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.     

The diurnal variation of atomic hydrogen

This note critically examines the relative importance of several effects which influence the diurnal variation of atomic hydrogen abundance near the critical level.It is pointed out that the neglect of exospheric hydrogen in a recent theoretical treatment causes an overestimation of the diurnal variation at high exospheric temperatures, and an underestimation at low exospheric temperatures. The fluxes due to lateral flow are large compared to other fluxes only to the extent that the actual diurnal variation is very different from the diurnal variation corresponding to zero net lateral flow, which does not seem to be the case in the real atmosphere. Two effects which are probably important are charge exchange reactions with thermal oxygen ions, resulting in a diurnal exchange with the plasmasphere; and charge exchange reactions with high velocity protons, resulting in enhanced escape and diurnal variation.     

Interpretation of OGO-5 line shape measurements of Lyman-α emission from terrestrial exospheric hydrogen

The velocity distribution of hydrogen atoms in the terrestrial exosphere was measured as a function of radial distance (up to 7 Earth radii, E_R) 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, n_c (density of exobase level), T_c (temperature at the exobase level), and r_cs (satellite critical radius). This predicted reduction factor R(p) is compared to the measured R_m(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 T_c = 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 n_c, 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.     

Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

The European Space Agency’s Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the H i Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet’s center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 Å is detected at altitudes of 400-1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of .     

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.     

Hydrogen density in the dayside venusian exosphere derived from Lyman-α observations by SPICAV on Venus Express

A series of observations of the venusian hydrogen corona made by SPICAV on Venus Express are analyzed to estimate the amount of hydrogen in the exosphere of Venus. These observations were made between November 2006 and July 2007 at altitudes from 1000 km to 8000 km on the dayside. The Lyman-α brightness profiles derived are reproduced by the sum of a cold hydrogen population dominant below ～2000 km and a hot hydrogen population dominant above ～4000 km. The temperature (～300 K) and hydrogen density at 250 km (～10⁵ cm^?3) derived for the cold populations, near noon, are in good agreement with previous observations. Strong dawn–dusk exospheric asymmetry is observed from this set of observations, with a larger exobase density on the dawn side than on the dusk side, consistent with asymmetry previously observed in the venusian thermosphere, but with a lower dawn/dusk contrast. The hot hydrogen density derived is very sensitive to the sky background estimate, but is well constrained near 5000 km. The density of the hot population is reproduced by the exospheric model from Hodges (Hodges, R.R. [1999]. J. Geophys. Res. 104, 8463–8471) in which the hot population is produced by neutral–ions interactions in the thermosphere of Venus.     

Effects of altitude on critical ionization velocity experiments in space

The efficiency with which critical ionization velocity (CIV) discharges can be generated in space experiments is affected by the altitude at which the experiments are conducted. At around 500 km higher plasma density enhances plasma lower hybrid instability, momentum coupling efficiency, and charge exchange which is needed for seed ionization. At higher altitudes where atomic hydrogen and helium become the dominant ambient neutral species, the conditions for CIV discharge may improve considerably because less energy is lost to atmospheric ionization, even though the ambient density is reduced.     

Titan’s atomic hydrogen corona

Based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini orbiter, Titan’s atomic hydrogen exosphere is investigated. Data obtained during the T9 encounter are used to infer the distribution of atomic hydrogen throughout Titan’s exosphere, as well as the exospheric temperature.The measurements performed during the flyby are modeled by performing Monte Carlo radiative transfer calculations of solar Lyman-α radiation, which is resonantly scattered on atomic hydrogen in Titan’s exosphere. Two different atomic hydrogen distribution models are applied to determine the best fitting density profile. One model is a static model that uses the Chamberlain formalism to calculate the distribution of atomic hydrogen throughout the exosphere, whereas the second model is a Particle model, which can also be applied to non-Maxwellian velocity distributions.The density distributions provided by both models are able to fit the measurements although both models differ at the exobase: best fitting exobase atomic hydrogen densities of n_H = (1.5 ± 0.5) × 10⁴ cm⁻³ and n_H = (7 ± 1) × 10⁴ cm⁻³ were found using the density distribution provided by both models, respectively. This is based on the fact that during the encounter, HDAC was sensitive to altitudes above about 3000 km, hence well above the exobase at about 1500 km. Above 3000 km, both models produce densities which are comparable, when taking into account the measurement uncertainty.The inferred exobase density using the Chamberlain profile is a factor of about 2.6 lower than the density obtained from Voyager 1 measurements and much lower than the values inferred from current photochemical models. However, when taking into account the higher solar activity during the Voyager flyby, this is consistent with the Voyager measurements. When using the density profile provided by the particle model, the best fitting exobase density is in perfect agreement with the densities inferred by current photochemical models.Furthermore, a best fitting exospheric temperature of atomic hydrogen in the range of T_H = (150-175) ± 25 K was obtained when assuming an isothermal exosphere for the calculations. The required exospheric temperature depends on the density distribution chosen. This result is within the temperature range determined by different instruments aboard Cassini. The inferred temperature is close to the critical temperature for atomic hydrogen, above which it can escape hydrodynamically after it diffused through the heavier background gas.     

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.     

Sun-as-a-Star Observation of Flares in Lyman α by the PROBA2/LYRA Radiometer

There are very few reports of flare signatures in the solar irradiance at H i Lyman α at 121.5 nm, i.e. the strongest line of the solar spectrum. The LYRA radiometer onboard PROBA2 has observed several flares for which unambiguous signatures have been found in its Lyman-α channel. Here we present a brief overview of these observations followed by a detailed study of one of them: the M2 flare that occurred on 8 February 2010. For this flare, the flux in the LYRA Lyman-α channel increased by 0.6 %, which represents about twice the energy radiated in the GOES soft X-ray channel and is comparable with the energy radiated in the He ii line at 30.4 nm. The Lyman-α emission represents only a minor part of the total radiated energy of this flare, for which a white-light continuum was detected. Additionally, we found that the Lyman-α flare profile follows the gradual phase but peaks before other wavelengths. This M2 flare was very localized and had a very brief impulsive phase, but more statistics are needed to determine if these factors influence the presence of a Lyman-α flare signal strong enough to appear in the solar irradiance.     

Decay of the magnetic storm ring current by the charge-exchange mechanism

Equatorial charge-exchange lifetimes of ring current protons are recalculated, and the decay of a collection of ring current protons trapped on an L-shell by the charge-exchange mechanism is determined using recent models of the hydrogen geocorona. Observational results pertaining to the decay of ring current energy are briefly discussed, as are a number of competing loss mechanisms. Since charge exchange is a simple physical process which is very efficient in removing ring current energy from L-shells near to the Earth (say, L < 4), it is suggested that it may well be the dominant loss mechanism in this region.     

The H Lyman-α emission line from the upper atmosphere of Jupiter: Parametric radiative transfer study and comparison with data

This study uses the adding-doubling radiative transfer method in which we take into account the curvature effect of the planetary atmosphere in order to test the sensitivity of the jovian Ly-α emission line in relation to H column density, eddy diffusion coefficient, frequency redistribution function for photon scattering, temperature vertical profile, and an added hot atomic H layer on the top of the atmosphere. We also focus here on developing new diagnostic tools that will help us to obtain more confidently the underlying thermospheric structure of Jupiter. First, using the brightness distribution for specific wavelength bands as proposed by Ben Jaffel et al. [Ben Jaffel, L., Magnan, C., Vidal-Madjar, A., 1988. Astron. Astrophys. 204, 319-326], we show that the spatial thickness of the atomic H layer above the homopause level can be measured directly as the separation between the vertical positions of respectively the line core and line wing optical limbs. This thickness also constrains the [H] column and the value KH of the eddy diffusion coefficient at the homopause level at the disc location under consideration. We also propose to refine the value of KH and [H], respectively, at a specific planetary latitude, using the Q ratio of the limb peak brightness to the intensity from other regions over the planetary disc. Finally, the relationship between the disc brightness distribution from specific wavelength bands of the emission line and the temperature gradient in the thermosphere is demonstrated, thus providing an accurate tool to access this key information from high resolution observations. Quick, preliminary comparisons with some existing HTS/STIS data show the H layer thickness at auroral latitudes (∼1700 km) is much smaller than at equatorial latitudes (∼3900 km). These results strongly support the existence of a gradient in both H density and KH versus latitude, with higher values of KH at high latitudes and higher values of the H density at the equatorial regions. Such a small H layer thickness at auroral latitudes is consistent with a high mixing in the atmosphere that brings the hydrocarbons upwards, reducing consequently the column of hydrogen that scatters photons. These preliminary results show the strength of the proposed approach and open new horizons to use strong resonant emission lines at high resolution as a diagnostic for the state and structure of planetary upper atmospheres.     

Lyman-alpha observations of venera-9 and 10 I. The non-thermal hydrogen population in the exosphere of venus

The dayside hydrogen exosphere was observed in October–November 1975 with a Lymanalpha photometer carried on board Venera-9 and 10. In addition to intensity measurements, the use of a hydrogen cell allowed for the first time linewidth measurements. Both intensity and linewidth measurements below 1500 km of altitude are well fitted by a single exospheric component (T_c = 500 ± 100 K, n_c = 1.5 × 10⁴ atom cm^?3at 250 km). Above 3000 km, the measured linewidth increased sharply, to decrease again above 4500 km. This feature is interpreted as the signature of an additional population of “hot” atoms circulating on satellite orbits, created just behind the bow-shock by charge-exchange collisions (with an efficiency of 4%) between the neutral atoms and the solar wind protons, which became turbulent after bow-shock crossing. The density ratio of “hot” to standard population is of the order of 10% around 3500 km of altitude.     

Ion temperature troughs induced by a meridional neutral air wind in the night-time equatorial topside ionosphere

A mathematical model has been developed to calculate consistent values for the O⁺ and H⁺ concentrations and field-aligned velocities and for the O⁺, H⁺ and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered.It is shown that the O⁺-H⁺ transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes.     

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.     

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.     

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.     

Hydrogen atoms and ions in the thermosphere and exosphere

The distribution of atomic hydrogen in the thermosphere and exosphere is computed taking into account the upward flow which balances the escape flux. Because of the upward flow the number-density gradient is much steeper than it would be in a static atmosphere. Attention is drawn to the fact that the ratio of the amount of hydrogen above the 100 or 110km levels to the amount of hydrogen above the 200 or 300 km levels is a sensitive measure of the temperature of the exosphere. The evidence on the absolute abundance of atomic hydrogen is examined. It is concluded that the number density at the 120km level is probably about 5 × 10⁵/cm³. The Ly. absorption line at this level is beyond the linear part of the curve of growth.

Consideration is also given to the steady-state distributions of O⁺ and H⁺ ions. In the lower part of the exosphere the number density of O⁺ ions falls with increase in altitude (the associated scale height being twice that of the O atoms) and the number density of H⁺ ions rises at the same rate (as was first pointed out by Dungey). The altitude at which the number densities of O⁺ and H⁺ ions become equal is calculated on various assumptions regarding the temperature and hydrogen content of the exosphere. It is found to be about 1200 km when the temperature is 1250° K and the hydrogen content corresponds to the number density cited near the end of the preceding paragraph. The gradient of the predicted electrondensity distribution at several Earth radii is much less than that deduced from whistler studies.

The passage from charge transfer to diffusive equilibrium is discussed in an Appendix.     

On the energy dependence of the ring current proton precipitation

Low altitude satellite measurements of protons in the 1–100 keV range indicate two energy dependent proton precipitation boundaries. At low invariant latitudes mostly below 60° there is a region of moderately weak proton precipitation. The poleward boundary of this region tends to be at higher latitudes for the high energy protons than for the low energy protons. At high invariant latitudes there is a region where both the low and high energy protons precipitate with an isotropic pitch-angle distribution. The equatorward boundary of this region tends to be at lower latitudes for protons with energy more than 100 keV than for those in the 1–6 keV range. This region with isotropic pitch-angle distribution is located well outside the plasmapause both for the 1–6 and 100-keV protons.Between these two precipitation zones there is a region where the proton pitch-angle distribution is highly anisotropic with almost no protons in the loss cone. This region tends to be wider and more pronounced in the 1–6 than in the 100-keV protons.These findings lend further support to the mechanism of ion-cyclotron instability as the cause of proton pitch-angle diffusion in the low and intermediate regions. The process responsible for the strong diffusion at auroral latitudes has not yet been identified.