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A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the 3P2, 1, 0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 10(exp 6)/cc at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

[1] X rays are produced throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and exospheric neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. We previously simulated X-ray images of the magnetosheath as seen from an observation point outside the geocorona for average solar wind conditions. The locations of the bow shock and magnetopause were evident in these images, but the cusps were not taken into account. For the current paper we used dynamic three-dimensional MHD simulations of the solar wind, magnetosheath, and magnetosphere for the 31 March 2001 geomagnetic storm. A sky map was generated of the expected X-ray emissions as seen by a hypothetical X-ray detector on the IMAGE spacecraft. Modeled images as seen from an observation point well outside the geocorona were also created. The cusps can clearly be detected in both types of simulated images. Images of the magnetosheath in energy neutral atoms (ENA) also show the cusps. X-ray imaging of the magnetosheath, revealing the structure of the magnetopause and the bow shock, if carried out, could potentially make a valuable contribution to our understanding of the solar wind interaction with the magnetosheath.

[1] The role of the auroral sources induced by the electron and proton precipitation in the formation of the hot oxygen corona in the polar upper atmosphere is studied. It is found that both electron precipitation through exothermic chemistry and proton precipitation through atmospheric sputtering significantly contribute to the population of the hot oxygen geocorona. It is also found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere. The exothermic chemistry induced by the electron precipitation and/or by the absorption of the solar UV radiation is operating continuously in the polar upper atmosphere and results in a steady population of the very near-Earth environment by suprathermal oxygen atoms with energies below a few eV. By contrast, atmospheric sputtering by magnetospheric protons provides a more variable contribution, strongly coupled with the cusp region. It produces the more energetic oxygen atoms that populate the external regions of the hot oxygen geocorona. The results of calculations are in a good agreement with the analysis of the low-latitude perigee Low Energy Neutral Atom (LENA) images showing that the instrument signal consists of low to medium energy (5–30 eV) oxygen atoms produced in and near the cusp region. The more energetic (>30 eV) fraction of energetic oxygen atoms produced by the ioninduced atmospheric sputtering could be responsible for the energetic neutrals observed by the instrument far away from the cusp or oval regions. The total escape flux of oxygen atoms associated with atmospheric sputtering by protons is found about 8 � 10 23 s � 1 ; therefore this mechanism may provide a substantial contribution to the magnetospheric oxygen population.

[1] Analysis of ENA data from the LENA instrument on the IMAGE spacecraft shows that the terrestrial atmosphere is a copious emitter of energetic neutral atoms (

[1] Radiative recombination of O+ generates a variety of O-atom Rydberg levels, which then radiatively cascade to the ground state. The last two steps in the cascade produce 844.6 nm and 130.2–130.6 nm radiation in the triplet set of Rydberg levels and 777.2–777.5 nm and 135.6 nm radiation in the quintets. These four emissions are well known from ionospheric observations, and the higher members of the cascades are sometimes seen in auroral spectra but rarely in the nightglow. Sky spectra taken in 2000 at the Keck II telescope with the ESI echelle spectrograph reveal 20 members of the Rydberg transitions in the 390–930 nm region not previously observed in the nightglow. Most of these are not seen in earlier Keck spectra, taken at solar minimum. We are able to quantify the relative intensities of the emissions across the entire wavelength region using standard star spectra, allowing comparisons to be made with theoretical calculations. As the atmosphere is optically thin for the quintet transitions, corrections for radiative scattering are not necessary, and relative agreement between theory and observations is excellent. For the triplets the data agree more closely with the optically thick calculations. The relationship between the intensities of the quintet 3d-3p (926–926.6 nm) and the triplet 3p-3s (844.6 nm) multiplets is investigated and found to be linear. Accurate wavelengths are determined for the Rydberg quintets on the basis of observations with the high-resolution (∼40,000) Keck I/HIRES system. For the triplets the multiplets are incompletely resolved. Examples of spectra are presented where the 844.6 nm multiplet is the only OI Rydberg emission observed. We argue that the cause is Bowen fluorescence, a consequence of photoexcitation of the OI 3d-2p transition by H Lyman-β radiation in the geocorona. The fact that so many of the oxygen Rydberg lines have been identified in this study and that their intensities are consistent with cascade models of radiative recombination is a strong confirmation of the importance of this process in the tropical nightglow near solar maximum.

[1] During the summer of 2000 the LENA instrument on the IMAGE spacecraft collected a unique data set of low-energy, neutral atom emissions. The data suggests an ever-present flux of low energy (

[1] X-rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and interstellar and geocoronal neutrals, respectively. Simple models of this X-ray emission have been presented in the past, but results from a more sophisticated model are described in this paper. In particular, to obtain the densities of interstellar neutrals our X-ray model uses to determine X-ray intensities, we use the Fahr [1971] hot model although with recently determined input parameters. The geocoronal X-ray model we use in this paper has also been improved, as described in a recent paper by Robertson and Cravens [2003]. In previous papers we demonstrated that there is significant correlation between the “long-term enhancement” part of the soft X-ray background measured by the Rontgen Satellite (ROSAT) and the solar wind proton flux. In the current paper we determine the steady-state X-ray intensities due to the interaction between the solar wind and both interstellar neutrals and the geocoronal neutrals as a function of look direction and time of year. X-ray intensity maps are shown for both a spherically symmetric solar wind and for a latitude-dependent solar wind (i.e., fast and slow solar wind regions). In all cases, the X-ray intensity is highest when the view direction is towards the Sun, but the intensity is also relatively high for view directions intersecting the gravitational focusing cone of interstellar helium. We also show a heliospheric/geocoronal X-ray intensity map for the conditions used by Snowden et al. [1995] in producing the 1/4 keV channel soft X-ray background map in galactic coordinates. Our preliminary conclusion is that very roughly 50% of the total background soft X-ray intensity in the galactic plane and 25% at high galactic latitudes can be attributed to the charge transfer process operating within the solar system, with the remaining emission coming from outside our heliosphere.

[1] In a previous study we performed a statistical analysis of the geosynchronous plasma environment from measurements of the magnetospheric plasma analyzer instruments aboard the Los Alamos geosynchronous spacecraft. In that study the mean spin-averaged particle fluxes were determined as a function of local time and Kp index, averaged over an entire year of observations. Particles on open drift trajectories should cross geosynchronous orbit twice during their drift from the nightside plasma sheet, through the inner magnetosphere, and out to the dayside magnetopause. The ratio of incoming plasma sheet phase space density and outgoing dayside phase space density of every drift path thus contains information about particle losses during the drift through the near-Earth region. For ions the losses are largely caused by charge exchange reactions with hydrogen atoms. Applying tomographic inversion techniques, we use the observed statistical losses inside the geosynchronous orbit region to infer the spatial distribution of exospheric neutral hydrogen. The particle drift paths from the nightside of geosynchronous orbit to the dayside are calculated from electric and magnetic field models. To test the sensitivity of the tomography to the convection model, we invert the geosynchronous particle observations using various field model combinations and compare the results. We find that the neutral hydrogen densities obtained from the inversion disagree with existing models of the exosphere, mainly in the near-Earth region. These differences are due to lower-than-expected losses of lower-energy particles that nominally drift through the inner region and/or particle sources not considered in the study.

Kinetic modeling results are analyzed to examine the transport of photoelectrons through the nightside inner magnetosphere. Two sources are considered, those on the dayside from direct solar illumination and those across the nightside from light scattered by the upper atmosphere and geocorona. A natural filter exists on the nightside for the dayside photoelectrons. Coulomb collisions erode the distribution at low energies and low L shells, and magnetospheric convection compresses the electrons as they drift toward dawn. It is shown that for low-activity levels a band of photoelectrons forms between L = 4 and 6 that extends throughout the nightside local times and into the morning sector. For the scattered light photoelectrons the trapped zone throughout the nightside is populated with electrons of E less than 30 eV. At high L shells near dawn, convective compression on the nightside yields an accelerated population with electrons at energies up to twice the ionospheric energy maximum (that is, roughly 1200 eV for dayside photoelectrons and 60 eV for scattered light electrons). Modeled energy and pitch angle distributions are presented to show the features of these populations.

Methods for Monte Carlo simulation of planetary exospheres have evolved from early work on the lunar atmosphere, where the regolith surface provides a well defined exobase. A major limitation of the successor simulations of the exospheres of Earth and Venus is the use of an exobase surface as an artifice to separate the collisional processes of the thermosphere from a collisionles exosphere. In this paper a new generalized approach to exosphere simulation is described, wherein the exobase is replaced by a barometric depletion of the major constitents of the thermosphere. Exospheric atoms in the thermosphere-exosphere transition region, and in the outer exosphere as well, travel in ballistic trajectories that are interrupted by collisons with the background gas, and by charge exchange interactions with ionospheric particles. The modified simulator has been applied to the terrestrial hydrogen exosphere problem, using velocity dependent differential cross sections to provide statistically correct collisional scattering in H-O and H-H(+) interactions. Global models are presented for both solstice and equinox over the effective solar cycle range of the F(sub 10.7) index (80 to 230). Simulation results show significant differences with previous terrestrial exosphere models, as well as with the H distributions of the MSIS-86 thermosphere model.

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non-thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision-dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.

The polarization of the 584- and 304-A emissions of helium that have been resonantly scattered in the earth's atmosphere and in the interplanetary medium is discussed. It is shown that in the geocorona the simultaneous measurement of altitude profiles of the 584-A He dayglow intensity and polarization will test the hypothesis that the 584-A dayglow is excited by resonance scattering of sunlight from geocoronal helium. If this is indeed the major source of the 584-A dayglow emission, a simultaneous measurement of the polarization and the altitude profile of the intensity in the exosphere will uniquely determine the helium number density distribution as a function of altitude, and in the thermosphere it will alow a study of nondiffusive equilibrium processes. An attractive feature of this method is that the interpretation does not require a knowledge of the solar 584-A line center flux or line profile.

Data on ion and electron temperatures and concentrations to several thousand kilometers of altitude were obtained from the Atmosphere Explorer C satellite for 1974 and to 850 km from Arecibo incoherent scatter radar measurements. These data were used to normalize diffusive equilibrium profiles. From these profiles and by using the neutral atmospheric model of Jacchia (1971) and a new hydrogen model, the charge-exchange-induced neutral hydrogen escape fluxes for equatorial and middle latitudes were calculated. The data confirm earlier estimates that the charge exchange loss is more important than Jeans escape for the earth. It is also found that inside the plasmapause this charge exchange process with hot plasmapheric ions is the major production and loss process for the satellite population in the hydrogen geocorona.

Measurements of polarization and absorption by a hydrogen cell of the Lyman alpha geocoronal emission, performed on board the French spacecraft D2-A Tournesol over the working period of the tape recorder (April 19 to May 2, 1971), are compared with theoretical predictions given by spherical models of transfer using Kockarts-Nicolet and Chamberlain profiles. From polarization measurements the value of the hydrogen density at the exobase ρ500 and its latitudinal-diurnal variation are inferred. From absorption measurements, by using the exospheric temperature Tex of Jacchia, another independent way of deducing ρ500 variations is presented. Owing to calibration problems, an absolute measurement of Tex derived from absorption measurements is no longer available. However, Tex departures from the Jacchia temperatures are deduced from departures between the two values of ρ500 deduced from absorption and polarization measurements. Data obtained over the considered period of time provided ρ500 measurements independent of the solar flux. For example, the ρ500 density at noon near the equator is equal to (10 ± 2) 104 atoms cm−3. Intensity measurements at Ly α and 1304 A are also reported and are compared with the Oso 4 and 5 results. Measurements of the extrageocoronal Ly α emission are first made from the core of the geocorona. The values obtained during a satellite spin vary between 430 and 780 R. These measurements permit a ρ500 midnight density near the equator equal to (11 ± 3)104 atoms cm−3 to be deduced. The measured latitudinal-diurnal variation of ρ500 over 24 hours LT is compared with the previous experimental and theoretical values. The agreement is roughly good over 16 hours between 0900 LT and 0100 LT: the maximum occurs around 0900 LT, the minimum occurs near 1800 LT, and the midnight to midday ratio is around 1.1. However, the maximum to minimum density ratio ρmax/ρmin is higher than the expected one: 2.7 ± 0.8 as deduced from absorption measurements and 3.8 ± 1.2 from polarization measurements. The discrepancy between these two values can be interpreted by a ∼50°K departure from the Jacchia 71 temperature at southern latitudes in the afternoon. In the second part of the night, between 0100 LT and 0900 LT, a steep and large depression is observed; it can be interpreted as a local one around 35°N or a remote one, above the northern geomagnetic pole. This northern depression, the weak values of densities measured at southern latitudes, and the high density ratio ρmax/ρmin suggest the existence of a hydrogen bulge whose axis seems to be located around the sun-earth axis and to be linked to depressions over the poles.

The average quiet time structure of energetic radiation belt protons can be explained as an equilibrium balance among radial diffusive transport from a proton source located just within the first closed field lines, losses due to Coulomb collisions, and charge exchange with the ambient neutral hydrogen geocorona. The mode of transport is diffusion due to substorm-associated fluctuations in the large-scale electric and magnetic fields. Attention is restricted to equatorially mirroring protons, and comparison is made between theoretical predictions of proton energy spectra at L values between 2 and 6.6 and in situ radiation belt ion observation on board the satellites Explorer 45 and ATS 6, both orbiting close to the equatorial plane. Good agreement between theory and observation suggests that the dominant ion population in the energy range 100– 1000 keV in the inner magnetosphere is indeed a proton population. Beyond this interval the major ions may be different from protons.

Lyman α imagery of the hydrogen geocorona was obtained from the lunar surface during the Apollo 16 mission. The images are of 20° diameter fields, with 2 arc min limiting resolution, centered on the earth and about 12° upsun of the earth. The data confirm that the hydrogen geocorona is detectable above the interplanetary Lyman α background to more than 15 RE in the upsun direction. In the antisolar direction there is a pronounced ‘geotail’ effect due to solar Lyman α radiation pressure, which is markedly asymmetric about the sun line. Comparison of the data with theoretical models shows quite good agreement, particularly in the vicinity of 3 RE. In agreement with other observations the hydrogen density is lower than the theoretical density in the far-upsun direction, and very close to the earth there is a buildup of hydrogen on the dark side.

Balmer alpha emission from Lyman alpha scattering in geocorona normalized to remove scatter geometry effects, discussing temporal variations

We computed the emission of He 584-A and He+ 304-A radiation from the geocorona, assuming resonance scattering of solar radiation. Comparison of these calculations with recent rocket experiments shows that (a) the Jacchia 1970 He model gives the observed variation of 584 A with altitude and solar zenith angle, but large discrepancies are evident in the absolute value of the emission rate; (b) both the magnitude and the spatial variation of 304 A can be explained by a model that constrains the helium ions to be within the plasmasphere; and (c) an extraterrestrial source of EUV radiation is implied by the inability of the models to predict the observed emission rate in the antisolar direction.

A rocket-borne high-resolution echelle spectrograph was used to measure the profile of the solar hydrogen-Lyman-alpha line. The profile was normalized to an integrated flux of 5.2 ergs/cm² sec, the value of which was measured with a nitric oxide ion chamber carried on the same flight. The flux available for scattering by hydrogen in the geocorona was 2.8 × 1011 photons/cm² sec. The optical depth at the center of the telluric absorption line was about 5.6 in the slant path at a 60° zenith distance. The general features of the profile are in agreement with earlier measurements.

Analysis of experiments bearing on Lyman α excitation in the night glow leaves two possible explanations. One is a combination of direct scattering in a distant hydrogen cloud following the earth and multiple scattering in the thermosphere and exosphere. There is a difficulty accounting for the presence of the hydrogen cloud or geocoma. The other explanation is that during the night the thermosphere and exosphere contain 4 to 5 times as much hydrogen per unit column above 120 km as during the day. Although considerably more hydrogen is expected at night because of the much lower temperature, it is questionable whether a large enough change can occur in half a day.

The spherical Lyman-β radiative transfer problem is solved for several theoretical models of the hydrogen geocorona. Comparisons of the computed intensities with the experimental observations of Lyman β and Balmer α airglows show that the geocoronal Balmer α contribution is superimposed on a uniform background of about 4 R. The 4 R background is probably galactic (or extra-galactic) in origin. The column density of hydrogen required to account for the telluric Balmer α component is about 5 × 1013 atoms per cm² above 650 km for a 950°K exosphere.

Observations of scattered solar hydrogen Lyman-α radiation made with detectors carried by satellites Vela 4A and 4B show a repetitive and marked asymmetry in the Lyman-α flux. The detectors look radially outward from the earth at an altitude of 110,000 km. The axis of the 34° angle of view lies in the plane of the circular satellite orbit, which is inclined 32° to the equator. The distinctive characteristic of the data for the period May, 13, 1967, to July 31, 1967, is a broad maximum in the flux. In a preliminary interpretation, we assumed the detectors were above the hydrogen geocorona and that the scattering hydrogen, which gives rise to the broad maximum, must be in interplanetary space. Detailed analysis of data for many orbits indicates that the most precisely located feature of the data, the leading edge of the broad maximum, moves in right ascension at about the same rate and in the same direction as the motion of the earth projected on the celestial sphere. Thus, a significant part of the scattering hydrogen moves with the earth, and the observations are interpreted as evidence for an aspherical geocorona with an increased density of hydrogen beyond 18 RE on the night side. The view direction is above the ecliptic plane when the maximum is observed. After an estimated interplanetary component is subtracted from the observed maximum flux, the scattered Lyman-α intensity from telluric hydrogen beyond 18 RE is about 150 rayleighs. The column density of hydrogen above 18 RE when the maximum is observed is 6 to 7 × 1010 atom cm−2 in the plane of the satellite orbit.

Lyman α intensity measurements (1216 A) were obtained by a photometer on board the Ogo 5 spacecraft outside the geocorona. The Lyman α emission originating from hydrogen atoms in the upper part of the geocorona was derived from the total measured intensity after subtraction of the extraterrestrial emission. The hydrogen density distribution between 5 and 16 RE was deduced. This distribution is consistent with the evaporative atmospheric model of J. W. Chamberlain on the night side. A depletion of atoms found above a distance of 6 RE on the day side could be accounted for by the action on satellite particles of charge exchange with solar wind protons and by the solar Lyman α radiation pressure, according to numerical computations of both effects. It was also calculated that orbits with a perigee of 2 RE are rapidly distorted by radiation pressure. The mean time spent in the exosphere by satellite particles on such orbits is only 5 × 105 sec, one-half the ionization lifetime.

The view is presented that the outer portion of the earth's ionosphere, through which radio whistlers propagate, has its origin in the earth's atmosphere rather than in interplanetary space. The outer portion of the earth's neutral-particle exosphere is dominated by neutral atomic hydrogen which is escaping steadily from the earth's atmosphere. Some of this neutral atomic hydrogen reacts with atomic oxygen ions near the base of the exosphere, giving rise to hydrogen ions. Diffusive equilibrium is the dominant factor controlling the relative concentration of ions and neutral particles near the base of the exosphere. At higher levels, the geomagnetic field exerts an influence in that it causes the ions and electrons to rotate with the earth, and the centrifugal force modifies the force of gravitational attraction appreciably. The ion electron plasma through which the radio whistlers propagate is hydrostatically supported within the magnetic tubes of force.

The interpretation that the nighttime Lyman α observed from rockets is solar Lyman α that has been transported to the night side by the earth's extensive hydrogen envelope (the ‘geocorona’) is considered in detail. The equations of radiation transfer are solved for several spherically symmetric models of the hydrogen distribution. The numerical results for the integrated Lyman α night glow are compared with the observations. Radiation transfer in models whose total hydrogen abundance is consistent with daytime absorption measurements is shown to be inadequate in accounting for the observed night glow features.

International audience; The University of Paris experiment, operating on board the Oso 5 spacecraft since January 22, 1969, observes the solar Lyman α line integrated over the whole solar disk. The hydrogen and deuterium resonance cells yield information about the solar flux at the center of the line and on the blue wing. The center of the solar line as observed from the spacecraft is strongly absorbed by the earth's geocorona. Therefore the hydrogen resonance data contain solar, as well as geocoronal, information. Comparison of the data with sophisticated exospheric models gives the following principal results: (1) A high correlation (0.87 coefficient) exists between the total Lyman α line flux and the center line flux. A 30% variation in the total flux corresponds to a 47% variation in the central flux. The average fluxes over 1969 and 1970 are 3 × 1011 ph cm−2 sec−1 total flux and 5 × 1011 ph cm−2 sec−1 A−1 central flux. (2) The absolute value of the hydrogen density at 500-km altitude (independent of the experiment's calibration) is of the order of 5 × 104 atoms cm−3 on the day side and 9 × 104 atoms cm−3 on the night side. (3) The earth's hydrogen exobase is controlled by the zero net ballistic flux condition (i.e., the outgoing ballistic flux is everywhere equal to the incoming ballistic flux). This is why an average ratio of 1.7 was found between the maximum and minimum densities at the exobase. A discrepancy appeared between the absolute values of the hydrogen density measured by this experiment and the corresponding absolute values predicted by the model of G. Kockarts and M. Nicolet (1963). In order to fit our data, the values of the model should be increased to the following hydrogen densities at an altitude of 100 km: 2.3 × 107 atoms cm−3 for low solar activity (Zurich number Rz ∼ 60); 2.5 × 107 atoms cm−3 for mean solar activity (Rz ∼ 120); and 4 × 107 atoms cm−3 for high solar activity (Rz ∼ 180).

Exospheric neutral hydrogen temperature diurnal variation from satellite resonance filter data, suggesting Lyman alpha source external to geocorona

A reanalysis of the high-altitude Lyman α experiment of Chubb et al. which measured the altitude variation of Lyman α brightness between 500 and 1100 km shows that the results are not inconsistent with as many as 15 × 1012 atoms/cm2 above 120 km. The altitude variation can be interpreted in terms of 0.7 × 10−3 erg/cm2 sec ster in a broad line originating outside the geocorona and 1.5 × 10−3 erg/cm2 sec ster from multiple scattering at 120 km. The extrageocoronal component is the same as that called for by the absorption cell measurement of Morton and Purcell, in which 15 per cent of the signal from overhead at 25° angle of solar depression passed through an absorption cell. This follows from the use of the theory of radiative transfer in a geocorona of optical thickness 2.3 and proper attention to the solar dip angle prevailing when the various experiments were performed. The result is that the model of a hydrogen geocorona containing 3 × 1012 atoms/cm2 in the daytime and 15 × 1012 atoms/cm2 at night can account for all Lyman α experiments, including the high-altitude and absorption cell experiments, if an external source of 0.7 × 10−3 erg/cm2 sec ster exists in the antisolar direction.

Hydrogen bulge above springtime pole indicated by Lyman alpha radiation in earth geocorona, discussing hydrogen horizontal transport and hydrogen compounds dissociation as causes

The absorption predicted by four current models of the earth's hydrogen geocorona is compared with the absorption obtained by analyzing the solar H Ly α spectrograms taken on an Aerobee rocket flight. Comparison shows that within the limits of observational error and theoretical assumptions, the data are consistent with all the models tested.

The transport of solar 584-A photons through the spherical, asymmetric helium geocorona is discussed in detail. The analysis shows that with a favorable geometry and a moderately optically thick atmosphere it is possible to determine the helium abundance from dayglow observations without knowing the incident solar flux or the instrument calibration. Comparison of theoretical intensities with 584-A airglow observations made from the STP 72-1 satellite shows that recent satellite-drag and mass spectrometer-incoherent scatter empirical models predict helium densities which have seasonal-latitudinal dependences which approximate the airglow-determined densities in shape, but in general are high by as much as a factor of 2 in absolute density. Airglow-determined densities in the polar regions suggest a decrease in the exospheric helium density relative to the empirical model estimates.

Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange. ENA observations during the recovery phase of two moderate geomagnetic storms are analyzed in detail: November 22–23, 1973, from IMP 8 at 33 RE, and December 17, 1977, from ISEE 1 at 20 RE. The three-dimensional viewing capability of the Medium Energy Particle Instrument (MEPI) on ISEE 1 provides coarse spatial information; i.e., the ENA emission fell off significantly beyond L ≃ 5 in the midnight and morning sectors of magnetic local time. The spectral resolution of the MEPI allowed us to deduce that oxygen atoms dominated the ENA fluxes at energies of ∼100 keV because the decay time constant (7.5 hours) for the ENA fluxes was identical in the two lowest energy channels over 10 hours. Combining the spatial and spectral information from ISEE 1, we estimate an ENA unidirectional differential flux jO ≃ 30 (cm² s sr keV)−1 for 100-keV oxygen atoms with an e-folding energy of 20 keV at the beginning of the recovery phase. This ENA flux implies a singly charged oxygen ion flux jO+ ≃ 3.5×104 (cm² s sr keV)−1 near the inner edge of the ring current region, and we estimate a corresponding decay time for geocorona H atoms ≃ 9 days at that time and location. Using a newly derived relation between the recovery rate of the ring current disturbance (dDst/dt) and the measured ENA flux, we show self-consistently that charge-exchange loss of O+ ions was the dominant mechanism in the recovery of Dst in both of the storms observed in ENA by IMP 8 and ISEE 1. Even though oxygen dominated the ENA flux and the ring current energy loss rate at ∼100 keV, H+ intensities at these energies could still have been comparable to those of O+ in the ring current (because the charge-exchange cross section for H+ is much smaller than that for O+ at 100 keV).

Summer hot oxygen densities of 10,000-30,000 per cubic cm at 1100 km (or 100,000-300,000 at 550 km) for low to moderate solar activities are inferred from differences between satellite drag and mass-spectrometer-based models. At high solar activities, densities at 925 km from 100,000-400,000 (or 5 x 10 to the 5th to 2 x 10 to the 6th at 550 km) are inferred from a limited number of Dynamics Explorer mass spectrometer measurements. The densities are of the same order as those reported previously from ground-based optical measurements. It is pointed out that a detailed theoretical calculation of the hot oxygen distribution consistent with the cool populations in mass-spectrometer-based models would provide the basis for a more complete interpretation of mass spectrometer data and of high-altitude differences between drag-based and mass-spectrometer-based models. It is concluded that hot oxygen could be providing the major contribution to satellite drag in the summer lower exosphere during low solar activity.

Theoretical line profiles, applicable to the analysis of geocoronal Hα profile measurements, are presented for illustrative cases. While retaining a number of simplifications (classical exobase and diffusive equilibrium plasmasphere conditions), distinctive spectral signatures of mechanisms governing the geocorona are isolated. Examining the consequences of solar radiation pressure dynamics is the main point here. In the prototype evaporative case, radiation pressure acts to form narrow profiles via the creation of an extensive quasi-satellite component. Comparison with a simple extension of the earlier analytic theory discloses the influence of an exopause in this regard. The main modifications to evaporative spectral shapes in the geocoronal application, for shadow heights greater than 2 RE, are predicted to be (1) a blueward “shift” or bias near line center, for look directions parallel to the antisolar axis, generated by loss mechanisms acting over the time of flight of exospheric constituents (for example, solar ionization) and (2) an enhanced red ward wing at spectral displacements exceeding that defined by the shadow height escape speed, produced by plasmaspheric charge exchange collisions. Implications of these results for recent observations of geocoronal Hα line profiles are briefly discussed.

At high-latitude regions, energetic oxygen atoms are produced by a nonthermal mechanism associated with precipitated energetic atomic oxygen ions during disturbed periods. The upward moving nonthermal atoms can redistribute themselves at higher altitudes. For the storm of December 17, 1971, assuming the energetic O(+) ions are uniformly precipitated between 50 deg and 70 deg magnetic latitudes, the nonthermal oxygen atoms become dominant over the ambient at about 700 km on the dayside and at about 600 km on the nightside at 60 deg magnetic latitude. Globally, a high-number-density atomic oxygen corona appears between 50 deg and 70 deg magnetic latitudes above the exobase, and the maximum is located at a few degrees poleward from the center of the band (63 deg magnetic latitude).

In situ measurements of H abundance between 73 and 93 km are reported for conditions of winter solstice, magnetic quiet, and a solar depression angle of 12 deg. The data were obtained by a rocket-borne instrument using the resonance fluorescence technique. A discharge source emitting photons at 1216 A was an integral part of the instrument. The instrument was radially deployed 80 cm by a boom from the front of the payload in order to avoid the shock created by the gas flow over the front of the payload. An attitude control system oriented the payload so that the gas flow was nearly perpendicular to the plane containing the incident and scattered photons, thus minimizing any correction for Doppler shift. The resonance radiation detector viewed a black backstop in order to minimize background radiation from the hydrogen geocorona; however, the background was not entirely eliminated. The signal-to-noise ratio was improved by summing the data in 1.8-s bins. The observed hydrogen concentration maximized at 85 km at 1.5 + or - 1.1 x 10 to the 8th atoms/cu cm.

Analysis of the geocoronal He-I 58.4-nm resonance radiation field with rocket borne gas absorption cell photometers has revealed singly scattered photons to attain steadily increasing importance in the radiation signals if photons close to the line center are effectively suppressed. In order quantitatively to confirm this, we have reinvestigated the radiation transport problem and have given the spectral intensity in terms of contributions from different scattering orders. In the formulation of these contributions we have used the concept of angle-dependent partial frequency redistribution in comparison with that of complete redistribution. The first and second order intensity contributions are studied in detail here with respect to their spectral profile, to their local and solar zenith angle dependence, and to their height profile. The second and higher order intensity contributions are shown to decrease steeply with increasing distance from the line center, such that for a specific observation a critical wavelength distance can be calculated beyond which only singly scattered photons contribute to the total intensity. The intensity due only to singly scattered photons is proven to be proportional to the helium column density on the line of sight within clearly determinable accuracies. Thus its height profile enables immediate deduction of local atmospheric parameters, as the atmospheric helium density and temperature, even under optically thick conditions.

The theory of planetary exospheres is extended to incorporate solar radiation pressure in a rigorous manner, and an evaporative geocoronal prototype (classical, motionless exobase) is constructed using Liouville's theorem. Model calculations for density and kinetic temperature at points along the earth-sun axis (solar and antisolar directions) reveal an extensive satellite component, comprising approximately 2/3 of the total hydrogen density near 10 earth radii, and a temperature profile suggestive of an isotropic quasi-Maxwellian velocity distribution for the bound component. A geotail is also evident as an enhancement of the density at local midnight compared to local noon that increases outward (from approximately 25 percent at 10 earth radii to over 60 percent at 20 earth radii). Additional mechanisms acting upon the geocorona alter the basic evaporative case in notable ways. Solar ionization has been included in a simple fashion; the effect is to partially deplete the density without otherwise altering the structure. Interaction with a simple plasmasphere via the Boltzmann equation results in 'heating' the geocorona and enhancing the escape flux at the expense of the density of the bound component, an effect not appreciated in earlier studies; the geotail survives this interaction.

The ultraviolet photometer of the University of Iowa spin-scan auroral imaging instrumentation on board Dynamics Explorer-1 has returned numerous hydrogen Lyman alpha images of the geocorona from altitudes of 570 km to 23,300 km (1.09 R sub E to 4.66 R sub E geocentric radial distance). The hydrogen density gradient is shown by a plot of the zenith intensities throughout this range, which decrease to near celestial background values as the spacecraft approaches apogee. Characterizing the upper geocorona as optically thin (single-scattering), the zenith intensity is converted directly to vertical column density. This approximation loses its validity deeper in the geocorona, where the hydrogen is demonstrated to be optically thick in that there is no Lyman alpha limb brightening. Further study of the geocoronal hydrogen distribution will require computer modeling of the radiative transfer.