A theoretical study of downward H+ field-aligned velocities at mid-latitudes

This paper examines the magnitude of downward H⁺ field-aligned velocities at mid-latitudes. For the isothermal, collisionless, steady-state case an analytical form is derived for the critical temperature, below which the plasma temperature must lie for there to be a possibility of supersonic H⁺ flow. Some simple equations are presented which yield velocity profiles and an aid to the understanding of energy distribution in the H⁺ gas. Very little gravitational potential energy is converted to kinetic energy. For the general case the situation is far more complicated, but we note in particular the importance of the O⁺ contribution to the electrostatic field. For the simpler case the flow is always subsonic regardless of the plasma temperature and it appears unlikely that supersonic flow will occur in the more general case.     

The topside equatorial ionosphere during daytime: Elevated plasma temperatures,the transequatorial O+ breeze and the dynamics of minor ions

Steady-state calculations are performed for the daytime equatorial F2-region and topside ionosphere. Values are calculated of the electron and ion temperatures and the concentrations and field-aligned velocities of the ions O⁺, H⁺ and He⁺. Account is taken of upward $\text{E × B}$ drift, a summer-winter horizontal neutral air wind and heating of the electron gas by thermalization of fast photoelectrons.The calculated plasma temperatures are in accord with experiment: at the equator there is an isothermal region from about 400–550 km altitude, with temperatures of about 2400 K around 800 km altitude. The transequatorial O⁺ breeze flux from summer to winter in the topside ionosphere is not greatly affected by the elevated plasma temperatures. The field-aligned velocities of H⁺ and He⁺ depend strongly on the O⁺ field-aligned velocity and on the presence of large temperature gradients. For the minor ions, ion-ion drag with O⁺ cannot be neglected for the topside ionosphere.     

Regions of He+ dominance in the high-latitude topside ionosphere

Observations of the occurrence of He⁺ dominance in the topside ionosphere are discussed. An earlier model of the behaviour of high-latitude H⁺ and O⁺ thermal plasma (Quegan et al., 1982) is extended to include He⁺ as a major ion. Calculations using the extended model show that plasma convection is likely to play a key rôle in producing regions of He⁺ dominance. Suggested conditions for He⁺ dominance are listed and their applicability to observed He⁺ behaviour is discussed.     

On the diurnal and seasonal variations of H+, He+, N+, O+, and Ne at 1400-km altitude

The diurnal and seasonal variations of H⁺, He⁺, N⁺, O⁺ and Ne are analyzed at 1400-km altitude. Using longitudinally averaged observations of ISIS-2 (April 1971 to December 1972), the ion and electron densities are decomposed via spherical harmonics and Fourier series into time-independent, seasonal and diurnal terms. The time-independent terms of H⁺ and He⁺ show a plateauor trough-like structure at medium to low latitudes and a strong decrease towards the poles; N⁺ and O⁺, on the other hand, yield an almost inverse picture with a density increase at high latitudes. All constituents, except He⁺, show at polar latitudes an enhancement during local summer conditions and a depletion during local winter conditions; He⁺, however, exhibits a winter bulge and a density minimum during local summer. The diurnal variations are strongly latitude dependent; while the amplitudes (relative) of H⁺, He⁺, and Ne are rather small, the heavier ions N⁺ and O⁺ show a deep minimum early in the morning and a high but flat maximum during daytime.     

Diffusion coefficients for three major ions in the topside ionosphere

Published experimental data on ion composition in the topside ionosphere are examined. For certain features (the light ion trough, the high-latitude trough, the high-latitude hole and the mid-latitude total ion concentration trough) it is pointed out that the number of major ions present may be 3 or more. Transport equations derived by Schunk and co-workers are extended to include the case of three major ions in the topside ionosphere. Specific calculations are made for the O⁺, H⁺ and He⁺ ions and the behaviour of the diffusion coefficients is discussed. From a model of the high-latitude ionization hole, described by Heelis et al., representative concentration and temperature profiles are obtained. These profiles are used to demonstrate further the behaviour of the ion diffusion coefficients.     

The dissociative recombination of O2+ in the ionosphere

Aeronomical determinations of the dissociative recombination reaction rate coefficient for O₂⁺, α, depend directly on a knowledge of the rate coefficient for the charge exchange of O⁺ with O₂, k. A re-evaluation of the aeronomical determination of α using Atmosphere Explorer satellite data is necessary in the light of a subsequent laboratory measurement of k. The results reported here are in reasonable agreement with laboratory determinations to within the uncertainty of the analysis for night-time conditions. However, for data obtained under sunlit conditions the aeronomical determination differs significantly from the laboratory measurements. The results imply that the state of the O₂⁺ molecule resulting from the major thermospheric processes requires further examination.     

Quenching of O+(2D) by electrons in the thermosphere

A major loss process for the metastable species, O⁺(²D), in the thermosphere is quenching by electrons

$\text{O}{}^{\mathrm{+}}\text{(}{}^2\text{D) + e → O}{}^{\mathrm{+}}\text{(}{}^4\text{S) + e}$

.To date no laboratory measurement exists for the rate coefficient of this reaction. Thermospheric models involving this process have thus depended on a theoretically calculated value for the rate coefficient and its variation with electron temperature. Earlier studies of the O⁺(²D) ion based on the Atmosphere Explorer data gathered near solar minimum, could not quantify this process. However, Atmosphere Explorer measurements made during 1978 exhibit electron densities that are significantly enhanced over those occurring in 1974, due to the large increases that have occurred in the solar extreme ultraviolet flux. Under such conditions, for altitudes ? 280 km, the electron quenching process becomes the major loss mechanism for O⁺(²D), and the chemistry of the N⁺₂ ion, from which the O⁺(²D) density is deduced, simplifies to well determined processes. We are thus able to use the in situ satellite measurements made during 1978 to derive the electron quenching rate coefficient. The results confirm the absolute magnitude of the theoretical calculation of the rate coefficient, given by the analytical expression k(T_e) = 7.8 × 10^?8 (T_e/300)^?0.5cm³s^?1. There is an indication of a stronger temperature dependence, but the agreement is within the error of measurement.     

Recombination of electrons with H3+ and H5+ ions

The dissociative recombination coefficients α for capture of electrons by H₃⁺ and H₅⁺ ions have been determined as a function of electron temperature T_e using a microwave afterglow-mass spectrometer apparatus. At ion and neutral temperatures T_u+ = T_n = 240 K, the coefficient α (H₃⁺) is found to vary slowly with T_e at first, decreasing from 1.6 × 10^?7 cm³/s at T_e = 240 K to 1.2 × 10^?7 cm³/s at T_e = 500 K, thereafter falling as T_e^?1 over the range 500 K ? T_e, ? 3000 K. These results, which have a ± 20% uncertainty, agree satisfactorily over the common energy range (0.03–0.36 eV) with the recombination cross sections determined in merged beam measurements by Auerbach et al. At T₊ = T_n = 128 K, the coefficient α(H₅⁺) is found to be (1.8 ± 0.3) × 10^?6 [T_e(K)/300]^?0.69 cm³/s over the range 128 K ? T_e ? 3000 K, with a more rapid decrease, as T_e^?1, between 3000 K and 5500 K. The implications of these results for modelling planetary atmospheres and interstellar clouds are briefly touched on.     

Diffusion and heat flow equations for the mid-latitude topside ionosphere

For application to the mid-latitude topside ionosphere, we have derived diffusion and heat flow equations for a gas mixture composed of two major ions, electrons and a number of minor ions. These equations were derived by expanding the velocity distribution of each constituent about its 13 lower order velocity moments. As a consequence, each constituent was allowed to have its own temperature and drift velocity. The restriction to mid-latitudes results because we have assumed that the species temperature and drift velocity differences were small. In deriving the diffusion and thermal conduction equations, we have discovered some new transport effects. For the major ions, we have found that: (1) a temperature gradient in either gas causes thermal diffusion in both gases; (2) a temperature gradient in either gas causes heat to flow in both gases; and (3) a relative drift between the major ion gases induces a heat flow in both gases. Similar transport effects have also been found for the minor ions.     

Changes in thermospheric composition inferred from twilight O+(2P) emission

Measurements of the twilight enhancement of airglow emission from O⁺(²P) near 7325 Å reveal major changes which accompany geomagnetic activity, no significant distance between evening and morning and an increase in brightness paralleling the approach to solar maximum. The principal source for O⁺(²P) is direct photoionization from O(³P) but at low solar activity there appears to be a contribution from another source in early twilight which may be local photoelectron ionization into O⁺(²P). The geomagnetic and solar effects appear to reflect changes in the O and N₂ density in the thermosphere; ground based twilight measurements of O⁺ emissions thus provide a simple means for monitoring thermospheric structure from 300 km to ～ 500 km at solar minimum and to ～600 km at solar maximum.     

Plasmaspheric hiss observed in the topside ionosphere at mid- and low-latitudes

Latitudinal characteristics of ELF hiss in mid- and low-latitudes have been statistically studied by using ELF/VLF electric field spectra (50 Hz-30 kHz) from ISIS-1 and -2 received at Kashima station, Japan from 1973 to 1977. Most ISIS ELF/VLF data observed in mid- and low-latitude include ELF hiss at frequencies below a few kHz. The ELF hiss has the strongest intensity among VLF phenomena observed by the ISIS electric dipole antenna in mid- and low-latitudes, but the ELF hiss has no rising structure like the chorus in the detailed frequency-time spectrum. The ELF hiss is classified into the steady ELF hiss whose upper frequency limit is approximately constant with latitude and the ELF hiss whose upper frequency limit increases with latitude. These two types of ELF hiss occur often in medium or quiet geomagnetic activities. Sometimes there occurs a partial or complete lack of ELF hiss along an ISIS pass.Spectral shape and bandwidth of ELF hiss in the topside ionosphere are very similar to those of plasmaspheric hiss and of inner zone hiss. The occurrence rate of steady ELF hiss is about 0.3 near the geomagnetic equator and decreases rapidly with latitude around L = 3. Hence it seems likely that ELF hiss is generated by cyclotron resonant instability with electrons of several tens of keV in the equatorial outer plasmasphere beyond L = 3.Thirty-seven per cent of ELF hiss events received at Kashima station occurred during storm times and 63% of them occurred in non-storm or quiet periods. Sixty-seven per cent of 82 ELF hiss events during storm times were observed in the recovery phase of geomagnetic storms. This agrees with the previous satellite observations of ELF hiss by search coil magnetometers. The electric field of ELF hiss becomes very weak every 10 s, which is the satellite spin period, in mid- and low-latitudes, but not near the geomagnetic equator. Ray tracing results suggest that waves of ELF hiss generated in the equatorial outer plasmasphere propagate down in the electrostatic whistler mode towards the equatorial ionosphere, bouncing between the LHR reflection points in both the plasmaspheric hemispheres.     

The influence of neutral winds on He+ distributions in the equatorial ionosphere

The effects of F-region neutral winds on the distribution of He⁺ in the equatorial ionosphere have been examined using a theoretical model and an observational data set. It is shown by the model that components of neutral wind in the magnetic meridian up to only 50 m s^? can produce He⁺ gradients in the northern and southern sectors of a flux tube that differ by more than 80%. This is associated with interhemisphere transport velocities of He⁺ as large as 15 m s^?1 at 800 km. A substantial latitude gradient in the He⁺ distribution across the dip equator also results from the redistribution of He⁺ The changes in the He⁺ concentration at the dip equator and the latitude distribution of He⁺ in response to different neutral wind components is determined from the model and used to construct longitude distributions of He⁺ to compare with observations made at equinox. Good agreement between the calculations and observations is obtained both at the geographic and geomagnetic equators using the relationship between neutral winds, interhemispheric transport velocity and He⁺ concentration derived from the model. If these relationships can be extrapolated to accommodate the different conditions expected during solstice, we can also discuss the He⁺ distributions expected during this season.     

Dissociative recombination of electrons with NO+ ions

Resonant scattering theory is applied to the calculation of the rate of dissociative recombination in e-NO⁺ collisions. For ground state ions at 300 K, the recombination rate is 4.3 × 10^?7 cm³ s^?1, in good agreement with the afterglow data. At higher electron temperatures, the calculated rate lies between the values measured in afterglow and trapped-ion experiments. A possible explanation of this discrepancy is offered. For vibrationally excited ions, the recombination coefficient is significantly lower than that of the ground state.     

Triple splitting with the F2-region of the ionosphere at high and mid-latitudes

The occurrence of the third (z-ray) component of the F2-trace on ionograms is investigated at high- and mid-latitudes. Diurnal variations show a systematic shift, with magnetic inclination, of the time of maximum occurrence. Seasonal variations show a winter maximum, and an inverse sunspot-cycle relationship exists. Maximum occurrence appears between a magnetic inclination of 70° and 80° with a fall-off either side.

Evidence is presented to suggest a z-ray association with “Spread-F” fronts, and a possible mechanism for the recording of the z-ray trace at the transmitter site is described. This involves longitudinal propagation of the o-mode at its normal reflection level, coupling at this point, and ultimate reflection for the z-ray mode as a result of sloping ionization contours belonging to “Spread-F” fronts extending in directions perpendicular to the magnetic meridian.

An association with V.L.F. emissions (“dawn-chorus”) is discussed.     

Aeronomical determination of the efficiency of O1D) production by dissociative recombination of O2+

Simultaneous measurements of the 6300 Å airglow intensity, the electron density profile, and F-region ion temperatures and vertical ion velocities taken at the Arecibo Observatory in March 1971 are utilized in the height integrated continuity equation to extract the number of photons'of 6300 Å emitted per recombination. After accounting for quenching of $\text{O}\text{(}{}^1\text{D}\text{)}$ and the electrons lost via NO⁺ recombination, the efficiency of $\text{O}\text{(}{}^1\text{D}\text{)}$ production by the dissociative recombination of O₂⁺ is determined to be 0.6 ± 0.2 including cascading from the $\text{O}\text{(}{}^1\text{S}\text{)}$ state. The uncertainty includes both random measurement errors and estimates of possible systematic errors.     

Additional ultra-high-resolution observations of Ca+ ions in the local interstellar medium

We present ultra-high-resolution (0.35 km s⁻¹ FWHM) observations of the interstellar Ca K line towards seven nearby stars. The spectral resolution was sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions ( b values). Absorption components with velocities similar to those expected for the Local Interstellar Cloud (LIC) and the closely associated 'G cloud' were identified towards six of the seven stars. However, in most cases the b values deduced for these components were significantly larger than the b  ≈ 2.2 km s⁻¹ (i.e. T _k ≈ 7000 K, v _t ≈ 1 km s⁻¹) expected for the LIC, and it is argued that this results from the presence of additional, spectrally unresolved, components having similar velocities and physical conditions. For two stars (δ Vel and α Pav) we detect interstellar components with much smaller b values (1.1 ± 0.3 and 0.8 ± 0.1 km s⁻¹, respectively) than are expected for low-density clouds within the Local Bubble. In the case of the narrow α Pav component, we also find an anomalously large Na  i /Ca  ii column density ratio, which is indicative of a relatively high density. Thus it is possible that, in addition to LIC-type clouds, the local interstellar medium contains a population of previously undetected cooler and denser interstellar clouds.     

Composition and temperatures of the topside ionosphere over Arecibo

Results of analysis of about 150 autocorrelation functions are presented for the period from about 2300 hr on 5 October to about 1200 hr on 7 October 1967. A large percentage concentration of helium ions are observed. It reaches a value as high as 50 per cent with a maximum at around 800 km. Downward heat fluxes deduced from the temperature variations yield a value of about 2–2.5 × 10⁹ eV cm^?2 sec^?1 during the period 1200–1600 hr and a value of about 1.5 × 10⁸ eV cm^?2 sec^?1 during the period 0100–0400 hr at night. These agree well with other measurements. The O⁺ ions are found not to be in diffusive equilibrium, and from the O⁺ fluxes and the electron density profiles, the O⁺ drift velocity has been estimated. It is found that the speed can be as high as 1–5 × 10³ cm sec^?1 even at altitudes as high as 700 km.     

Turbulent transport and heating in the auroral plasma of the topside ionosphere

Using plasma parameters from a typical stormtime ionospheric energy balance model, we have investigated the effects of plasma turbulence on the auroral magnetoplasma. The turbulence is assumed to be comprised of electrostatic ion cyclotron waves. These waves have been driven to a nonthermal level by a geomagnetic field-aligned, current-driven instability. The evolution of this instability is shown to proceed in two stages and indicates an anomalous increase in field-aligned electrical resistivity and cross-field ion thermal conductivity as well as a decrease in electron thermal conductivity along the geomagnetic field. In addition, this turbulence heats ions perpendicular to the geomagnetic field and hence leads to a significant ion temperature anisotropy.     

The broadening of strong lines of Ca+, Mg+ and Ba+ by collisions with neutral hydrogen atoms

The theory of Anstee &38; O'Mara is extended to the line broadening of transitions of singly ionized atoms by collisions with neutral hydrogen atoms. The theory is used to calculate broadening cross-sections for strong lines of singly ionized calcium, magnesium and barium. The broadening cross-sections calculated are compared with both theoretical and empirical results of other workers.