Ion Composition Measurements Research Articles

The O I (λ5577 Å) airglow: Observations and excitation mechanisms

Analysis of the O I (λ5577 A) airglow observations conducted on board the Atmosphere Explorer C satellite yields the following results when combined with simultaneous measurements of ion and neutral composition. A volume emission rate profile derived after sunset requires the production of an O(¹S) atom in 8% of the O2+ recombinations. Photoelectron impact on atomic oxygen and the dissociative recombination of O2+ are important sources of O(¹S) in the dayglow; however, the airglow measurements require a large additional production of O(¹S) below 200 km in addition to these. The reaction N + O2+ → NO+ + O(¹S) can provide the missing O(¹S) source if the rate coefficient is near 2.5 × 10−11 cm³ s−1.

Satellite measurements of ion composition and temperatures in the topside ionosphere during medium solar activity

Information on both the ion density and temperature is obtained from the analysis of Retarding Potential Analyser data from OGO-4 and Explorer-31 satellites. Results obtained from September–October 1967 data during medium solar activity in the altitude range of 700–2000 km are presented. An attempt is made to describe the major altitudinal variations of ion densities and temperatures in middle and low latitudes. The transition heights, where the heavier and lighter ions are equal, are found to be about 1600 and 1300 km in the middle and low latitudes respectively for daytime and 700 km at night for middle latitudes. Based on the observed data and using diffusive equilibrium as a first order approximation, topside ionospheric composition models are given for the medium solar activity.

NO + and water cluster ions in the D-region

A positive-ion model for the daytime D-region is used to examine the formation of water cluster ions when NO + is the major ion produced by photoionization. Particular attention is paid to the role of NO. +CO 2 and NO. +N 2 in the NO + hydration processes and to the importance of collisional dissociation of these ions. It is found that with rate coefficients for these dissociation processes based on laboratory measurements, the computed results are not consistent with measurements of ion composition and electron concentrations.

A study of F2 region daytime vertical ionization fluxes at Millstone Hill during 1969

A study has been undertaken of the vertical fluxes of ionization in the F2 region over Millstone Hill ( L = 3.2) utilizing incoherent scatter measurements of electron density, electron and ion temperatures, ion composition and vertical velocity, made over 24-hr periods twice per month during 1969. The paper presents the results for all these parameters on five representative days, and discusses the distribution of the vertical flux observed during the daytime at other times during the year. Near noon the downward flux reached a peak near 300 km with an average value of ∼3 × 10 9 el/cm 2/sec in winter and ∼1.6 × 10 9 el/cm 2/sec in summer. The difference is thought to be real and be caused by the higher loss rates prevailing in summer. Above 550 km there is usually a transition to upward flux, which appears to be fully established by 700 km and has an average value of the order of 5 × 10 7 l/cm 2/sec. From ion composition measurements, it appears that this flux is carried almost entirely by O + ions to at least ∼900 km, as the H + ion concentration is small (<2% at ∼775 km altitude) in this region by day. While the value of the escape flux appears in fair agreement with theoretical estimates of the limiting flux for this portion of the sunspot cycle, the extremely low H + concentrations do not appear to be in accord with existing models. The diurnal variation of the upward flux through 650 km exhibits an abrupt onset close to the time of sunrise at the 200 km level ( χ = 103°). A reversal to downward flux usually begins before sunset, often in the early afternoon.

In situ measurements of plasma drift velocity and enhanced NO+ in the auroral electrojet by the Bennett Spectrometer on AE‐C

Simultaneous measurements of ion composition and plasma drift velocity by the Bennett mass spectrometer on the Atmosphere Explorer‐C satellite reveal a direct correlation between enhancements in NO+ concentration and ion drift velocity in the southern auroral oval. Low altitude (137 to 250km) data obtained between 1700 and 2400 hr magnetic local time on October 22, 1974 reveal a region of westward plasma flow at velocities up to 1.3km/s between 62° and 68° invariant latitude, with corresponding NO+ enhancements of up to a factor of 20. A narrow region of reverse flow at ∼0.9km/s was also measured. These drift observations are consistent with convective flow patterns derived from electric field measurements, and their correlation with NO+ appears to support the suggestion that NO+ enhancements would be expected in regions of drift owing to the dependence on ion energy of the reaction O+ + N2 → NO+ + N.

Mass spectrometer measurements of the positive ion composition in the D- and E-regions of the ionosphere

On 14 December 1971, during the maximum of the Geminid Meteor Shower, the positive ion composition was measured in the D- and E-regions above Sardinia. The payload was launched at 12:11 UT, and measurements were made between 68.5 and 152 km altitude. A magnetic sector type mass spectrometer with dual collector and a liquid helium cryopump was used. The instrument covered the mass range from 11 to 73 AMU and had a resolution at the 1 % level of M ΔM = 60 . In the E-region two distinct metal ion layers were observed, centred at 95 and 119 km, respectively. In the lower layer Fe + and Mg + were the most abundant metal ions, and in the upper layer Si + was dominant. Si + ions were conspicuously absent in the lower layer ( Si + Mg + < 2 × 10 −3 ). This particular behaviour of Si could be due to the inability of atomic oxygen to reduce SiO, whereas in the upper layer Si +ions might be formed directly by the charge rearrangement reaction SiO + O + → Si + + O 2. In addition, Na +, Al +, K +, Ca +, Ti +, Cr +, Ni + and Co + were also identified. The metal oxide ions AlO + and SiO + were detected, and probably also MgO + and SiOH +. The concentrations of NO + and O 2 + show a deep minimum at the maximum of the lower metal ion layer. A very high neutral metal density of 6 × 10 7 cm −3 would be required to explain this feature as resulting from charge transfer reactions between the molecular and metal ions Such a high metal density is in contradiction to direct measurements and to cosmic dust influx rates. The isotopic ratios of Mg +, Si +, and of the major isotopes of Fe + and Ni + were measured, some of them with an accuracy of a few per cent ( 25 Mg + 24 Mg + = 0.124 ± 0.006 ; 26 Mg + 24 Mg + = 0.139 ± 0.008 ; 29 Si + 28 Si + = 0.050 ± 0.004 ; 54 Fe + 56 Fe + = 0.069 ± 0.005 ; 57 Fe + 56 Fe + = 0.029 ± 0.004 ; 60 Ni + 58 Ni + = 0.31 ± 0.12 ). The isotopic ratios agree within the experimental errors with the corresponding terrestrial ratios, thus giving evidence that these elements have the same isotopic composition in the Geminid meteors as in the Earth's crust, in chrondrites, and in lunar material. In the D-region the ions Na +H 2O, Na +(H 2O) 2, NaO + and NaOH + were tentatively identified. Below 95 km altitude the relative abundances of the ions 32 +, 33 + and 34 + deviate from the values expected for molecular oxygen isotopes. Their abundances can not be explained by the presence of S-ions only, and we conclude that HO 2 + and H 2O 2 + are present. The ion density profiles of the major D-region constituents show some remarkable deviations from typical D-region conditions. These deviations are related to the winter anomaly in ionospheric absorption observed over Spain during the launch day, and our data represent the first ion composition measurements during such conditions. In particular, H +(H 2O) 2 is the major ion only up to 77 km, and at 80 km altitude the NO + concentration exceeds the total water cluster ion density by almost two orders of magnitude. An increase of the mesospheric NO, O 3 and O concentrations as well as of the O H 2 O ratio could explain the observed ion profiles. The low NO + O 2 + ratios of approximately unity measured in the E-region are in agreement with a strong downward transport of NO and/or O into the mesosphere during the launch day. A simple four-ion model was used to interpret our D-region data. The calculated neutral NO concentration increases from about 2 × 10 7 cm −3 at 85 km to 5 × 10 7 cm −3 at 80 km. In addition, evidence for an increased O 2 + production rate above 83 km was found, probably due to an enhanced O 3 concentration. We conclude that our data strongly support vertical transport of minor neutral consituents as cause of the winter anomaly.

A discussion of possible negative ion detachment mechanisms in the sunrise D region

Many recent measurements of photochemical rate constants and atmospheric ion composition have changed our understanding of D‐region sunrise processes. In this paper several new sunrise mechanisms are suggested in the light of the recent data. It is concluded that very stable hydrated negative ions will probably be converted by chemical reaction at sunrise into less stable hydrates which can be photodetached.

Ion composition in an IBC class II aurora: 2. Model

Relatively good agreement is obtained between the ion composition measurements for a steady class II aurora and a simple auroral model. Better agreement would require a more precise neutral model for N2, O2, O, and NO. Auroral ion composition model computations were performed by using the mean Cira (1965) and the Jacchia (1971) 800°K thermopause neutral models as extremes. The computations suggest that the true N2, O2, and O concentrations lie between these two models in contrast to a previous study of a class I aurora, which suggested a higher O/O2 ratio than either model. The required O/O2 ratio is found to increase in proportion to the fraction of atomic oxygen ions formed in metastable states. The peak NO concentration, about 4×108 cm−3; near 110 km, is seen to be much less sensitive to the major neutral gas model employed in the computations. The deduced nitric oxide profile agrees with the profile deduced earlier for the class I aurora within the estimated uncertainties for both profiles.

Ion composition and drift observations in the nighttime equatorial ionosphere

The first in situ measurements of ion composition in the nighttime equatorial E and F region ionospheres (90-300 km) are presented and discussed. These profiles were obtained by two rocket-borne ion mass spectrometers launched from Thumba, India on March 9-10, 1970 at solar zenith angles of 112 deg and 165 deg. Ionosonde data established that the composition was measured at times bounding a period of F region downward drift. During this period the ions O(+) and N(+) were enhanced by one to three orders of magnitude between 220 and 300 km. Below the drift region (200 km), O(+) ceased to be the major ionic constituent, but the concentrations of O(+) and N(+) remained larger than predicted from known radiation sources and loss processes. Here also, both the O2(+) and NO(+) profiles retained nearly the same shape and magnitude throughout the night in agreement with theories assuming scattered UV radiation to be the maintaining source. Light metallic ions including Mg(+), Na(+) and possibly Si(+) were observed to altitude approaching 300 km, while the heavier ions Ca(+) and K(+) were seen in reduced quantity to 200 km. All metal ion profiles exhibited changes which can be ascribed to vertical drifting.

Ion composition in the E- and lower F- region above kiruna during sunset and sunrise

A magnetic type mass spectrometer has been flown on two ESRO sounding rockets from ESRANGE (Kiruna 68°N) on February 25 and 26, 1970. The first launch was at sunset (16:33 UT) and the second the next morning, during sunrise (04:47 UT). For both flights the solar zenith angle was approximately 98°. The instrument was measuring simultaneously the neutral gas and positive ion composition and the total ion density. In this paper the results of the ion composition measurements are presented. For both flights the main ion constituents measured between approximately 110–220 km were O +, NO + and O 2 +. Only at sunset were N + and N 2 + detected above 200 km. In spite of the identical solar UV-radiation, pronounced sunset/sunrise variations in the positive ion composition were found. The total ion densities at sunrise were between 5×10 3 and 5 × 10 4 ions cm −3 and therefore too high to be explained without a night-time ionization by precipitated particles. At sunrise the NO + and O 2 + profiles show a correlated wavelike structure with three pronounced almost equally spaced layers in the E-region. Only the highest layer is present in the O + profile. Locally enhanced field aligned ionization originated by particle precipitation and an E × B instability are the most likely source for this structure. In the E- and lower F-regions the NO + O 2 + ration increased overnight from values around 7 at sunset to 15 at sunrise, correlated with an increase of the local magnetic activity index K from 0 + to 2°. This could be explained if the NO density and magnetic activity are correlated.

Metallic ions in the equatorial ionosphere

Four positive ion composition measurements of the equatorial E region at Thumba, India, are presented. During the day, the major ions between 90 and 125 km are NO+ and O2+. The relative concentrations are similar to those observed at midlatitudes but exhibit unusual structural behavior with altitude. A metallic ion layer centered at 92 km is found to contain Mg+, Fe+, Ca+, K+, Al+, Na+, and possibly Si+ ions. The layer is explained in terms of a similarly shaped altitude distribution of neutral atoms that are photoionized and charge exchanged with NO+ and O2+. Three-body reactions form molecular metallic ions that are rapidly lost by dissociative ion-electron recombination. Unfortunately reactions that create molecular ions from Na+ and K+ using CO2 as a third body to form cluster ions are too slow to account for the observed alkali metal ion distribution. Nighttime observations show downward drifting of the metallic ion layer caused by equatorial dynamo effects. The increased ion-electron recombination, induced by the downward drifting electrons, produces an observed depletion of NO+ and O2+ within the metallic ion region. The inclusion of drift effects is found to be necessary in producing a proper description of metal ion behavior at the magnetic equator. Similar considerations are probably appropriate at other latitudes.

The light-ion trough, the main trough, and the plasmapause

Extensive observations of midlatitude depletions in electron and total ion density by both direct and indirect techniques have prompted numerous studies of the possible association between these troughs, observed both in the F region and in the topside ionosphere, and the plasmapause. Within this work, one basic problem arises in that, although the plasmapause has been detected as a global phenomenon by both VLF and ion composition measurements, the electron and ion density troughs have been identified primarily as nightside features. This problem, as well as the difficulty in explaining various inconsistencies in relating the position of the plasmapause and the ionization trough, is explained by a close examination of the ion composition, generally unavailable in previous trough studies. In particular, ion composition results from the polar-orbiting Ogo satellites identify the persistence of a pronounced light-ion trough (LIT) in H+ and He+, often identified by order-of-magnitude decreases in the light-ion concentrations, that occurs within a few degrees of latitude and reaches residual concentration levels of 10²–10³ ions/cm³ near 60° dipole latitude. The LIT, observed both within the thermosphere and at high latitudes in the magnetosphere, has been correlated directly with the VLF whistler identification of the plasmapause. On the nightside near 1000 km a complex relationship is observed between the dominant ions O+ and H+. The rapidly changing mean ion mass observed at the plasmapause typically produces a resultant total ion density distribution of complexity sufficient to confuse correlative studies with the plasmapause, even though the LIT is very sharply defined within the ion composition. On the dayside, where O+ is typically the predominant ion near 1000 km, LIT structure is often hidden beneath the O+ layer, which may exhibit little or no depletion even though H+ and He+ show trough structure at the plasmapause. This analysis of the midlatitude topside ion composition indicates the need for caution both in studies of the plasmapause per se and in phenomenological studies such as the relationship between the subauroral red arc and the plasmapause, which are most likely dependent on the detailed information available within the ion composition.

Dayglow [O I] λλ6300 and 5577 A lines in the early morning ionosphere

Data on a developing morning ionosphere have been obtained by rocket measurement of the atomic oxygen red- and green-line airglow emissions and of the composition of the neutral and ionized constituents of the upper atmosphere. The solar zenith angle was 86°. The use of tilting-filter photometers for the optical measurements resulted in reliable and accurate height profiles of the airglow intensity from below 100 to 245 km. The roles of the excitation of both O(¹D) and O(¹S) atoms by the dissociative recombination of O2+ and electron impact were studied by using the λλ6300 and 5577 A data in conjunction with the ion composition measurements. The component of photodissociative excitation of O(¹D) by solar radiation in the Schumann-Runge continuum of O2 absorption was assessed as being approximately twice that determined for a previous rocket experiment. The ion chemistry analysis provided good agreement between the calculated height profile of O(¹S) excitation and the λ5577 A data above 170 km. but, below this height, significant production of O(¹S) through the photodissociation of O2 by solar radiation at wavelengths shorter than 1334 A was apparent.

The relationship of theory and experiment in the D-region

For any physical system, one seeks to monitor its descriptive parameters and construct a model which predicts their temporal and spatial variations. This is a particularly difficult task for the lower ionosphere. In a given situation simultaneous measurements should be conducted of ionization sources, the altitude distribution of all neutral atmospheric constituents, the identification, number density and altitude distribution of different species of positive and negative ions and electrons. These results are compared with theoretical predictions to serve as boundary conditions and guides to modification of the theory. It has not been possible to realize this goal in lower ionosphere research because of the large number of parameters and since certain parameters, as for example, positive ion species distribution and nitric oxide, can only be measured with the aid of sounding rockets. To illustrate the difficulty, recent positive ion composition measurements will be compared with predictions utilizing clustering of ions and neutrals to form hydrated ions.

Ion composition measurements in the lower ionosphere during the November 1966 and March 1970 aolar eclipses

Positive ion composition measurements in the D- and lower E-regions were performed on three rocket flights during the November 1966 solar eclipse program conducted at Cassino, Brazil. Progressing into totality, the E-region results showed that NO + and O 2 + decreased in concentration while the ratio NO + O 2 + increased. Long-lived meteoric atomic-ions appeared to be unaffected within the short period of the eclipse. A meteoric ion layer that was submerged prior to totality, became prominent as the molecular ions decayed and produced a sporadic E-layer at 105 km near totality. The D-region results provided evidence for the existence of a fast process for the conversion of NO + to water cluster ions. The decrease in water cluster ion concentration at totality was probably less than a factor of four in the vicinity of 80 km. Negative ion composition measurements were obtained over Wallops Island, Virginia between 70 and 111 km close to totality of the 7 March 1970 solar eclipse. Heavy negative cluster ions were predominant below 92 km and were restricted to below this altitude. Between 90 and 98 km, and only on rocket descent, there were relatively large concentrations of ions with masses near 16, 32, 46 and 62 amu, probably O −, O 2 −, NO 2 − and NO 3 −. Such large concentrations were not measured in earlier flights in the undisturbed ionosphere and cannot be explained with presently known processes.

The electron heating rate and ion chemistry in the thermosphere above Wallops Island during the solar eclipse of 7 March 1970

An identical pair of thermosphere probes measured the N 2 concentration and temperature, the ion composition and concentrations and the electron temperature up to 290 km about 30 min and 5 min before totality during the 7 March 1970 eclipse at Wallops Island. The rockets travelled similar trajectories thus permitting the purely temporal changes between nights to be resolved. The neutral temperature and N 2 concentration changed little but the electron temperature decreased by as much as 20 per cent in the lower F-region. The ion concentration decreased by about 30 per cent in the F-region and about 50 per cent in the E-region, with little change in relative ion composition. The electron cooling rates decreased by a factor of 6 in the lower F-region, approximately in proportion to the change in visible solar disc. A smaller than expected decrease in the cooling rate below 150 km between the two flights indicates a hardening of the solar spectrum and suggests a significant heat contribution from the solar corona near totality. The ion composition measurements were consistent with solutions of the ion continuity equations. A proper fit required a factor of three enhancement of the flux below 200 Å, an amount also consistent with the electron heat balance analysis. Reactions involving the minor ions N + and N 2 + were found to be important for the ion chemistry of the major ions O 2 + and NO +, especially at the time of eclipse. The negligible response of the neutral atmosphere to the eclipse is reasonable considering the long time constants for the conductive and convective transport processes and the local nature of the disturbance.

Contribution to the planning of the station network for measuring the chemical composition of precipitation

The data on the chemical composition of precipitation obtained during the period from 1958 through 1965 at 28 stations situated on the European Territory of the USSR (ETU) are used to calculate correlation functions of Ca··, Na·, SO4 ion and ion sum concentrations in precipitation and deposition. Correlation distances for various ions are found to be in the range of 90 to 450 km. The mean square random errors in ion composition measurements (35–45%) are estimated using calculated correlation functions. It is shown that the mean station separation of about 100 km gives the error of optimum interpolation to the point for the total ion concentration in precipitation equal to the random error of observations. The optimum averaging of the deposition value over some area can give the same error, 2–3 stations being in the square of 300 times 300 km2.

D‐region ion chemistry

The status of D‐region positive‐ and negative‐ion chemistry is reviewed. In the case of the positive‐ion chemistry, the presently accepted ion production rates and ion‐neutral reaction scheme are incompatible with the observed qualitative ion composition measurements. The observations indicate that water‐ion clusters, H3O+(H2O)n, particularly H5O2+, are dominant below 80 km, whereas calculations predict that NO+ and its hydrates would be dominant. It seems rather likely that an essential element of the physics involved has so far escaped attention. In the case of the negative ions, the few observations only recently obtained are insufficent to critically test the detailed reaction schemes available from laboratory studies. Uncertainties in the concentrations of minor neutral constituents severely limit the ability to predict theoretical negative‐ion profiles at present. The first rocket negative‐ion observations do support the laboratory‐derived prediction of a dominant role for NO3− and its hydrates in the D‐region.

Discussion of the formation of major positive and negative ions up to the 50 km level

Tremendous progress has been made in recent years in understanding positive ion formation in the D-region. The results of direct measurements (Narcisi, 1969) of D-region ion composition can now be understood on the basis of a reaction model calculated with measured reaction rate constants (Fehsenfeld and Ferguson, 1969; Ferguson and Fehsenfeld, 1969). Similar progress has been made on explaining the D-region negative ion chemistry, and some very successful results have been obtained in recent months (Ferguson, private communication).

Seasonal variation of the O/N2ratio in theF1region

Incoherent scatter measurements of the altitude variation of the ratio of atomic ions (O+) to electrons are presented that supplement earlier ion composition measurements made in the region 130–240 km at the Millstone Hill radar. In this paper, we use an improved technique for recovering the ratio ρ = [O+]/Ne from the observations. No assumptions need be made about the electron temperature, and we allow for the variation of the measured parameters within the height interval occupied by the exploring pulse. As in earlier measurements at Millstone Hill, a definite seasonal variation in ρ was found with O+, extending to lower altitudes in winter than in summer. These ratios are used, together with measurements of the electron density and relevant theory, to derive the corresponding seasonal variation in the ratio of O to N2. Finally, the seasonal variation in O/N2 obtained here is compared with measurements of O/N2 made by rocket- and satellite-borne mass spectrometers.