models based on geographic coordinates. Atomic oxygen and helium densities and a parameter related to the molecular nitrogen density were analyzed. The long‐term averages of these quantities are presented in graphical form and as analytic functions to provide models of these thermospheric parameters. The atomic oxygen and helium densities show minima at high latitudes in the postmidnight sector for nearly all seasons and magnetic activity levels. The exospheric temperature inferred from the N2 density increases toward high latitudes for all seasons and all magnetic activity levels. This inferred temperature is about 300° K higher at the summer magnetic pole than at the winter magnetic pole. This article contains supplementary material.

F layer positive response to a geomagnetic storm – June 1972

A circulation model of neutral thermosphere‐ionosphere coupling is used to interpret in situ spacecraft measurements taken during a topside mid‐latitude ionospheric storm. The data are measurements of electron density taken along the circular polar orbit of Ariel 4 at 550 km during the geomagnetically disturbed period June 17–18, 1972. We infer that collisional momentum transfer from the disturbed neutral thermosphere to the ionosphere was the dominant midday process generating the positive F layer storm phase in the summer hemisphere. In the winter hemisphere the positive storm phase drifted poleward in apparent response to magnetospheric drifts. A summer F layer positive phase developed at the sudden commencement and again during the geomagnetic main phase; a winter F layer positive phase developed only during the geomagnetic main phase. The observed seasonal differences in both the onsets and the magnitudes of the positive phases are attributed to the interhemispheric asymmetry in thermospheric dynamics.

Nighttime thermospheric winds at high latitudes

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

Neutral thermospheric winds deduced above Millstone Hill, 2. Seasonal wind variations, 1970–1971

Millstone Hill (42.6°N) incoherent scatter data gathered during 1970–1971 were used as input for a local model of the neutral thermosphere. The pressure gradients in the model were adjusted so as to match the exospheric temperature and neutral wind parallel to the magnetic field observed above the radar. Large seasonal variations in the diurnal average horizontal winds were deduced. Annual average zonal velocities were 5 m/s toward the west with an annual variation of ±43 m/s. Meridional velocities were 21 m/s toward the equator in the annual average with annual variations of ±52 m/s. The mean winds were toward the southwest in the summer and the northeast in the winter and tended to be stronger in 1971 than in 1970. The average vertical velocities were found to be generally downward throughout the year. The mean equatorward winds, large annual variations, and average downward velocities are consistent with a global circulation model presented by Dickinson et al. (1977) and require the existence of significant heating of the thermosphere at high latitudes. Seasonal variations calculated using the MSIS (mass spectrometer/incoherent scatter) model pressure gradients were similar, except that in contrast to the observed behavior the intensity of the annual circulation appeared to decrease from 1970 to 1971, reflecting the decline in the solar flux. Since the magnetic activity tends to lag the sunspot cycle by about a year and remained at about the same level over the 2 years, it is suggested that this difference may be accounted for if the influence of the high‐latitude heating is increased.

Atomic oxygen emission at 63 µm as a cooling mechanism in the thermosphere and ionosphere

FAR IR emission by atomic oxygen fine structure transitions is considered an important cooling mechanism in the Earth's upper atmosphere and ionosphere as well as in several astronomical objects. The cooling rates so far have only been calculated theoretically on the basis of local thermodynamic equilibrium. No experimental or theoretical determination of the fine structure excitation or quenching cross sections for mutual neutral atomic oxygen collisions has been reported. We report here the first spectrometrically measured atomic oxygen 63-µm emission profile at thermospheric altitudes (90–180 km). An energy-averaged quenching cross section for the O(3P1) level was deduced. The importance of the 63-µm emission as a cooling process for both the neutral thermosphere and the ionosphere is much less than predicted.

Semi-empirical models of the neutral atmosphere based on turbopause height and exospheric temperature variations

Based on mass spectrometer observations of the satellites Ogo 6 and Esro 4 the long periodic density variations of He, O, N 2 and Ar are interpreted to be a result of combined variations of the exospheric temperature and the turbopause height. Only these two parameters are used in the models. Thus in comparison with previous models the number of parameters needed is significantly reduced. This simplification does not diminish the accuracy of the analytic representation of the observations and provides new physical insight into the processes which govern the dynamic behaviour of the neutral thermosphere. The annual variation of the turbopause height is found to cover the height range between 90 and 115 km and is approximately equal for both the Ogo 6 and the Esro 4 data.

Direct measurements of nighttime thermospheric winds and temperatures, 2. Geomagnetic storms

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), to determine the nighttime thermospheric temperatures and winds during geomagnetic storm periods from the line profiles and Doppler shifts of the (O I) 15867-K (6300 A) line emission. Data obtained during four geomagnetic storm periods in 1974 are presented: March 20–22, October 15–20, June 29 to July 8, and April 16–27. The data are compared to temperatures determined from the Ogo 6 global empirical model of neutral composition and temperature and to winds calculated from a semiempirical dynamic model of the neutral thermosphere. The results show considerable variations in the dynamic character of the thermosphere which are also related to the intensity of geomagnetic activity. During geomagnetic storms the nighttime equatorward winds are greatly enhanced from their quiet time values with a maximum measured velocity of 640 m s−1 during a Kp = 9 storm. The zonal winds generally develop a westward component relative to the geomagnetic quiet zonal winds. During intense geomagnetic activity the zonal winds are westward at 100–200 m s−1 in the early evening hours, flowing in the direction of magnetospheric convection and opposite of model predictions. The meridional winds measured south of the site are generally smaller than those measured to the north. Sometimes the meridional winds measured north and south of the station flow in opposite directions. The nighttime neutral gas temperatures are observed to increase from their geomagnetic quiet values during the storm and are in general concurrence with the Ogo 6 model predictions.

Direct measurements of nighttime thermospheric winds and temperatures, 1. Seasonal variations during geomagnetic quiet periods

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory (39.9°N, 105.5°W) to determine the nighttime thermospheric temperature and winds from November 1973 through February 1975. The thermospheric temperature and winds at a height near the F2 peak are determined from the Doppler broadening and shift, respectively, of the atomic oxygen line emission at 15867.852 K (6300.308 Å). The experimentally derived temperatures and winds during geomagnetically quiet conditions are compared with a three-dimensional semiempirical model of the neutral thermosphere. This model in turn uses global empirical models of neutral temperature, composition, and electron density to determine the pressure forces and ion drag which generate and modify the wind fields. The large-scale details of measured and calculated nighttime meridional wind components are in general agreement, showing maximum equatorward winds during the summer months. Measured and calculated zonal winds agree for the equinoctial and winter months; however, the measured nighttime zonal winds are westward during summer months in contrast to model calculations that indicate a midnight eastward to westward transition. The measurements also indicate that both the temperature and the equatorward component of the meridional wind measured north of the observing station are greater than those measured south of the station. These positive latitudinal temperature and wind gradients are greatest during the summer months.

The intensity variation of the atomic oxygen red line during morning and evening twilight on 9–10 April 1969

During the evening of 9 April and the morning of 10 April 1969, the twilight zenith intensity of the atomic oxygen red line OI( 3P- 1D) at 6300 Å was measured at the Blue Hill Observatory (42°N, 17°W). At the same time incoherent scatter radar data were being obtained at the Millstone Hill radar site 50 km distant. We have used a diurnal model of the mid-latitude F-region to calculate the ionospheric structure over Millstone Hill conditions similar to 9–10 April 1969. The measured electron temperature, ion temperature, and electron density at 800 km are used as boundary conditions for the model calculations. The diurnal variation of neutral composition and temperature were obtained from the OGO-6 empirical model and the neutral winds were derived from a semiempirical three-dimensional dynamic model of the neutral thermosphere. The solar EUV flux was adjusted to yield reasonable agreement between the calculated and observed ionospheric properties. This paper presents the results of these model computations and calculations of the red line intensity. The 6300 Å emission includes contributions from photoelectron excitation, dissociative recombination, Schumann-Runge photodissociation and thermal electron impact. The variations of these four components for morning and evening twilight between 90–120° solar zenith angles, and their relative contributions to the total 6300 Å emission line intensity, are presented and the total is compared to the observations. For this particular day the Schumann-Runge photodissociation component, calculated using the solar fluxes tabulated by Ackermann (1970), is the dominant component of the morning twilight 6300 Å emission. During evening twilight it is necessary to utilize a lower O 2 density than for the morning twilight in order to bring the calculated and observed 6300 Å emission rates into agreement. The implication that there may be a diurnal variation in the O 2 density at the base of the thermosphere is discussed in the light of available experimental data and current theoretical ideas.

The calculated and observed diurnal variation of the ionosphere over Millstone Hill on 23–24 March 1970

A numerical model of current F-region theory is use to calculate the diurnal variation of the mid-latitude ionospheric F-region over Millstone Hill on 23–24 March 1970, during quiet geomagnetic conditions. From the solar EUV flux, the model calculates at each altitude and time step primary photoelectron spectra and ionization rates of various ion species. The photoelectron transport equation is solved for the secondary ionization rates, photoelectron spectra, and various airglow excitation rates. Five ion continuity equations that include the effects of transport by diffusion, magnetospheric-ionospheric plasma transport, electric fields, and neutral winds are solved for the ion composition and electron density. The electron and ion temperatures are also calculated using the heating rates determined from chemical reactions, photoelectron collisions, and magnetospheric-ionospheric energy transport. The calculations are performed for a diurnal cycle considering a stationary field tube co-rotating with the Earth; only the vertical plasma drift caused by electric fields perpendicular to the geomagnetic field line is allowed but not the horizontal drift. The boundary conditions used in the model are determined from the incoherent scatter radar measurements of T e , T i and O + flux at 800km over Millstone Hill (Evans, 1971a). The component of the neutral thermospheric winds along the geomagnetic field has an important influence on the overall ionospheric structure. It is determined from a separate dynamic model of the neutral thermosphere, using incoherent scatter radar measurements. The calculated diurnal variation of the ionospheric structure agrees well with the values measured by the incoherent scatter radar when certain restrictions are placed on the solar EUV flux and model neutral atmospheric compositions. Namely, the solar EUV fluxes of Hinteregger (1970) are doubled and an atomic oxygen concentration of at least 10 11 cm 3 at 120 km is required for the neutral model atmosphere. Calculations also show that the topside thermal structure of the ionosphere is primarily maintained by a flow of heat from the magnetosphere and the night-time F2-region is maintained in part by neutral winds, diffusion, electric fields, and plasma flow from the magnetosphere. The problem of maintaining the calculated night-time ionosphere at the observed values is also discussed.

Transport equations for aeronomy

In this paper we present results for a general system of transport equations appropriate to a multi-constituent gas mixture. This system includes a continuity, momentum, internal energy, pressure tensor and heat flow equation for each species. The results can be applied to both collision dominated and collisionless plasmas with there being explicit limits derived for the validity of the various expressions. In the limit of very frequent collisions the pressure tensor and heat flow equations give the usual Navier-Stokes results for the viscous stress tensor and heat flow vector. Furthermore, the momentum equation includes thermal diffusion and thermoelectric transport coefficients equivalent to the second approximation of Chapman and Cowling. The basic system of equations has been applied to different regions of the ionosphere and neutral atmosphere. It is found that: (1) The viscous stress tensor and heat flow expressions used in previous studies of the neutral thermosphere may not be appropriate; (2) The transport coefficients normally used for mid-latitude F2-region and topside studies seem to be adequate; (3) The high speed flow of plasma in the polar topside ionosphere is likely to be strongly affected by stresses and heat flow; and (4) E- and F-region ionization at high latitudes is substantially affected by stresses and heat flow.

Diurnal variation of the neutral thermospheric winds determined from incoherent scatter radar data

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density, and vertical plasma drift are used to determine the diurnal variation of the neutral thermospheric winds. The ionospheric properties needed to determine the neutral winds are (1) the horizontal neutral wind component along the magnetic declination, SD (determined from plasma drift measurements and F region theory) and (2) the longitudinal derivative of the neutral exospheric temperature, ∂T∞/∂γ (derived from ion and electron temperatures and density measurements above 300 km). The atmospheric response to various forcing functions is determined from a three-dimensional dynamic model of the neutral thermosphere, the ion drag being specified by the electron density measurements. Harmonic components of the pressure force in terms of the latitudinal and longitudinal variations of exospheric temperature are used to determine the SD response coefficients from the model. A least squares fit of the measured and calculated diurnal variation of SD and ∂T∞/∂γ gives the Fourier coefficients necessary to construct the appropriate latitudinal and longitudinal pressure gradients. The pressure gradients are then used to calculate the diurnal winds for two geomagnetically quiet days over Millstone Hill during equinox. Amplitudes of 200 m s−1 during the night and 50 m s−1 during the day are obtained. The diurnal winds determined by using three different empirical models of the thermospheric structure are also calculated and compared with the winds determined from the least squares fit.

The effect of displaced geomagnetic and geographic poles on the thermospheric neutral winds

A three-dimensional, semi-empirical dynamic model of the neutral thermosphere is used to examine the effect of the displaced geomagnetic and geographic poles on the daily variation of neutral gas motion. The global-scale pressure distribution to drive the neutral gas motion is derived from the empirical model of Jacchia (1965). The ionization distribution is obtained from the Pennsylvania State M.K 1 model ionosphere using the first few longitudinal Fourier coefficients of the ionization distribution. The calculations were made at various latitudes at equinox and solstice and for various values of solar activity. The results show that the calculated neutral winds for the case where the geomagnetic and geographic poles are coincident differ at most only a few per cent from the winds calculated assuming the poles displaced. With the poles coincident, longitude and local time are interchangeable, and one dimension in any dynamic model of the thermosphere may be eliminated.

Response of Dayside Thermosphere to an Intense Geomagnetic Storm

INCREASES in the density of the neutral thermosphere associated with geomagnetic activity were first noted by Jacchia1. Although the thermosphere will not be in equilibrium, and acceleration by electrodynamic forces may be involved, on balance the increased density must reflect additional heating. Further studies2–4 by orbital drag analysis from observations of artificial Earth satellites have led to a complex and somewhat confusing picture. The heating seems to be slightly greater3,4 at high latitudes and, for larger disturbances, sometimes much greater2,3. Also, the heating seems to be slightly greater3,4 on the nightside, but possibly with the opposite trend3 for larger disturbances (Kp > 5). Jacchia et al.3 also found that the time delay between the peak of the storm, as measured by the planetary 3-h indices Kp or ap, and the maximum response was a little shorter at high latitudes; on the other hand Roemer4 concluded that the time delay was independent of latitude, height or local time.

Magnetic control of the near equatorial neutral thermosphere

Neutral density data between 400 and 500 km obtained with the Ogo 6 neutral mass spectrometer indicate that at all longitudes the daytime N2 and O densities have latitudinal minimums at the magnetic equator that are about 20 and 10%, respectively, below maximums observed at ±20° magnetic latitude. Our calculations suggest that this anomaly could result from latitudinal variations in ion drag associated with the ‘equatorial anomaly’ in F region ionization. At latitudes of enhanced electron density the energy flow from the dayside to the nightside of the earth is damped, and thus the amplitude of the temperature and density variations is enhanced, and the time of maximum is shifted to later local times.

A neutral‐atmosphere composition experiment for the Atmosphere Explorer‐C, ‐D, and ‐E

The neutral‐atmosphere composition experiment instrumentation is designed to obtain in‐situ measurements of neutral thermosphere composition from Atmosphere Explorer‐C, ‐D, and ‐E. The system is based on previously flown OGO‐6 and San Marco‐3 composition instruments. The mass‐spectrometer sensor includes a gold‐plated thermalizing chamber and ion source, a hyperbolic rod quadrupole analyzer, and an off‐axis electron multiplier. Automatic ion‐source sensitivity control and pulse‐counting techniques provide density measurement capability from approximately 125 to 1000 km altitude. The normal operating mode includes measurement at all masses in the range 1 to 44 amu with emphasis on hydrogen, helium, oxygen, nitrogen, and argon. Additional operational modes are optimized for minor constituent studies of any individual gas in this mass range.

Neutral thermosphere temperatures from density scale height measurements

During each 4-min satellite interrogation period the Explorer 32 density gages measured the atmospheric density approximately every 2 sec. Over certain segments of the satellite orbit these measurements determined the neutral atmospheric density scale height. The scale heights measured at an altitude of 400 (plus or minus 50) km have been analyzed to infer thermospheric temperatures. The results confirm an earlier conclusion from the density data of the same experiment that the diurnal temperature variation is latitude dependent.

Is there enough solar extreme ultraviolet radiation to maintain the global mean thermospheric temperature?

The global mean temperature profile of the neutral thermosphere above 120 km is calculated using the solar EUV flux tabulation of H. E. Hinteregger (1970) for wavelengths less than 1310 A and the data of K. G. Widing et al. (1970) for wavelengths greater than 1310 A. With a heating efficiency of 0.33, the calculated global mean exospheric temperature is 700°K. This temperature does not lie within the range of the 900° to 1050°K global mean temperature obtained from the model of L. G. Jacchia (1971) for the range of solar F10.7 emission suggested by H. E. Hinteregger (1970) as appropriate to his data. The electron temperature in the ionosphere is also calculated using the solar EUV flux and is compared with the data of J. V. Evans (1971). The calculated electron temperature is also considerably lower than the measured values. Calculations show that a doubling of the solar EUV flux for wavelengths less than 1310 A is required to bring both the neutral and electron temperatures into reasonable agreement with observations.

Photochemical heating of the mesosphere and lower thermosphere

A diurnally varying photochemical-diffusive model of a neutral oxygen-hydrogen mesosphere and lower thermosphere has been used to calculate the heating rates resulting from the absorption of solar ultraviolet radiation. The model extended from 60 to 160 km and permitted a coherent study to be made of this region, unlike previous investigations. In the thermosphere, where most of the absorption is by O 2 , only about 35 % of the solar energy input appeared as local heating. The remainder of the input was primarily removed as chemical energy in the form of a downward flux of atomic oxygen into the mesosphere. This chemical energy was converted into heating by reactions occurring near the mesopause. In the mesosphere, where absorption by O 3 becomes important, a significant amount of the heating is realized indirectly by chemical reactions. An important consequence of this indirect heating is that it continues at night between about 80 and 100 km, unlike the situation in the thermosphere where there is almost no nocturnal heating in the model. Airglow emissions, primarily by electronically excited O 2 in the thermosphere and especially by vibrationally excited OH in the mesosphere, represent important cooling mechanisms in the model. In particular it is estimated that at 88 km. the OH airglow emission could remove about 30 % of the local energy input averaged over 24 hours. DOI: 10.1111/j.2153-3490.1972.tb01532.x

Response of the Neutral Thermosphere at F-Layer Heights to Interaction of a Global Wind with Anomalies of Ionization

Departures from a mean global-scale ionization distribution are commonly found in the ionospheric F region. Global-scale winds experience an acceleration where the ion drag is locally less than its global-scale smooth value and, they likewise, experience a deceleration where the ion drag is locally greater. Thus, a perturbation in the horizontal flow is set up in response to this ion-drag momentum source. A two-dimensional, steady-state dynamic model of the neutral thermosphere, incorporating thermal conduction, viscosity and ion drag, is used to calculate the temperature perturbation and circulation pattern caused by these ion-drag anomalies. The forcing is given by a momentum source which depends on the interaction of a basic-state neutral wind with the anomaly. For horizontal-scale anomalies of a few hundred kilometers, such as the electron density depression within the stable auroral red arc, the momentum source due to perturbation ion drag is almost completely balanced by a perturbation pressure force. The perturbation temperature and circulation responses are, therefore, negligibly small. For horizontal-scale anomalies of the order of a few thousand kilometers, such as the day-night electron density variation at sunset, the force exerted by the perturbation pressure is not able to cancel the addition of momentum by the ion-drag anomaly. Thus, such a momentum source produces a significant perturbation in the horizontal velocity, vertical motion, and temperature field.