On the global mean temperature of the thermosphere

The solar extreme ultraviolet (e.u.v.) flux and solar ultraviolet (u.v.) flux in the Schumann-Runge continuum region have been measured by spectrometers on board the Atmosphere Explorer satellites from about 1974 to 1981. The solar flux spectra measured on 23 April 1974 (a day the Atmosphere Explorer satellite reference spectrum was obtained), 13–28 July 1976 (a period of spotless conditions near solar cycle minimum), and 19 February 1979 (a day near solar cycle maximum) are used to examine the global mean temperature structure of the thermosphere above 120 km. The results show that for solar cycle minimum the calculated global mean exospheric temperature is in agreement with empirical model predictions, indicating that the energy absorbed by the thermosphere is balanced by downward molecular thermal conduction. For solar cycle maximum the energy absorbed by the thermosphere is not balanced by downward thermal conduction but agreement between the calculated and observed temperature is obtained with the inclusion of 5.3μm radiational cooling by nitric oxide. Model calculations of the minor neutral constituents in the thermosphere show that about three times more nitric oxide is produced during solar cycle maximum than solar cycle minimum conditions. The results suggest that nitric oxide cooling is small during solar cycle minimum, because of low nitric oxide densities and low thermospheric temperatures, but it becomes significantly larger during solar cycle maximum, when nitric oxide densities and thermospheric temperatures are larger.23 April 1974 was a moderately disturbed day and the results of the global mean temperature calculation indicate that it is necessary to consider a high latitude heat source associated with the geomagnetic activity to obtain agreement between the calculated and observed global mean temperature structure.     

ESRO-1 AND ESRO-4: A model of the U.T. effect in electron density at middle latitudes of the southern hemisphere

The electron density observations made using ESRO-1 and ESRO-4 near solar maximum and solar minimum, respectively, show a strong longitudinal variation at middle latitudes in the southern hemisphere. The peak of this sinusoidal variation occurs at around 7 hr U.T. and decreases exponentially in size from about 300 km (depending on local time, season, solar flux) with increasing or decreasing altitude. During local summer conditions the amplitude is larger than during local winter conditions and particularly high values occur near the solar maximum. Selecting data from magnetically quiet periods, a quantitative model is constructed of the UT-eflect in the topside electron densities.     

Observed heating effects of conjugate photoelectrons

A gridded spherical electrostatic analyzer aboard Injun 5 has been used to measure fluxes of thermal and hyperthermal electrons at subauroral latitudes in the midnight sector of the northern ionosphere between altitudes of 2500 and 850 km. Due to the offset between the geomagnetic and geographic poles hyperthermal fluxes, consisting of energetic photoelectrons that have escaped from the sunlit southern hemisphere are observed along orbits over the Atlantic Ocean and North America but not over Asia. The ambient electron temperatures (T_e) near 2500 km have their highest values at trough latitudes for all longitudes. At altitudes near 1000 km elevated electron temperatures in the trough were not a consistent feature of the data. Equatorward of the trough, in the longitude sector to which conjugate photoelectrons have access, T_e ～ 4000 K at 2500 km and ～ 3000 K at 1000 km. For regions with the conjugate point in darkness T_e ? 2300 K over the 1000–2500 km altitude range. The effective thermal characteristics of conjugate photoelectrons are studied as functions of altitude and latitude. The observations indicate that (1) at trough latitudes elevated electron temperatures in the topside ionosphere are mostly produced by sources other than conjugate photoelectrons, and (2) at subtrough latitudes, in the Alantic Ocean-North American longitude sector, conjugate photoelectrons contribute significantly to the heating of topside electrons. Much of the conjugate photoelectron energy is deposited at altitudes >2500 km then conducted along magnetic field lines into the ionosphere.     

Thermospheric no profiles observed at the diminishing phase of solar cycle 21

Two density profiles of the thermospheric nitric oxide were obtained by means of the γ(1,0) band airglow measured with rocket-home radiometers flown from Uchinoura, Japan (31°N) at around autumnal equinoxes in 1982 and 1983. The peak densities were found at altitudes of 105–110 km and are 9 × 10⁷ and 7 × 10⁷ cm⁻³, respectively. They are well reproduced by the variation of solar activity in terms of a one-dimensional photochemical-diffusive model, but the densities above 140 km under moderate solar activity differ considerably from the model prediction. A similar discrepancy has already been found in the NO density profile obtained by our previous experiment at solar maximum. These discrepancies infer a possibility either that our understanding of thermospheric nitrogen chemisty includes a serious error, or that the meridional circulation affects considerably the NO density profile even at altitudes above 140 km and at low latitudes.     

Some aspects of a global thermospheric density model deduced from the analysis of the orbit of Intercosmos 13 rocket (1975-22B)

An empirical density formula is explored as a practical model for atmospheric variations and satellite drag analyses. Expanding neutral air density as a series of spherical harmonics and normalizing to a fixed height, an analytical expression for the rate of change of the mean motion is developed for an oblate atmosphere with density scale height varying linearly with altitude. A subset of the coefficients in the density expansion is determined by least-squares adjustment to the observed orbital decay rate of Intercosmos 13 rocket (1975-22B) for the period May 1975–December 1979. Comparisons against four thermospheric models are undertaken for the solar activity effect and the diurnal and semi-annual variations. Given the even spread of data and the increase in solar activity from low to moderate, the air density variation with solar activity is particularly well determined. The results support the “J77” model revealing a greater increase in density with the daily solar index than either the “MSIS” or “DTM” thermospheric models near the solar minimum. Analyses of the diurnal and semi-annual variations are less exact.     

A model of thermospheric temperature and composition

An empirical model of thermospheric temperature (T_∞T₁₂₀, and s) and composition (H, He, N, O, N₂, O₂, and Ar) was derived from measurements of 8 satellites (AE-C, AE-E, AEROS-A, AEROS-B, ARIEL-3, ESRO-4, OGO-6, and SAN MARCO-3) and 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, and St Santin). The altitude covered extends from 120 km up to about 600 km over the time period 1967 to 1976. The analytical framework used in the model resembles closely the MSIS setup: time independent terms, solar flux terms, geomagnetic activity (K_p) effect, annual (semiannual) and diurnal (semidiurnal, terdiurnal) variations, longitudinal terms, the U.T. effect, and corrections compensating for deviations from diffusive equilibrium at altitudes below 200 km. The model describes quiet to medium disturbed geomagnetic conditions (K_p ? 4) at solar fluxes (10.7cm) ranging from 60 to 180 × 10^?22 Wm^?2Hz^?1. To get an impression of the accuracy presently obtained, the model is compared with MSIS, Jacchia (1977), and the models of Thuillier (T_∞ and Engebretson (N). The best agreement is found for the temperature and the constituents He, O, and N₂ with increasing deviations in the order of H, N, Ar, and O₂.     

The diurnal and solar cycle variation of the charge exchange induced hydrogen escape flux

Using ion temperature and density data at specific points and times in June 1969 provided by the OGO 6 satellite, and altitude profiles of the ion and electron temperature and concentration provided by the Arecibo radar facility over the period February 1972–April 1974, the diurnal and solar cycle variation of the charge exchange induced hydrogen escape flux was investigated. It was calculated that for low to moderate solar activity at Arecibo, the diurnal ratio of the maximum-to-minimum charge exchange induced hydrogen escape flux was approximately 6 with a peak around noon and a minimum somewhere between 0100 and 0300 h L.T. This study of a limited amount of OGO 6 and Arecibo data seems to indicate that the charge exchange induced hydrogen escape flux increases as the F_10.7 flux increases for low to moderate solar activity.     

Global distribution of thermospheric heat sources: EUV absorption and joule dissipation

The global distribution and temporal variations of thermospheric heating due to Joule dissipation of measured ionospheric electric fields are computed. It is shown that the volume Joule dissipation rate at high and middle latitude is similar in magnitude and altitudinal profile to the global solar EUV absorption rate discussed in the previous papers. Thus, Joule dissipation contributes significantly towards reconciling the quantitatively known sources of thermospheric heat input and that required to maintain the normal thermosphere. The combined heat source due to EUV absorption and Joule dissipation varies with the annual cycle in a manner closely resembling that of the thermospheric density.     

A rocket measurement of thermospheric temperatures derived from molecular rotational intensity distributions in the aurora

The vertical distribution of thermospheric temperature was measured from molecular rotational intensity distributions, in a rocket flight through the aurora at Fort Churchill, Canada. Both the N₂⁺ (1NEG; 0–1) and O₂ (ATM ; 0-0) bands were used and a stepping mask photometer was employed to locate each filter passband at seven points on each molecular band. The N₂⁺ temperature follows the appropriate Jacchia (1971) model fairly closely at all altitudes but is higher in temperature by about 60 K. The O₂ temperatures follow the model results more closely but one cannot be sure whether the differences between the two sets of measured temperatures are real.     

The topside ionosphere above Arecibo at equinox during sunspot maximum

The coupled time-dependent O⁺ and H⁺ continuity and momentum equations and O⁺, H⁺ and electron heat balance equations are solved simultaneously within the L = 1.4 (Arecibo) magnetic flux tube between an altitude of 120 km and the equatorial plane. The results of the calculations are used in a study of the topside ionosphere above Arecibo at equinox during sunspot maximum. Magnetically quiet conditions are assumed.The results of the calculations show that the L = 1.4 magnetic flux tube becomes saturated from an arbitrary state within 2–3 days. During the day the ion content of the magnetic flux tube consists mainly of O⁺ whereas O⁺ and H⁺ are both important during the night. There is an altitude region in the topside ionosphere during the day where ion-counterstreaming occurs with H⁺ flowing downward and O⁺ flowing upward. The conditions causing this ion-counterstreaming are discussed. There is a net chemical gain of H⁺ at the higher altitudes. This H⁺ diffuses both upwards and downwards whilst O⁺ diffuses upwards from its solar e.u.v. production source which is most important at the lower altitudes. During the night the calculated O⁺ and H⁺ temperatures are very nearly equal whereas during the day there are occasions when the H⁺ temperature exceeds the O⁺ temperature by about 300 K.     

Location and source of ionospheric high latitude troughs

One prominent feature of the high latitude topside ionosphere is the existence of sharp latitudinal depletions in the total ion (electron) concentrations within the auroral/cusp regions. These high latitude troughs, as seen by the Bennett ion mass spectrometer observations on the satellite OGO 6 at altitudes between 400 and 1100km correspond to depletions in the atomic ions which are accompanied by localized enhancements of the minor molecular ion densities. All of the high latitude troughs traversed by OGO 6 (1969–1970) were recorded and the average invariant latitude-magnetic local time (M.L.T.) distribution was determined. The troughs on the average were found at all local times to be in the vicinity of the auroral oval and to move equatorward in response to increasing magnetic activity. The average trough location was compared to the average polar cap boundary as defined by the convection electric field reversal and the electron trapping boundary as well as to the maximum horizontal magnetic disturbance associated with the large scale field aligned currents. The high latitude troughs on the average best followed the maximum magnetic disturbance distribution. It is concluded that the troughs are the result predominantly of enhanced chemical 0⁺ losses in regions with high convection velocities.     

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.     

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.     

Continuous monitoring of nightside upper thermospheric mass densities in the martian southern hemisphere over 4 martian years using electron reflectometry

Details are presented of an improved technique to use atmospheric absorption of magnetically reflecting solar wind electrons to constrain neutral mass densities in the nightside martian upper thermosphere. The helical motion of electrons on converging magnetic field lines, through an extended neutral atmosphere, is modeled to enable prediction of loss cone pitch angle distributions measured by the Magnetometer/Electron Reflectometer (MAG/ER) experiment on Mars Global Surveyor at 400 km altitude. Over the small fraction of Mars' southern hemisphere (∼2.5%) where the permanent crustal magnetic fields are both open to the solar wind and sufficiently strong as to dominate the variable induced martian magnetotail field, spherical harmonic expansions of the crustal fields are used to prescribe the magnetic field along the electron's path, allowing least-squares fitting of measured loss cones, in order to solve for parameters describing the vertical neutral atmospheric mass density profile from 160 to 230 km. Results are presented of mass densities in the southern hemisphere at 2 a.m. LST at the mean altitude of greatest sensitivity, 180 km, continuously over four martian years. Seasonal variability in densities is largely explained by orbital and latitudinal changes in dayside insolation that impacts the nightside through the resulting thermospheric circulation. However, the physical processes behind repeatable rapid, late autumnal cooling at mid-latitudes and near-aphelion warming at equatorial latitudes is not fully clear. Southern winter polar warming is generally weak or nonexistent over several Mars years, in basic agreement with MGS and MRO accelerometer observations. The puzzling response of mid-latitude densities from 160° to 200° E to the 2001 global dust storm suggests unanticipated localized nightside upper thermospheric lateral and vertical circulation patterns may accompany such storms. The downturn of the 11-year cycle of solar EUV flux is likely responsible for lower aphelion densities in 2004 and 2006 (Mars years 27 and 28).     

Radial variation of differential rotation in the solar electron corona

Long-lived brightness structures in the solar electron corona persist over many solar rotation periods and permit an observational determination of coronal magnetic tracer rotation as a function of latitude and height in the solar atmosphere. For observations over 1964–1976 spanning solar cycle 20, we compare the latitude dependence of rotation at two heights in the corona. Comparison of rotation rates from East and West limbs and from independent computational procedures is used to estimate uncertainty. Time-averaged rotation rates based on three methods of analysis demonstrate that, on average, coronal differential rotation decreases with height from 1.125 to 1.5 R _S. The observed radial variation of differential rotation implies a scale height of approximately 0.7 R _S for coronal differential rotation.Model calculations for a simple MHD loop show that magnetic connections between high and low latitudes may produce the observed radial variations of magnetic tracer rotation. If the observed tracer rotation represents the rotation of open magnetic field lines as well as that of closed loops, the small scale height for differential rotation suggests that the rotation of solar magnetic fields at the base of the solar wind may be only weakly latitude dependent. If, instead, closed loops account completely for the radial gradients of rotation, outward extrapolation of electron coronal rotation may not describe magnetic field rotation at the solar wind source. Inward extrapolations of observed rotation rates suggest that magnetic field and plasma are coupled a few hundredths of a solar radius beneath the photosphere.     

Low density regions observed at high altitudes and their connection with equatorial spread F

Observations from the high resolution spherical electrostatic analyzer experiment aboard ISIS 1 have been used to study large amplitude irregularities at low latitudes in the tipside ionosphere. The irregularities appeared as plasma depletions near the magnetic equator and were observed up to satellite apogee (3500 km). The altitude local time distribution of the depletions was such that those at altitudes greater than 2000 km were found only in the post-midnight sector. This result agrees with the predictions of a model for plasma bubbles drifting under the influence of gravity-buoyancy forces. Evidence is presented that the initial steep gradients observed at low altitudes are reduced by anomalous diffusion due to drift waves.     

The composition,structure, temperature and dynamics of the upper thermosphere in the polar regions during October to December 1981

During the period October to December 1981, the Dynamics Explorer-2 (DE-2) spacecraft successively observed the South polar and the North polar regions, and recorded the temperature, composition and dynamical structure of the upper thermosphere. In October 1981, perigee was about 310 km altitude, in the vicinity of the South Pole, with the satellite orbit in the 09.00–21.00 L.T. plane. During late November and December, the perigee had precessed to the region of the North Pole, with the spacecraft sampling the upper thermosphere in the 06.00 18.00 L.T. plane. DE-2 observed the meridional wind with a Fabry-Perot interferometer (FPI), the zonal wind with the wind and temperature spectrometer (WATS), the neutral temperature with the FPI, and the neutral atmosphere composition and density with the neutral atmosphere composition spectrometer (NACS). A comparison between the South (summer) Pole and the North (winter) Pole data shows considerable seasonal differences in all neutral atmosphere parameters. The region of the summer pole, under similar geomagnetic and solar activity conditions, and at a level of about 300 km, is about 300 K warmer than that of the winter pole, and the density of atomic oxygen is strongly depleted (and nitrogen enhanced) around the summer pole (compared with the winter pole). Only part of the differences in temperature and composition structure can be related to the seasonal variation of solar insolation, however, and both polar regions display structural variations (with latitude and Universal Time) which are unmistakeable characteristics of strong magnetospheric forcing. The magnitude of the neutral atmosphere perturbations in winds, temperature, density and composition within both summer and winter polar regions all increase with increasing levels of geomagnetic activity.The UCL 3-dimensional time dependent global model has been used to simulate the diurnal, seasonal and geomagnetic response of the neutral thermosphere, attempting to follow the major features of the solar and geomagnetic inputs to the thermosphere which were present during the late 1981 period.In the UCL model, geomagnetic forcing is characterized by semi-empirical models of the polar electric field which show a dependence on the Y component of the Interplanetary Magnetic Field, due to Heppner and Maynard (1983), It is possible to obtain an overall agreement, in both summer and winter hemispheres, with the thermospheric wind structure at high latitudes, and to explain the geomagnetic control of the combined thermal and compositional structure both qualitatively and quantitatively. To obtain such agreement, however, it is essential to enhance the polar ionosphere as a consequence of magnetospheric particle precipitation, reflecting both widespread auroral (kilovolt) electrons, and “soft” cusp and polar cap sources. Geomagnetic forcing of the high latitude thermosphere cannot be explained purely by a polar convective electric field, and the thermal as well as ionising properties of these polar and auroral electron sources are crucial components of the total geomagnetic input.     

The consequences of high latitude particle precipitation on global thermospheric dynamics

A previous comparison of experimental measurements of thermospheric winds with simulations using a global self-consistent three-dimensional time-dependent model confirmed a necessity for a high latitude source of energy and momentum acting in addition to solar u.v. and e.u.v. heating. During quiet geomagnetic conditions, the convective electric field over the polar cap and auroral oval seemed able to provide adequate momentum input to explain the thermospheric wind distribution observed in these locations. However, it seems unable to provide adequate heating, by the Joule mechanism, to complete the energy budget of the thermosphere and, more importantly, unable to provide the high latitude input required to explain mean meridional winds at mid-latitudes. In this paper we examine the effects of low energy particle precipitation on thermospheric dynamics and energy budget. Modest fluxes over the polar cap and auroral oval, of the order of 0.4 erg cm ⁻²/s, are consistent with satellite observations of the particles themselves and with photometer observations of the OI and OII airglow emissions. Such particle fluxes, originating in the dayside magnetosheath cusp region and in the nightside central plasma sheet, heat the thermosphere and modify mean meridional winds at mid-latitudes without enhancing the OI 557.7 line, or the ionization of the lower thermosphere (and thus enhancing the auroral electrojets), neither of which would be consistent with observations during quiet geomagnetic conditions.     

λ 5303 Fe XIV density models of the inner solar corona

The Sacramento Peak Observatory's 40 cm coronagraph was used with an emission line photometer to observe the distribution of 5303 Fe XIV brightness as a function of position angle, height above the limb, and time. These data were used to construct models of the volume emissivity as a function of solar latitude and longitude. These models in turn yield estimates of the distribution of electron density in the lower solar corona as a function of latitude and longitude for several specific periods in 1973 and 1975. Three observational results are obtained. An upper limit for the inferred electron density in coronal hole regions is set at log N _e = 7.4 for an altitude of 1.15R . Density models from late 1973 demonstrate an evolutionary trend toward a rather regular four-lobed appearance of coronal material; models from 1975 suggest that this characterization persisted for at least 27 solar rotations. A decrease in the total integrated 5303 intensity of a factor of 2.9 is inferred to have taken place between 1973 and 1975.The National Center for Atmospheric Research is sponsored by the National Science Foundation.