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Ionic reactions in the laboratory and in planetary atmospheres
H. Massey, F. R. S.
To cite this version:
H. Massey, F. R. S.. Ionic reactions in the laboratory and in planetary atmospheres. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-21-C7-35. �10.1051/jphyscol:19797426�. �jpa-00219428�
Ionic reactions in the laboratory and in planetary atmospheres
H. S. W. Massey, F. R. S.
Department of Physlcs and Astronomy, University College London, U.K.
Abstract. - A detailed understanding of the ionospheres of the Earth and planets requires knowledge of the rates of many ionic reactions as well as of electron recombination coefficients and photoionization cross sections. These must be obtained primarily by laboratory measurements but it is often difficult to provide data applicable under the atmospheric conditions, especially of translational and vibrational temperature. An account is given of the present situation for the main terrestrial ionosphere taking account of the availability of extensive data from the Atmospheric Explorer satellites, for the ionospheres of Venus and Mars in the light of recent data obtained with planetary probes and for the lower ionosphere of the Earth taking account of rocket data.
Introduction. - Very soon after the discovery of the earth's ionosphere it was realized that a know- ledge of the rates of certain atomic collision processes would be required in order to understand the beha- viour of this region. It was early realized that at least a large fraction of the free electrons which were responsible for reflecting radiowaves back to earth were produced through photoionization by ultra- violet, and possibly even shorter, electromagnetic radiation from the Sun. To determine the electron concentration at any time and place, it was necessary to know also what processes are responsible for loss of free electrons. The choice seemed to be between recombination to positive ions or attachment to neutral molecules - massive negative ions would have little effect on the transmission of radio waves.
As at the time, in the early 1930's, little or nothing was known about the rates of these processes, the matter was fiercely debated. However, it was shown in 1942 [Massey and Bates, 19421 that photodetach- ment by sunlight is so rapid that, in the main iono- sphere, negative ions must be of negligible importance during the day. The search for a sufficiently fast recombination process took some time until Bates and Massey in 1947 produced arguments to show that it must be dissociative recombination. At that time there was little theoretical and no experimental evi- dence about this process but it is now very well esta- blished as we shall see. Since recombination is only effective with positive molecular ions it follows that much depends on the rates of ionic reactions which take place after the initial process of photoionization.
The importance of reliable information about these rates for an understanding of the behaviour of the terrestrial ionosphere is clear. This is true not only for the main ionosphere but also a fortiori for the lower fringe, the D region and below, which extends down to quite low altitudes. Here the pressure is so high that many more reaction possibilities arise involving negative as well as positive ions.
Similar considerations apply to the ionospheres of Mars and Venus which are already being investigated from planetary probes. In the near future there will be a demand for information about reaction rates
involving more exotic molecular neutral and ionized systems in order to interpret data about the Jovian ionosphere. Before much longer this will extend to still other cases including Saturn, certain planetary satellites such as Titan, and certain comets.
The problem of carrying out experimental measure- ments of electronic and ionic collision rates for application to these large scale atmospheric environ- ments is by no means straightforward. It is essential that the conditions under which the laboratory data are obtained reproduce those prevailing in the regions of the ionosphere to which they are being applied. To achieve this is much more difficult than would appear at first and it is largely responsible for the fact that, although a considerable fraction of research in atomic and molecular physics since the last war has been especially devoted towards obtaining such data, it is only now that the informa- tion is sufficiently comprehensive and reliable for use with confidence.
The dynamical temperature Tg of the neutral gas in the earth's atmosphere increases with altitude in the main part of the ionosphere from about 200 K at 100 km to over 1 000 K at 300 km. Moreover, the electron and ion temperatures T , and T i respec- tively at any particular height are in general substan- tially different from each other and from Tg. This means that the rates of the relevant ionic reactions are needed as functions of these temperatures over quite a wide range, extending well beyond room temperature. While this is difficult in any case it is aggravated by the fact that, when molecular systems are involved, internal excitation of the reactants is likely to have a major effect on the reaction rate. It cannot be presumed that the vibrational temperature of ambient ions will be the same as the translational.
In the main terrestrial ionosphere the atmospheric tides produce ionospheric motion and the consequent interaction with the earth's magnetic field leads to drift motions of the ions which may affect the ener- getics of the reactions involved as well as compli- cating the analysis of observed data in terms of electron loss rates.
A further important matter which is particularly
3
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797426
C7-22 H. S. W. MASSEY
significant in dealing with relatively high pressure situations is the state of aggregation of the ambient ions. Cluster formation was the bugbear of early measurements of ionic mobilities and it is very important in the lower ionosphere.
Advances in technique which we shall shortly summarize have made it possible to provide usefully applicable information but the whole situation has been greatly simplified by the development of space research techniques. These have made possible in situ measurements of ionospheric properties in so compre- hensive a fashion that a self-consistent check of reaction rates derived from laboratory experiment is
of ion composition. It is then relatively easy to check for self-consistency. This is the situation reached at present with the main region of the earth's iono- sphere, at least in the mean as we have defined it.
To interpret the variable aspects of the main iono- sphere is more complicated because it is necessary to take account of ionization due to solar corpuscular streams and to variations in the solar XUV radiation.
Although we shall not discuss these aspects for the main ionosphere we shall say something about the relation of the disturbances observed in the lower ionosphere to the various relevant reaction rates.
already practicable, at least for the main ionosphere. Releysnt ionospheric lneasurements by space tefh- The combination of laboratory and space techniques niques. - Because of the variability of the ionosphere is a very powerful One and, as we see, a remar- and indeed ofthe factors producing it, in situ measure- k a b l ~ picture
''
the mean behaviour of ments should be made at the same time and place of the main regions of the ionosphere in terms of ionic aN the relevant quantities. Figure shows a mean and dissociative recombination reaction rates can ionospheric profile of electron concentration. Parti- now be built up. cular interest attaches to the region below 250 kmRequirements for a complete theory of the mean ionosphere. - The ionosphere varies with latitude, longitude, time of day, epoch in the solar cycle and condition of the Sun. We shall not attempt to discuss the variability in any detail but refer to a mean iono- sphere, at a temperate latitude and quiet phase in the solar cycle and usually in day time equilibrium.
We shall say only a little about the behaviour at night.
For a complete theory of the behaviour of this mean ionosphere we need t o know :
(a) the composition and concentration of the neu- tral atmosphere as a function of height,
(b) the spectrum of the solar radiation in the XUV region which is of sufficient quantum energy to produce photoioniza'tion of atmospheric atoms and/or molecules, and
(c) the drift velocities and diffusion rates of the ions as functions of height.
In principle given these data and knowledge of the cross sections for ionization of atmospheric atoms and molecules by XUV radiation and by electrons with energies characteristic of the photoelectrons produced in these processes, for dissociative recom- bination of electrons with atmospheric molecular ions as a function of electron and gas temperature, and for all relevant reactions involving neutral and ionized atmospheric molecular systems as functions of translational and vibrational temperatures.
It would be possible to calculate the electron and ion concentrations and temperatures and the ion compo- sition as a function of height. In fact laboratory data are always to some extent incomplete and it is much more effective if information is available by direct in situ measurements using space techniques of electron and ion concentration and temperature and
as it is in this region that photochemical equilibrium,
-
in day time, is relatively unaffected by drift and dif- fusion effects. However the lifetime of an artificial satellite decreases rapidly as the perigee distance
Electron concentration /c.c.
Fig. 1. - Schematic representation of the variation of the electron concentrat~on with height in the undisturbed terrestrial iono- sphere.
falls below this value. For this reason it is only recently that continuous and comprehensive data have become available from satellite-borne instruments. This has been achieved through the Atmospheric Explorer satellites (AE) C, D and E.
These satellites have been launched into highly eccentric orbits, the apogee distance being of the order 4 000 km 'and the perigee close to 150 km.
With this perigee distance the apogee distance would decrease at a rapid rate (initially about 300 km a month) but by means of auxiliary propulsion this is avoided so that the satellite lifetime is adequate.
Excursions to lower perigee distances for a limited number of orbits is also possible. The limit in this case is the maximum pressure at which the onboard instruments can function effectively.
The second important feature of the Atmospheric Explorer is the comprehensive range of instruments carried so that simultaneous measurements may be made continuously of all important ionospheric quantities. Thus AEC, the first to be launched (Dec. 1973), carries an open p i e r et al., 19731 and a closed [Pelz et al., 19731 source neutral mass spectro- meter to measure the composition of the neutral atmosphere in the mass range from 1 to 46 a.m.u., a velocity distribution analyser [Spencer et al., 19731 to measure the neutral gas temperature, a cold- cathode gauge and capacitance manometer [Rice et al., 19731 to measure the total atmospheric density, accelerometers [Champion and Marcos, 19731 to measure the total atmospheric density a t altitudes below 400 km, a magnetic [Hoffman et al., 19731 and a Bennett type [Brinton et al., 19731 ion mass spectrometer to measure ion composition and concen- tration up to 46 a.m.u., a retarding potential ana- lyser [Hanson et al., 19731 to measure ion mass, temperature, concentration and drift velocity, a cylindrical Langmuir probe [Brace et al., 19731 to measure electron temperature and concentration as well as ion concentration, a solar EUV spectrophoto- meter [Hinteregger et al., 19731 to measure the solar energy in the wavelength range 14 to 185 nm, a solar EUV filter photometer [Heath and Osantowski, 19731 to measure the solar energy in bands from 4 to 135 nm, a UV spectrometer [Barth et al., 19731 to measure the altitude distribution of NO between 80 and 250 km, a photoelectron spectrometer [Doering et al., 19731 to measure the flux of photoelectrons and fast electrons with energies between 2 and 500 eV, a visible airglow photometer [Hays et al., 19731 to measure the intensities of the airglow at wavelengths of 630,557.3,427.8, 337.1, 52.0 and 731.9 t o 733.0 nm, a low energy ion and electron detector [Hoffman et al., 19731 to measure particle fluxes in the energy range 0.2 to 25 key, as well as a three-axis fluxgate magnetometer [Armstrong and Zmuda, 19731.
With this remarkable automatic laboratory in space not only are all the important ionospheric parameters observed continuously but there is a
considerable degree of redundancy so that measure- ments made by one instrument may be checked or calibrated against those made by others. The only special problem which is at all important for appli- cations to ionospheric photochemistry is that of measuring the concentration of atomic and molecular oxygen. Above 100-120 km the atmospheric oxygen is mainly monatomic and hence highly reactive.
With a closed source mass spectrometer most of the atomic oxygen will react on the surfaces before ioniza- tion. There is strong evidence that if the surfaces to which the incoming atoms are exposed are gold- plated quantitative conversion to 0, occurs, so that the sum [O,]
+ 3
[O] is measured, [ ] denoting concentration oJ In an open source instrument the position is not so clear. It seems, however, that in this case with surfaces only briefly exposed to low pressure conditions, as in rocket flights, most of the atomic oxygen reaches the ionizing chamber without reaction. For surfaces exposed over a long period as in satellite observations this no longer applies and effectively all of the atoms recombine t o form 0, just as in the closed source case. This means that, at altitudes well above 120 km both types of instru- ment yield [0] with reasonable accuracy because [O,]is relatively small. However it is important also to determine [O,]. [Nier et al., 19741 were able to adapt their open source instrument to do this, distinguishing the ambient 0, from that derived by recombination of ambient 0 , by the possession of a velocity relative to the collector equal to the satellite velocity. This was only possible when the satellite was operated in
a spinning mode.
Remarks on the measurement of ionic reaction rates.
- A great step forward in experimental techniques for measurement of ionic reaction rates under thermal conditions was taken with the introduction of the flowing afterglow technique [Ferguson et al., 19641.
This method has been applied with great success to the measurement at room temperature of a great variety of reaction rates of importance in planetary ionospheres. Recently a further variant known as the selected ion flow tube (SIFT) technique has been introduced [Adams and Smith, 1976al. In this the ions under study are not produced by an electric discharge in the flowing gas but by injection of a mass-sele6ted beam of ions into the flow tube. This has the advantage of eliminating a number of other ionic species as well as electrons, photons and excited atoms and molecules from the reacting system. The identification of product ions and the determination of branching ratios is thereby facilitated while it becomes possible to study reactions involving ions in metastable states.
Both the original flowing afterglow technique and this modification are very effective at room tempe- ratures but it is more difficult to extend the tempe- rature range of the observations. With some diffi-
C7-24 H. S. W. MASSEY
culty, in a flowing afterglow tube measurements may be made up to a temperature of 900 K, but not higher.
For these reasons a great deal of effort has been devoted to the use of drift tube techniques in which the ions are accelerated to suberthermal energies by appli- cation of an electric field. By an ingenious extension of techniques for measurement of ion mobilities McDaniel and his collaborators [McDaniel, 19681 have been able to measure ionic reaction rates k(T,, FIN) as a function of the ratio FIN of electric field strength F to the concentration N of the main (buffer) gas through which the ions drift, a t room temperature T,. The problem is to convert k(Tg, FIN), to k(T) which is the reaction rate when all the degrees of freedom of the reactants are in thermal equilibrium a t temperature T.
The usual procedure is to assume that the mean kinetic energy El,, of the ions is given by Wannier's expression (Wannier, 1953), valid when the collision frequency is constant,
E,,, =
M,
v,2+ $
M, uz+
$ K T (1)where Mi, M , are the masses of the ion and neutral gas atom respectively, v, is the ion drift velocity and T the gas temperature. The mean kinetic energy Em, in the CM system, for the interacting systems is given by
where
02
and v," are the mean square thermal velocities of the ions and neutral atoms respectively.Since
v," = 3 KTIM, , v; = 2 E,,,IMi , (3)
k(Tg, F/N) is then taken to be k(Teff) where
Strong support for the validity of this procedure, when both reacting systems are atomic and the variation of the reaction rate with temperature is not too rapid, has recently been provided by a theo- retical study due to Viehland and Mason (1977).
They develop a systematic procedure for determining T,,,. The first approximation in this procedure is a good one under the conditions stated and it is just (5) with Ec, given by (4). At the same time Lin and Bardsley (1977) carried out a Monte Carlo simulation of ion motion in drift tubes which again showed that (5) is a good approximation under these condi- tions. On the other hand, their results showed clearly that indiscriminate use of the approximation could lead to unsatisfactory data about the variation of the reaction rate coefficient with temperature. A particular example of interest in the present context is the reaction
This was studied experimentally by Allbritton et al.
(1977) together with two other reactions with the special aim of checking the conclusions of Viehland and Mason and of Lin and Bardsley. For this purpose they used a flow drift tube [McFarland et al., 1973al which combines the versatility of the flowing after- glow technique with the drift tube for obtaining higher energy ions. They measured rate coefficients, k(Tg, FIN) using two different buffer gases He and Ar.
From these results they derived k(Teff) using ( 5 ) . Figure 2 shows that different results are obtained for the two buffer gases arising from differences in the ion velocity distributions in the two cases. This is confirmed by using the velocity distributions calcu- lated by Lin and Bardsley (1977) first to unfold the reaction cross sections as a functhn of ion energy for the helium data and then to derive the reaction rate for argon as a function of Teff. This agrees well with the observed results (see Fig. 2). The helium observations also agree with those calculated assuming Maxwellian distributions about Teff (Fig. 2).
KE,, (eV)
Fig. 2. - Rate coefficients for the reaction 0 +
+
N, -t NO++
N measured by Albritton et al. (1977) using the flow drift technique with diierent gases as indicated. o A argon at pressures of 0.1 12 and 0.206 torr respectively; r A helium at pressures of 0.206, 0.395, 0.410 and 0.810 torr respectively; x measured by the flowing afterglow method at room temperature (Dunkii et al., 1968). The rate coefficients are given using E,, as defined by (4).This reaction is an extreme case in which the varia- tion with temperature is rapid. An experimental check on the validity of (5) in other cases may be carried out by verifying that the resultant k(Teff)
curve is independent of the buffer gas. If not it is necessary to use higher approximations in the Vieh- land and Mason procedure to obtain the correct curve.
Strictly speaking these considerations only apply when both reactants are atomic so no complications arise from internal degrees of freedom. So far it has been necessary to assume in the case of molecular reactants that they are in their lowest vibrational states. As the reaction rate may well depend quite strongly on the degree of vibrational excitation this may introduce considerable uncertainty into the application of experimental data. Attention must always be paid to this question, particularly when the ions are molecular because their mode of pro- duction may well lead preferentially t o formation in excited vibrational states. In such cases the reaction rate will depend on the age of the ions and this may be used to check whether excited states are contributing to the results. The problem of application to large scale systems may be aggravated by absence of knowledge of the state of internal excitation of some of the molecular constituents in such systems.
Incidentally, a considerable step forward has recently been made in solving the related problem of determining the degree of excitation of the products.
Thus Bierbaum et al. (1 977) have succeeded in observ- ing infrared radiation arising from vibrationally excited CO, produced in the reaction
C O + O - + C 0 2 + e . (7) From observations of this kind the vibrational energy distribution of the CO, may be obtained.
Measurement of dissociative recombination coeffi- cients. - Although electron-ion recombination is not usually thought of as an ionic reaction it plays so central a role in atmospheric photochemistry that we must pay some attention here to the problems8 involved in its measurement.
Until the introduction [Biondi and Brown, 19491 of the microwave probing of discharge afterglows to monitor the rate of loss of electrons there was no direct experimental information available about recombination rates. To obtain definite results for ions in well defined states, as a function of electron and of ion temperature, proved to be a very difficult task. Particular interest for atmospheric applications attaches to recombination to N:, 0: and N O f .
In experiments involving discharge afterglows one major problem is that of ensuring that the ions to which the electrons are recombining are indeed those which are under study and not other more complex ions which are readily formed in afterglow conditions. This must be achieved while a t the same time ensuring that recombination and not diffusion to the container walls is the dominant loss process for electrons.
Kasner and Biondi (1968) found these conditions were achieved for N z ions in an afterglow in neon
at a pressure between 15 and 40 torr containing a partial pressure of N2 between 5 x l o p 5 and torr.
However the state of vibrational excitation of these ions remained uncertain. Measurements were then made of the recombination coefficient at room tempe- rature and extended by Mehr and Biondi (1969) to electron temperatures as high as 4 000 K by using microwave heating. These latter measurements conti- nued to refer, however, to neutral gas a t room tempe- rature.
Merging beam techniques have not yet proved accurate enough at low relative kinetic energies to provide a check on afterglow measurements but Walls and Dunn (1974) have used the ion storage technique to measure recombination cross sections for slow electrons with NOf and 0: which may be compared with the afterglow measurements for these ions. Walls and Dunn contained the ions under study in a cylindrical quadrupole ion trap [Byrne and Farago, 19651, the containing fields being a large homogeneous magnetic field parallel to the axis and an electrostatic field of potential proportional to r2 - 2 z2 where r, z are the usual cylindricaI coordi- nates. In such a combination of fields each ion has a characteristic frequency of oscillation in the z direction and this was used t o detect each ion species through the noise power spectrum of the image currents induced at one end of the trap [Dehmelt and Walls, 19681. Electrons of well defined energy from 0.045 eV up to 10 eV were fired into the trap along magnetic field lines and the rate of loss of ions observed.
With residence times attainable for NO+ of 40 min it seems very probable that the recombination mea- surements referred to ions in their ground electronic and vibrational states. For 0: on the other hand the vibrational lifetimes are expected t o be very much longer and the distribution of vibrational excitation is likely to be that determined by their mode of formation.
Walls and Dunn measured cross sections as func- tions of electron energy and these must be averaged over Maxmillian energy distributions to obtain recom- bination coefficients as functions of electron tempe- rature. Some ambiguity arises in this procedure because of the uncertain extrapolation of the cross section data to zero electron energy. This is unim- portant for 0; and figure 3 shows remarkably good agreement with the results obtained in afterglow experiments over a very wide range of electron tempe- ratures. The indications are that, despite all the uncertainties, the 0; under study has indeed been in the ground electronic and vibrational states.
For NOf there is greater ambiguity in deriving recombination coefficients from the cross section data as seen from figure 4. Nevertheless there is quite good agreement with the afterglow data, sup- porting again the assumption that the ions are in their ground states.
C7-26 H. S. W. MASSEY
TEMPERATURE ( O K )
1c6 -
Fig. 4. - Rate coefficients for dissociative recombination of electrons to NOC measured as functions of electron temperature by different observers. 0 Weller and Biondi (1968);
0
Guntonand Shaw (1965); x Young and St-John (1966) ; 0 Mahdavi et al.
(1971) ; S Stein et al. (1964) ; L Lin and Teare (1963). The full line curves are derived from the cross section measurements of Walls and Dunn (1974) using different extrapolations of these
-
4 5 --
I- $ 16'r
U
LL LL
1 2 3 4
measurements to lower (- and -) and higher (- and -) energies -
.:*,
1 -
B M
respectively. --- calculated by Bardsley (1968) ;
.
. . . calculated by Michels (1973).:
UW -
I- 4
a 2 -
16'- lo2
2
T E M P E R A T U R E ( O K ) --+
Fig. 3. - Rate coefficients for dissociative recombination of
electrons to O:, measured as functions of electron temperature lo2 101 10' 105 by different observers. x Mehr and Biondi (1969); Kasner and n+(~rn-~)
Biondi (1968); Plumb et al. (1972); Cunningham and Hobson
(1972) ; stein et al. (1964); ~ ~ het d ~(1971). ~ - i derived Fig. 5. - Variation with altitude in the earth's ionosphere of the from the cross section measurements of walls and D~~~ (1974). concentrations of electrons and of the main varieties of positive ions near sunspot minimum, derived from analysis over many years of observations made with rocket borne instruments.
Ionic reactions and dissociative recombination in the ionosphere. - Figure 5 shows the height distri- bution of electrons and of the main positive ions in the terrestrial ionosphere near sunspot minimum derived from analysis of observations over many years with rocket-borne instruments. It will be seen that NOf is dominant at altitudes below 180 km whereas NO is a minor constituent. The reverse
situation applies to N l which is a minor ion derived from a major neutral constituent.
Between altitudes of 130 and 250 km the major neutral constituents are N, and 0. Thus a t 160 km the concentrations of 0, N, and 0, are typically 4 x lo1', 9 x lOI5 and 6 x 1014 m-3 while at 220 km the corresponding values are 7.5 x loi4, 6 x 1014 and 3 x 1013. The ions primarily produced by the solar radiation are, in this altitude range, very largely N: and Of. It is clear by reference to figure 5 that ionic reactions must play a vital part in redistributing this ionization.
A great deal of analysis of these reactions is now possible by working with a combination of laboratory data with data obtained from the Atmospheric Explorer satellites. We shall analyse in this way the equilibrium concentrations of O:, :N and NO', checking particularly the self-consistency of the schemes developed.
The 0; Chemistry. - The AEC data were first discussed in this connexion by Oppenheimer et al.
(1976a).
In the neighbourhood of the 200 km level, where the concentration of 0, is relatively small, the main source of 0: is the charge transfer reaction
direct production by photoionization of 0, being very small. At the same time the only important loss process is dissociative recombination
The equilibrium concentration [O:] of 0: is then determined from
where k , is the rate coefficient for (8) and a is the recombination coefficient for (9). The latter is thus given by
Torr et al. (1977) applied these considerations to data obtained from AEC in the day time at altitudes between 190 and 240 km for which they should be valid. For k , they assumed the value measured by McFarland et al. (1973) using the flow drift tube method, namely
T being the reduced temperature
Mi, Mn being respectively the ion and neutral mole- cule masses, Ti, Tn the corresponding temperatures.
[O'] and [O:] were obtained directly from the mass spectrometer measurements and [O,] from the open source neutral mass spectrometer using the technique developed by Nier et al. (1 974) referred to earlier. The total ion density and electron tempe- rature were measured by the cylindrical electrostatic
a (cm3 sec-'1
Fig. 6. - Rate coefficient for dissociative recombination of elec- trons with 0: as a function of electron temperature,
-
derivedfrom the measurements of Walls and Dunn (1974) (see Fig. 3) and 0 obtained by Torr et al. (1976) from analysis of Atmospheric Explorer C data.
probe, and neutral temperatures inferred from the scale height of the N,.
To avoid any contribution to ionization via auroral excitation data used were limited to latitudes less than 550.
Figure 6 shows the derived value of cc as a function of electron temperature compared with the measure- ment of Walls and Dunn (1974) discussed earlier.
The agreement is remarkably good and suggests that the measured values of both k , and cc are close to the truth. It is interesting to note that the analysis is independent of knowledge of the EUV spectrum of the sun.
Oppenheimer et al. (1976) were primarily concerned with deriving [O,] from observed concentrations of [Oil and they took into account a number of minor sources and sinks for O:, using experimental values for reaction rates. The 0, concentrations which they derived were checked against measured data and found to be consistent.
The N; Chemistry. - In the daytime N: is pro- duced primarily through photoionization, ionization by energetic photoelectrons and by charge transfer reactions with metastable 0' ions, namely
Loss of N: occurs mainly through the ionic reactions N:
+
O - + N O + + N ( k 4 ) , (1 5 4 N,f+
0 -+ O f+
N, (k,), (1 5b) and by dissociative recombinationThe equilibrium concentration [N:] of N5 is then given by
q, and q, are the respective rates of production by photoionization and by photoelectron ionization of N,.
Oppenheimer et al. (1976) made a preliminary comprehensive study in which they considered in detail the sources and sinks of the metastable 0' ions over the altitude range from 160 to 380 km.
Below 240 km the only important sinks are found to be the reactions (13) and (14) respectively but at greater altitudes the quenching reactions
become important.
C7-28 H. S. W. MASSEY
Using available measured values for rate constants, supplemented by information on quenching rates available from measurements of the airglow emission at 731.9 nm p a l k e r et al., 1975 ; Rusch et al., 19751 and theoretical values for rates of electron deactiva- tion of the metastable ions, together with AEC data on the EUV solar spectrum and laboratory and theo- retical information about photoionization and photo- electron ionization rates, Oppenheimer et al. (1976) derived [Nil as a function of altitude for a number of orbital passes. Their results are compared in figure 7 with observed values for a particular orbit.
The calculated results depend, above 240 km, on assumptions made about the unmeasured reaction rates for the reactions (18), (19). For 0+('P) the best fit was obtained with reaction rates for quenching by N2 and by 0 respectively of 4.0 x 10-l6 and 1.0 x 10-l6 m3 s-' and for quenching of 0+('D) by 0 of 1.0 x 10-l6 m3 s-'. Although these are very preliminary and probably not definite values they show that, with further refinement, data on reactions very difficult to study in the laboratory may be obtained from observations in space.
i I 1 _1 1 1 , 1 1 1 I 1 1 1 1 1 1 1 1 I
lo2 l o3
DENSITY ( ~ r n ' ~ )
Fig. 7. - Variation ofN: concentration with altitude :*observed with magnetic ion mass spectrometer on Atmospheric Explorer C during the upleg of a particular orbit; x calculated by Oppen- heimer e t al. (1976) from the ion chemistry of N,'.
Torr et al. (1977) took advantage of the fact that the reactions (18) and (19) are unimportant below 240 km. In that case all O+('P) and 0 f ( 2 D ) primarily formed by photoionization will be ultimately con- verted to N:. Furthermore, below 210 km the contri- bution to N l loss by recombination is small. Under these conditions (17) reduces to
Here q, and q4 are the rates of production of O+('P)
and 0 + ('D) by photoionization and photoelectron ionization respectively and
P
= [O]/m,].Using measured values for [N,] and [O] made by the open source mass spectrometer, values were found for k4 and k, increasing gradually from 1.03 x 10-l6 m3 s-' at an ion temperature of 600 K to 1.19 x 10-l6 m3 s-' a t 900K. The lowest value agrees very well with that measured by McFarland et al. (1974) using the flow drift tube method, but the laboratory values decrease as the effective tempe- rature rises. The opposite result found by Torr et al.
can be ascribed to neglect of recombination which becomes increasingly important at the higher altitudes as may be seen from figure 8. Detailed analysis of the recombination contribution is complicated by inadequate evidence about the influence of vibra- tional excitation on the rate but the order of magni- tude seems to be quite close to that measured by Mehr and Biondi (1969).
Fig. 8. - Comparison of loss rates of N i in the ionosphere at different altitudes : - due to dissociative recombination;
--- due to the ionic reaction N i
+
0 + NOC+
N (from Torr et al., 1977a).The NOf Chemistry. - NO+ is produced, at altitudes above 150 km, mainly through the reactions, O f
+
N2 -+ NO++
N (k7), (21)N z
+
0 -+ NO++
N (k8), (22)while appreciable contributions also come from N f + 0 2 - + N O f + O ( k 9 ) (23) 0; + N O - + 0 2 + N O + , (24) and, below 200 km, from
0; + N - , N O + + O . (2 5 ) Dissociative recombination (coefficient a) is the only effective loss process at these altitudes.
At altitudes above 240 km there is no significant During day time the production of NO+ is so much contribution from (24) or (25) so that in equilibrium, greater that in any case the influence of drift motion
in day time, on the NO+ distribution is relatively unimportant.
[NO
+I
= k,[O+I
[N,1+
~,[N:IP I +
+
k,[N+I [O,ll~(T,) [el.
(26) [Torr et al., 1977~1 used this rklation to analyse AEC data for altitudes below 350 km so as to avoid compli- cations due to vertical transport of NO'. k, was obtained from the analysis of the [N:] chemistry.Conditions were chosen in which the contribution from (22) was negligible while a major contribution comes from (21). It was found that, over the ion temperature range from 500 to 1 200 K, good agree- ment with (26) is obtained if k, is taken to be that measured by Albritton et al. (1977) as described earlier and a(T,) is taken from the observations of Walls and Dunn [1974].
As an interesting illustration of the importance of ion drift motion under certain conditions Torr et al.
also analysed night time data for which case no contribution comes from (22) or (23). Figure 9 shows a comparison between the values of k, consistent with the night time data and those derived from the day time observations. It is seen that, at ion tempe- ratures greater than 750 K, the night time values rise rapidly with increasing ion temperature whereas for the day time,
ih
agreement with laboratory experi- ment, they continue to fall slowly, at least up to 1 000 K. The difference can be ascribed to the pos- session by the ions at night of a large drift velocity, as measured by the retarding potential analyser.Ionic reactions in the ionospheres of Mars and Venus.
- A considerable amount of information is already available about the atmosphere of Mars and about the Martian ionosphere. The Viking project which successfully landed two instrumented packages on the Martian surface in July 1976 greatly expanded the information already available from the earlier Mariner missions. During entry through the atmo- sphere observations were made with a mass spectro- meter of the atmospheric composition [Nier and McElroy, 19771, and with a retarding potential ana- lyser of ionospheric properties [Hanson et al., 19771, including the ion composition, concentration and temperature.
The atmosphere of Mars is much more tenuous than that of the earth, the pressure a t ground being only 6 x lo-' times as large. It is composed very largely of CO,. Measurements with the mass spectro- meters on the two Viking landers show that a t 130 km altitude the concentrations of CO,IN,, CO and 0, are roughly, in m-3, 3 x loi6, 1015, 4 x loi4 and 1014 respectively. At 160 km the values differ somewhat between the two landers. From the one which landed on 3 September 1976, the respective values are 5 x 1014, 5 x 1013, 2 x 1013 and 10".
The concentrations of atomic oxygen were not measured but from estimates based on photochemical information it is likely that at 130 km the concen- tration is of the order 2 x 1014 m-3, falling by a factor of ten in the next 40 km.
Measurements with the retarding potential ana- lysers on the two landers show that the peak electron concentration in day time is very close to 10'
'
mV3, occurring at an altitude of 130 km, falling to 10, in the next 120 km. The main positive ion over this range is 0:. The next most abundant ion, CO;, is about ten times less abundant at altitudes above the maximum and increasingly less so at lower altitudes - at 110 km its concentration is only about 1%
of that of the 0;.The ion chemistry responsible for this situation is quite simple. CO; ions primarily produced by solar photoionization are converted to 0: through the reaction
Fig. 9. - The rate coefficient for the reaction the rate of which has been measured at room tempe-
0+ + N , - + N O f f N , rature by Fehsenfeld et al. (1970) as
observed as a function of ion temperature Ti by Torr et al. (1977), 1.0 x lo-'6 m3 S-'
.
from observations made by instruments aboard Atmospheric
Explorer C : derived from day time observations ;
2
derived MCElroy e l al. (1977) have considered other reactionsfrom night time observations. The numbers refer to the number in a preliminary analysis which yields values for the
of samples used. Curves 1, 2, 3, calculated for the ionospheric
condltlons using t h e rneasuremcnts 01' A l b r ~ t t o n cr trl. (1'977) and concentrations of 0: and CO: in quite good agree-
different assumed ion velocity distributions. ment with the observed data.
C7-30 H. S. W. MASSEY
An unexpected major difference between the ter- restrial and Martian ionospheres is that the latter exhibits no F region (see Fig. 1). This region arises because at altitudes from 300 to 800 km or more the main neutral constituent in the earth's atmosphere is atomic oxygen. The primary 0' ions can only recombine through conversion, in some ionic reaction, to a molecular ion, a process which must necessarily be very slow at altitudes where few neutral molecules are present. This means that the effective rate of loss of electrons is very low and hence the equilibrium electron concentration is large.
It was expected that a similar situation would apply to Mars. Just as the terrestrial 0, is dissociated by sunlight and at sufficiently great height the atomic oxygen floats above the denser N,, so on Mars the CO, should be photodissociated to form CO and 0.
I n this case once again the 0 should dominate in the neutral atmosphere at high altitudes so that recom- bination would be very slow and the equilibrium electron concentration high. It is still not clear why this does not occur.
We must expect that the ion chemistry of an atmo- sphere which is predominantly composed of CO, will also be appropriate for the ionosphere of Venus.
The atmosphere of this planet is known from early probing missions to be composed mainly of CO, with about a 5
%
admixture of N,, a result confirmed by the recent Pioneer and Venera lander and orbital missions. On the other hand the pressure at ground level is about 90 times larger than on the earth. The electron concentration-altitude profile of the iono- sphere [Bauer et al., 19771 has been derived from observations made in the Mariner 10 occultation experiment. The day time maximum of 4 x 10'' m-3 occurs at 130-140 km altitude. Remarkably enough, considering the very different pressure and dynamical conditions compared to Mars, the Venus ionosphere also shows no F region. Information about the ion composition is just now becoming available from observations made in the latest U.S. and Russian missions. Taken together with data obtained in the same missions on the composition of the neutral atmosphere, sufficient material will be available for an analysis of the ion chemistry taking account of laboratory data.A remarkable feature of the ionosphere of Venus is the persistence of an electron concentration which, at the maximum, is of order of 10" m-3 on the night side. This is despite the fact that the rotation period of the planet is as long as 244.3 days. Any explanation must clearly depend in some form or other on ionic reactions.
It is clear that there will be demands for new mea- surements to be carried out in the laboratory on reactions which have only become important in the atmospheric context because of the possibility of extending the terrestrial studies to other planets.
These demands will increase and embrace even more
exotic reactants in the near future when data on the composition of the atmospheres and ionospheres of the outer planets become available. Already electron concentration profiles of the Jovian ionosphere have been obtained. Saturn is well within range as is also its remarkable satellite Titan which certainly possesses an atmosphere. In addition to the planets many problems arise in connexion with the ionospheres at the cores of comets [Mendis and Ip, 19771. The posi- tion here is that even the source of the ionization is not known. A cometary fly-by is likely to help in resolving such questions and raising others about the origin of the constituent positive ions which will certainly call for further laboratory studies of ionic reactions.
The lower ionosphere of the earth. - We now consider the reactions involved in the region of the earth's atmosphere at heights below 100 km. Although this represents only a lower fringe of the ionosphere it is of considerable importance in practice. Thus, because of the relatively high collision frequency at these altitudes, electromagnetic waves suffer absorp- tion which can become serious for radio propagation if the electron concentration is not too low. In fact the concentration is subject to considerable variations not only due to solar flares but also at certain periods during the year, as we shall see.
It is also of intereit to trace out the downward prolongation of the ionosphere into the stratosphere and below. Thus it is conceivable that some ionic reactions may be of importance in the chemistry of minor constituents in these atmospheric regions, a subject which is now actively studied in relation to environmental problems. Again it may well be that ions form the nucleation centres for the noctilucent clouds observed in middle latitudes in the region of the temperature minimum near 85 km.
In many ways the problem of understanding the ion chemistry of the atmosphere below 100 km is more difficult than for the upper atmosphere. As the pressure increases many more reactions become possible while minor neutral constituents such as H,O, NO, metastable 0, ('A,) molecules, CO,, O,, etc., play important, and sometimes major, roles. At the same time the altitudes are too low for artificial satellites to circulate for useful times.
However, vertical sounding rockets may still be used to provide uniquely valuable data albeit dis continuously and at a much slower rate. I t is perhaps ironic that the bugbear of early experimental work on ionic mobilities, cluster formation of ions with water molecules, is a matter of much importance and interest at altitudes below 85 km. This has led to a revival of the study of ion clustering and presents many challenging experimental problems.
Below 70 km account must be taken of negative ions which at higher altitudes are quite unimportant.
Figure 10 shows the temperature distribution in the atmosphere below 100 km with the various regions indicated. The temperature minimum near 85 km is noteworthy.
THERMOSPHERE
160 200 240 280 TEMPERATURE ( K )
below 85 km ions of mass 19 and 37 a.m.u., as well as with mass 55 a.m.u., become dominant. In all cases these ions may be identified as hydrated pro- tons H,Of, H,O+.H,O and H 3 0 + . 2 H 2 0 respec- tively.
Later observations have confirmed these results and indicate that even heavier clusters may be present.
It is difficult to detect these because the last water molecules are so weakly bound as to be lost in the sampling process.
As mentioned earlier, negative ions predominate over electrons at altitudes below 70 km but analysis of the negative ion composition with rocket-borne equipment is very difficult. This is because, in flight, the rocket charges up negatively with respect to the ambient atmosphere so that the problem of drawing in negative ions in a non-selective manner for sampling is a complicated one. However, some observations have been made. Narcisi et at. (1971, 1972) found, in particular, among the dominant species, a series beginning at mass 80 a.m.u. and extending up to 152 a.m.u. in stages of 18 a.m.u. These were identified as NO;(H,O), with n ranging from 0 to 5. Ions with mass 60 and 76 were probably CO; and COT.
Arnold et al. (1971) on the other hand observed ions with masses 11 1 & 1 and 125
t.
1 as the most abun- dant. These were identified tentatively as COT (H20),, or NO; (HNO,) H,O, and NO; (HNO,) respectively.Unclustered CO;, NO;, HCO;, 0; and C1- were
Fig. 10. - Variation of the temperature with altitude in the middle and lower atmosphere, in equatorial regions.
Figure 11 shows the observations made by Narcisi et a1. (1971) of the positive ion composition at alti- tudes below 90 km using a mass spectrometer. Above
85 km the main ions, of mass 32 and 30 a.m.u., h
80
are 0; and NO' respectively, but very shortly
I:
1~Y
- UNCLUSTERED IONS
' \ NEGATIVE IONS
- I I
I I
- I
6 0
!
2 5 10 I 2 I 5 6I 2I 5 1 0 ' 2 I I 5 10' IELECTRON AND ION
Ion dens~ty- Ions, cme3
CONCENTRATION (Arb units)
Fig. 11. - Ion composition in the terrestrial atmosphere at
altitudes below 90 km observed by Narcisi et al. (1971) using F I ~ 12. - Schemat~c representation of the variation of electron rocket borne instruments. The numbers give the ion mass in a.m.u. and ol'pos~~ive 1011 (-) concentrat1011 w ~ t h altitude below 100 km.
C7-32 H. S. W. MASSEY
also observed but none of the clustered ions observed by Narcisi et al.
Figure 12 shows schematically the different regions of ionization below 100 km on the basis of rocket and other observations. It must be remembered that this diagram as well as most of the remarks above refer to the undisturbed region. We shall conclude the discussion of the lower ionosphere with some brief description of the disturbed conditions as far as they are concerned with ionic reactions.
Discussion of the ion chemistry in the undisturbed lower ionosphere must begin by considering the source of the ionization. Solar radiation with quantum energy sufficient to ionize the main atmospheric constituents is absorbed above 90 km. However Ly cr radiation with a quantum energy of 10.4 eV can penetrate through an absorption window down to 80 km and even lower. This radiation can ionize NO, which while a minor constituent is present in sufficient concentration to provide the observed NO+ as a primary ion. This conclusion has been derived from rocket measurements of the NO concentration and the solar spectrum, taken together with laboratory measurement of the recombination rate to NO+.
The source of the 0: ions is less obvious but it is more generally ascribed to ionization of 0, ('A,) metastable molecules by radiation in the wavelength range between 102.7 and 1 1 1.8 nm.
At altitudes below 70 km most of the ionization must be due to cosmic radiation, with N l and 0;
as the primary ions.
Given that NOf and 0: are the primary ions at the altitude where hydronium cluster formation first occurs and that NO+ is produced at the more rapid rate what reactions are responsible for producing these clusters ?
A great amount of laboratory effort has been spent in the past few years in measuring rates of cluster formation and break-up. There seems to be no difficulty in tracing out the sequences which lead from 0: to the hydronium clusters but for NO+
the situation is still not clear. In considering possible sequences account must be taken of the rate at which cluster ions recombine. Measurements by Lin et al.
(1973) of dissociative recombination coefficients to water cluster ions show that values as high as
are to be expected.
For formation from O;, 0: .H,O is first formed, either through the three body reaction,
or through the sequence
Subsequently the Oz.H,O reacts either directly with water to produce H 3 0 +
or, more probably as judged from measured reaction rates, via
followed by
H 3 0 f . 0 H
+
H,O -+ H 3 0 f .H,O+
O H . (32) Further three body reactions with H,O then produce higher degrees of hydration.Measurements of reaction rates by Fehsenfeld et al. (1971) and Good et al. (1970) using quite different techniques indicate that these reaction sequences are acceptable.
With NO+ the basic difficulty is that the process analogous to (28)
which could initiate the sequence, is far too slow (Fehsenfeld et al., 1971). Alternative suggestions involving the formation of intermediate cluster ions NO +
.
N, and/or NOf .CO, were made [Dunkin et al., 1971 ; Heimerl and Vanderhoff, 1971 ; Niles et al., 19721.It seems likely that these intermediates do play a key role but there is still no certainty about the relative importance of the different possibilities. The sequence
NO+
+
N2+
N, -+ NO+ .N,+
N2,
(34a)NOf.N2
+
H 2 0 - + N O + . H 2 0+
N , , (34b) does not seem likely because of the high rate of the backward reaction in (34a), N 2 being only weakly bound to N O f . An alternative possibility involves the reactionNOf .N2
+
COz -+ NO+.CO,+
N , , (35) followed byNO+.C02
+
H,O -+ NO+ . H 2 0+
CO,.
(36) A still further possibility involves NO+.CO, alone viz :NO+
+
CO,+
N, -+ NO+ .C02+
N,,
(37a) NO+. C 0 2+
H,O -+ NO +.
H,O+
CO,.
(37b)The measurement of reaction rates by flowing afterglow drift tube and SIFT methods is very difficult because of the low binding energies of some of the complexes. This requires that a sufficient concen- tration of a highly reactive scavenger is present to react totally with the weakly bound complex and produce a more stable detectable ion before the
weakly bound complex can be broken up. Values of the rate coefficient for (37a) at 200 K differ by a factor of ten. The most recent measurements [Smith
e t al., 19771 using the SIFT method and paying
special attention to the scavenging technique yield a high value. If this is correct then both the sequences (35), (36) and (37a), (37b) are likely to be significant in producing N O f . H,O.
Even when this stage is reached it is far from clear what series of reactions leads thence to hydrated protons.
Until these complex reaction sequences are uns- crambled it is not possible to develop a detailed ion chemistry at those altitudes for which cluster ions are dominant.
Considerable attention has also been paid to determining the reaction paths followed in producing negative ions which predominate over the electrons below 7Q km. The primary ion in these cases is almost certainly 0; formed by the three body reaction
A series of ionic reactions ultimately leads to the production of NO; which, because of the high electron affinity of NO, [3.8 eV, Refaey and Franklin, 1974; Ferguson e t al., 19721, is the terminal ion.
One sequence which proceeds via 0; and one via 0, are as follows
0,
+
0, -+ 0;+
0, ,0;
+ co,
-+co; +
0, ,0; + N O + N O ; + O , COT
+
NO + NO,+
CO, , \No;
+ o3
-+ NO3+
0 , .0; 4- 0,
+
0, + 0,+
0,.oq +
NO -t NO;+
0, ,0,
+ co,
-+ CO,'+
0, , (40)CO,
+
NO + NO;+
CO,.
The rates of these reactions have been measured by flowing afterglow techniques [Massey, 19761 and they provide at least plausible reaction paths. How- ever, the further stages which lead to hydration are less well understood and progress is hindered by the paucity of in situ observations of the negative ion composition.
In the region in which negative ions predominate over electrons ionization loss is by mutual neutra- lization. A number of measurements of mutual neutralization rates have been made [see for example Church and Smith, 1977 ; Smith et al., 1976, 19771 using the flowing afterglow technique. The rate coefficient is not sensitive to the nature of the reactant ions being close to 5 x 10- l4 m3 s-
'
at room tempe- rature for both clustered and unclustered ions.Turco (1977) has considered in some detail the production of C1- ions in the lower ionosphere and finds that these ions are likely to depend strongly on the local concentrations of NO and H,O. Under certain conditions they may well constitute as much as 10
%
of the negative ion population. This is consis- tent with the in situ observations of Arnold et al.(1 97 1) referred to earlier.
Although a great deal of work has been done in measuring rates of relevant ionic reactions the situa- tion is so complex that much remains to be done before the ion chemistry is thoroughly understood.
The disturbed lower ionosphere. - At the time of a solar flare the X-ray emission from the Sun is enhanced, especially at the short wave end of the spectrum. These hard X-rays penetrate to the lower ionosphere where they ionize the main gases N, and 0,. A similar increase in ionization at these levels occurs at polar latitudes p o l a r Cap Absorp- tion, PCA, events) when the flux of energetic charged particles entering the atmosphere from the sun is increased. It is found that during these disturbances the level at which ion clustering becomes important is depressed by more than 10 km during the day time.
Mitra (1975) has considered how the ion composi- tion changes during flares and PCA events in terms of ionic reaction rates and showed that the reduction in the altitude at which clustering occurs would be expected in both cases.
The same feature is present during the so-called winter anomaly [Appleton, 19371 in ionospheric absorption - on most winter days at latitudes above 350 the h.f. ionospheric absorption is often larger than on summer days at midday. Measurements from rockets have shown that this is due to increased electron concentration in the height range 80-90 km [Mechtly and Sharke, 1968; Beynon and Williams, 1970 ; Dickinson et al., 19761. Beynon et al. (1976) have found, in further rocket experiments, that during an anomaly day the concentration of NOf is increased relative to 0: so that it is by far the most abundant ion above 72 km, well below the altitude at which ion clusters normally dominate. It appears that the anomaly is due to increase in the NO concen- tration in the lower ionosphere.
Noctilucent clouds. - The highest cloud forma- tions observed are the so called noctilucent clouds which are visible at high latitudes for a few months in the summer near twilight. As long ago as 1888 Jesse showed by triangulation that these clouds are located at altitudes close to 85 km, the height of the tempera- ture minimum in the mesosphere, which is at its lowest in the summer at these latitudes. Recently, evidence from satellite observations [Donahue et al.,
19721 suggests that the noctilucent cloud displays are a nearly permanent feature at high latitudes in summer.
