Atmosphere Explorer Satellites Research Articles

Perspective on Energetic and Thermal Atmospheric Photoelectrons

Atmospheric photoelectrons are central to the production of planetary ionospheres. They are created by photoionization of the neutral planetary atmosphere by solar EUV and soft X-ray irradiance. They provide the energy to heat the thermosphere. Thermalized photoelectrons permeate magnetospheres creating polarization electric fields and plasma waves as they interact with ions to maintain charge neutrality. Energetic photoelectrons (>1 eV) have a distinctive energy spectral shape as first revealed in data from the Atmosphere Explorer satellites. Energetic photoelectrons escaping the ionosphere follow local magnetic fields illuminating the planet's magnetic topology. Current models using state-of-the-art EUV observations accurately capture their production and transport. However, in spite of 60 years of space research the electron thermalization processes occurring below 1 eV at low altitudes in planetary thermospheres are not understood quantitatively. Results from event analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission are not consistent with current models of photoelectron thermalization. The lack of quantitative understanding reflects the complexity of the physics and the lack of a large data base of simultaneous neutral, ion, and electron densities and temperatures in lower planetary thermospheres.

Comparative Analysis of Satellite Aerodynamics and Its Application to Space-Object Identification

The accuracy of atmospheric density measurements inferred from satellite drag is limited by errors in drag coefficient estimates. In this work, we use a unique opportunity in which the Drag and Atmospheric Neutral Density Explorer satellite and three Polar Orbiting Passive Atmospheric Calibration Spheres are deployed from a common launch vehicle. Each object flies through similar atmospheric conditions but has a different area-to-mass ratio. This allows aerodynamic analysis that is independent of atmospheric density via comparisons of measured and modeled ballistic coefficient ratios. A test particle method combined with a satellite energy accommodation model is used to model the aerodynamics of these objects. Fitted ballistic coefficients computed as a result of special-perturbations orbit analysis are then compared to the model results. The drag coefficient model and observations agree at the 1–2% level when coefficient ratios are compared. Comparisons of an additional shape with model predictions are made possible when one of the satellites shed its launch adapter. This work validates the aerodynamic model used here and can be applied in future research to improve models of atmospheric density and predictions of satellite drag. Beyond potential improvements to atmospheric models, the technique presented here is shown to identify objects based on their aerodynamic signature.

An empirical topside electron density model for calculation of absolute ion densities in IRI

An empirical model of the electron density in the topside ionosphere for solar maximum and minimum is introduced. This model is intended for use in combination with the relative ion composition (TTS) model that will be included in the forthcoming version of the IRI code to calculate the absolute ion composition in the topside ionosphere. The validation of the model using satellite data from five different satellites for different altitudes and high and medium solar activity is presented. The proposed model is based on the Langmuir Probe measurements onboard the Atmosphere Explorer satellites for low solar activity and Retarding Potential Analyzer measurements onboard the Intercosmos satellites for high solar activity. Examples of vertical profiles of the absolute density of the most important ions following from the mentioned models are shown.

Manifestation of solar activity in the global topside ion composition − a study based on satellite data

Abstract. The solar cycle variation of the most important parameters characterizing the ion composition in the topside ionosphere is studied. For this purpose data from the ACTIVE mission (the IK-24 satellite) for the maximum of solar cycle 22 (aver F10.7~200), complemented by data available from the Atmosphere Explorer (AE) satellites, for the minimum of solar cycle 21 (average F10.7~85), were processed. OGO-6 data from the low maximum of solar cycle 20 (average F10.7~150) were used for medium solar activity conditions. The results for the equinox from the recently developed empirical model of ion composition are analyzed and presented, and typical vertical profiles from solar maxima and minima are shown. It was found that the logarithm of the O+, H+, He+, and N+ densities in the topside ionosphere at a fixed altitude, latitude, and local time is, in the first approximation, a linear function of solar activity characterized by the daily F10.7. On the other hand, the upper transition height is generally a non linear function of the daily F10.7, the deviation from linear dependence increases with latitude. Keywords. Ionosphere (Plasma temperature and density; Ion chemistry and composition; Modeling and forecasting)

Detailed simulation of surface chemistry leading to spacecraft glow

Glow brightness around the Atmosphere Explorer satellite over the altitude range 140 km to 300 km is calculated using a multi-step approach. The flow field around the satellite is simulated using the direct simulation Monte Carlo method. An overlay technique is used to simulate rare atmospheric species with acceptable statistical accuracy. Both non-reactive and reactive surface events are modeled by a system of ordinary differential equations. A simple analytical model is then used to convert a surface flux to glow brightness. This study is the first in which a comprehensive set of production mechanisms is included in the calculation of glow brightness. The effects of satellite rotation are investigated and found to be most significant at higher altitudes. Sensitivity to the surface event parameters is assessed and the critical production reaction for spacecraft glow is identified. The importance of the free stream concentration of nitric oxide is demonstrated. Very good agreement with experimental data is achieved.

Development of a Monte Carlo Overlay Method with Application to Spacecraft Glow

Brightness calculations for visible emissions from nitrous oxide, known as a spacecraft glow, around the Atmospheric Explorer (AE) satellite are presented over an altitude range of 140 ‐ 180 km. The e owe eld is determined using the direct simulation Monte Carlo method. A novel overlay technique is developed to capture the microscopic behavior of rare atmospheric species, such as nitric oxide. A simple model for glow is proposed, identifying nitric oxide as the important species in the gas phase controlling e ow brightness. Sensitivity to altitude and to different chemical reaction cross sections for nitric oxide production is assessed. The importance of nitric oxide production is found to be greatest at lower altitudes. At higher altitudes, the ambient concentration of nitric oxide is the critical factor in determining glow brightness. Comparison of the results with measurements from the AE indicates an efe ciency of approximately 0.01 for producing the excited state of nitrous oxide from nitric oxide impacting on the vehicle surface.

Instrument for measuring spacecraft potential

A high-resolution, low-energy electron spectrometer can be deployed on rockets or satellites to measure absolute spacecraft floating potential relative to true (Earth) ground with an accuracy of ±0.2 V. Features due to the photoionization of nitrogen and atomic oxygen by an extremely sharp solar extreme ultraviolet line (304 A, He II) appear in the electron spectra collected by the instrument. At altitudes where the photoelectron production is local (below 250 km), the peaks that appear in the spectrum are created by electrons of a known kinetic energy: the energy of the ionizing photon minus the energy needed to ionize either atomic oxygen or molecular nitrogen. The spacecraft potential is determined from the apparent shift in the energy of the photoelectron spectral peaks. The instrument is best suited for use in the daytime atmosphere at altitudes between 150 and 250 km. The spectrometer is based on an earlier design, that of the photoelectron spectrometer of the Atmosphere Explorer satellites. Enhancements of the earlier design, including a threefold greater throughput and a specialized scan mode, greatly improve the determination of spacecraft potential. The placement of the instrument on the spacecraft is critical to the success of the measurements.

A steady-state F-region model and its use for satellite data analysis

A steady-state mathematical model of the Earth's upper ionosphere and plasmasphere is presented. In the model the equations of continuity, momentum, and energy balance for O+, H+, and He+ ions are solved numerically along dipole magnetic field lines. As an extension of the model, a searching method is developed for de- termination of the boundary values in a self-consistent manner. Model results are compared with Atmosphere Explorer satellite measurements.

Analysis of electromagnetic wave scattering by ionospheric irregularities

A quasi‐particle approach is used to analyze the effect of bottomside sinusoidal irregularities on the propagation of beacon satellite signals through the ionosphere. In this work, the radio wave is treated as a distribution of quasi‐particles described by a Wigner distribution function, and the irregularities are modeled as a two‐dimensional density modulation on an uniform background plasma. The collision of quasi‐particles with the irregularities modifies the quasi‐particle distribution and gives rise to the wave‐scattering phenomenon. The multiple scattering process is generally considered in this deterministic analysis of radio wave scattering off the ionospheric density irregularities. The analysis shows that this two‐dimensional density grating effectively modulates the intensity of the beacon satellite signals. This spatial modulation of the wave intensity is converted into time modulation due to the drift of the ionospheric irregularities, which then results in the scintillation of the beacon satellite signals. The modulation is due mainly to the transverse (to the wave propagation) grating. The results of the analysis agree well with the existing experimental data on the scintillation of Atmosphere Explorer satellites signal (Basu et al., 1986). The model is further based on to predict the scintillation of radio signals for their long distance propagation. For a fixed scintillation index (S4) the predicted propagation distance (d) versus the wave frequency (ƒ) follows a power law, namely, d∝ƒK, where k ≈ 2.

Empirical global model of upper thermosphere winds based on Atmosphere and Dynamics Explorer satellite data

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been used to generate an empirical wind model for the upper thermosphere, analogous to the MSIS model for temperature and density, using a limited set of vector spherical harmonics. The model is limited to above approximately 220 km where the data coverage is best and wind variations with height are reduced by viscosity. The data base is not adequate to detect solar cycle (F10.7) effects at this time but does include magnetic activity effects. Mid‐ and low‐latitude data are reproduced quite well by the model and compare favorably with published ground‐based results. The polar vortices are present, but not to full detail.

Mesospheric 5577Å green line and atmospheric motions—Atmosphere explorer satellite observations

Photometric measurements of the 5577Å O( 1S) green line mesospheric emission obtained by the Visible Airglow Experiment (VAE) on board the Atmosphere Explorer ( AE) satellite have been analyzed. The inverted volume emission rate profiles showed a peak at approximately 96–97 km with a half-width of ∼8 km. The diurnal variation of the intensity indicates the presence of a wave component with 10 ∼ 12 h period, probably of solar semi-diurnal tide. Shorter time scale variations due to the presence of travelling waves were also observed.

On the global mean structure of the thermosphere

A self‐consistent global average model of the coupled thermosphere and ionosphere has been developed and used to examine the global mean structure of these regions for solar minimum and maximum conditions. The model uses a parameterized version of the solar EUV and UV flux measured from the Atmospheric Explorer satellite to calculate photoionization rates of the major ion species and photo‐dissociation rates for O2 and N2. The ionospheric calculation solves for the vertical distributions of O+, including ambipolar diffusion, and of NO+, O2+, N2+, and N+, assuming photochemical equilibrium with respect to O+. It also solves the electron and ion energy equations as well as the coupled minor neutral constituent diffusion equations for N(²D), N(4S), and NO. The neutral gas energy equation is solved considering heating from photoelectrons, O2 absorption in the Schumann‐Runge continuum and bands, excess energy from exothermic ion‐neutral and neutral‐neutral chemical reactions, thermal electron, ion, and neutral collisions, O3 absorption in the Hartley bands, and atomic oxygen recombination and cooling from molecular and eddy thermal conduction and NO and CO2 IR radiation. The O2 photodissociation rates are used in a model of major species diffusion to solve for the O, O2, and N2 number densities. From specified lower‐boundary conditions and arbitrary initial conditions, both the thermospheric and the ionospheric structure evolve self consistently with time for specified solar flux values. The calculated globally averaged thermospheric and ionospheric structure compares well with the globally averaged structure predicted by the thermospheric and ionospheric empirical models based on data for both solar minimum and maximum conditions by considering solar EUV and UV radiation and aurora particle precipitation and joule heat sources to represent high‐latitude processes. Total joule heating is 7 × 1010 W for geomagnetic quiet conditions for both solar minimum and maximum. The model results show that the dominant heat sources for the thermosphere are collisions between ambient electrons, ions, and neturals above about 250 km, exothermic ion‐neutral chemical reactions between 200 and 250 km, exothermic neutral‐neutral chemical reactions between 200 and 150 km, and heating in the Schumann‐Runge continuum and bands below 150 km. Dominant cooling mechanisms are downward molecular thermal conduction above 150 km and NO and CO2 radiation below 150 km. The results also show that NO cooling must be considered in the thermal energy balance if the effect of minor neutral constituent chemical heating is considered in a definition of the neutral gas solar heating efficiency. Otherwise, the heat input into the thermosphere is overestimated.

Scintillations associated with bottomside sinusoidal irregularities in the equatorial F region

A new category of equatorial F region plasma irregularities characterized by nearly sinusoidal waveforms in the ion number density Ni observed by the Atmosphere Explorer satellites has recently been discussed by Valladares et al. (1983) and Cragin et al. (1985). We present multisatellite scintillation observations made at Huancayo, Peru, and spaced‐receiver drift measurements made at Ancon, Peru, associated with such irregularities observed by AE‐E on a few nights in December 1979. Utilizing ray paths to various geostationary satellites, it is found that the irregularities grow and decay almost simultaneously in long‐lived patches extending at least 1000 km in the E‐W direction. The scintillations continue for a period of almost 6 hours, at a level that varies from moderate to fairly intense (S4 = 0.1–0.8 at 250 MHz), and these S4 fluctuations are quite well correlated, even over a distance of 1000 km. The irregularities constituting the large patch are found to drift eastward at a velocity of approximately 140 m/s. This and other such events are accompanied by the frequency spread signatures on Huancayo ionograms, as previously reported. The unique feature of the Fourier spectra associated with such bottomside sinusoidal (BSS) irregularities is the presence of Fresnel oscillations, which allow a determination of the velocity of the diffraction pattern perpendicular to the direction of the ray from the satellite to the ground station. The velocity so determined agrees well with the results of simultaneously performed spaced‐receiver drift measurements. The presence of Fresnel oscillations indicates that the BSS irregularities occur in a relatively thin layer. However, while the scintillation data indicate a high‐frequency roll‐off with a spectral index of the order of −3 to −4, the in situ data tend to indicate that the index is of the order of −5 to −6. Modeling studies are necessary to resolve this difference.

The source of midlatitude metallic ions at F-region altitudes

A survey of metallic ions detected by the Bennett Ion Mass Spectrometers flown on the Atmosphere Explorer satellites, including both circular and eccentric orbital configurations, shows that patches of these ions of meteoric origin are frequently present during magnetically active periods on the bottomside of the F-layer at middle and high latitudes. In particular the F-region metals statistically tend to appear at night in the vicinity of the main ionospheric trough (in a band of invariant latitudes approx. 10 degrees wide) and on the day side of the polar cap. These distributions were previously associated with the expected dynamics of ions in the F-region above 140 km where meridional neutral wind drag and convection electric fields are the dominant ion transport mechanisms. However, the main meteor deposition layer—the presumed source region of the metals—is located below 100 km where these transport mechanisms do not prevail. It is demonstrated that the Pedersen ion drifts driven by intense electric fields such as those associated with sub-auroral ion drifts (SAID) are sufficient to transport the long-lived metallic ions upward from the main meteor layer to altitudes where the drag of equatorial directed neutral winds and electric field convection can support them against the downward pull of gravity and transport them to other locations. The spatial and temporal distribution of the middle and high latitude F-region metals are consistent with the known characteristics of the electric fields and with the expected F-region ion dynamics.

Ionization frequencies for solar cycle 21: Revised

Detailed spectra of the extreme ultraviolet solar flux at the earth were provided by instruments on the Atmosphere Explorer satellites. These data have been used for aeronomical purposes in a large number of studies. An important parameter for such studies is the rate of production of various ions through the photoionization process. This parameter, known as an ionization frequency, is the integral over wavelength of the product of the solar flux and the cross section for the ionization of the particular constituent. Thus the determination of the ionization rate is dependent on a good knowledge of the solar EUV intensities for the solar period in question. Over the past few years the EUV solar spectra that have been developed for use by aeronomers as reference spectra for such photochemical and ionospheric studies have been improved. In this paper we report the results of a redetermination of the most important ionization frequencies using the revised solar fluxes. The impact is found to be more significant at solar maximum, amounting to a reduction of 12–21% in the ionization frequencies of the major terrestrial thermospheric constituents for solar minimum conditions and 21–33% for solar maximum conditions. The corrections are apart from the ongoing debate concerning the absolute intensity of EUV solar flux measurements for the solar cycle 21 maximum period.

Global distribution of helium in the upper atmosphere during solar minimum

Helium in the Earth's thermosphere traces the dynamical systems that redistribute energy and mass. Measurements of the global helium distribution in the thermosphere, using Atmosphere Explorer satellite C. ( AE- C), show a gradual seasonal change in the number density of helium for all latitudes. The enhancement in helium over the winter pole (the helium bulge) changes in magnitude slowly as seasons progress. The bulge builds and recedes following the progression of winter North to South and back again. This progression of the winter helium enhancement is presented in this paper using latitudinal profiles of helium number density for each month during the year. The absolute magnitude of the winter helium enhancement in the auroral regions is affected by auroral heating at low altitudes. The reduction in the winter helium bulge at low altitudes shown in AE- C data can be traced to this localized heating. The gradual variation in helium concentration measured at many latitudes for all seasons of the year implies that global thermospheric wind systems change gradually with the seasons.

Characteristics of the spacecraft optical glow

The optical glow induced by spacecraft‐environment interaction under daytime conditions is analyzed using photometric data obtained by the Visible Airglow Experiment on board the Atmosphere Explorer Satellite. Because of the increased ambient oxygen density, the glow is more intense in the daytime. The daytime production efficiency, measured in Rayleighs per oxygen atom, is similar to the nighttime value. No dependence of the glow intensity on the ambient plasma density was found. The production efficiency increased with time throughout the duration of the spacecraft in orbit.

Results of a comprehensive study of the photochemistry of N2+ in the ionosphere

The disagreement between model calculations of ionospheric N2+ concentrations and those measured by the Atmosphere Explorer (AE) satellite has been controversial for several years. The model calculations of N2+ require an additional loss of the order of ∼10² ions cm−3 s−1 at ∼250 km in order to account for the observed N2+ concentrations. After analysis of many orbits of AE data we have drawn the conclusion that such a loss rate could only be provided by either (1) enhancing the ionospheric dissociative recombination rate coefficient for N2+ by a factor of 2–3 over its currently accepted value if this is not precluded by laboratory data or (2) enhancing of the charge exchange of N2+ with atomic oxygen. This could be achieved by two mechanisms which either independently or combined can account for the observed N2+ concentrations. The first is the enhanced destruction of N2+ by charge exchange of vibrationally excited N2+ with O (enhancement of the branch to NO+ only transfers the problem to NO+). The second mechanism is the accidentally energetically resonant reaction O+(²D) + N2 ⇄ N2+(X)υ=5 + O. In spite of the valid theoretical arguments against the plausibility of these two mechanisms, without any viable alternatives we feel compelled to present our results. The best agreement between theory and data is obtained when these two mechanisms are combined with the rate coefficients for charge exchange of vibrationally excited N2+ with O increased by a factor of 50 and N2 set equal to 2.5×10−10 cm³ s−1 and where N2+(X)υ>0 is quenched by N2 at a rate of ∼5×10−10 cm³ s−1. An important aspect of the chemical scheme is negligible quenching of O+(²D) by O. We estimate an upper limit for the rate coefficient of this process to be ∼5×10−12 cm³ s−1.

Observation of orbiting vehicle induced luminosities on the STS‐8 mission

During the STS‐8 space shuttle mission the vehicle glow was observed and photographed with an image intensified photographic camera. The altitude of the shuttle was 120 n.m.(222 km) and high intensity glow was observed, which was easily visible to the crew by naked eye. Three material test samples were mounted on the vehicle to test the dependance of the glow intensity on the nature of the surface material. These samples were exposed in the ram direction during the night side of the orbit. The glow intensity was found to depend on the nature of the surface. The mass loss suffered by the samples did not correlate with the intensity of the observed glow. The proper orientation of the shuttle with respect to the velocity vector and a camera objective grating gave a good objective spectrum of the glow in the visible region. From this it is clear that with the limited spectral resolution of the objective spectrum (about 150 A) the vehicle glow on the tail appears as continuum emission. The intensity of the tail glow becomes detectable at 4600 A and increases towards the red. Between 7000 A 8000 A a slight decrease in the glow intensity is inferred. The absolute intensity of the obliquely viewed tail glow was about 450 Rayleighs per Angstrom at around 7000 A and about 1 MR in the range within the transmission band of the image intensifier camera. The spectral distribution measured here seems to agree fairly well with that of the vehicle glow measured on the atmospheric explorer satellite.

The global distribution of thermospheric odd nitrogen for solstice conditions during solar cycle minimum

A two‐dimensional model of the minor neutral constituents N(²D), N(4S), and NO from 70 to 360 km is formulated and used to investigate the global distributions of these species for December solstice conditions during solar cycle minimum. For background properties the zonally averaged circulation, temperature, and compositional structure of major neutral constituents in the thermosphere are obtained from the coupled dynamical chemical model calculations of Roble and Kasting (this issue). Transport of the minor neutral constituents by both the meridional and vertical circulation and vertical molecular and eddy diffusion are considered in the model. The calculated distributions of the minor neutral constituents for solar ionization only and for solar plus auroral ionization are considered. The results show that the summer‐to‐winter pole meridional circulation transports both NO and N(4S) across the solar terminator into the polar night region where there is a downward vertical transport toward the mesosphere. The calculated latitudinal distribution of NO in the lower thermosphere for the solar‐ionization‐only case does not agree with the latitudinal NO distribution usually measured by the Atmosphere Explorer satellites. To obtain agreement between the calculated and observed structure, it is necessary to include ionization and dissociation sources due to auroral particle precipitation. The temperature and compositional structure and the circulation changes caused by high‐latitude heating and the particle‐induced production of N(²D) and N(4S) all combine to bring the calculated NO structure into better agreement with observations. The choice of the eddy diffusion coefficient critically controls the downward flow of thermospheric nitric oxide into the mesosphere. The model shows that the odd nitrogen densities at high winter latitudes are entirely controlled by particle precipitation and transport processes.