Latitude variations of exospheric hydrogen and the polar wind

Several satellite experiments have measured the solar Lyman-α line, either in scattering from upper atmospheric atomic hydrogen (the Lyman-α airglow) or directly at line center (which determines the hydrogen column density along the line of sight). Recent analyses of data from the above experiments consistently reveal the presence of an atomic hydrogen depletion at high latitudes. In situ determinations of hydrogen at lower altitude show no evidence of such behaviour. This has led us to postulate two mechanisms which may be more effective in reducing the high-latitude density at the high altitudes of the exospheric measurements (500–2000 km). The first is the polar wind loss of protons, which depletes atomic hydrogen through a charge exchange reaction. The second is a high-latitude magnetospheric heating of protons, followed by charge exchange. Opposing the above loss mechanisms are the influences of ballistic lateral flow and mean meriodional winds. We have shown by means of a three-dimensional exospheric transport model that none of the above mechanisms can reconcile the disparate results in the two altitude regimes, nor can they provide the large outward hydrogen fluxes and the correct seasonal variations observed at high latitudes.     

Auroral zone effects on hydrogen geocorona structure and variability

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 of neutral H should be produced by this phenomenon in the auroral 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.     

Density and temperature distributions in non-uniform rotating planetary exospheres with applications to earth

Using an exosphere model which includes the effects of rotation and temperature and density variations at the exobase, we determine kinetic temperature and density distributions for planetary exospheres in general and terrestrial O, He and H in particular, the latter being based on empirical models for density and temperature variations at exobase altitudes. We examine the effects of energy flow and confirm Fahr's suggestion that the lateral energy flow at the exobase should be important for the temperature distributions above the base. Considering uniform density and sinusoidal temperature variations at the base, we find that temperatures decrease with altitude above the diurnal temperature maximum Tmax at the base. On the other hand, above the diurnal temperature minimum Tmin at the base, the temperatures increase from the base to peak values (except for low values of mMG/kT₀) and then decrease above the peaks, tending to approach the values above Tmax. The corresponding densities near the base, above Tmin, decrease with altitude more rapidly than above Tmax but exhibit considerable increases in their scale heights in the vicinity of their temperature peaks, at which points the densities begin to approach those above Tmax. In the converse case, with uniform base temperature and sinusoidal base density variations, the exospheric density and temperature distributions above the diurnal density maximum Nmax and minimum Nmin at the base result in similar characteristics to those above Tmax and Tmin, respectively. Applying the model to terrestrial O, He and H, we find that multiple exospheric temperatures should occur wherein temperatures above Tmax decrease less rapidly with altitude for increasing species mass. On the other hand, O and He temperatures increase with altitude above Tmin to peak values near 5000 km and then decrease above the peaks while H temperatures decrease with altitude throughout. We also examine the effects of the terrestrial exospheric H temperature distribution on optical depths for Lyman alpha absorption and find that such temperature variation may be important for radiative transfer calculations when the depths are greater than unity and satellite orbits are unimportant.     

The influence of thermospheric winds on exospheric hydrogen on Venus

Monte Carlo models of the distribution of atomic hydrogen in the exosphere of Venus were computed which simulate the effects of thermospheric winds and the production of a “hot” hydrogen component by charge exchange of H⁺ and H and O in the exosphere, as well as classic exospheric processes. A thermosphere wind system that is approximated by a retrograde rotating component with equatorial speed of 100 m/sec superimposed on a diurnal solar tide with cross-terminator day-to-night winds of 200 m/sec is shown to be compatible with the thermospheric hydrogen distribution deduced from Pioneer Venus orbiter measurements.     

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

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

Nonthermal escape of hydrogen and deuterium from Venus and implications for loss of water

Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H⁺ with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O⁺ and H₂. However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 10¹⁰ cm^?2 sec^?1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10^?3. This corresponds to an H₂O mixing ratio of 1 × 10^?3 at the cold trap and ～15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ～1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H₂O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ～4.0 × 10^?6.     

The terrestrial hydrogen problem

Current theoretical models do not satisfactorily explain observed variations of the global exospheric atomic hydrogen density distribution. Differences between the mesospheric upward flux and the Jeans escape mechanism indicate that other escape fluxes affect hydrogen density. Observations of latitudinal depletions and an early morning trough in the exosphere can be attributed to the influence of additional escape mechanisms. These variations of the exospheric hydrogen density distribution seem to be correlated with other observed variations in the atmosphere; however, no straightforward explanation has been proposed to date.     

The effect of the diurnal variation in temperature on densities derived from near-circular satellite orbits,with applications to Skylab 1 (1973-27A)

Satellites in almost circular paths experience appreciable drag throughout the entire orbit; the localised effect being intrinsically related to the global distribution of exospheric temperature. To normalise the density values derived from such orbits to a fixed temperature, an effective exospheric temperature is required. In this paper a “pseudo” exospheric temperature is determined analytically such that, by assuming the atmosphere is held constant at this temperature, the same perturbation in the semi-major axis is achieved as that by a satellite moving in an atmosphere exhibiting a realistic approximant to the measured diurnal variation in temperature. The theory is applied to data and densities derived from orbital analysis of Skylab 1 and the course of the semi-annual variation is retraced for 1974–1976.     

Propagation of the geomagnetic density and temperature effect into the exosphere

The propagation of the geomagnetic effect into the exosphere is investigated based on a free-flight particle kinetic model of exospheric densities and temperatures. Exobasic neutral gas conditions and their variations during a geomagnetic storm occurrence are adopted as given by the OGO-6 model. The contributions of particles originating at different exobasic locations to the density and temperature at exospheric regions are taken into account according to the time needed to reach these regions. A short-time geomagnetic variation of exobasic conditions is simulated by a Gaussianshaped A_p -index variation with an FWHM of 20 min. It is then shown that the relative amplitude and the half width of the geomagnetic density variation increase strongly with exospheric heights. The density peak and the main temperature peak are shown to be delayed by more than one and two hours, respectively, at heights above 10,000 km. The temperature variation changes from a singlepeaked to a double-peaked structure at greater exospheric heights. It is shown that the exospheric density response to geomagnetic disturbances is detectable in observations of the geocoronal He-1-584 Å resonance radiation.     

Planetary ENA Imaging: Venus and a comparison with Mars

We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.     

Upper atmospheric temperatures from Doppler line widths—V. Auroral electron energy spectra and fluxes deduced from the 5577 and 6300 Å atomic oxygen emissions

This is a report upon further data obtained from the auroral OI 5577 Å emission with a Wide Angle Michelson Interferometer (WAMI), and upon our first observations made with it on the 6300 Å emission. The method used for converting emission intensities and temperatures to auroral electron fluxes and energy spectra is described. Data for the 5577 Å emission are presented for the (lack of) heating in auroral forms, vertical temperature profiles in aurora, electron flux and energy spectrum variations in pulsating aurora, and a ‘cold’ subvisual auroral arc. Data from the OI 6300 Å emission are presented for the diurnal variation of exospheric temperature and for the thermalization of $\text{O}\text{(}{}^1\text{D)}$ in the F-region.     

The helium drag effect on the orbit of Explorer 19

The orbit of Explorer 19 (1963-53A) has been determined at 60 epochs between February 1976 and October 1976 from over 3000 observations. Using values of the orbital decay rate corrected for the effects of solar radiation pressure, 58 values of air density at a height of 900 km have been evaluated. After correcting for solar and geomagnetic activity and seasonal-latitudinal and diurnal variations in the exospheric temperature, the residual variation exhibited modulations associated with the ‘winter helium bulge’.An examination of three different models of the helium variation has indicated a procedure, which combines distinct features of the CIRA (1972) and Jacchia (1977) model atmospheres, for determining the atmospheric drag effect on Explorer 19. It is proposed that this technique may be equally applicable to any satellite in near-polar orbit at an equivalent height.     

The greenhouse of Titan

Both non-gray radiative equilibrium and gray convective equilibrium calculations for Titan indicate that the discrepancy between the equilibrium temperature of an atmosphereless Titan and the observed infrared temperatures can be explained by a massive molecular hydrogen greenhouse effect. The convective calculations indicate a probable minimum optical depth of 14, corresponding to many tens of km-atm of H₂, and total pressures of ～0.1 bar. The tropopause is several hundred km above the Titanian surface and at a temperature of about 90°K. Methane condensation is likely at this level. Such an atmosphere is unstable against atmospheric blow-off unless typical mesosphere scale heights are < 25km, an unlikely situation. Blow-off can also be circumvented by exospheric temperatures near the freezing point of hydrogen. It is considered more plausible that the present atmosphere is in equilibrium between outgassing and blow-off of the one hand and accretion from protons trapped in a hypothetical Saturnian magnetic field on the other; or exhibits uncompensated blow-off of outgassing products. To maintain the present blow-off rate without compensation for all of geological time requires an outgassing equivalent to the volatilization of a few km of subsurface ices. Photo-dissociation of these volatilized ices produces the observed high abundance of H₂ as well as large quantities of complex organic chromophores which may explain the reddish coloration of the Titanian cloud deck. An extensive circum-Titanian hydrogen corona is postulated. Surface temperatures as high as 200°K are not excluded. Because of its high temperatures and pressures and the probable large abundance of organic compounds, Titan is a prime target for spacecraft exploration in the outer solar system.     

Spatial and temporal variations of the Lyman-alpha airglow and related atomic hydrogen distributions

Spatial variations of the atomic hydrogen Lyman-alpha airglow were observed at 550 km from the OSO-4 spacecraft. Near the subsolar position, the zenith, horizon and nadir emission rates were 27,50 and 35 kR, respectively, while near the antisolar position, the respective emission rates were 2·0, 2·7 and 1·2 kR on 27 October 1967. Analyses of these results shows that the mean vertical optical depth above satellite altitude is in the range 0·87–3·5 with 1·3 as the preferred value. Atomic hyrdogen undergoes a diurnal density variation of the order of a factor of 1·7 with a maximum in the early morning and a minimum in the afternoon. A hydrogen altitude profile corresponding to 1100 K exospheric temperature is adequate to explain the data. Temporal variations of the airglow observed from OGO-4 show that the solar line-center Lyman-alpha flux should typically increase by 50 per cent for an increase in Zurich sunspot count from 30 to 210.     

Field-aligned suprathermal electron fluxes below 270 km in the auroral zone

Observations are reported of field aligned etectron fluxes in the energy range 50–500 eV at altitudes below 270 km from two rocket flights in the auroral zone. The regions of field aligned suprathermal electrons occurred in bursts of a few seconds duration, and in some instances the energy of the peak field aligned flux was in the range 100–500 eV. Theoretical calculations of the pitch angle distribution were made using the Monte Carlo technique for two model atmospheres having exospheric temperatures of 750 and 1500 K bracketing the expected auroral zone exospheric temperature. The calculations were made for the case of incident field aligned suprathermal fluxes with no local parallel electric field and also for the case of a local constant parallel electric field. Comparison of theoretical and experimental pitch angle distributions showed that in one case at 270 km a parallel electric field of 1–2 mV/m fitted the data whereas another burst at 210 km required a parallel electric field of about 10 mV/m to produce a field aligned distribution of 230 eV electrons as pronounced as was observed. Furthermore in this latter case the lack of strong field alignment at 500 eV pointed to localisation of the parallel electric field to an altitude range of 20–30 km about the rocket altitude.     

Effects of charge exchange involving H and H+ in the upper atmosphere

It is argued that there is a terrestrial loss of hydrogen as ions which includes the polar wind but extends effectively down to a latitude in the range 45–50° invariant. In daytime and for much of the night-time the flux is close to the limiting value for H⁺ flow through the topside ionosphere. It is argued that the flux decreases rapidly with increasing solar activity, following the decrease in neutral hydrogen concentration. It has been found that as solar activity increases the Jeans escape flux increases, and the charge exchange escape flux increases until moderate solar activity levels are reached. As solar activity increases from moderate to high levels, the charge exchange escape may decrease again. A new budget for terrestrial hydrogen loss over the solar cycle is given. The global flux of hydrogen ions outward from the ionosphere is comparable with estimates of the plasma sheet loss rates, and this flux, together with some solar wind plasma, is an attractive source for the plasma sheet.The energetic neutrals produced from the charge exchange of ring current ions with thermal-energy neutrals in the exosphere produce the optical emission of the equatorial aurora, which can be related to ion production rates near and above the E-region. The ionization production is adequate to explain the enhancements in ion production observed during magnetic storms at Arecibo.     

Solar system X‐rays from charge exchange processes

While X‐ray astronomy began in 1962 and has made fast progress since then in expanding our knowledge about where in the Universe X‐rays are generated by which processes, it took one generation before the importance of a fundamentally different process was recognized. This happened in our immediate neighborhood, when in 1996 comets were discovered as a new class of X‐ray sources, directing our attention to charge exchange reactions. Charge exchange is fundamentally different from other processes which lead to the generation of X‐rays, because the X‐rays are not produced by hot electrons, but by ions picking up electrons from cold gas. Thus it opens up a new window, making it possible to detect cool gas in X‐rays (like in comets), while all the other processes require extremely high temperatures or otherwise extreme conditions. After having been overlooked for a long time, the astrophysical importance of charge exchange for the generation of X‐rays is now receiving increased general attention. In our solar system, charge exchange induced X‐rays have now been established to originate in comets, in all the planets from Venus to Jupiter, and even in the heliosphere itself. In addition to that, evidence for this X‐ray emission mechanism has been found at various locations across the Universe. Here we summarize the current knowledge about solar system X‐rays resulting from charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Helium escape from the Earth''s atmosphere: The charge exchange mechanism revisited

We have studied the escape of neutral helium from the terrestrial atmosphere through exothermic charge exchange reactions between He⁺ ions and the major atmospheric constituents N₂, O₂, and O. Elastic collisions with the neutral background particles were treated quantitatively using a recently developed kinetic theory approach. An interhemispheric plasma transport model was employed to provide a global distribution of He⁺ ions as a function of altitude, latitude and local solar time and for different levels of solar ionization. Combining these ion densities with neutral densities from an MSIS model and best estimates for the reaction rate coefficients of the charge exchange reactions, we computed the global distribution of the neutral He escape flux. The escape rates show large diurnal and latitudinal variations, while the global average does not vary by more than a factor of three over a solar cycle. We find that this escape mechanism is potentially important for the overall balance of helium in the Earth's atmosphere. However, more accurate values for the reaction rate coefficients of the charge exchange reactions are required to make a definitive assessment of its importance.     

Global atomic hydrogen density derived from OGO-6 Lyman α measurements

Measurements of the Lyman α airglow intensity were made between June 1969 and June 1970 by a u.v. photometer experiment on the OGO-6 satellite. The data for the zenith intensity at altitudes between 400 and 1100 km were fitted to theoretical airglow models to derive atomic hydrogen density n_c at a reference altitude, taken to be 500 km. n_c was determined for each of 286 orbits throughout the year. The mean exospheric temperature T∞(J) during this period varied from 900 to 1300 K according to the Jacchia model. The solar Lyman α flux at line center F₀ was also determined over each 90-min orbit in the model-fitting procedure. F₀ was found to be correlated with sunspot number, in agreement with previous results. A nearly-exact linear relationship was found for F₀, when averaged over ‘bins’ which are 20 sunspot numbers in width. n_c was found to be inversely correlated with T∞(J); however the dependence is not that predicted by steady-state models whose only escape mechanism is Jeans evaporative escape. Unless the total atmospheric loss rate depends upon 27-day changes in the solar EUV, which is unlikely, an additional upper atmospheric loss is required in order that the total loss remain constant with T∞(J). This extra loss may be largely due to charge-exchange reactions in the exosphere, wherein energetic protons are converted to fast hydrogen atoms, as suggested previously by a number of authors. An additional result is suggested by the apparent spherical symmetry of the inferred density, namely that the familiar diurnal variation of hydrogen is absent at the high latitudes preferentially sampled by the OGO-6 data.     

Fractionation of hydrogen and deuterium on Venus due to collisional ejection

The fractionation factor f is important for interpreting the current escape fluxes of H and D on Venus and how the D/H ratio has evolved. The escape flux is currently governed by the two processes of charge exchange and collisional ejection by fast oxygen atoms. Using a best-fit parameterized equation for the O-H scattering angle phase function, more accurate branching ratios for the oxygen ion dissociation and including the effects of the initial energy and momentum of the ions and electrons, as well as for the hydrogen and deuterium gas, we have reanalyzed the collisional ejection process. Our analysis produces improved values for the efficiency of H and D escape as a function of the ionospheric temperature. From our results we propose the reduction of the hydrogen flux for collisional ejection from 8 to 3.5 x 10(6) cm-2 s-1. Assuming that collisions leading to escape occur mostly in the region between 200 and 400 km, the revised D/H fractionation factor due to collisional ejection is 0.47, where previously the process had been considered completely discriminating against deuterium escape (or f approximately 0.) The resulting deuterium flux is 3.1 x 10(4) cm-2 s-1, roughly 6 times the flux due to charge exchange, making collisional ejection the dominant escape mechanism for deuterium on Venus.