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Dynamics of the F-region observed with Thomson scatter—II Influence of neutral air winds on the ionospheric F-region
Journal of Atmospheric and Terrestrial Physics. 1970,vol.32,pp. 775-787.Pergamon Press. Printed inNorthern Irelaud
Dynamics of the F-region observed with Thomson scatter-II Mlaence of neutral air winds on the ionospheric F-region G. VASSEUR C’.N.E.T./R.S.R., 3 Avenue de la Republique, 92, Issy-les-Moulineaux, France (Received
20 September
1969)
Ah&&-In a previous paper the author described diurnal variations of the meridional component of neutral air winds which were derived from Thomson scatter measurements of ion drifts in the F-region. In this paper solutions of the F-layer continuity equation are presented which take account of the effects of these wind-induced vertical drifts. Various F-region phenomena including the evening enhancement of n, and the diurnal variation of the height of the F&-layer peak are shown to be associated with neutral sir winds. There are some discrepanciesbetween the calculated and observed behaviour of the F-region which suggest that there is a night-time ion production source in Winter and a seasonal v&&ion of the neutral atmosphere composition. 1. INTRODUCTION (a) A summary of previous work
Since June 1966, the French Thomson scatter sounder (Saint-Santin-NanCay) has been used to measure the component of F-region ion drift in a direction which lies very nearly along a magnetic field line. In a previous paper (VASSEUR, 1969; hereafter referred to as GV l), the author discussed ion velocity measurements in the F-region. The velocities observed were interpreted as being the resultant of (a) the diffusion velocity and (b) the ion velocity induced by neutral air winds. Both these components are along the Earth’s magnetic field line and it was assumed that the wind-induced motion was due to the North-South component, ‘v, of the neutral air wind. Diurnal variations of ?‘7, (for Winter, 1966-67 and Summer, 1967) were described; these were inferred by subtracting from the total observed ion motion, an estimate of the diffusion velocity which was obtained from the Thomson scatter measurements of ion concentration and temperature. The values of V, were found to be directed equatorwards during the night, and polewards during the day. When compared with various theoretical calculations (KOHL and Krxa, 1967 ; GEISLER, 1966), these values of V, show a general agreement but some discrepancies remain. In particular, during Summer, the time at which V, becomes equatorwards occurred in the early afternoon rather than in the evening as the theoretical calculations suggest. (b) Purpose of the present study Various authors (HANSON and PATTERSON, 1964;
RISHBETH, 1967, KOHL and
KING, 1967 ; KOHL, KING and ECCLES, 1968) have suggested that some peculiarities of the ionospheric F-region can be attributed to the effects of neutral air winds; the aim of this paper is to use the values of Vv, previously derived from observations (GV 1) to check whether they can indeed account for the various phenomena taking
place in the F2-region
of the ionosphere. 775
G. VASSEUR
776
In order to calculate the effects of neutral air winds on the ionosphere, it is necessary to solve the F-layer continuity equation and to include a term representing the wind-induced ion motion. The assumptions made and the method used to solve t,his equation are outlined in Section 2. In Section 3 the results of the calculations for Summer and Winter conditions are compared with the observed ion concentrat’ions and velocities. It will be shown that the neutral air winds cannot account for all the observed phenomena and additional mechanisms are suggested to account for the discrepancies. 2. THE F-LAYER
CONTINUITY EQUATION
(a) Assumptions
The F-layer continuity
equation may be written as ;
= Q -
L -
V(nV)
(1)
where n is the ion concentration, Q is the ion production rate, L is the chemical loss rate, a,nd V is the ion velocity. In the atmospheric model used, it was assumed that the concentration of each constituent remained constant at 200 km altitude and that the atmosphere was isothermal above this level. A model of this nature depends entirely on the exospheric temperature (which can be derived from Thomson scatter measurements) and, for F-region heights, it agrees well with the model suggested by NICOLET (1960). The following assumptions were made for the various terms in the equation: (1) Ion production. The production of N,+ ions was neglected ; O+ ion production was assumed to be given by,
&(x9h) = Plea,
exp [-arch(x) j)N
dh]
where [0] is the atomic oxygen concentration, x the solar zenith angle, ch(X) the Chapman function, G and G’ the ionization and absorption cross-sections respectively of 0 and S, the flux of incident photons. The parameter cr’ was adjusted so that Q(0, h) was a maximum at about 160 km; the value of 0 assumed was such that Q(0, 225 km) N log m3 see-l as derived from Thomson scatter measurements (VASSEUR and WALDTEUFEL, 1968). (2) Ion loss. The loss term, L, was assumed to arise from an ion-molecule charge exchange process between the 0+ ions and N, molecules, with the formation of short lived NO+ ions. A similar reaction is possible with 0, molecules but this was ignored. The loss term is thus ‘linear’ and is given by, L = /?n = y[NJn. The value of y used was 1.6 x lo-l2 cm3 set-l. It was adjusted in order to give &225 km) N 0.25 x 1O-2 set-l as derived from Thomson scatter measurements (VASSEUR and WALDTEUFEL, 1968). (3) Ion
transport.
Only vertical
gradients
were considered
which reduce
the
777
P-region winds deduced from Thomson scatter
flux divergence
term, to
V(nV) = y In the absence of an East-West J”‘, may be written (cf. GV 1) as,
.
electric field, the total vertical ion drift velocity v = w, + w,
(2)
where W, is the ambipolar diffusion velocity and was calculated (see GV 1, equation (13)), by ignoring the effects of temperature gradients. The ion and neutral temperatures were assumed to be equal, but the difference between the electron and ion temperatures, as derived from the Thomson scatter data, was taken into account. The value of the ambipolar diffusion coefficient was calculated from the expression given by DALGARNO (1964). W, is the vertical component of the wind-induced ion velocity and is given by
W, = V, sinIcos1 where V, is the meridional neutral air wind velocity, being positive when directed equatorwards, and I is the magnetic dip angle. Vertical gradients of W,, were neglected. (b) Solutions of the F-layer continuity equation With the assumptions described, equation (1) becomes a partial derivative equation in n(TL,t). The following boundary conditions were made: n(h,t)-+Owhenh+-co n(h, t) varies like exp [--h/H(l
+
T,/T]
when h +
+co
where H is the scale height of atomic oxygen and T, and T are the electron and The high altitude boundary condition means neutral temperatures respectively. that diffusive equilibrium is reached. The following transformations were used : II: = exp [--h/H(l
+ T,/T]
and
n = x . y.
The numerical solution of the equation was obtained by replacing derivatives by finite differences using an implicit method. The initial condition was the equilibrium solution at mid-day and the integration was continued, with step widths of At = O-2 hr until a periodic solution was obtained to give n(TL,t) between 0 and 2000 km. The values of W,, obtained from equation (2), by estimating W, and measuring V, were used in these solutions of the P-layer continuity equation; the new values of W, derived in these calculations were combined with W, to obtain theoretical values of ‘v. Comparisons between these theoretical and observed values of I’ give an indication of how reliable the technique is for deriving W, from the total ion drift motion measured by the Thomson scatter sounder. 3. RESULTS OF THE CALCULATIONS As in the first part of the article (GV l), two typical solstice periods were considered; Winter 1966-67 and Summer 1967. In GV 1, diurnal variations of W,, were
778
G. VABSEUR
obtained as a mean from several periods of observation during Summer 1967 and Winter 196667. In this paper, however, two particular days (one for each season) are considered for which there were data available for the F-region from 200 to 400 km. The two days are lo-11 January 1967 and 18-19 July 1967 when the mean 10.7 cm flux values were 142 and 126 1O-22 Wm-2 Hz-l respectively. These particular days were magnetically quiet, and were considered as typical of the two solstice periods. A description is given of diurnal variations of T, T, and W, which were obtained from the Thomson scatter measurements and which were used to solve the F-layer continuity equation. (a) Summer results (1) Diurnal variations ofT,T,, W,. The neutral temperature can be derived from measurements of ion temperature, electron temperature, and electron concentration, by solving the energy transfer equation for ions, but at about 275 km it is reasonable to assume that the ion temperature is very close to the exospheric neutral temperature. Figure 1 (a) therefore shows the diurnal variation of the neutral temperature using this approximation. Figure 1 (b) shows the diurnal variation of electron temperature T, for several altitudes near the peak of the F2-layer. Vertical gradients of T, have been neglected and the Thomson scatter results at various altitudes show that, in fact, near the peak of the FB-layer, T, does not vary appreciably with altitude. The diurnal variation of W, was derived from the observed ion velocity I’(cf. GV 1, equation (22)), by calculating the diffusion velocity W, from h, and T. Figure 1 (c,d,e) shows the diurnal variations of I’ and jL, which were obtained from Thomson scatter measurements, and also the derived variation of W,. (2) Diurnal variations of thx F2-layer. Figure 2 (a,b,c) shows a comparison between the observed and computed diurnal variations of (a) h,, the height of the FB-layer, (b) n,, the peak electron concentration, and (c) V(h,), the vertical component of the total ion velocity at the peak of the F&layer. It can be seen that when the ion drift due to neutral air winds are taken into account, the theoretical diurnal variation of hm is in fairly good agreement with the observations. Between 1300 UT and 2000 UT, however, the computed values of h, are somewhat higher than those observed. The theoretical diurnal variation of n, (Fig. 2 (b)) shows a decrease at about 1200 UT, followed by a large enhancement at 1900 UT very similar to the one observed. The presence of this evening enhancement is mainly due to an upward wind-induced ion drift which raises the height of the F2-layer. However, it can be seen from Fig. 2 (b) that the variation of n, which was computed without including the effects of winds also exhibits a maximum at about 1700 UT. This maximum is due to the decrease of T, after 1500 UT ; a decrease of this kind is accompanied by an increase of n,, as can be deduced from theoretical computation of a stationary layer (THOMAS, 1966). It appears that the presence of this evening enhancement arises from the simultaneous effects of neutral air winds and of electron temperature variations. During the afternoon however, the computed values of n, and h, are somewhat too high. This could be explained by the presence of an electrodynamic drift directed
P-region winds deduced from Thomson scatt’er
00
06
12
18
24
LT
Fig. 1. Diurnal variations on 18th July 1967 of, (a) the ion temperature Ti at 275 km altitude assumed to be equal to the exospheric temperature Tsz; (b) the electron temperature T, observed at three ahitudes (250, 300, 350 km); (c) the observed vertical ion drift velocity V in the altitude range 200-400 km; (d) the height of the P2-layer peak, h,, rss derived from Thomson scatter measurements; (e) the component TV,, of the vertical ion drift velocity attributed to horizontal neutral air winds V,.,. Also shown is the evaluated diffusion velocity W, using the values of T,, and h, shown in (a) and (d).
779
G. VAYSEUR
780
km
a
18.07.67 1011m-3
nm
m
C
Fig. 2. Diurnal variations on 18 July 1967 of, (a) the height of the P2-layer peak, h,; (b) the electron concentration m, at the peak of the PB-layer; (c) the total vertical ion drift velocity, V, at the peak of the P2-layer. Dots (.) represent ) are the curves calculated neglecting the observed values; thin lines (---) are the curves calculated by effects of wind-induced drifts; thick lines (including the effects of wind-induced drifts; arrows (I) indicate the times of sunrise and sunset at ground level.
downwards lowering h, and reducing n,. MAEDA (1963) predicted the existence of such a drift from the dynamo theory. Moreover, in a previous publication (VASSEUR and WALDTEUFEL, 1968), evidence was presented which indicated the presence of such a drift in the F-region. During the night, the observed value of n, is higher than given by the calculations, in spite of the F2-layer being lifted upwards into a region where the recombination rate is slower. Figure 2 (c) shows the computed and observed total vertical ion drift at the height Except for around sunrise, there is good agreement between the of the FZ-layer. During the sunrise period, however, the calculated ion two diurnal variations. concentration depends critically on the diffusion velocity which was calculated only The agreement between the observed and the computed values of the approximately. total ion drift velocities during the major part of the day indicates that the method
P-region winds deduced from Thomson scatter
781
used to separate the diffusion and the neutral air wind components of the observed ion motion is valid for this period. It may therefore be concluded that the wind-induced drifts can account for some of the major Summer F-region phenomena: they can explain very satisfactorily the diurnal variation of h, and the presence of the evening enhancement of n,. Some discrepancies between the observed and the computed values of n, remain, particularly during the night. (3) Vertical distributions of electron concentration. In Fig. 3 an observed vertical distribution of electron concentration is compared with two computed height
1*&I
2
5
10
m- 3
Fig. 3. Comparison of observed and calculated vertical electron concentration distributions. Dots (.) represent the observed values and the continuous curve ) the interpolated profile; the dashed curve (- - - - -) is the profile (p calculated by including the effects of electron temperature T,; the dotted curve (..... .) is the profile calculated assuming T, = T.
profiles ; one was computed using the observed value of T, and the other assuming T, = T. Above It,, the slope of the observed profile agrees better with the one computed using the observed values of T,. It therefore appears that in order to obtain a realistic model of the F-region the electrons and ions cannot be assumed Below h,, the computed profile appears to disagree to be in thermal equilibrium. with the one observed. This could be due to the neglect of some important chemical processes at low altitudes. (b) Winter results (1) Diurnal variations of T, T,, W,. Figure 4 (a) shows the diurnal variation of T as derived from the ion temperature at 275 km: T is lower during this period than during Summer 1967. In the atmospheric model assumed this seasonal temperature variation results in a seasonal variation of the neutral atmosphere density. For the electron temperature model the values of T, at the F&-layer peak were used, whereas in fact the height variation of T, may be important in winter as can be inferred from the measurements made at the different altitudes shown in Fig. 4 (b). The influence of these electron temperature gradients will be discussed later. During Winter 1966-67, the ion drift measurements were poor and it was not
G. VASSEUR
b
c
c
12
l6
L'T
c
Fig. 4. Diurnal variations on 10 January 1967 of, (a) the ion temperature T, at 275 km altitude assumed to be equal to the exospheric temperature T,,; (b) the electron temperature T, observed at three altitudes (250, 300, 350 km). The continuous curve represents the function T,(t) which was used in the computer program; (c) the component W, of the vertical ion drift attributed to horizontal neutral air winds, V,. A best fit analytical curve was assumed.
possible to derive a diurnal variation of W, for a particular day, as was the case for Summer 1967. Therefore, a mean diurnal variation of W, was used as obtained in the first part of this paper (GV 1) and is reproduced in Fig. 4 (c). (2) Diurnal variations of the F2-layer. Figure 5 (a,b,c) shows the observed and calculated diurnal variations of (a) h,,,, (b) 12, and (c) V(h,). Error bars have been added to the observed values of h,,, owing to the uncertainty of the night-time observations. The calculated variation of h, (Fig. 5 (a)) which included the effects of winds, agrees quite well with the observed data between 0000 and 0600 UT, but is significantly too high during the day.
F-region
winds deduced
from Thomson
783
scatter
b
msec -1 _: ‘.
0
h ‘t :
1’\
-20
C
‘t _
LT Fig.
5. Diurnal variation on 10 January 1967 of, (a) the height of the F&layer (b) the electron concentration, la,, at the peak of the FZ-layer; (c) the peak h,; total vertical ion drift velocity, V, at the peak of the F.&layer. Dots (.) represent the observed values. When necessary, error bars have been added; thin lines ) are the curves calculated neglecting the effects of wind-induced drifts; () are the curves calculated by including the effects of windthick lines (induced drifts; dashed lines (- - - - -) are the curves calculated by including the effects of wind-induced drifts, a small night-time ion production source QN and a 50 per cent Winter decrease in the molecular nitrogen concentration; arrows (I) indicate time of sunrise and sunset at ground level.
The calculated variation of the total ion drift velocity, VT,at the peak of the Flayer agrees well with the mean diurnal variation derived from observations made on four days during Winter 1966-67 (Fig. 5 (c)). However, the calculated values of n, (Fig. 6 (b)) which included the effects of winds are too small during the day due to the wind-induced drifts being downwards. At sunset, because the drifts are still downwards, n, decreases very rapidly reaching
784
G. VASSEUR
extremely low values of some 10 cm-3 just before sunrise even though the drifts at this time are upwards. These two main discrepancies correspond to two important ionospheric phenomena namely, (a) The maintenance of the night-time F-layer and (b) the mid-day n, Winter anomaly so called because the observed noon values of nnz are larger in Winter than in Summer. To obtain more satisfactory results, changes to the model assumed in the calculations are necessary such as, (a) a night-time source of ion production to account for the ionization found at night, and (b) seasonal changes of one or several ionospheric parameters to increase the daytime values of n,. (3) Modijication of the model. (3a) Seasonal changes in the composition of the neutral atmosphere. During Summer the model of the F-region given above accounts satisfactorily for the observed diurnal variation of electron concentration. The question of finding new values for these parameters for the Winter solstice now arises. It does not seem likely that the various cross-sections should exhibit a noticeable seasonal variation ; a variation of this kind can only arise from a seasonal temperature variation, and the temperature difference between the two seasons is quite small. The solar radio flux, as an index of the solar U.V. radiation does not change sufficiently to account for the seasonal difference. The seasonal variation of the ionospheric parameters (production and recombination rates) can therefore only be primarily due to variations in the composition of the neutral atmosphere. In particular a decrease in the molecular nitrogen concentration [NJ corresponds to a smaller recombination coefficient and results in a larger value of n, and a lower value of h,. Therefore it was assumed that, during Winter, the N, concentration is 50 per cent smaller than the Summer value. Such a seasonal variation of the neutral atmosphere does not seem unreasonable, and has often been put forward to explain the seasonal variation of the ionosphere (BECKER, 1966). However direct measurements of atmospheric composition do not give clear evidence for a variation of this kind. (3b) Night-time ion production. The existence of an ion production source during the night has often been suggested. Its origin might be either a corpuscular ionization phenomenon (IVANOV-KHOLODNY, 1963)) or a downward ionization flux coming from the protonosphere (HANSON and PATTERSON, 1964). Therefore. in the new winter model, a small ion production source QN has been added to the solar uv. production Q(x, h). Its expression is similar to equation (2) but does not depend upon x and is only 2 per cent of Q(0, h). During the day time, QAyis much smaller that the solar production but becomes the only production source at night. (3~) Diurnal variations of the FB-Zczyer. The dashed curves in Fig. 5(a,b.c) show the diurnal variations of h,, n, and V(h,) respectively, and were calculated by including the effects of wind-induced drifts and also by incorporating the modifications to the model mentioned above. During the day, h,,, (Fig. 5(a)) has been lowered by some 20 km when compared with the previous model, but the values are still somewhat higher than those observed. There is good agreement between the calculated variation of n, and that observed ; the reduction of [NJ increases the day time values of n, and, during the
F-region winds deduced from Thomson scatter
786
night, because of the night-time production source combined with the upward windinduced drifts, a weak secondary maximum of ‘12,is reproduced. This maximum is similar to the one observed, but occurs somewhat later. The changes to the model do not appear to appreciably alter the calculated variation of V&J. On the whole, this new model accounts satisfactorily for the observed Winter behaviour of the FZ-layer. (4) Vertical d~t~but~~s of electmn co~centrut~on. Figure 6, shows a comparison between an observed and calc~a~d vertical distribution of electron concentration.
‘\
Fig. 6. Comparison of observed and calculated vertical electron concentration distributions. Dots (.) represent the observed values and the continuous curve (------- ) is the interpoIated profde; the dashed curve (- - - - -) is the profhe calculated by including the effects of wind-induced drifts and a 50 per cent Winter decrease in the molecular nitrogen concentration.
Above the peak of the F2-layer the observed electron concentration decreases more rapidly with height than the calculated one. This effect can be interpreted as being caused by the important height gradient of T, which sometimes can be about 4°K km-l. Such a gradient of !Z’,would produce an additional component to the diffusion velocity which was neglected in GV I. At 250 km this additional component is negligible because of the small diffusion coefficient, but at 350 km it can be as high as 15 m see--l which could be sufficient to modify the vertical distribution of ionization to be in better agreement with the observed n(n) profile. Amore satisfactory model for the Winter FZ-layer should therefore take into account the observed temperature gradients in the F-region. 4.
CONCLUSION
Vertical ion drifts, obtained from Thomson scatter measurements and interpreted as being caused by neutral air winds, have been used in solutions of the F-layer continuity equation. The computed results when compared with the observed values of ion concentration and velocity, suggest that some of the problems associated with the behaviour of the FZ-layer can be explained. (1) Sunarmerresults. The calculated and observed diurnal variations of h,,,, n, a’nd J’ agree quite well; in particular the ‘mid-day bite out’ and the ‘evening enhancement’ of n, can be attributed to ion drifts induced by neutral air winds. The
786
G. VASSEUR
night-time maintenance of ionization is related to the presence during the late afternoon and night of an upward drift of ionization which decreases the ionization ~~omb~ation rate at the peak of the P&layer. The calc~a~d and observed vertical distributions of electron concentration are very similar, particular above the peak of the layer. 2. Winter results. The calculated and observed diurnal variations of h, agree during the night but not during the day. The rapid decrease of n, after sunset seems to be associated with a downward drift of connation which increases the recombination rate at the peak of the layer. However there are many problems which cannot be explained solely by the effects of neutral air winds, and other factors must be considered. (a) Phenomena not explained by the effects of neutral air winds. (1) The amplitude of the diurnal ~~~~at~on n, dying Summer. The calculated values of nsn were too high during the afternoon and too low during the night. The effect of an East-West electric field could account for the afternoon discrepancy, and a diurnal variation of the recombination rate could explain the high electron concentration observed at night. However, no attempt has been made here to evaluate quantitatively their effects. (2) The mu~ntenunce of the night time ~~~~~t~~ dur~~ Winter. This phenomenoI1 may be accounted for by the simultaneous action of neutral air winds, driving ionization upwards, and a night time source of ionization with a production rate amounting to 2 per cent of the day time solar U.V. production rate. (3) The mid day n, Winter anomaly. The calculations in which the same neutral atmosphere model was used for both seasons, did not account for the observed mid-day n, seasonal variation. In order to explain this phenomenon, a seasonal variation of the neutral atmosphere composition must be considered; a decrease of the N, concentration by 50 per cent gave satisfactory results. (4) Vertical distributions of electron concentration during Winter. With the simple model used? the calculated values of electron concentration decreased with height The height gradients of !P, which sometimes appears less rapidly than observed. too be important during Winter, could account for the observed behaviour. (b) Discussions and extensions The preceding results have been obtained by means of several simplifying assumptions and the following improvements could be made. (I) ~eterrn~~~on of the wind induces drifts. For the calculation of W,, an approximate evaluation of the diffusion velocity has been used. Moreover, altitude gradients of W, have been neglected. A more precise evaluation of W, will soon be possible due to improvements in the accuracy and the temporal resolution of the Thomson scatter equipment. However it must be emphasised that the evaluation of the total ion velocities from the integration of the F-layer continuity equation were in good a~eement with the values observed. This seems to favour the method used for deriving values of W, particularly as the values of the horizontal wind velocity assumed to produce W, were in close agreement with those derived theoretically from horizontal pressure gradients in the atmosphere.
F-region winds deduced from Thomson scatter
787
(2) Model of the F-region. Some of the assumptions concerning the various terms of the continuity equation need some discussion. The simple atmospheric model is not valid at low altitudes and a more refined one ought to be used. However, the actual seasonal and diurnal behaviour of the atmosphere is still uncertain and the choice of a realistic model is a difficult problem. The choice of values for some of the various cross-sections may be erroneous. In particular, the recombination coefficient may vary with electron temperature. The electron (and ion) temperature gradients have been neglected whereas, on some occasions, they could play an important role in the behaviour of the F-region. X realistic calculation should include their effects. (3) Measurements of the ion velocity. If there is a transverse electric field in the ionosphere, movements of the ionization perpendicular to the direction of observation may take place. These components of the ion velocity could be observed by using a multistatic Thomson scatter sounder. It would seem highly desirable to have this facility available. Acknowledgements-1 would like to thank Dr. M. PETIT, M. H. CARRU and M. P. WALDTEUFEL for fruitful discussions and Dr. J. 1%‘. KING and Mr. D. ECCLES for their helpful remarks. The assistance of Mrs. GUERARD for the computation program and Mrs. BOUSQTTET for data processing is gratefully acknowledged. The Thomson scatter facility of Saint-Santin-Nancay was built with funds of the Centre Sa,tional d’Etudes Spatiales and is supported by the Centre National de la Recherche Scient ifiqur. REFERENCES 1966
DALGARNO A. GEISLER J. E. HANSON W. B. and PATTERSON T. N. L. IVANOV-KHOLODNY G. S. KOHL H. and KING J. W. KOHL H., KING J. W. and ECCLES D. MAEDA H.
1961 1966 1964
SI~OLET M.
1960
RISRBETH H. THOMAS L. VASSEUR 0. and WALDTEUFEL P. MASSEUR G.
1967 1966 1968
1963 1967 1968 1963
Electron Density Profiles in Ionosphere and Exosphere, p. 217. North-Holland, Amsterdam. J. Atmosph. Terr. Phys. 26 939. J. Atmosph. Terr. Phys. 28, 703. Planet. Space Sci. 12, 979. Planet. Space Sci. 10, 219. J. Atmosph. Terr. Phys. 29, 1045. J. Atmosph. Terr. Phya. 30, 1733. Proc. Conf. Ionosph. London Institute of Physics and the Physical Society, London. Space Research I, p. 46. North-Holland, Amsterdam. J. Atmosph. Terr. Phys. 29, 225. J. geophya. Res. 71, 1357. J. Atmoaph. Terr. Phys. 30, 779. J. Atmosph. Terr. Phys. 31, 397.
Dynamics of the F-region observed with Thomson scatter-II Mlaence of neutral air winds on the ionospheric F-region G. VASSEUR C’.N.E.T./R.S.R., 3 Avenue de la Republique, 92, Issy-les-Moulineaux, France (Received
20 September
1969)
Ah&&-In a previous paper the author described diurnal variations of the meridional component of neutral air winds which were derived from Thomson scatter measurements of ion drifts in the F-region. In this paper solutions of the F-layer continuity equation are presented which take account of the effects of these wind-induced vertical drifts. Various F-region phenomena including the evening enhancement of n, and the diurnal variation of the height of the F&-layer peak are shown to be associated with neutral sir winds. There are some discrepanciesbetween the calculated and observed behaviour of the F-region which suggest that there is a night-time ion production source in Winter and a seasonal v&&ion of the neutral atmosphere composition. 1. INTRODUCTION (a) A summary of previous work
Since June 1966, the French Thomson scatter sounder (Saint-Santin-NanCay) has been used to measure the component of F-region ion drift in a direction which lies very nearly along a magnetic field line. In a previous paper (VASSEUR, 1969; hereafter referred to as GV l), the author discussed ion velocity measurements in the F-region. The velocities observed were interpreted as being the resultant of (a) the diffusion velocity and (b) the ion velocity induced by neutral air winds. Both these components are along the Earth’s magnetic field line and it was assumed that the wind-induced motion was due to the North-South component, ‘v, of the neutral air wind. Diurnal variations of ?‘7, (for Winter, 1966-67 and Summer, 1967) were described; these were inferred by subtracting from the total observed ion motion, an estimate of the diffusion velocity which was obtained from the Thomson scatter measurements of ion concentration and temperature. The values of V, were found to be directed equatorwards during the night, and polewards during the day. When compared with various theoretical calculations (KOHL and Krxa, 1967 ; GEISLER, 1966), these values of V, show a general agreement but some discrepancies remain. In particular, during Summer, the time at which V, becomes equatorwards occurred in the early afternoon rather than in the evening as the theoretical calculations suggest. (b) Purpose of the present study Various authors (HANSON and PATTERSON, 1964;
RISHBETH, 1967, KOHL and
KING, 1967 ; KOHL, KING and ECCLES, 1968) have suggested that some peculiarities of the ionospheric F-region can be attributed to the effects of neutral air winds; the aim of this paper is to use the values of Vv, previously derived from observations (GV 1) to check whether they can indeed account for the various phenomena taking
place in the F2-region
of the ionosphere. 775
G. VASSEUR
776
In order to calculate the effects of neutral air winds on the ionosphere, it is necessary to solve the F-layer continuity equation and to include a term representing the wind-induced ion motion. The assumptions made and the method used to solve t,his equation are outlined in Section 2. In Section 3 the results of the calculations for Summer and Winter conditions are compared with the observed ion concentrat’ions and velocities. It will be shown that the neutral air winds cannot account for all the observed phenomena and additional mechanisms are suggested to account for the discrepancies. 2. THE F-LAYER
CONTINUITY EQUATION
(a) Assumptions
The F-layer continuity
equation may be written as ;
= Q -
L -
V(nV)
(1)
where n is the ion concentration, Q is the ion production rate, L is the chemical loss rate, a,nd V is the ion velocity. In the atmospheric model used, it was assumed that the concentration of each constituent remained constant at 200 km altitude and that the atmosphere was isothermal above this level. A model of this nature depends entirely on the exospheric temperature (which can be derived from Thomson scatter measurements) and, for F-region heights, it agrees well with the model suggested by NICOLET (1960). The following assumptions were made for the various terms in the equation: (1) Ion production. The production of N,+ ions was neglected ; O+ ion production was assumed to be given by,
&(x9h) = Plea,
exp [-arch(x) j)N
dh]
where [0] is the atomic oxygen concentration, x the solar zenith angle, ch(X) the Chapman function, G and G’ the ionization and absorption cross-sections respectively of 0 and S, the flux of incident photons. The parameter cr’ was adjusted so that Q(0, h) was a maximum at about 160 km; the value of 0 assumed was such that Q(0, 225 km) N log m3 see-l as derived from Thomson scatter measurements (VASSEUR and WALDTEUFEL, 1968). (2) Ion loss. The loss term, L, was assumed to arise from an ion-molecule charge exchange process between the 0+ ions and N, molecules, with the formation of short lived NO+ ions. A similar reaction is possible with 0, molecules but this was ignored. The loss term is thus ‘linear’ and is given by, L = /?n = y[NJn. The value of y used was 1.6 x lo-l2 cm3 set-l. It was adjusted in order to give &225 km) N 0.25 x 1O-2 set-l as derived from Thomson scatter measurements (VASSEUR and WALDTEUFEL, 1968). (3) Ion
transport.
Only vertical
gradients
were considered
which reduce
the
777
P-region winds deduced from Thomson scatter
flux divergence
term, to
V(nV) = y In the absence of an East-West J”‘, may be written (cf. GV 1) as,
.
electric field, the total vertical ion drift velocity v = w, + w,
(2)
where W, is the ambipolar diffusion velocity and was calculated (see GV 1, equation (13)), by ignoring the effects of temperature gradients. The ion and neutral temperatures were assumed to be equal, but the difference between the electron and ion temperatures, as derived from the Thomson scatter data, was taken into account. The value of the ambipolar diffusion coefficient was calculated from the expression given by DALGARNO (1964). W, is the vertical component of the wind-induced ion velocity and is given by
W, = V, sinIcos1 where V, is the meridional neutral air wind velocity, being positive when directed equatorwards, and I is the magnetic dip angle. Vertical gradients of W,, were neglected. (b) Solutions of the F-layer continuity equation With the assumptions described, equation (1) becomes a partial derivative equation in n(TL,t). The following boundary conditions were made: n(h,t)-+Owhenh+-co n(h, t) varies like exp [--h/H(l
+
T,/T]
when h +
+co
where H is the scale height of atomic oxygen and T, and T are the electron and The high altitude boundary condition means neutral temperatures respectively. that diffusive equilibrium is reached. The following transformations were used : II: = exp [--h/H(l
+ T,/T]
and
n = x . y.
The numerical solution of the equation was obtained by replacing derivatives by finite differences using an implicit method. The initial condition was the equilibrium solution at mid-day and the integration was continued, with step widths of At = O-2 hr until a periodic solution was obtained to give n(TL,t) between 0 and 2000 km. The values of W,, obtained from equation (2), by estimating W, and measuring V, were used in these solutions of the P-layer continuity equation; the new values of W, derived in these calculations were combined with W, to obtain theoretical values of ‘v. Comparisons between these theoretical and observed values of I’ give an indication of how reliable the technique is for deriving W, from the total ion drift motion measured by the Thomson scatter sounder. 3. RESULTS OF THE CALCULATIONS As in the first part of the article (GV l), two typical solstice periods were considered; Winter 1966-67 and Summer 1967. In GV 1, diurnal variations of W,, were
778
G. VABSEUR
obtained as a mean from several periods of observation during Summer 1967 and Winter 196667. In this paper, however, two particular days (one for each season) are considered for which there were data available for the F-region from 200 to 400 km. The two days are lo-11 January 1967 and 18-19 July 1967 when the mean 10.7 cm flux values were 142 and 126 1O-22 Wm-2 Hz-l respectively. These particular days were magnetically quiet, and were considered as typical of the two solstice periods. A description is given of diurnal variations of T, T, and W, which were obtained from the Thomson scatter measurements and which were used to solve the F-layer continuity equation. (a) Summer results (1) Diurnal variations ofT,T,, W,. The neutral temperature can be derived from measurements of ion temperature, electron temperature, and electron concentration, by solving the energy transfer equation for ions, but at about 275 km it is reasonable to assume that the ion temperature is very close to the exospheric neutral temperature. Figure 1 (a) therefore shows the diurnal variation of the neutral temperature using this approximation. Figure 1 (b) shows the diurnal variation of electron temperature T, for several altitudes near the peak of the F2-layer. Vertical gradients of T, have been neglected and the Thomson scatter results at various altitudes show that, in fact, near the peak of the FB-layer, T, does not vary appreciably with altitude. The diurnal variation of W, was derived from the observed ion velocity I’(cf. GV 1, equation (22)), by calculating the diffusion velocity W, from h, and T. Figure 1 (c,d,e) shows the diurnal variations of I’ and jL, which were obtained from Thomson scatter measurements, and also the derived variation of W,. (2) Diurnal variations of thx F2-layer. Figure 2 (a,b,c) shows a comparison between the observed and computed diurnal variations of (a) h,, the height of the FB-layer, (b) n,, the peak electron concentration, and (c) V(h,), the vertical component of the total ion velocity at the peak of the F&layer. It can be seen that when the ion drift due to neutral air winds are taken into account, the theoretical diurnal variation of hm is in fairly good agreement with the observations. Between 1300 UT and 2000 UT, however, the computed values of h, are somewhat higher than those observed. The theoretical diurnal variation of n, (Fig. 2 (b)) shows a decrease at about 1200 UT, followed by a large enhancement at 1900 UT very similar to the one observed. The presence of this evening enhancement is mainly due to an upward wind-induced ion drift which raises the height of the F2-layer. However, it can be seen from Fig. 2 (b) that the variation of n, which was computed without including the effects of winds also exhibits a maximum at about 1700 UT. This maximum is due to the decrease of T, after 1500 UT ; a decrease of this kind is accompanied by an increase of n,, as can be deduced from theoretical computation of a stationary layer (THOMAS, 1966). It appears that the presence of this evening enhancement arises from the simultaneous effects of neutral air winds and of electron temperature variations. During the afternoon however, the computed values of n, and h, are somewhat too high. This could be explained by the presence of an electrodynamic drift directed
P-region winds deduced from Thomson scatt’er
00
06
12
18
24
LT
Fig. 1. Diurnal variations on 18th July 1967 of, (a) the ion temperature Ti at 275 km altitude assumed to be equal to the exospheric temperature Tsz; (b) the electron temperature T, observed at three ahitudes (250, 300, 350 km); (c) the observed vertical ion drift velocity V in the altitude range 200-400 km; (d) the height of the P2-layer peak, h,, rss derived from Thomson scatter measurements; (e) the component TV,, of the vertical ion drift velocity attributed to horizontal neutral air winds V,.,. Also shown is the evaluated diffusion velocity W, using the values of T,, and h, shown in (a) and (d).
779
G. VAYSEUR
780
km
a
18.07.67 1011m-3
nm
m
C
Fig. 2. Diurnal variations on 18 July 1967 of, (a) the height of the P2-layer peak, h,; (b) the electron concentration m, at the peak of the PB-layer; (c) the total vertical ion drift velocity, V, at the peak of the P2-layer. Dots (.) represent ) are the curves calculated neglecting the observed values; thin lines (---) are the curves calculated by effects of wind-induced drifts; thick lines (including the effects of wind-induced drifts; arrows (I) indicate the times of sunrise and sunset at ground level.
downwards lowering h, and reducing n,. MAEDA (1963) predicted the existence of such a drift from the dynamo theory. Moreover, in a previous publication (VASSEUR and WALDTEUFEL, 1968), evidence was presented which indicated the presence of such a drift in the F-region. During the night, the observed value of n, is higher than given by the calculations, in spite of the F2-layer being lifted upwards into a region where the recombination rate is slower. Figure 2 (c) shows the computed and observed total vertical ion drift at the height Except for around sunrise, there is good agreement between the of the FZ-layer. During the sunrise period, however, the calculated ion two diurnal variations. concentration depends critically on the diffusion velocity which was calculated only The agreement between the observed and the computed values of the approximately. total ion drift velocities during the major part of the day indicates that the method
P-region winds deduced from Thomson scatter
781
used to separate the diffusion and the neutral air wind components of the observed ion motion is valid for this period. It may therefore be concluded that the wind-induced drifts can account for some of the major Summer F-region phenomena: they can explain very satisfactorily the diurnal variation of h, and the presence of the evening enhancement of n,. Some discrepancies between the observed and the computed values of n, remain, particularly during the night. (3) Vertical distributions of electron concentration. In Fig. 3 an observed vertical distribution of electron concentration is compared with two computed height
1*&I
2
5
10
m- 3
Fig. 3. Comparison of observed and calculated vertical electron concentration distributions. Dots (.) represent the observed values and the continuous curve ) the interpolated profile; the dashed curve (- - - - -) is the profile (p calculated by including the effects of electron temperature T,; the dotted curve (..... .) is the profile calculated assuming T, = T.
profiles ; one was computed using the observed value of T, and the other assuming T, = T. Above It,, the slope of the observed profile agrees better with the one computed using the observed values of T,. It therefore appears that in order to obtain a realistic model of the F-region the electrons and ions cannot be assumed Below h,, the computed profile appears to disagree to be in thermal equilibrium. with the one observed. This could be due to the neglect of some important chemical processes at low altitudes. (b) Winter results (1) Diurnal variations of T, T,, W,. Figure 4 (a) shows the diurnal variation of T as derived from the ion temperature at 275 km: T is lower during this period than during Summer 1967. In the atmospheric model assumed this seasonal temperature variation results in a seasonal variation of the neutral atmosphere density. For the electron temperature model the values of T, at the F&-layer peak were used, whereas in fact the height variation of T, may be important in winter as can be inferred from the measurements made at the different altitudes shown in Fig. 4 (b). The influence of these electron temperature gradients will be discussed later. During Winter 1966-67, the ion drift measurements were poor and it was not
G. VASSEUR
b
c
c
12
l6
L'T
c
Fig. 4. Diurnal variations on 10 January 1967 of, (a) the ion temperature T, at 275 km altitude assumed to be equal to the exospheric temperature T,,; (b) the electron temperature T, observed at three altitudes (250, 300, 350 km). The continuous curve represents the function T,(t) which was used in the computer program; (c) the component W, of the vertical ion drift attributed to horizontal neutral air winds, V,. A best fit analytical curve was assumed.
possible to derive a diurnal variation of W, for a particular day, as was the case for Summer 1967. Therefore, a mean diurnal variation of W, was used as obtained in the first part of this paper (GV 1) and is reproduced in Fig. 4 (c). (2) Diurnal variations of the F2-layer. Figure 5 (a,b,c) shows the observed and calculated diurnal variations of (a) h,,,, (b) 12, and (c) V(h,). Error bars have been added to the observed values of h,,, owing to the uncertainty of the night-time observations. The calculated variation of h, (Fig. 5 (a)) which included the effects of winds, agrees quite well with the observed data between 0000 and 0600 UT, but is significantly too high during the day.
F-region
winds deduced
from Thomson
783
scatter
b
msec -1 _: ‘.
0
h ‘t :
1’\
-20
C
‘t _
LT Fig.
5. Diurnal variation on 10 January 1967 of, (a) the height of the F&layer (b) the electron concentration, la,, at the peak of the FZ-layer; (c) the peak h,; total vertical ion drift velocity, V, at the peak of the F.&layer. Dots (.) represent the observed values. When necessary, error bars have been added; thin lines ) are the curves calculated neglecting the effects of wind-induced drifts; () are the curves calculated by including the effects of windthick lines (induced drifts; dashed lines (- - - - -) are the curves calculated by including the effects of wind-induced drifts, a small night-time ion production source QN and a 50 per cent Winter decrease in the molecular nitrogen concentration; arrows (I) indicate time of sunrise and sunset at ground level.
The calculated variation of the total ion drift velocity, VT,at the peak of the Flayer agrees well with the mean diurnal variation derived from observations made on four days during Winter 1966-67 (Fig. 5 (c)). However, the calculated values of n, (Fig. 6 (b)) which included the effects of winds are too small during the day due to the wind-induced drifts being downwards. At sunset, because the drifts are still downwards, n, decreases very rapidly reaching
784
G. VASSEUR
extremely low values of some 10 cm-3 just before sunrise even though the drifts at this time are upwards. These two main discrepancies correspond to two important ionospheric phenomena namely, (a) The maintenance of the night-time F-layer and (b) the mid-day n, Winter anomaly so called because the observed noon values of nnz are larger in Winter than in Summer. To obtain more satisfactory results, changes to the model assumed in the calculations are necessary such as, (a) a night-time source of ion production to account for the ionization found at night, and (b) seasonal changes of one or several ionospheric parameters to increase the daytime values of n,. (3) Modijication of the model. (3a) Seasonal changes in the composition of the neutral atmosphere. During Summer the model of the F-region given above accounts satisfactorily for the observed diurnal variation of electron concentration. The question of finding new values for these parameters for the Winter solstice now arises. It does not seem likely that the various cross-sections should exhibit a noticeable seasonal variation ; a variation of this kind can only arise from a seasonal temperature variation, and the temperature difference between the two seasons is quite small. The solar radio flux, as an index of the solar U.V. radiation does not change sufficiently to account for the seasonal difference. The seasonal variation of the ionospheric parameters (production and recombination rates) can therefore only be primarily due to variations in the composition of the neutral atmosphere. In particular a decrease in the molecular nitrogen concentration [NJ corresponds to a smaller recombination coefficient and results in a larger value of n, and a lower value of h,. Therefore it was assumed that, during Winter, the N, concentration is 50 per cent smaller than the Summer value. Such a seasonal variation of the neutral atmosphere does not seem unreasonable, and has often been put forward to explain the seasonal variation of the ionosphere (BECKER, 1966). However direct measurements of atmospheric composition do not give clear evidence for a variation of this kind. (3b) Night-time ion production. The existence of an ion production source during the night has often been suggested. Its origin might be either a corpuscular ionization phenomenon (IVANOV-KHOLODNY, 1963)) or a downward ionization flux coming from the protonosphere (HANSON and PATTERSON, 1964). Therefore. in the new winter model, a small ion production source QN has been added to the solar uv. production Q(x, h). Its expression is similar to equation (2) but does not depend upon x and is only 2 per cent of Q(0, h). During the day time, QAyis much smaller that the solar production but becomes the only production source at night. (3~) Diurnal variations of the FB-Zczyer. The dashed curves in Fig. 5(a,b.c) show the diurnal variations of h,, n, and V(h,) respectively, and were calculated by including the effects of wind-induced drifts and also by incorporating the modifications to the model mentioned above. During the day, h,,, (Fig. 5(a)) has been lowered by some 20 km when compared with the previous model, but the values are still somewhat higher than those observed. There is good agreement between the calculated variation of n, and that observed ; the reduction of [NJ increases the day time values of n, and, during the
F-region winds deduced from Thomson scatter
786
night, because of the night-time production source combined with the upward windinduced drifts, a weak secondary maximum of ‘12,is reproduced. This maximum is similar to the one observed, but occurs somewhat later. The changes to the model do not appear to appreciably alter the calculated variation of V&J. On the whole, this new model accounts satisfactorily for the observed Winter behaviour of the FZ-layer. (4) Vertical d~t~but~~s of electmn co~centrut~on. Figure 6, shows a comparison between an observed and calc~a~d vertical distribution of electron concentration.
‘\
Fig. 6. Comparison of observed and calculated vertical electron concentration distributions. Dots (.) represent the observed values and the continuous curve (------- ) is the interpoIated profde; the dashed curve (- - - - -) is the profhe calculated by including the effects of wind-induced drifts and a 50 per cent Winter decrease in the molecular nitrogen concentration.
Above the peak of the F2-layer the observed electron concentration decreases more rapidly with height than the calculated one. This effect can be interpreted as being caused by the important height gradient of T, which sometimes can be about 4°K km-l. Such a gradient of !Z’,would produce an additional component to the diffusion velocity which was neglected in GV I. At 250 km this additional component is negligible because of the small diffusion coefficient, but at 350 km it can be as high as 15 m see--l which could be sufficient to modify the vertical distribution of ionization to be in better agreement with the observed n(n) profile. Amore satisfactory model for the Winter FZ-layer should therefore take into account the observed temperature gradients in the F-region. 4.
CONCLUSION
Vertical ion drifts, obtained from Thomson scatter measurements and interpreted as being caused by neutral air winds, have been used in solutions of the F-layer continuity equation. The computed results when compared with the observed values of ion concentration and velocity, suggest that some of the problems associated with the behaviour of the FZ-layer can be explained. (1) Sunarmerresults. The calculated and observed diurnal variations of h,,,, n, a’nd J’ agree quite well; in particular the ‘mid-day bite out’ and the ‘evening enhancement’ of n, can be attributed to ion drifts induced by neutral air winds. The
786
G. VASSEUR
night-time maintenance of ionization is related to the presence during the late afternoon and night of an upward drift of ionization which decreases the ionization ~~omb~ation rate at the peak of the P&layer. The calc~a~d and observed vertical distributions of electron concentration are very similar, particular above the peak of the layer. 2. Winter results. The calculated and observed diurnal variations of h, agree during the night but not during the day. The rapid decrease of n, after sunset seems to be associated with a downward drift of connation which increases the recombination rate at the peak of the layer. However there are many problems which cannot be explained solely by the effects of neutral air winds, and other factors must be considered. (a) Phenomena not explained by the effects of neutral air winds. (1) The amplitude of the diurnal ~~~~at~on n, dying Summer. The calculated values of nsn were too high during the afternoon and too low during the night. The effect of an East-West electric field could account for the afternoon discrepancy, and a diurnal variation of the recombination rate could explain the high electron concentration observed at night. However, no attempt has been made here to evaluate quantitatively their effects. (2) The mu~ntenunce of the night time ~~~~~t~~ dur~~ Winter. This phenomenoI1 may be accounted for by the simultaneous action of neutral air winds, driving ionization upwards, and a night time source of ionization with a production rate amounting to 2 per cent of the day time solar U.V. production rate. (3) The mid day n, Winter anomaly. The calculations in which the same neutral atmosphere model was used for both seasons, did not account for the observed mid-day n, seasonal variation. In order to explain this phenomenon, a seasonal variation of the neutral atmosphere composition must be considered; a decrease of the N, concentration by 50 per cent gave satisfactory results. (4) Vertical distributions of electron concentration during Winter. With the simple model used? the calculated values of electron concentration decreased with height The height gradients of !P, which sometimes appears less rapidly than observed. too be important during Winter, could account for the observed behaviour. (b) Discussions and extensions The preceding results have been obtained by means of several simplifying assumptions and the following improvements could be made. (I) ~eterrn~~~on of the wind induces drifts. For the calculation of W,, an approximate evaluation of the diffusion velocity has been used. Moreover, altitude gradients of W, have been neglected. A more precise evaluation of W, will soon be possible due to improvements in the accuracy and the temporal resolution of the Thomson scatter equipment. However it must be emphasised that the evaluation of the total ion velocities from the integration of the F-layer continuity equation were in good a~eement with the values observed. This seems to favour the method used for deriving values of W, particularly as the values of the horizontal wind velocity assumed to produce W, were in close agreement with those derived theoretically from horizontal pressure gradients in the atmosphere.
F-region winds deduced from Thomson scatter
787
(2) Model of the F-region. Some of the assumptions concerning the various terms of the continuity equation need some discussion. The simple atmospheric model is not valid at low altitudes and a more refined one ought to be used. However, the actual seasonal and diurnal behaviour of the atmosphere is still uncertain and the choice of a realistic model is a difficult problem. The choice of values for some of the various cross-sections may be erroneous. In particular, the recombination coefficient may vary with electron temperature. The electron (and ion) temperature gradients have been neglected whereas, on some occasions, they could play an important role in the behaviour of the F-region. X realistic calculation should include their effects. (3) Measurements of the ion velocity. If there is a transverse electric field in the ionosphere, movements of the ionization perpendicular to the direction of observation may take place. These components of the ion velocity could be observed by using a multistatic Thomson scatter sounder. It would seem highly desirable to have this facility available. Acknowledgements-1 would like to thank Dr. M. PETIT, M. H. CARRU and M. P. WALDTEUFEL for fruitful discussions and Dr. J. 1%‘. KING and Mr. D. ECCLES for their helpful remarks. The assistance of Mrs. GUERARD for the computation program and Mrs. BOUSQTTET for data processing is gratefully acknowledged. The Thomson scatter facility of Saint-Santin-Nancay was built with funds of the Centre Sa,tional d’Etudes Spatiales and is supported by the Centre National de la Recherche Scient ifiqur. REFERENCES 1966
DALGARNO A. GEISLER J. E. HANSON W. B. and PATTERSON T. N. L. IVANOV-KHOLODNY G. S. KOHL H. and KING J. W. KOHL H., KING J. W. and ECCLES D. MAEDA H.
1961 1966 1964
SI~OLET M.
1960
RISRBETH H. THOMAS L. VASSEUR 0. and WALDTEUFEL P. MASSEUR G.
1967 1966 1968
1963 1967 1968 1963
Electron Density Profiles in Ionosphere and Exosphere, p. 217. North-Holland, Amsterdam. J. Atmosph. Terr. Phys. 26 939. J. Atmosph. Terr. Phys. 28, 703. Planet. Space Sci. 12, 979. Planet. Space Sci. 10, 219. J. Atmosph. Terr. Phys. 29, 1045. J. Atmosph. Terr. Phya. 30, 1733. Proc. Conf. Ionosph. London Institute of Physics and the Physical Society, London. Space Research I, p. 46. North-Holland, Amsterdam. J. Atmosph. Terr. Phys. 29, 225. J. geophya. Res. 71, 1357. J. Atmoaph. Terr. Phys. 30, 779. J. Atmosph. Terr. Phys. 31, 397.