N. W. Spencer

An equatorial temperature and wind anomaly (ETWA)

Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE-2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl-shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion-neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

Composition and Structure of the Martian Atmosphere: Preliminary Results from Viking 1

Results from the aeroshell-mounted neutral mass spectrometer on Viking I indicate that the upper atmosphere of Mars is composed mainly of CO2 with trace quantities of N2, Ar, O, O2, and CO. The mixing ratios by volume relative to CO2 for N2, Ar, and O2 are about 0.06, 0.015, and 0.003, respectively, at an altitude near 135 kilometers. Molecular oxygen (O2+) is a major component of the ionosphere according to results from the retarding potential analyzer. The atmosphere between 140 and 200 kilometers has an average temperature of about 180� � 20�K. Atmospheric pressure at the landing site for Viking 1 was 7.3 millibars at an air temperature of 241�K. The descent data are consistent with the view that CO2 should be the major constituent of the lower martian atmosphere.

Neutral temperature anomaly in the equatorial thermosphere - A source of vertical winds

Data obtained from the WATS (Wind and Temperature Spectrometer) instrument on DE-2 (Dynamics Explorer) during high solar activity, show new evidence for the presence of vertical winds of a significant magnitude in the equatorial thermosphere. They reveal a latitudinal structure that can be related to the recently discovered phenomena of the Equatorial Temperature and Wind Anomaly (ETWA). In the local evening hours, the vertical winds usually are downward around the dip equator and collocated with the temperature minimum of ETWA. In general, they are upward at about 24° dip latitude away from the dip equator and are collocated with the ETWA temperature crests. The magnitude of the vertical winds is in the 10–40 m/s range. It is proposed that the temperature and pressure ridges, formed by the excess ion drag on the zonal winds around the two crests and ordered by the relatively lower ion drag at the trough of the well known Equatorial Ionization Anomaly (EIA), drive a new wind system in the meridional plane and that the measured vertical winds form part of this wind system.

A theoretical and empirical study of the response of the high latitude thermosphere to the sense of the “Y” component of the interplanetary magnetic field

Abstract The strength and direction of the Interplanetary Magnetic Field (IMF) controls the transfer of solar wind momentum and energy to the high latitude thermosphere in a direct fashion. The sense of “ Y” component of the IMF (BY) creates a significant asymmetry of the magnetospheric convection pattern as mapped onto the high latitude thermosphere and ionosphere. The resulting response of the polar thermospheric winds during periods when BY is either positive or negative is quite distinct, with pronounced changes in the relative strength of thermospheric winds in the dusk-dawn parts of the polar cap and in the dawn part of the auroral oval. In a study of four periods when there was a clear signature of BY, observed by the ISEE-3 satellite, with observations of polar winds and electric fields from the Dynamics Explorer-2 satellite and with wind observations by a ground-based Fabry-Perot interferometer located in Kiruna, Northern Sweden, it is possible to explain features of the high latitude thermospheric circulation using three dimensional global models including BY dependent, asymmetric, polar convection fields. Ground-based Fabry-Perot interferometers often observe anomalously low zonal wind velocities in the (Northern) dawn auroral oval during periods of extremely high geomagnetic activity when BY is positive. Conversely, for BY negative, there is an early transition from westward to southward and eastward winds in the evening auroral oval (excluding the effects of auroral substorms), and extremely large eastward (sunward) winds may be driven in the auroral oval after magnetic midnight. These observations are matched by the observation of strong anti-sunward polar-cap wind jets from the DE-2 satellite, on the dusk side with BY negative, and on the dawn side with BY positive.

Thermospheric gravity waves: observations and interpretation using the transfer function model (TFM)

Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.

The high latitude circulation and temperature structure of the thermosphere near solstice

Abstract The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCARs thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F -region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E -region ionosphere. The results of several TGCM runs are used to show that at F -region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid. The F -region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer ( DE −2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s −1 ) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s −1 ). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE −2 satellite measurements.

A comparison of wind observations of the upper thermosphere from the dynamics explorer satellite with the predictions of a global time-dependent model

Abstract Seven polar passes of the NASA Dynamics Explorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of the data from the remote-sensing Fabry-Perot interferometer (meridional wind) and the in situ wind and temperature spectrometer (zonal wind) and are compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Model A) and a model of global electron density which excludes the effects of high-latitude geomagnetic processes. A second version of the global dynamic model includes a theoretical model of the high-latitude ionosphere which is self-consistent and reflects the enhancement of ionization due to magnetospheric phenomena acting in addition to solar e.u.v. photo-ionization, including the interactive processes which occur between ionization and high latitude ion convection and thermospheric winds. This second dynamical model shows an improved comparison with the structure and magnitude of polar cap and auroral oval winds at times of other than extremely low geomagnetic activity, when the first model appears a better match. An improved empirical description of the complex magnetospheric processes exciting the thermosphere in the vicinity of the dayside polar cusp and an empirical description of storm-time electric fields will be required for a quantitative explanation of the polar thermospheric winds during geomagnetic substorm events.

A computer model of global thermospheric winds and temperatures

A computer model of the global, time-dependent, thermospherichorizontal vector neutral wind and neutral temperature fields has been constructed based on output fromthe NCAR thermosphericgeneralcirculation model (NCAR-TGCM).The wind field is represented by a vector spherical harmonic (VSH)expansion in the horizontal, a fourier expansion in Universal Time, anda polynomial expansion in altitude. The global temperature field representation differs in that a scalar spherical harmonic expansionis used in the horizontal and a Bates model temperature profile is used in altitude. A set of suitably-truncatedspectral coefficients contains the wind and temperature description for a diurnally-reproduciblerun of the NCAR-TGCM. The VSH model is coded in a FORTRANsubroutine that returns vector wind and temperature valuesfor a given UT, geographic location, and altitude. The model has applicability for studies of thermospheric and/or ionospheric physics where reasonable timedependent neutral wind and temperature values are of interest. The routine is novel since portable computer models of thermospheric wind fields have not previously been available to researchers. The current version of the model is valid for solar maximum, December solstice only, although themodel can beextended to anyseason andspecific set of geophysical conditions for whichTGCM results are available. Results from the VSHcomputermodel are presentedto compare with global-scale wind measurements from the Dynamics Explorer (DE-2) satellite. The agreementbetween the computer model results and data from individual orbits of DE-2 is good, indicating that the model provides reasonable wind values, having the appropriate characteristic latitudinal, diurnal, and Universal-Time-dependentsignatures observedfrom the satellite at upper thermospheric altitudes. The VSH thermospheric temperature values are in general agreement with MSIS-83 temperatures but illustrate smaller-scale horizontal temperature structures than are resolved by MSIS-83,owing to the larger numberof spectralharmonics retained. I. INTRODUCTION Significant progress hasbeen madeover the last several yearsin the modelingand empirical descriptionof the global thermospheric neutral wind and temperature system. The Dynamics Explorer (DE 2) spacecraft, in particular, was instrumented to measure the thermospheric vector wind and temperature along the track of the polar-orbiting spacecraft /1-3/. Published results from this mission have served to characterize the global-scale thermosphericwind field for the solar maximumconditions pertaining to the 1981-1983period /3-9/. In addition to the newinformation providedby the DE2 spacecraft andother experimental techniques, the theoretical understanding of thermospheric motionshas progressed rapidly. Varioustheoretical modelsexist that can simulate the dynamical response of the upper atmosphere for a variety of geophysical conditions. In particular, two numerical general circulation models, the NCAR-TGCM/10, 11/and the UCL-TGCM/12/have had a large measure of success in calculating wind and temperature fields similar to those observed from DE 2 /6, 9, 13-17/. The spectralmodel of Mayr et al. / 18/ has also provided significantadditional insight intoglobal-scale thermospheric dynamics. In spite of these recent theoretical modeling efforts, no simple “user-friendly” computermodel of the global thermospheric wind field has been previously constructed to enable neutral winds to be conveniently used in other theoretical studies or in straightforwardcomparisons with newdatasets. The underlyingreasonsfor this situation involve the sophistication andcomplexity of the TGCMsand the large physical size of the data arrays necessary to contain the numerically-simulatedwind fields. Moreover, the fragmentary nature of the global-scale wind measurementscollated to date from all experimental sources has postponed the construction of a purely empirical model. For thermospheric temperatures, the situation is better in that semi-empirical models, such as the MSIS-83 model of Hedin /19/, have, for many years, provided researchers with reliable values for thermospheric temperatures, incorporating explicit dependences on geomagnetic activity, solar activity, and season.

Detailed behaviour of the midlatitude ionosphere from the explorer XVII satellite

Abstract Measurements of electron temperature and ion density by electrostatic probes on the Explorer XVII satellite have revealed the detailed diurnal and latitudinal behaviour of the summer ionosphere near the altitude of the F 2 maximum over the eastern United States at the time of solar minimum. The electron temperature at 40° north magnetic latitude is observed to rise rapidly from a nighttime value of about 1100°K to a mid-morning maximum of 2700°K followed by an afternoon plateau of 2000°K. The electron temperature always exceeds accepted values of neutral gas temperature and thus reflects the existence of heat sources in both the daytime and nocturnal ionosphere. The ion density displays a maximum value about three hours after local noon. A strong degree of latitude control, evident near the F 2 maximum, causes the temperature to increase and the density to decrease with increasing latitude. The electron temperature is relatively independent of altitude between 260 and 450 km in the forenoon but displays a slight increase with altitude at night. The diurnal variations found over stations at 10 and 60° north magnetic latitude display general characteristics similar to those found at 40°N and also reveal the inverse gradients of temperature and density with latitude. Calculations based on the measurements show that the ion temperature begins to exceed the neutral temperature significantly above 300 km and approaches the electron temperature near 600 km. The ratio of electron to ion temperature at 300 km is between 2 and 3 in the daytime and about 1.5, but highly variable, at night. The measurements are also employed to calculate the amount of electron heating in the F -region. It appears that heating by solar ultraviolet radiation is adequate to explain the daytime results below 300 km, and that the available flux of fast photoelectrons is adequate to account for the heating at higher altitudes. The heating at night is consistent with a corpuscular flux of soft electrons with a total energy of about 1 × 10 −2 ergs cm −2 sec −1 .

Venus upper atmosphere neutral gas composition - First observations of the diurnal variations

Measurements of the composition, temperature, and diurnal variations of the major neutral constituents in the thermosphere of Venus are being made with a quadrupole mass spectrometer on the Pioneer Venus orbiter. Concentrations of carbon dioxide, carbon monoxide, molecular nitrogen, atomic oxygen, and helium are presented, in addition to an empirical model of the data. The concentrations of the heavy gases, carbon dioxide, carbon monoxide, and molecular nitrogen, rapidly decrease from the evening terminator toward the nightside; the concentration of atomic oxygen remains nearly constant and the helium concentration increases, an indication of a nightside bulge. The kinetic temperature inferred from scale heights drops rapidly from 230 K at the terminator to 130 K at a solar zenith angle of 120�, and to 112 K at the antisolar point.