Neutral and ion-exospheres in the solar wind with applications to mercury

A model of planetary neutral and ion-exospheres in the solar wind is formulated for weak or lunar like solar-wind interaction with a planet. The neutral exosphere model allows for density and temperature variations and for rotation at the exobase. The ion-exosphere is produced by ionization of the neutral exosphere in the solar wind and its density distribution is obtained by solving the continuity equation in the drift approximation. Applying to Mercury a surface temperature distribution inferred from infra-red data and a vanishing bound neutral flux at the base, He and He⁺ density distributions are found. When the He atmosphere of Mercury is due entirely to the surface bombardment by solar wind He⁺⁺, the resulting He⁺ density is found to vary from 1.5 × 10⁻¹ to 10⁻³ cm⁻³ over the range 1.5–5 planetocentric radii on the dayside. These densities are found to be detectable by typical solar-wind plasma instruments. The possible effects of cyclotron-resonant scattering by interplanetary magnetic field fluctuations are examined and shown to be negligible. An electromagnetic plasma instability, triggered by the birth of ions in the exosphere, is shown to be important for the thermalization of the energy mode transverse to the interplanetary magnetic field, allowing more ions to be detected by solar-wind ion probes.     

Satellite beacon measurements of protonospheric fluxes

The ionospheric and protonospheric regions of the plasmasphere, which are dominated by the O⁺ and H⁺ ionic species, respectively, interact by means of proton fluxes within tubes of magnetic force. The present study is concerned with the determination of these fluxes by the beacon satellite technique as used in the ATS-6 experiment in relation to three observing sites: Boulder, Colorado; Lancaster, U.K.; and Fairbanks, Alaska. From plasmasphere models based on solutions of the time dependent O⁺ and H⁺ momentum and continuity equations, it is shown that the time differential of the “residual content” as measured at Lancaster, provides a good estimate of the protonospheric flux at 4000km altitude in the L = 1.8 magnetic shell under quite geomagnetic conditions. The effect of the neutral thermospheric wind on the protonospheric flux is also investigated. Fluxes determined by the beacon technique for the period from September 1975 to July 1976 are shown, and these are compared with typical results derived from other techniques.     

A theoretical study of the time-dependent behaviour of O in the mid-latitude plasmasphere

The behaviour of O²⁺ at L = 3 in the plasmasphere is studied. Starting with a low O²⁺ flux-tube content to characterize post-magnetic-storm conditions the time-dependent equations of continuity and momentum for O²⁺ are solved to give densities and fluxes for a period of several days using both sunspotmaximum and sunspot-minimum parameters. Our results show large amounts of O²⁺ near the equator at sunspot maximum but relatively little at sunspot minimum, and emphasize the key role of the collisional process between O²⁺ and O⁺. It is the combined effects of O²⁺---O ⁺ collisions and thermal diffusion that lead to the large O²⁺ densities near the equator at sunspot maximum. Both of these mechanisms have less influence at sunspot minimum. At sunspot maximum the O⁺ layer acts as a collisional barrier below the O²⁺ production region preventing O²⁺ from sinking towards regions of high recombination rate. In this production region the effects of thermal diffusion are small and upward flow of O²⁺ results from the action of the O²⁺ pressure gradient and the polarization electric field. When the upward flowing O²⁺ reaches regions in which thermal diffusion has a strong influence it is accelerated to even higher altitudes. The O ⁺ barrier is so effective that the diurnal variation of the O⁺ layer is reflected in the diurnal variation of O²⁺ near the equator at sunspot maximum. Our sunspot maximum results also indicate that certain types of temperature profiles are more likely to enhance equatorial O²⁺ densities. The existence of large temperature gradients below 1000 km altitude does not help the flow of O²⁺ towards the equator. The associated changes in the O⁺ layer lead to more O²⁺-O ⁺collisions and a smaller O²⁺ thermal-diffusion coefficient, the latter being sensitive to the ratio n(H⁺)/n(O⁺).     

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.     

Pc 1–2 observations of heavy ion effects by synchronous satellite ATS-6

Ion cyclotron waves generated in the magnetosphere by the ion cyclotron instability of protons are thought to be the origin of Pc 1–2 geomagnetic pulsations. Propagation characteristics of these waves have been measured using ATS-6 synchronous satellite magnetometer wave data. Of particular interest are the wave spectra, polarization properties, and wave diagnostics; all are characteristic of propagation in a cool ambient magnetospheric plasma containing He⁺ and O⁺ heavy ions.     

Effects of greatly increased O loss in the ionospheric F-region

A model of the ionosphere and plasmasphere is used to study some of the signatures of an idealized SAID (subauroral ion drift) event in the nightside ionosphere. A closed subauroral tube of plasma is considered under solar maximum atmospheric conditions and the westward velocity of 3 km s⁻¹ persists for 30 min. By pursuing, in turn, calculations in which plasma diffusion is suppressed and in which chemical loss of O⁺ is suppressed, the signatures of O⁺ chemistry alone and of field-aligned diffusion alone during the SAID event can be elucidated. Both chemical loss and transport contribute to the decay of the F-layer. Results from full calculations (including both chemistry and transport) demonstrate strong chemical-transport interaction.     

Formation and structure of neutrino astronomical objects

Neutrinos with non-zero mass could gather to form a new kind of astronomical bodies: the Neutrino Astronomical Objects (NAO). We have investigated the mechanism of their formation and the relation of this formation to that of the galaxies, ascertained their e, p, He⁴ content, whose presence should produce a series of observable effects. NAOs are a peculiar kind of heavenly bodies with many new properties. They have a linear size of the order of 100 pc, a total neutrino content of the order of 10¹⁴M and an e, p, He⁴ content of the order of 10⁹M.     

Transient emissions on the wavelength of helium I, 5876 Å recorded during auroral break-up

During the break-up phase of two strong auroral events, emissions of short duration on the wavelength of He I, 5876 Å have been observed. These records are accurate within 0.5 Å and intensities of up to 120 R have been measured. This high value is not consistent with the theoretical limit suggested by other authors. Simultaneous observations of H, 6563 Å show that the He I and H emissions are not closely related to each other with time, which may be one reason for explaining the discrepancy with the predicted intensity derived from observed ratios of He⁺⁺/H⁺ in the solar wind. The emission on 5876 Å has only been detected at the lowest border of very intense ray bundles towards north but not yet in auroral arcs and diffuse glow. It is suggested that two principally different helium events in aurora may be observable, one resulting in a low level He emission lasting for longer time and another in a stronger He emission of short duration. The observational difficulties caused by the presence of OH bands are discussed.     

A study of F2 region night-time vertical ionization fluxes at millstone hill

Vertical fluxes of ionization in the F2 region have been measured by the incoherent scatter technique over Millstone Hill in 1969. The results obtained near midnight for the region above h_maxF2 have been examined to determine whether there is a significant flux of ionization from the magnetosphere to the ionosphere that serves to maintain the F-layer. It is found that H⁺ ions are a minor constituent over the altitude range in which useful measurements can be made, so that any conclusion must rest upon properly interpreting the observed O⁺ fluxes. By selecting periods when the layer did not appear to be decaying rapidly it was hoped to find cases where the O⁺ flux did not vary with altitude in the range 500 h 800 km (i.e. where losses are unimportant), since this would imply that the flux is of magnetospheric origin.

While three cases exhibited this behaviour, the majority exhibited a decrease in the O⁺ flux with height, indicating that the layer was descending. Attempts to correct for this were made, and the average flux from the magnetosphere was estimated as 3 × 10⁷ el/cm²/sec. This is in fair agreement with other recent estimates, and implies that at this latitude the ionosphere is not maintained solely by the magnetospheric flux. Moreover, large increases in flux that could give rise to nocturnal increases in the total content of the layer do not appear to have been seen.     

Laboratory studies of helium ion loss processes of interest in the ionosphere

Preliminary measurements of reaction rates for loss of thermal helium ions in reaction with molecular oxygen and nitrogen establish rather conclusively that the helium ions in the ionosphere will be lost in reaction with molecular nitrogen rather than molecular oxygen in contrast to previous assumptions based on theoretical considerations. The rate of thermal He⁺ loss in reaction with N₂ is measured to be 1·2 ± 0·3 times the rate for reaction with O₂. This conclusion is of considerable significance to atmospheric physics because the oxygen loss process contained the possibility of leading to terrestrial helium escape and therefore the possibility of a steady state helium atmosphere. The nitrogen loss process does not have this possibility so that no satisfactory mechanism has yet been proposed which will allow a steady state helium atmosphere. The interpretation of recent atmospheric helium ion profiles obtained by rocket borne mass spectrometers appear to be inconsistent with the laboratory loss rate constants and current atmospheric theory.     

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O₂⁺, NO⁺ and N₂⁺ using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [NO⁺]/[O₂⁺] ratio. In the region of most intense electron fluxes (20 erg cm⁻² sec⁻¹) at 280 km, the N density is found to be between 2 and 7 × 10⁷ cm⁻³. The resulting N densities are found to account for approx. 60% of the production of N⁺ through electron impact on N and the resonant charge exchange of O⁺(²P) with N(⁴S). This reaction also provides a significant source of O(¹S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O⁺(²P).     

Chemistry of the nightside ionosphere of Venus

We have constructed a one-dimensional model of the nightside ionosphere of Venus in which it is assumed that the ionization is maintained by day-to-night transport of atomic ions. Downward fluxes of O⁺, C⁺ and N⁺ in the ratios measured on the dayside at high altitudes are imposed at the upper boundary of the model (about 235 km). We discuss the resulting sources and sinks of the molecular ions NO⁺,CO⁺,N₂⁺,CO₂⁺ and O₂⁺. As the O⁺ flux is increased, the peak density of O⁺ increases proportionally and the altitude of the peak decreases. The O₂⁺ peak density is approximately proportional to the square root of the O⁺ flux and the peak rises as the O⁺ flux increases. NO⁺ densities near the peak are relatively unaffected by changes in the O⁺ flux. If the ionosphere is maintained mostly by transport, the ratio of the peak densities of O⁺ and O₂⁺ indicates the downward flux ofO⁺, independent of the absolute magnitudes of the densities. The densities of mass-28 ions are, however, still considered to be the most sensitive indicator of the importance of electron precipitation. We examine here the inbound and outbound portions of six early nightside orbits with low periapsis and use data from the Pioneer Venus orbiter ion mass spectrometer, the retarding potential analyzer and the electron temperature probe to determine the relative importance of ion transport and electron precipitation. For most of the orbits, precipitation is inferred to be of low to moderate importance. Only for orbit 65, which was the first nightside orbit published by Taylor et al. [J. geophys. Res. 85, 7765 (1980)] and for the inbound portion of orbit 73 does the ionization structure appear to be greatly affected by electron precipitation.     

Detection of a strong transient blueshifted absorption component in the β Pictoris disc

We present high-resolution spectra (1.0 km s⁻¹ FWHM) of the circumstellar Ca K line towards β Pictoris obtained on 1997 June 19 and 20. On the former date a strong absorption component was found at a heliocentric velocity of v _helio = +8 km s⁻¹, that is blueshifted by 14 km s⁻¹ with respect to the main, 'stable', circumstellar component at v _helio = +22 km s⁻¹. To our knowledge, this is the first detection of a blueshifted Ca  ii component with a strength comparable to the more frequently observed redshifted events. On the following night a blueshifted component was still present, but its strength had decreased significantly; in addition, a strong redshifted component had appeared at v _helio = +54 km s⁻¹ which was absent on the previous night. The implications of these observations for the evaporating 'comet' model of spectral variations in the β Pictoris disc are discussed.     

C-type shocks in the interstellar medium: profiles of CH+ and CH absorption lines

We have computed optical absorption-line profiles of CH⁺ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C⁺(H₂, H)CH⁺. Whilst CH⁺ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH⁺₃(e⁻, H₂)CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH⁺ and CH line profiles is found to be no more than approximately 2 km s⁻¹, which is smaller than has previously been assumed. We also investigate OH and HCO⁺, finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model.     

Deactivation of N2A Σumolecules in the aurora

Recent rocket observations of the N₂ V-K (Vegard-Kaplan) system in the aurora have been reinterpreted using an atmospheric model based on mass spectrometer measurements in an aurora of similar intensity at the same time of year. In contrast to the original interpretation, we find that population by cascade from the C³Π_u and B³Π_g states in the A³Σ_u⁺v=0,1 levels, as calculated using recently measured electron excitation cross sections, accurately accounts for the observed relative emission rates (IV-K/12PG_0.0). In addition there is no need to change the production rate of A ³ Σ _u⁺ molecules relative to that of C³Π_uv=0 as a function of altitude in order to fit the profile of the deactivation probability to the atmospheric model. Quenching of A ³ Σ _u⁺ molecules at high altitudes is dominated by atomic oxygen. The rate constants for the v=0 and v=1 levels are 8 × 10⁻¹¹ cm³ sec⁻¹ and 1.7 × 10⁻¹⁰ cm³ sec⁻¹ respectively, as determined using the model atmosphere mentioned above. Recent observations with a helium cooled mass spectrometer suggest that conventional mass spectrometer measurements tend to underestimate the atomic oxygen relative concentration. The rate coefficients may therefore be too large by as much as a factor of 3. Below 130 Km we find that it is possible to account for the deactivation in bright auroras by invoking large nitric oxide concentrations, similar to those recently observed mass spectrometrically and using a rate constant of 8 × 10⁻¹¹ cm³ sec⁻¹ for both the v=1 levels. This rate constant is very nearly the same as that measured in the laboratory (7 × 10⁻¹¹ cm³ sec⁻¹). Molecular oxygen appears not to play a significant role in deactivating the lower A ³ Σ _u⁺ levels.     

The winter anomaly in ionospheric absorption and the D-region ion chemistry

A detailed analysis of the D-region ion composition measurements performed by Zbinden et al. (1975), during a winter day of high ionospheric absorption, has been carried out. The study examines the interactive mesosphere-D-region processes which occur in such anomalous conditions and their implication for water cluster ion chemistry. Two clustering regimes for NO⁺ have been observed in the data. Association with N₂ is identified as the dominant process below 76 km. Between 76 and 78 km altitude the effective loss rate of NO⁺ drops by two orders of magnitude. Above 77 km, the three-body reaction NO⁺ + CO₂+M→NO⁺CO₂+M seems to be the main NO⁺ loss. A mesospheric temperature profile could be derived from the ion composition data. This indicates the presence of a strong inversion above 76 km altitude. The wavelike structure obtained, is shown to be consistent with in situ winter temperature measurements. The sharp suppression of the N₂ association reaction could, thus, be explained by an increase in the collisional break-up of the NO⁺N₂ ion because of the enhanced temperature. In conclusion, our study indicates that, besides the increase in the production of NO⁺ and O₂⁺, due to an enhancement in the minor ionizable constituents, an additional thermal mesosphere-D-region interaction seems necessary to explain this winter anomalous ion composition data.     

A measurement of the O2 ultra-violet nightglow emission

New measurements of the Herzberg I emission height profile in the night airglow are reported and indicate a peak emission height near 96 km in agreement with previous measurements. Using an atomic oxygen concentration profile determined from the oxygen green line profile measured on the same rocket it is concluded that the O₂(A³Σ_u⁺) state is not excited in the direct three body recombination of atomic oxygen. It is suggested that the excitation mechanism is a two step process, similar to the Barth mechanism for the atomic oxygen green lineand that the excited intermediate state is C³Δ_u.     

Twilight effects of solar ionizing radiation

The absorption of solar ionizing radiation during twilight is investigated. Ion production rates are obtained as a function of altitude and twilight intensities and altitude profiles of emissions arising from the fluorescence of solar ionizing radiation are calculated for various solar depression angles. For an atmosphere with an exospheric temperature of 750°K, the predicted overhead intensity from fluorescence of the O⁺(²P — ²D) lines at 7319–7330 diminishes from 175 R at dusk to 10 R at a solar depression angle of 10°. The predicted overhead intensities from fluorescence of the N₂⁺ Meinel and first negative systems are respectively about 175 R and 20 R at dusk diminishing to respectively 1.5 R and 0.1 R at a solar depression angle of 10°.

It is suggested that a charge transfer reaction of O⁺²D in N₂ is a significant source of N₂⁺ ions. This reaction offers a possible explanation for the high apparent rotational temperatures in the first negative system observed by Broadfoot and Hunten. Other excitation and ionization mechanisms are briefly discussed.     

Calculated spectra for HeH+ and its effect on the opacity of cool metal-poor stars

The wavelength and Einstein A coefficient are calculated for all rotation–vibration transitions of  ⁴He¹H⁺, ³He¹H⁺, ⁴He²H⁺  and  ³ He²H⁺  , giving a complete line list and the partition function for  ⁴HeH⁺  and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH⁺ has a visible but small effect for very low densities  (ρ≤ 10⁻¹⁰ g cm⁻³)  , at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH⁺ becomes important for higher densities  (ρ≤ 10⁻⁶ g cm⁻³)  , its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.     

On the diurnal period variation of midlatitude ULF pulsations

Magnetometer studies of the periods of mid-latitude ULF pulsations have produced conflicting results on the variation of the pulsation periods with both latitude and local time. Since the mid-latitude geomagnetic field is not expected to be significantly distorted by the solar wind, the observed diurnal period variations should be determined by changes in the ambient plasma density. We have applied a physically realistic plasmasphere model to the determination of pulsation eigenperiods over a 24-h interval at L=2.3 (appropriate to Wellington, New Zealand). The resulting model pulsation eigenperiods are largest during the day, with minimum and maximum values at 05.00 and 18.00 L.T. respectively. The model predicts a general increase in the eigenperiods during the replenishment of the protonosphere after a period of geomagnetic activity.