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International audience; The analysis of Lyman α observations in the exosphere of Mars has become limited by the assumption of spherical symmetry in the modeling process, as the models are being used to analyze increasingly detailed measurements. In order to overcome this limitation, a two-dimensional density model is presented, which better emulates the density distribution of deuterium and hydrogen atoms in the exosphere of Mars. A two-dimensional radiative transfer model developed in order to simulate multiple scattering of solar Lyman α photons by an asymmetric, non-isothermal hydrogen exosphere, is also presented here. The models incorporate changes in density and temperature structure of the martian atmosphere with radial distance and solar zenith angle. The 2-D models were applied to the MAVEN-IUVS echelle observations of deuterium and hydrogen Lyman α as well as HST Lyman α observations of hydrogen at Mars. The asymmetric 2-D model provided better fits to the data and smaller thermal escape rates in comparison to the symmetric 1-D model for the exosphere of Mars. However, intensity differences between both models became small above ~2.5 martian radii indicating that the exosphere of Mars approaches spherical symmetry at higher altitudes, in agreement with earlier studies. In addition, a new cross calibration of the absolute sensitivities of two instruments on the Hubble Space Telescope and the MAVEN-IUVS echelle mode is presented based on near-simultaneous observations of the geocorona and Mars.

Recent studies suggest that the hydrogen (H) density in the exosphere and geocorona might differ from previously assumed values by factors as large as 2. We use the SAMI3 (Sami3 is Also a Model of the Ionosphere) and Comprehensive Inner Magnetosphere-Ionosphere models to evaluate scenarios where the hydrogen density is reduced or enhanced, by a factor of 2, relative to values given by commonly used empirical models. We show that the rate of plasmasphere refilling following a geomagnetic storm varies nearly linearly with the hydrogen density. We also show that the ring current associated with a geomagnetic storm decays more rapidly when H is increased. With respect to these two space weather effects, increased exosphere hydrogen density is associated with reduced threats to space assets during and following a geomagnetic storm.

The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation (PROCYON) spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled out

Circumterrestrial Lyman-α column brightness observations from 3–8 Earth radii (Re) have been used to study temporal density variations in the exospheric neutral hydrogen as response to geomagnetic disturbances of different strength, i.e., Dst peak values between −26 and −147 nT. The data used were measured by the two Lyman-α detectors (LAD1/2) onboard both TWINS satellites between the solar minimum of 2008 and near the solar maximum of 2013. The solar Lyman-α flux at 121.6 nm is resonantly scattered near line center by exospheric H atoms and measured by the TWINS LADs. Along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H column density, assuming optically thin conditions above 3 Re. In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % (equal to the same increase in H column density ΔnH) compared to the undisturbed case short before the storm event. Even weak geomagnetic storms (e.g., Dst peak values ≥ −41 nT) under solar minimum conditions show increases up to 23 % of the exospheric H densities. The strong H density increase in the observed outer exosphere is also a sign of an enhanced H escape flux during storms. For the majority of the storms we found an average time shift of about 11 h between the time when the first significant dynamic solar wind pressure peak (pSW) hits the Earth and the time when the exospheric Lyman-α flux variation reaches its maximum. The results show that the (relative) exospheric density reaction of ΔnH have a tendency to decrease with increasing peak values of Dst index or the Kp index daily sum. Nevertheless, a simple linear correlation between ΔnH and these two geomagnetic indices does not seem to exist. In contrast, when recovering from the peak back to the undisturbed case, the Kp index daily sum and the ΔnH essentially show the same temporal recovery.

Accepted: 2017-01-11, 資料番号: SA1160265000

International audience; The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the SWAN/SOHO instrument in January 1996, 1997 and 1998 (low solar activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth Radii (RE) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of LAICA imager (∼50 RE), and encompassing the orbit of the Moon (∼60 RE). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the day side, producing a bulge of H density between 3 and 20 RE which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to OGO‐5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 RE are larger than LAICA/OGO‐5 by factors 1.1‐2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to non‐thermal atoms (not in the model).

(Abridged) The existence of an extended neutral hydrogen exosphere around small planets can be used as an evidence for the presence of water in their lower atmosphere but, to date, such feature has not been securely detected in rocky exoplanets. Planetary exospheres can be observed using transit spectroscopy of the Lyman-$\alpha$ line, which is limited mainly by interstellar medium absorption in the core of the line, and airglow contamination from the geocorona when using low-orbit space telescopes. Our objective is to assess the detectability of the neutral hydrogen exosphere of an Earth-like planet transiting a nearby M dwarf using Lyman-$\alpha$ spectroscopy and provide the necessary strategies to inform future observations. The spatial distribution in the upper atmosphere is provided by an empirical model of the geocorona, and we assume a velocity distribution based on radiative pressure as the main driver in shaping the exosphere. We compute the excess absorption in the stellar Lyman-$\alpha$ line while in transit, and use realistic estimates of the uncertainties involved in observations to determine the observability of the signal. We found that the signal in Lyman-$\alpha$ of the exosphere of an Earth-like exoplanet transiting M dwarfs with radii between 0.1 and 0.6 R$_\odot$ produces an excess absorption between 50 and 600 ppm. The Lyman-$\alpha$ flux of stars decays exponentially with distance because of interstellar medium absorption, which is the main observability limitation. Other limits are related to the stellar radial velocity and instrumental setup. The excess absorption in Lyman-$\alpha$ is observable using LUVOIR/LUMOS in M dwarfs up to a distance of $\sim$15 pc. The analysis of noise-injected data suggests that it would be possible to detect the exosphere of an Earth-like planet transiting TRAPPIST-1 within 20 transits., Comment: 12 pages, 13 figures, accepted for publication in Astronomy & Astrophysics

Geocoronal H emission data acquired by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission are analyzed to quantify the H density distribution over the entire magnetosphere ionosphere thermosphere region in order to investigate the response of the atmospheric system as a whole to geomagnetic storms. It is shown that at low and mid latitudes the H density averaged over storm times in the thermosphere-exosphere transition region decreases by ∼30%, while the H density at exospheric altitudes above ∼1-2RE increases by up to ∼40% relative to quiet times. We postulate that enhanced ion-neutral charge exchange in the topside ionosphere and inner plasmasphere is the primary driver of the observed H redistribution. Specifically, charge exchange reactions between H atoms and ionospheric/plasmaspheric O+ leads to direct H loss, while those between thermal H and H+ yield kinetically energized H atoms which populate gravitationally bound satellite orbits. The resulting H density enhancements in the outer exosphere would enhance the charge exchange rates in the ring current and the associated energetic neutral atom production. Regardless of the underlying mechanisms, H redistribution should be considered as an important process in the study of storm-time atmospheric evolution, and the resultant changes in the geocoronal H emissions potentially could be used to monitor geomagnetic storms.

[1] Satellite-based measurements of geocoronal Lyman α (Lyα) emission at 121.6 nm, created through multiple scattering of solar Lyα photons by atomic hydrogen, offer a valuable means of inferring the hydrogen abundance, [H], in the terrestrial thermosphere and exosphere on a global, long-term basis. We present initial results from an analysis of Lyα radiance measurements acquired across the Earth's limb from 2002 to 2007 by the Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. This data spans nearly half of a solar cycle, and both the absolute Lyα radiance as well as its relative variation across the limb are shown to exhibit a significant dependence on solar activity. We describe sensitivities of a forward radiative transport (RT) model to key parameters governing the [H] distribution in order to assess implications for [H] estimation from the GUVI limb scan data throughout the solar cycle. Based on data-model comparisons, we conclude that the observed solar cycle variability is indicative of a decrease in dayside H density at the exobase with increasing solar activity. These results, along with additional forward RT modeling based on NRLMSISE-00 model specification of [H], are also used to assess contemporary semiempirical model accuracy.

[1] The dominant neutral constituent in Earth's upper exosphere, atomic hydrogen (H), resonantly scatters solar Lyman-alpha (121.567 nm) radiation, observed as the geocorona. We report here observations of an exospheric response to geomagnetic storms obtained using measurements of the geocorona by Lyman-alpha detectors on the Two Wide-angle Imaging Neutral-atom Spectrometers mission. We introduce a new parameter, NH, the number of H atoms in the spherical shell from a geocentric distance of 3 to 8 Earth radii, to quantitatively characterize in a simplified way global exospheric conditions. Five geomagnetic storms observed during three months in the second half of 2011 are accompanied by abrupt temporary increases, spikes, of NH from 6% to 17%, lasting not longer than a day. These increases seem to show some correlation with the minimum Dst index reached during the peak of each storm.

X-ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X-ray emission can account for roughly half of the observed soft X-ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X-ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X-ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X-ray intensity observed at Earth for the time periods 1990–1993 and 1996–1998. Measured solar wind proton fluxes are also directly compared with the “long-term enhancement” part of the soft X-ray background measured by the Rontgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X-ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.

We present results from EUVE measurements obtained during the all-sky survey of interplanetary and geocoronal He I 584-A emission. The data consist of count rates from the long wavelength spectrometer and the long wavelength photometric band (520–750 A) of scanner C obtained over a 1-year period from July 1992 to July 1993. During this period, EUVE was in survey mode so that the scanners made 5° × 360° sweeps of the sky in a plane perpendicular to the Sun-Earth line, while the spectrometers were aligned with the antisolar direction. The interplanetary He I signal is morphologically consistent with previous observations with similar observing geometry, such as Prognoz 6 [Dalaudier et al., 1984]. However, unlike the Prognoz 6 data, the EUVE measurements were made from low Earth orbit (520 km) and so contain geocoronal emission as well. As a result, along sight lines where the relative speed between the interplanetary wind and the Earth is at a minimum, extinction of the interplanetary signal through resonance scattering by the He geocorona occurs. We believe this to be the first detection of line extinction of the local interstellar He wind emissions by the geocorona. We find that the geocoronal extinction signatures provide a new means of determining the interstellar He wind vector and emission line profile and add further constraints on the values of other interplanetary and solar He parameters and the morphology of the He geocorona. On the basis of model fits of the observed interplanetary emission and geocoronal extinction, we determine values for the interplanetary wind ecliptic longitude λ = 76.0 ± 0.4°, latitude ϕ = −5.4 ± 0.6° (downwind direction), speed υ0 = 26.4 ±1.5 km s−1, and temperature T0 = 6900 ± 600 K. In addition, assuming an interplanetary He density of 0.01 cm−3, we determine an average solar He I 584-A line center flux of 1.4 ± 0.3 × 1010 photons cm−2 s−1 A−1 for the data analyzed here.

Energetic neutral atoms are created when energetic magnetospheric ions undergo charge exchange with cold neutral atoms in the Earth's tenuous extended atmosphere (the geocorona). Since they are unaffected by the Earth's magnetic field, these energetic neutrals travel away in straight line trajectories from the points of charge exchange. The remote detection of these particles provides a powerful means through which the global distribution and properties of the geocorona and ring current can be inferred. Due to its 2 × 9 RE polar orbit, the Polar spacecraft provides an excellent platform from which to observe ENAs because it spends much of its time in the polar caps which are usually free from the contaminating energetic charged particles that make observations of ENAs more difficult. In this brief report, we present the first ENA imaging results from Polar. Storm-time ENA images are presented for a northern polar cap apogee pass on August 29, 1996 and for a southern polar cap perigee pass on October 23, 1996. As well, we show with a third event (July 31, 1996) that ENA emissions can also be detected in association with individual substorms.

The Hydrogen Deuterium Absorption Cell (HDAC) is part of the Ultraviolet Imaging Spectrograph (UVIS) experiment aboard the Cassini spacecraft. During Cassini’s Earth swingby on August 18, 1999, HDAC was used as a photometer to measure solar Lyman-α radiation scattered by the neutral hydrogen atoms of the geocorona. These data provide information about the hydrogen column density along HDAC’s line of sight during the Earth swingby. The data cover a large part of the Earth’s exosphere and enable us to determine the distribution of hydrogen atoms in this region. The results are compared to predictions of models providing hydrogen densities of the Earth’s exosphere/geocorona. We found that the models overestimate the decrease of hydrogen density for increasing distance from the Earth.

A model of the oxygen geocorona near the exobase solving the nonlinear Boltzmann equation with a Monte Carlo method is used to calculate the distribution of the hot oxygen atoms during geomagnetically disturbed nighttime conditions. The precipitation of energetic O+ ions and the subsequent enhancement of the hot O corona at high latitudes is simulated for the September 17, 1971, storm conditions. It is found that in such circumstances, the O+ precipitation is a significant source of superthermal O atoms leading to important perturbations of the velocity distribution of the bulk oxygen population. The effective gas temperature near the exobase is similar to that in the undisturbed atmosphere, but the hot O density rises considerably over the quiet condition values.

Exothermic reactions involving metastable neutrals and ions were recently proposed as sources of hot oxygen atoms in addition to the classical O2+ and NO+ dissociative recombination. The Boltzmann equations for thermal and nonthermal populations of O atoms are solved with a Monte Carlo stochastic simulation method. It is shown that the calculated energy distribution functions of O atoms are significantly in nonequilibrium in the transition region between the thermosphere and the exosphere. It is found that the inclusion of additional sources leads to stronger disturbances of the energy distribution function and, as a consequence, increases the nonthermal fraction of hot O atoms. The variation of the vertical distribution of hot O between solar maximum and minimum conditions is also evaluated and shows good agreement with the available experimental evidence.

[1] High spectral resolution line profile observations indicate a reproducible semi-annual variation in the geocoronal hydrogen Balmer α effective temperature. These observations were made between 08 January 2000 and 21 November 2001 from Pine Bluff Observatory (WI) with a second generation double etalon Fabry-Perot annular summing spectrometer operating at a resolving power of 80,000. This data set spans sixty-four nights of observations (1404 spectra in total) over 20 dark-moon periods. A two cluster Gaussian model fitting procedure is used to determine Doppler line widths, accounting for fine structure contributions to the line, including those due to cascade; cascade contributions at Balmer α are found to be 5 ± 3%. An observed decrease in effective temperature with increasing shadow altitude is found to be a persistent feature for every night in which a wide range of shadow altitudes were sampled. A semiannual variation is observed in the column exospheric effective temperature with maxima near day numbers 100 and 300 and minima near day numbers 1 and 200. Temperatures ranged from ∼710 to 975 K. Average MSIS model exobase temperatures for similar conditions are approximately 1.5× higher than those derived from the Balmer α observations, a difference likely due to contributions to the observed Balmer α column emission from higher, cooler regions of the exosphere.

[1] Exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation, observed as the geocorona. Measurements of scattered solar photons allow one to probe time-varying three-dimensional distributions of exospheric H atoms. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission images the magnetosphere in energetic neutral atom (ENA) fluxes and additionally carries Lyman-α detectors (LADs) to investigate exospheric atomic hydrogen. Knowledge of exospheric properties is essential for the interpretation of magnetospheric images in ENA fluxes produced in charge exchange between energetic ions and H atoms. We describe the process of obtaining exospheric distributions and present, as an example, a global H number density distribution for 11 June 2008. The TWINS LAD experimental data are especially sensitive to atomic hydrogen at geocentric distances from 3 to 8 Earth radii. The distribution reveals asymmetries from day to night, dawn to dusk, and north to south. We discuss the available data sets and their coverage, limitations, and promise for a study of exospheric response to seasonal, solar, and geomagnetic variations.

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.

Development of the low energy neutral atom (LENA) imager was originally motivated by a need to remotely sense plasma heating in the topside ionosphere, with the goal of greatly enhanced temporal resolution of an otherwise familiar phenomenon. During ground test and calibration, the LENA imager was found to respond to neutral atoms with energies well above its nominal energy range of 10–750 eV, up to at least 3–4 keV, owing to sputtering interactions with its conversion surface. On orbit, LENA has been found to respond to a ubiquitous neutral atom component of the solar wind, to the neutral atoms formed by magnetosheath interactions with the geocorona during periods of high solar wind pressure, and to the interstellar neutral atoms flowing through the heliosphere during the season of maximal relative wind velocity between spacecraft and interstellar medium. LENA imaging has thus emerged as a promising new tool for studying the interplanetary medium and its interaction with the magnetosphere, in addition to the ionospheric heating and outflow that result from this interaction. LENA emissions from the ionosphere consist of a fast component that can be observed at high altitudes, and slower components that evidently create a quasi-trapped extended superthermal exosphere. The more energetic emissions are responsive to solar wind energy inputs on time scales of a few minutes.

[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.

Solar wind charge exchange (SWCX) X-rays are emitted when highly charged solar wind ions such as O{sup 7{sup +}} collide with neutral gas, including the Earth's tenuous outer atmosphere (exosphere or geocorona) and hydrogen and helium from the local interstellar medium drifting through the heliosphere. This geocoronal and heliospheric emission comprises a significant and varying fraction of the soft X-ray background (SXRB) and is seen in every X-ray observation, with the intensity dependent on solar wind conditions and observation geometry. Under the right conditions, geocoronal emission can increase the apparent SXRB by roughly an order of magnitude for an hour or more. In this work, we study a dozen occasions when the near-Earth solar wind flux was exceptionally high. These gusts of wind lead to abrupt changes in SWCX X-ray emission around Earth, which may or may not be seen by X-ray observatories depending on their line of sight. Using detailed three-dimensional magnetohydrodynamical simulations of the solar wind's interaction with the Earth's magnetosphere, and element abundances and ionization states measured by ACE, we model the time-dependent brightness of major geocoronal SWCX emission lines during those gusts and compare with changes in the X-ray background measured by the Chandra X-ray Observatory.more » We find reasonably good agreement between model and observation, with measured geocoronal line brightnesses averaged over 1 hr of up to 136 photons s{sup –1} cm{sup –2} sr{sup –1} in the O VII Kα triplet around 564 eV.« less

There is no “edge of space,” no sharp demarcation where the Earth's atmosphere ends and outer space begins. Instead, there is the fuzzy, progressively thinning exosphere, a region filled mostly by neutral hydrogen atoms that stretches from an altitude of 500 kilometers to hundreds of thousands of kilometers above the Earth's surface. So far, the outer detectable limit of the exosphere is around 15.5 Earth radii—roughly half the distance between the Earth and the Moon.

The Venus Lya corona is caused by resonance scattering of the solar 1215.67 A Lya line by hydrogen atoms in the Venus upper atmosphere. The atmospheric atomic hydrogen content is probed remotely via Lya observations. On 1990 February 10 the Galileo spacecraft flew by Venus, obtaining a series of Venus scans with the Ultraviolet Spectrometer Experiment. The Pioneer Venus (PV) Orbiter Ultraviolet Spectrometer obtained Venus Lya images approximately weekly throughout its 14 year mission (1978–1992), spanning the 11 year solar cycle. I develop a morphology of the behavior of the Venus Lya signal with respect to location on Venus and solar activity level. The predawn hydrogen density enhancement extends to high latitudes (160 ). I find an equatorial minimum of hydrogen and evidence for a polar hood of enhanced hydrogen abundance. This hood may be produced by a hydrogen influx from the predawn region that is not removed as efficiently as hydrogen in the subsolar region. All features examined persist throughout the solar cycle and increase in hydrogen abundance with solar activity. A search for small-scale (1000 km) features produced a null result. However, the timescale for ballistic transport of atomic hydrogen in the exosphere is ∼8 minutes, too short for the detection of nonequilibrium density features. I analyze the data using a two-dimensional nonisothermal complete-frequency-redistribution multiple-scattering code modified from the LYAB code provided by James Bishop for the geocorona. I employ the VTS3 neutral thermosphere model (A. Hedin, H. Niemann, W. Kasprzak, & A. Seiff, J. Geophys. Res., 88, 73 [1983]) and calculate diffusive profiles for the vertical distribution of atomic hydrogen, characterized by the hydrogen number density and vertical flux at the exobase n f 0 0 (L. Paxton, D. Anderson, & A. Stewart, J. Geophys. Res., 93, 1766 [1988]). The flux parameter regulates the amount of hydrogen in the lower thermosphere, and the exobase density controls the amount in the upper thermosphere and exosphere. The almost linear relationship between the PV Langmuir probe photoelectron current and the observed solar Lya output provides a daily proxy for solar illumination of Venus hydrogen. I determine the parameter values that best fit the data for selected segments of the sunlit disk and examine their behavior with respect to location and solar activity. Numerical results are limited to the region of spatially uniform temperature in the exosphere and thermosphere. For high solar activity ( ), subsolar values are F ∼ 200 n (5.6 10.7 0 cm 3 and cm 2 s , cor4 7 0.8) # 10 f (5.8 0.9) # 10 0 responding to hydrogen column densities of (2.0 0.3) # cm 2 above 200 km and between 135 12 13 10 (1.0 0.2) # 10 km and the CO2 Lya absorbing layer at 112 km. These values agree with the subsolar work of Paxton et al. and with densities derived from in situ measurements by H. Brinton, H. Taylor, H. Niemann, H. Mayr, A. Nagy, T. Cravens, & D. Strobel (Geophys. Res. Lett., 7, 865 [1980]). Exobase density, , den0 creases from a local solar time (LST) of 10 hr to 15 hr by to cm . At 12 hr LST, increases from the 4 4 7 # 10 4 # 10 n0 equator to 60 latitude by to cm . This exo4 4 4 # 10 7 # 10 base density minimum at low latitudes on the dayside is consistent with solar EUV heating driving the transport removal of hydrogen atoms from the dayside upper atmosphere. The hydrogen abundance in the lower thermosphere, , does not f0 vary significantly. However, because of the opacity of the overlying upper thermosphere and exosphere, the variation of hydrogen column abundance for the values observed would n0 not be detectable in . Both and increase with solar f n f 0 0 0 activity, and there is evidence suggesting solar cycle phase dependence. A detailed interpretation of these quantitative results requires a photochemical model of the Venus upper atmosphere. The details of suprathermal hydrogen atom production are essential to understanding the transport behavior of hydrogen in the upper thermosphere and exosphere.

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 idea of escape of light atmospheric gas is older than the kinetic theory, having been introduced by J. L. Waterston in 1846 (Chamberlain 1963). The thermal escape concept was revived by G. J. Stoney around the turn of the century and . given a definitive form by Jeans (1925), whose name is usually associated with it. Spitzer (1952) was the first to realistically incorporate atmospheric structure into the problem. Observation in 1955 of Lyman-IX scattering from the geocorona, the atomic-hydrogen cloud around the Earth, and many more recent satellite and space-probe measurements have led to a number of interpretive and theoretical papers, which Chamberlain ( 1963) and Tinsley (\974) have reviewed. Composition and structure of other atmospheres are discussed by H unten ( 1971 b) and Ingersoll & Leovy ( l97\). Escape from a planet is intimately linked to the nature of the exosphere, the region in which the mean free path is so long that collisions can be neglected for many purposes. Most atoms in the exosphere are in free ballistic orbits; a few of the fastest ones may be on escape orbits. The bottom of the exosphere is the critical level, or exobase, defined as where the mean free path (in the horizontal direction) is equal to the scale height. The conventional approximation of the critical level is that the atmosphere is fully collisional below it and collisionless above; Chamberlain (1963) discusses the validity of this approximation. The planetary corona can be defined as the hydrogen component of the exosphere. Global averages are used throughout most of the present review. The exosphere is actually asymmetric to a considerable degree, as are the escape processes. Neverthe­ less, the long mean free path permits a great deal of lateral flow, which helps to justify the approximation of spherical symmetry (McAfee 1967; Vidal-Madjar & Bertaux 1972; Tinsley 1974; Tinsley, Hodges & Strobel 1975). Atmospheric escape is an interesting phenomenon, particularly for its effect on

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.

While the Doppler profiles of Balmer-alpha and -beta are in principle analyzable in order to derive orbital data concerning both escaping and satellite geocoronal particles, interpretations are in practice hampered by a lack of understanding of profile properties apart from such geocoronal features as an asymmetry, caused by the absence of escape velocity-exceeding, downward-directed particles. Attention is accordingly given to the profiles of H atoms which, while emitting radiation from a complete Maxwellian distribution, also exhibit an H-alpha fine structure-generated asymmetry which is a predictable function of the excitation mechanism.

Several series of auroral and geocoronal images from th Dynamics Explorer Mission are presented and analyzed. It is found that the motion of the transpolar arc of a theta aurora is correlated with the y-component of the IMF; the arc motion is in the general direction of the y-component. In addition, a sequence of global images of a small auroral substorm shows the initial development of intense luminosities in a relatively small spatial region or 'bright spot' in the premidnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at low latitudes and contiguous to the midnight boundary of the bright spot. Finally, a series of images of the geocorona in scattered solar Ly-alpha emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the earth's exosphere; a Chamberlain model provides an adequate fit to radial distances of 4.5 earth-radii, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly-alpha intensities in the antisolar direction.

The velocity distribution of hydrogen atoms in the terrestrial exosphere was measured as a function of radial distance (up to 7 Earth radii, ER) with the help of a Lyman-α hydrogen absorption cell, flown in 1968 on board the OGO-5 satellite. This paper contains the final analysis of the measurements. As a basis of comparison, the theory for the calculation of projected velocity distribution along a line of sight is established for the theoretical exospheric model of Chamberlain (1963). Self-absorption of Lyman-α photons along a line of sight is included to derive Lyman-α line profiles emerging from the geocorona. The effect of the hydrogen absorption cell, measured by the reduction factor R(p) is predicted as a function of impact parameter p of the line of sight, for various values of the parameters of a Chamberlain's model, nc (density of exobase level), Tc (temperature at the exobase level), and rcs (satellite critical radius). This predicted reduction factor R(p) is compared to the measured Rm(p), with the following findings: the Ly-α line width decreases with radial distance, as expected from the “evaporation and escape” theory of Chamberlain; the measured temperature Tc = 1080 K is in very good agreement with the exospheric temperature prediction from satellite drag data. An upper limit of 8 × 104at. cm−3 is imposed on nc, regardless of photometric absolute calibration. A good fit to data requires the presence of atoms in satellite orbits, distributed in a different fashion than that described by the concept of satellite critical radius. Lyman-alpha radiation pressure is thought to be the cause of this departure from the exospheric theory of Chamberlain (1963), otherwise perfectly confirmed. The same scientific rationale will be applied to exospheric hydrogen of the planets Mars and Venus in subsequent papers.

The instantaneous structure of planetary exospheres is determined by the time history of energy dissipation, chemical, and transport processes operative during a prior time interval set by intrinsic atmospheric time scales. The complex combination of diurnal and magnetospheric activity modulations imposed on the Earth's upper atmosphere no doubt produce an equally complex response, especially in hydrogen, which escapes continuously at exospheric temperatures. Vidal-Madjar and Thomas (1978) have discussed some of the persistent large scale structure which is evident in satellite ultraviolet observations of hydrogen, noting in particular a depletion at high latitudes which is further discussed by Thomas and Vidal- Madjar (1978). The latter authors discussed various causes of the H density depletion, including local neutral temperature enhancements and enhanced escape rates due to polar wind H + plasma flow or high latitude ion heating followed by charge exchange. We have reexamined the enhancement of neutral escape by plasma effects including the recently observed phenomenon of low altitude transverse ion acceleration. We find that, while significant fluxes ofneutral H should be produced by this phenomenon in the aurora1 zone, this process is probably insufficient to account for the observed polar depletion. Instead, the recent exospheric temperature measurements from the Dynamics Explorer-2 spacecraft suggest that neutral heating in and near the high latitude cusp may be the major contributor to depleted atomic hydrogen densities at high latitudes.

The studies on the hydromagnetic interactions of the ionic exosphere with the geomagnetic field are reviewed. Considering the various hydromagnetic interactions, it is shown that the geomagnetic dipole field and the earth's atmosphere are confined within the cavity with the radius of the about from six to ten earth's radii, and hydromagnetics in the outer atmosphere are reduced to the electrodynamics for the thin transition layer between the interplanetary gas and the earth's exosphere.

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.

An attempt was made during 1965 to observe the diurnal and seasonal variations of the Hα emission from the geocorona with a very simple photometer. Because of its simplicity the instrument was susceptible to disturbing radiations, so the effects upon it of light from sources other than the geocoronal hydrogen are discussed in some detail. The observational results, which were very dependent upon the geometry of the Earth's shadow are believed to be a good indication of the variation of the Hα emission. The build-up of hydrogen content during the night is demonstrated and discussed, the time at which it reached its maximum diurnal value being inferred to be soon after midnight. It is suggested that the distribution with altitude of the hydrogen in the thermosphere and exosphere changes markedly towards dawn, the gas rising to great heights due to an increasing exosphere base temperature before base dawn. No strong evidence for a concentration of hydrogen towards the plane of the ecliptic was seen, neither was there any convincing indication of a Brandt geocoma. The absolute intensity of the Hα emission was inferred from observations of the Orion nebula, and was found to be several times greater than would be expected from the reported intensity of the solar Lβ line.

A comprehensive theory is presented for the region of a planetary atmosphere where collisions are rare and where the controlling factors are gravitational attraction and thermal energy conducted from below. Although the subject of this article originated literally with the kinetic theory itself, until recently attention has been confined to atmospheric evaporation. The early sections (1–4) are developed on the classic assumption of a sharply defined critical level, above which the atmosphere is completely free of collisions. Throughout, the different types of particle orbits are treated separately; coronal particles are either ballistic (meaning captive particles whose orbits intersect the critical level), satellite (captive particles orbiting above the critical level), or escaping. Liouville's equation leads to exact expressions for the density distributions and escape flux. The latter is a simple analytic expression, but the density integrals are more complex and numerical evaluations have been provided in tables. At large distances from the planet all the integrals may be evaluated in their asymptotic limits. Expressions are given for the different density components, the kinetic temperature, and heat flux. The latter two cannot generally be evaluated with the thermal conductivity as normally computed. The integrated density in a column above a specified height and orientated in a specified direction is given particular attention, because it fixes the critical level and because it is essentially an observable quantity. Numerical calculations are given for relating such observations in either the radial or transverse directions to the basic parameters of the corona (the criticallevel density, temperature, and height above the planet). The radial integrated density at the critical level is shown to deviate sensibly from the simple expression applicable to a planeparallel atmosphere unless the gravitational energy is very much greater than the mean thermal energy. Section 5 utilizes results of the preceding theory to justify the adoption in the first place of the concept of a critical level. The principal question focuses on the outward flux of escaping particles. A detailed treatment of escape as governed by collisions agrees with earlier similar analyses in showing that (with certain approximations) the critical-level concept predicts the accurate flux. However, it is noted that this result, except for confirming the height of the critical level, is trivial: it is a consequence of an assumed Maxwellian distribution and the special properties accruing to it. Section 6 treats a variety of related problems dealing with production and loss mechanisms, after some orbital properties and flight times are investigated. The critical-level theory cannot cope with the important question of the abundance of satellite orbits. Here it is shown that the satellite contribution may be included throughout the theory by omitting the expressions for the satellite component as such and evaluating the ballistic component, not with the critical level, but with a higher satellite critical level instead. For the Earth's hydrogen corona this level is near 2.5 Earth radii. The effect of photoionization on the density distribution of escaping orbits is also treated and shown to be negligible for the Earth. The concentration of hot interplanetary gas around a bare planet offers an interesting effect in that screening of orbits by the planet may more than counteract condensation due to the gravitational attraction. A hot, stationary interplanetary gas intermingled with a planetary corona will not have any appreciable effect on the coronal density or temperature distributions. Specifically, it is inappropriate to regard heat as being “conducted” (in the usual meaning of the word) from the interplanetary medium through the corona. Doppler profiles of coronal particles in a column along a specified direction have been examined in Section 7. Asymptotic expressions are given along with the general formulae, which will have to be evaluated numerically if and when the pertinent spectral observations are made. Even at large distances from the planet where the population consists largely of escaping particles, the shape of the spectral profile does not deviate very seriously in radially outward velocities from the profile with the Maxwellian distribution at the critical level. Section 8 examines the basic assumption of the entire preceding theory: the establishment of a Maxwellian distribution near the critical level. The collision treatment here is based on the Boltzmann equation, and while it is crude, it is not trivial. The conclusion is that the Maxwellian distribution in the upward direction is depleted beyond the escape velocity by several per cent, but this depletion is generally negligible when compared with errors produced by small uncertainties in temperature. The deviations would be important only were it possible to make measurements affected by the detailed shape of the velocity distribution curve. Real coronae are summarized briefly in Section 9. While more and better observations are needed, it appears now that the hydrogen geocorona may become seriously perturbed into a geocoma, as Brandt labelled it, by solar particles flowing past the Earth. Attention is also directed to observations of the helium geocorona as a means of clarifying the now highly confused “helium problem”. Some estimates of the features of the Martian oxygen corona are made. Venus, as the Earth's sister planet, is an enigma, but coronal observations may point the way to an explanation of the distinctive differences between the evolution of the atmospheres of Venus and the Earth.

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 experiments relating to the Lyman α radiation observed from rockets in the night sky are reviewed. In part the radiation appears to be solar Lyman a transported by the exosphere below 3 earth radii. However, an important fraction must be attributed also to single scattering in a very extensive envelope of hydrogen moving with the earth out to 50–100 earth radii. The amount of hydrogen needed is about 50 times as much as should be present in the escape component of the exosphere required. The principal problem at present is to account for this hydrogen.

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.

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.

At present, there are some theories being able to explain the frequency characteristic of dawn chorus. In these theories, the natural thermal noise excited in a manner similar to the operation of a travelling-wave tube, the Doppler shifted proton cyclotron radiation and the Cerenkov radiation are considered as the origin of V. L. F. noise in the earth's exosphere respectively. This paper indicates that the Cerenkov radiation will be the most effective with respect to the emission energy in the above mentioned mechanisms in the earth's exosphere and that Cerenkov radio emissions generated by high speed protons (1.4×104km/sec) precipitating into the ionized exosphere along the earth's magnetic field may be excited simultaneously by the operation of the mechanism similar to a travelling-wave tube due to charged particles different from those causing the Cerenkov radiation relatively near the earth less than about 4 earth radii from the center of the earth and then propagate toward the earth in the whistler mode.

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