Ionospheric constraints of mesospheric nitric oxide

Journalof Atmospheric andTerrestrial Physics,1974,Vol. 36,pp. 1797-1808. Pergamon Press. Printedin NorthernIreland

Ionospheric constraints of mesospheric nitric oxide A. P. MITRA Radio Science Division, National

Physical Laboratory,

New Delhi-12,

India

and J. N. ROWE Ionospheric

Research Laboratory,

Pennsylvania

State University,

(Received 4 February

Pennsylvania,

U.S.A.

1974)

Ah&r&---In view of the unreliabilities of y-band estimates of mesospheric nitric oxide, ionospheric measurements of different types, including some controlled experiments specifically undertaken for this purpose, have been used to define an acceptable range of NO concentration. The ionospheric observations used are (i) the level of ionization reversal with solar activity (MITRA, 1966), (ii) the gradual loss of solar control in the diurnal variation of electron density at levels below 70 km (MITRA, 1969), (iii) magnitude and changes in the ratio of the two molecular ions NO+ and O$. The controlled experiment was the continuous monitoring of changes in mesospheric ionization profiles with the high power wave interaction facility at Pennsylvania State University during selected solar flare events. The flare measurements provide an NO profile similar to, but displaced downwards in height, from that given by Meira’s y-band measThe sharp ledge in electron density observed around 85 km requires a sharper urements. valley in NO distribution at this height and the observed electron and positive ion densities at 70 km require that Meira’s NO concentration should be increased by a factor between 5 and 7 at 70 km.

1. INTRODUCTION DOUBTS have been raised concerning the reliability of any nitric oxide determination in the lower mesosphere from observations of NO emission rate in the y( 1, 0) band, and in particular on the distribution obtained by MEIRA (1971), for several years accepted as the standard nitric oxide model. For one thing, the major contribution in the y( 1, 0) band emission comes from above 90 km, and consequently Meira’s observations are really not sensitive to NO concentrations below this height. Secondly, theoretical estimates by STROBEL (1972), and more recently by BRA~SEUR and NICOLET (1973), show that use of low eddy diffusion coefficients and inclusion of the photodissociation of nitric oxide give a rapid decrease in NO concentration in the mesosphere to values significantly lower than Meira’s (in the extreme cases of low 7c,as much as two orders of magnitude) and a distribution substantially different from Meira’s giving a minimum in NO concentration near 70 km in contrast to Meira’s minimum around 85 km. We have, therefore, come back a full circle to the pre-Meira situation of conflicting views on NO distributions with the only direct estimates from observations on y( 1,0) band still largely suspect. Under these circumstances we consider it desirable, and even necessary, to fall including some controlled experiments back upon ionospheric measurements, specifically undertaken for this purpose, to see if we cannot define acceptable ranges of NO concentration. In several earlier works, the possibility of using information on the level of ionization reversal with solar activity (MITRA, 1966), the gradual loss 1797

1798

A. P. MITR~and J. N. Rowz

of solar control in the diurnal variation of electron density at levels below 70 km (MITRA, 1969), and the magnitude and changes in the ratio of the two molecular ions [NO+l/P$l (MITRA,1969; NARCISIet al., 1972) have been discussed. It is important here to mention that although ionospheric estimates are necessarily approximate, it is almost always possible to define an upper limit, and that estimates of such upper limit have in the past acted as a restraint on hasty interpretations of rocket observations of y( 1, 0) band. The new observations we referred to before include: (i) controlled experiments using the high power wave interaction facility at Pennsylvania State to monitor, with high time and height resolution, changes in mesospheric ionization during solar flares along with simultaneous satellite-borne observations on the time changes in X-ray flux and its spectral structure, (ii) results of ten rocket flights during quiet and active sun periods by the Illinois Group all conducted at about the some solar zenith angle (x = 60”) and all from the same station (Wallops Island) providing a more reliable result on the solar cycle variation in ionization and of the ionization reversal level (MECHTLYet al., 1972); (iii) greater reliability and overall agreement in the measurements of electron density profiles, especially over the heights 70-80 km ; (iv) availability of larger number and more reliable observations of the melecular ion ratio mO+]/[O,+], and (v) greater reliability in positive ion measurements. It is the purpose of this article to examine (i), (ii) and (iii) in detail, to use the results of NARCISI et al. (1972) and CIUKRAVARTYand MITRA (1973) concerning (iv) and of HALE (1973) concerning (v), and to arrive at a final set of values, with ranges, that would be in agreement with all ionospheric measurements. 2. CURRENTESTIMATESOF MESOSPHERIC NITRIC OXIDE Both experimental (direct only) and theoretical estimates are discussed here. Meira’s estimates were based on two rocket flights made of 31 January 1969 and 6 February 1969 in which the emission rate profiles for (1, 0)~ band of NO were measured as a function of height over the range 60-115 km. The NO distributions derived by Meira are given in Fig. 1 (curves 1 and 2). That the NO profiles so estimated are not unique, was shown by STROBEL(1972) on theoretical grounds from the fit with observational data of a very different NO distribution and normalised to the observed emission rate at 90 km. It is important to point out that Strobe1 made a substantial correction to the data for Rayleigh scattering, of amounts roughly 20 and 400 per cent of the NO emission intensity at 80 and 60 km, respectively. The theoretical curve of Strobe1 is one of a family of curves obtained with an eddy diffusion coefficient k of approximately lo6 cm2 set-l at turbopause level which he considers more appropriate than his earlier value of 4.5 x lo6 cm2 see-1 (STROBEL,1971),in view of the mass-spectrometric observations of [O]/[O,] ratio by OFFERMANN and VON ZA~N (1971) coupled with the theoretical results of boundary value of 3 x lO’cm--3 at 90 km. More recently, BRASSEURand NICOLET(1973) have given a set of theoretical NO profiles for two distributions of k (kdn and km,, used previously by NICOLETand VENISON, 1971), with an upper boundary condition of lOBNO molecules (oonsistentwithMeira’s) at 100 km, and including the effect of photodissociation process that leads to a rapid decrease of the nitric oxide concentration

Ionospheric oonstreints of mesospheric nitric oxide

1799

in the mesosphere. The results of additional production of atomic nitrogen (Brasseur and Nicolet use P(N) = 1 and 10 cm-Ssec-l) have strong effects on NO distribution for kmin distribution but relatively little effects for &,, distribution for which transport effects are more dominant. The dotted curves in Fig. 1 give Brasseur

110100 -

E x .

z

.a

go 60-

2

?O-

6050 10-G

I

I

I

IO’

IO8

109

NO Concentration,

cti3

Fig. 1. Nitric oxide distributions in the mesosphere obtained with rockets (Me&, shaded curve), distribution deduced from the constraints of electron density profiles (solid curve) and theoretical profiles deduced by Braaseur and Nicolet.

and Nicolet’s theoretical curves for km, profiles for P(N) = 1 and 10 cm-3 see-1 respectively. It is important to note that not only do these theoretical profiles give NO concentrations significantly lower than Meira’s below 80 km, the shapes are also different. Brasseur and Nicolet’s distributions for km,, have minima around 70 km, considerably lower than the 85 km given by Meira. 3. CONSTRAINTS ON NITRIC OXIDE CONCENTRATION FROM OBSERVEDELECTRONDENSITY PROFILES Electron density in the D- and the lower E-regions can be conveniently classified into four regions, as shown in Fig. 2. Region I is the one above the sharp gradient in electron density ledge (Region II) occurring usually between 82 and 85 km and coinciding with height with the level where water cluster ions suddenly disappear. Region III includes the region between this ledge and 70 km, is controlled almost entirely by the photoionization of NO by L,, and during daytime is not influenced by negative ions except at and near the lower limit. This is also the region where electron density measurements by different authors and by different techniques (both ground-based and rocket-borne) agree to a surprising degree. Region IV is the negative ion-dominated region below 70 km ; this is also the region where there is no general agreement on the nature and magnitude of N, values, although there is some evidence that there is a residual bank of ionization in excess of 10 cm-s at 60 km. Amongst these, the most reliable constraints on NO can be derived from Region III N, measurements only. For Region IV, however, no constraints are available

A. P. MITRAand J. N. ROWE

1800

HO Colrstroint 6-

t

!

60

IO4

IO3

Electron

density,

cme3

Fig. 2. Experimental electron density distributions in the D- and the lower E-regions. Note that the lower ionosuhere is classified into four regions I. II. III and IV.

(1) BAIN and HARRISON(1973) (a/s summer noon) (2) Illinois active sun (x = 60’) MEcarLY et al (1972)

(3) Illinois quiet sun (x = 60’) I (4) VLF (5) Partial reflection F,,., = 100, BELROSEand SEGAL (1973) (corrected) I (6) BREMERet al. (1973) (x = 60’) (7) Penn. State crossmodulation (x = 60’) Solar Activity Reversal Region

from a different type of N, information; e.g. ratio (N,),/(N,), giving the variation of electron density with solar activity, and in particular the reversal in the trend in ionization charge as one moves downwards from La-dominated region to cosmic ray dominated region. This is discussed in Section 5. The procedure for determining NO constraints is implicit in an earlier paper (ROWE et al., 1973) giving our approach to a semi-empirical D-region modelling. The starting point is the laboratory values of the dissociative recombination coefficients of the molecular ions NO+ and O,+ and the various hydrates H+(H,O), (n = l-6) (BIONDI et al., 1972) and a power law dependence of T-o.6 of the coefficients at the low mesospheric temperatures. In other words, we are taking the stand that large values of around 1O-4 ems/s obtained by REID (1971) and others, on an assumed NO distribution, are not acceptable, and that in the region where 1& 1, which should obtain at least between 75 and 82 km, a is between (5 and 7) x 1O-s ems/s, its actual value dependent upon the Hf(H,O), population. This means that in the height range 75-80 km (A < 1) the constraints on nitric oxide concentrations are given primarily by the ranges in observed electron densities and thus by the curves bounded by 1 and 5 in Fig. 2. Below 75 km the constraints are modified by L distributions.

Ionospheric constraints of meaospheric nitric oxide

1801

In Fig. 3 the observed electron density profiles are given along with electron densities calculated from above considerations with a water cluster population H+(H,O), determined by the chemical-decomposition equation of KEBARLE et al.

$-

76

g a

6H CM EEL ILL

74

l3AIN tHARRISTON(l972) FERRARO et 01 (1972) BELROSEtSEGAL (1973) MECHTLY etaI (1972)

72 NO Adjusted 11111

70 IO’

IO’

IO3 Ne,

1

to 5x10’ ,

ci3

ILIllll

at 70km 1

I

I

I04

cmb3

Fig. 3. Experimental electron density profiles in the region ‘70-85 km (below ledge region) with theoretical curves calculated with nitric oxide distributions of Meira, Brasseur and Nicolet, and the results of adjustments of nitric oxide distribution.

(1967) (which determines the distribution of the resulting dissociative recombination coefficient) and the NO distribution of Meira. There are two remarkable features in this diagram: the first is the close similarity between the profiles, although they are derived by different authors and through different techniques and the second is the very slow variation with altitude. Theoretical curves calculated with Meira’s experimental profiles and with the theoretical proG.les of Brassuer and Nicolet show a larger altitude gradient, although the electron density values at 80 km agree, even for the lower limit of 1 variation which should theoretically give the slowest altitude variation. Even for il = 0 the calculated slope is too large. The adjustment needed is an increase by a factor of about 7 from Meira’s values at 70 km. The third shaded area in the diagram is a result of adjustment of NO concentration; the adjusted NO distribution is given in Fig. 4. Some changes in Meira NO distribution also becomes necessary at the level of the electron density ledge. With Meira NO or Nicolet’s theoretical curves, the observed sharp gradient in N,, such as that shown by the Illinois results for active sun period does not appear in the theoretical profiles I and II in Fig. 1. The necessary adjustment in the N, gradient will, therefore, have to be made through one or more of the of the NO distribution in this region, following parameters : (a) adjustment (b) a sharper fall in M, or (c) an adjustment of q(02+) relative to Q(NO+). Of these three, we consider that only marginal adjustments are possible in (b) and (c). The indication is that the appropriate adjustment has to be in the electron production rate CJ,and since at these heights and for the active sun period and x = 60°, NO+ production rate greatly exceeds O,+ production rate, it appears that the adjustment 7

A. P. MITRA and J. N. ROWE

1802

ILL-MECHTLY et01(I9721 BH-BAINod HARRISON (1972) TH-SATYAPIAKASH et01(1973)

IO’

I04

IO’

Ne,

cm-3

Fig. 4. Experimental electron density profiles in the ledge region alongwith the calculated profiles with different nitric oxide adjustments.

has to be in the NO distribution. The SO-85 km part of the solid curve in Fig. 4 shows the results of such adjustments. 4.

NO CONSTRAINTSFROM ION CORIPOSITION OBSERVATIONS

NO concentrations can also be divided from NO+ ions (or from the ratio [NO+]/ [O,+]) such as those now available from Narcisi and Bailey’s group (White Sands, New Mexico), Aikin and Goldberg (Wallops Island, U.S.A.; Thumba, India; and Spain), Krankowsky et al. and the Zurich Group. NO and NO+ concentrations are related through the equation:

A(x, O,+) NO = a,(NO+)[NO+]iV, - I 10-15W,1Pz+11

- JW,IOMW

with and

(1)

A&, O,+) = y(x) + 3 x l@-‘O LO,+] @0,/O)

= [I +

m/~w-‘.

Direct photoionization of NO ceases to be important in NO+ chemistry above 80 km, but the neutral nitric oxide can still control NO+ through process (O,+ + NO). Excepting for conditions of enhanced nitric oxide, the left hand side is negligible above about 100-120 km, and consequently the method ceases to be valid above this height. One may also consider the ratio [NO+]/[O,+], in which case one uses: [NO+]

N

=

co,+1 B

8

x 10-l’ adNO+) (

cNol

I

INI 4 1*

(2)

DANILOV (1972) found that while [NO+]/[O,+] varies by a large amount with time of the day, the parameter [NO+]/[O,+] N, is nearly constant at 100, 120 and 130 km. Danilov concludes that this remarkable diurnal constancy can only arise

Ionospheric constraints of mesospheric nitric oxide

1803

from NO, and on this assumption derives NO concentrations to be about 3 x lo6 cm-3 at 110 km, 1 x lo8 cm-3 at 120 km, and 1.5 x lo* at 130 km (reducing to O-5 x lo* cm-3 sunset for 130 km). Danilov’s determinations are shown in Fig. 5. 160. 150-

I

I

I

From ion composition s 2

IO6

107

IO6

Nitric oxide concentration,

Fig.

I09

IO'0 cms3

5. Nitric oxide distributions obtained from ion composition data.

A more detailed study has recently been made by CEAERAVARTY and MITRA (1973) using all available ion composition measurements (over midlatitudes as represented by White Sands and Wallops Island, U.S.A. ; over aurora1 latitudes as represented by Andoya, Norway ; and over equatorial latitudes as represented by Thumba, India). In this work soundings made during low solar activity period are separated from those made during high solar activity period, and the NO estimates separately for these two periods are given in Fig. 5. The solar cycle variation is remarkably large ; from about 5 x 10’ cm-3 to about 2 x 108 cm-3 at 140 km, and about 4 x 10’ cm-3 to 2 x lo8 cme3 at 100 km. This variation (by a factor of about 4) continues down to 90 km, below which the two curves appear to merge. Estimates made below 90 km with equation (2) can be suspect since we are then entering into a region where NO+ ions are rapidly converted into water cluster ions through channels not yet firmly established but some tentative calculations using a channel: NO+ :

H+(H,O),

have been made in a companion paper by Zalpuri and Somayajulu. In the water cluster-dominated region reliable estimates are, however, still possible from equation (2) under conditions when cluster ions virtually disappear as during PCA. Estimates made during PCA of November 1969 by NARCISI et al. (1972) by day, night and sunset conditions in the heights 89-90 km show remarkable agreement with Meira’s values, including an agreement in the height of the level of minimum NO concentration.

A. P. MITRA and J. N. ROWE

1804

All estimates of NO including some made during an aurora1 from observations [NO+], [O,+] and their ratio are given in Fig. 5.

of

5. NO CONSTRAINTSFROM SOLAR ACTIVITY REVERSAL IN ELECTRON DENSITY BELOW 70 km The method based on the solar cycle variation in electron density profiles was first used by MITRA (1966). The method makes use of the fact that the solar cycle variation in the three major ionizing sources for the D-region (namely, X-rays below lOA, L, and cosmic rays) are dissimilar. While the first increases about 100-fold from sunspot minimum to maximum, the second is nearly constant, while the third, unlike the other two, decreases with solar activity. There is, consequently, in the total production rate a solar cycle variation which charges with height not merely in magnitude but also in sense, as one moves from a region of cosmic ray dominated ionization to one controlled by L,. The first series of observations that showed that this does, in fact, occur are the extensive VLF/LF measurements (16-200kHz) in Cambridge. The original electron density profiles deduced by DEEKS (1965) indicated that the transition occurs between 71 and 74km for equinox midday conditions in middle latitudes. A later correction lowered the transition level by about 6 km to a height around 67 km. Mitra showed that the ratio Nh2/N,2( = t) as a function of height can provide a distribution of NO concentration through the equation :

and his results with the original Deeks profiles, as well as the later corrections Bain and May, are shown in Fig. 6.

by

-84 65

90-

0 E E x

Bo-~

-74

c s

60 Id4

g f

c3

10-2

IO-’

IO0 0

q&J 2

ccAo;WEBBER 60

54 0

(1962)

01 ,-. I 1

I

I

I

0.5

I.0

I.5

2.0

C=[Ne2]. max./ [Ne’lmin

Fig. 6. Nitric oxide determination from solar cycle varietion in mesospheric electron density. Data used ere the electron density profIles of Deeks. The new experimental results that one can use with advantage are the ten rocket profiles obtained by the Illinois University for low and active sun periods, all carried out over Wallops Island and for x = 60”. There is quite a distinct and unambiguous

Ionospheric constraints of mesosphericnitric oxide

1805

(N,/N,l”

2.P

70

f 60

IO"

IO'

IO‘ Ne

105

Fig. 7. Rocket results of representative electron density profiles in the mesosphere during high and low solar activity. Note the decrease in electron density during high solar activity below about 64 km.

evidence of reversal in ionization in these profiles shown in Fig. 7 which indicates by shades, the total range in the profile variations. The figure also gives the smoothed variation of the quantity Nh2/NI 2. The transition height (no solar cycle variation of E = 1) occurs in this case at roughly 64 km, even lower than the Bain-May case of Fig. 5. The region of L, dominance-in this case between 70-78 km-is quite clearly evident. Above about 80 km, there is a sudden sharp peak, decreasing to a constant value of about 2 above 90 km. The latter can be attributed to soft X-rays, and the former to specific emission lines, such as CVI at 33.78. At the level below 65 km where reversal in ionization occurs, there is an anomalous turn in below 60 km which we believe to arise from increased detachment from negative ions. For our present purpose we are concerned principally with the region between 60 and 78 km. In Fig. 8 (N,/N,)2 for this height range has been plotted, and from the smoothed variation of this parameter q(L,)/q(CR) has been derived. This is also shown in Fig. 8. Above 68 km q(L,) rapidly overshadows q(CR), so that at 72 km it is 10 times larger and at 73 km 100 times larger. The curve giving q(LJ/q(Clz) in turn allows an estimate of NO profile which is also shown in Fig. 8. The rather curious feature of the NO distribution is the distinct minimum around 70 km. 6. NO CONSTRAINTS FROM FLARE OBSERVATIONS OF ELECTRON DENSITY PROFILES

The ease with which even a minor X-ray enhancement produces an SID does not allow NO concentration much in excess of 10’ cm-3 at 80 km. An accurate measurement of the changes in N, during a flare at any fixed level coupled with a knowledge of the X-ray production rate at that time provides an estimate of NO distribution. If one has complete information on the ionizing spectrum (especially for wavelengths below 3 A) so that Aq can be computed with some reliability, then one can estimate NO concentration by: (a) comparing peak values of Aq/q and AN/N, or (b) more properly, by comparing the entire time sequence of q and N.

A. P. MITRA and J. N. ROWE

1806

NOConcentmtion,

cm-3

Fig. 8. Estimation of the relative contribution of nitric oxide ionization and cosmic ray ionization in the mesosphere deduced from solar activity veriation in mesospheric electron density.

Controlled experiments for the determination of flare time mesospheric ionization profiles were carried out in the Pennsylvania State University since the present high power wave interaction equipment was established in November 1967. Soundings have been made for more than a dozen flares. The experiment produces profiles of N,, over the range 55-90 km, at the rate of about one profile every 3 or 4 min. In some of these cases continuous time variations of the X-ray flux in several wavelength bands was available from NRL. A preliminary estimate of NO distribution from such analysis is given in Fig. 9. There are two major sources of errors in such estimates: one is the difficulty of building spectral distributions during solar flares for wavelengths below 5 A, and the other is the drastic change in a that occurs during an SID. The latter must be independently evaluated. 7. SUMMARY

OF DIFFERENTIONOSPHERIC APPROACHES

Profiles obtained from different ionospheric approaches and discussed in the earlier sections are summarised in Fig. 9, along with the y-band profiles of Meira and Tisone and the theoretical profiles of Brasseur and Nicolet. The different ionospheric approaches include :

(9 Estimates (ii) (iii) (iv) (v)

from rocket mass-spectrometric observations of [NO+] and [NO+]/ [O,+] curves 2(a), 2(b), 4 and 5. From lV, constraints: curve 7. From solar cycle reversal in iV,: curve 6. From simultaneous measurements of flare time N,--t values and solar X-ray observations : curve 1. From measurements of N+: curve 3.

Ionospheric constraints of mesosphericnitric oxide

1807

110

100

90

E

x

c’

SO

.c

2

70 1

.sF 60

c

50 I05

\

and HALE

I

I

I

I

I

I06

107

IO8

IO9

10’0

Particle

density,

cmm3

Fig. 9. Summary of nitric oxide profiles obtained from different ionospheric approaches and comparison with the rocket profiles.

(1) Flare (MITRAand ROWE) (24 NO+ Distr. (MITRA) (2b) NO+ Distr. (NARCISIet al.) (3) (4) (5) (6) (7)

Ions, Positive (HALE) NO+/O$ (DANILOV) NO+/O$ ( CHA~RAVARTY and MITRA) From ionization reversal (this work) From Ne Constraint

Above 90 km, where the theoretical curves accept Meira’s distributions, ionospheric estimates are in agreement with y-band measurements, except that the latter fall somewhat intermediate between the low and high solar activity values. An increase in NO concentration from low to high solar activity by a factor of 4 above this height is indicated. The evidence of quiet-day midlatitude profiles between 70 and 80 km requiring a concentration of 5 x lo* cm-a at 70 km is rather strong; however, in sharp contrast, both flare and solar cycle reversals in N, show very low IV, values around 70 km. A minimum around 70 km is indicated by not only these measurements, but also the analysis of positive ion concentrations by Hale. One should also note that inclusion of photo-dissociation of NO by Braeseur and Nicolet also lowered the minimum to around 70 km. A very strong case can, therefore, be made for the 70 km minimum in NO distribution. These must, however, be reconciled with the 80 km minimum indicated by (i) y-band measurements of Meira, (ii) N, constraints and (iii) diurnal variation in NO+ ions discussed in the companion paper by Zalpuri and Somayajulu. REFERENCES BRASSEURG. and NICOLETM. BIONDIM. A., LEU M. T. and JOHSENR.

1973 1971

6%. 21, 925. COSPAR Symposium on D- and ERegion Chemistry. Aeronomy Rept. No. 98, University of Illinois, 1 June 1972.

Planet. SW

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A. P. MITRA and J. N. ROWE

DANILOV A. D.

1972

Space Research XII, Verlag, Berlin.

DEEKS D. 0. KEBARLE P., SE~RLES S. K., ZOLLA A., SCARBOROUQHJ. and ARSHADI M. MEIRA L. G. MITRA A. P. MITRA A. P.

1966 1967

Proc. R. Sot. A!291, 413. J. Am. them. Sot. 89,6393.

1971 1966 1969

J. geophys. Res. 76, 202. J. atrnos. terr. phys. 28, 945.

p. 1299. Akademie-

Meteorological and Chemical Factors in D-Region Aeronomy. Record of Third Aeronomy Conference, Aeronomy Rept. No. 32, University of Illinois, 1 April 1969. J. geophys. Res. W, 1332.

NARCISI R. S., PHILBRICK C. R., ULWICK J. C. and GARDNER M. E. REID G. C.

1972 1971

Mesospheric Models and Related Experiments (Edited by G. FIOCCO). Reidel,

ROWE J. N., FERRORO A. J., LEE H. S., KREPLIN R. W. and MITRA A. P. ROWE J. N., MITRA A. P., FERRARO A. J. and LEE H. S. STROBEL D. F. TISONE G. C.

1970

J. atmos. tew. Phys. 32, 1609.

1974

J. atmos. terr. Phys. (in press).

1972 1973

J. geophys. Res. 77, 1337. J. geophys. Res. 78, 746.

Dordrecht.