Altitude profile of H in the atmosphere of Venus from Lyman α observations of Venera 11 and Venera 12 and origin of the hot exospheric component

zcArus 52, 221-244 (1982)

Altitude Profile of H in the Atmosphere of Venus from Lyman oL Observations of Venera 11 and Venera 12 and Origin of the Hot Exospheric Component 1 J. L. BERTAUX AND V. M. LEPINE Service d'A~ronomie du CNRS, Verri~res-le-Buisson, 91370 France AND

V. G. KURT AND A. S. SMIRNOV Institute of Cosmic Investigation ( IK1), Academy of Sciences, 88 Profsoyouznaya Moscow, 117485 USSR Received March 25, 1982; revised July 19, 1982 Two extreme ultraviolet (EUV) spectrophotometers flown in December 1978 on Venera 11 and Venera 12 measured the hydrogen Lyman a emission resonantly scattered in the atmosphere of Venus. Measurements were obtained across the dayside of the disk, and in the exosphere up to 50,000 km. They were analyzed with spherically symmetric models for which the radiative transfer equation was solved. The H content of the Venus atmosphere varies from optically thin to moderately thick regions. A shape fit at the bright limb allows one to determine the exospheric temperature Tc and the number density nc independently of the calibration of the instrument or the exact value of the solar flux. The dayside exospheric temperature was measured for the first time in the polar regions, with T¢ = 300 _+ 25°K for Venera 11 (79°S) and Tc = 275 -+ 25°K (59°S) for Venera 12. At the same place, the density is nc = 4"_~ × 104 atom.cm -3, and the integrated number density Nt from 250 to 110 km (the level of CO2 absorption) is 2.1 × 1012atom.cm -2, a factor of 3 to 6 lower than that predicted in aeronomical models. This probably indicates that the models should be revised in the content of H-bearing molecules and should include the effect of dynamics. Across the disk the value of Nt decreases smoothly with a total variation of two from the morning side to the afternoon side. Alternately it could be a latitude effect, with less hydrogen in the polar regions. The nonthermal component if clearly seen up to 40,000 km of altitude. It is twice as abundant as at the time of Mariner 10 (solar minimum). Its radial distribution above 4000 km can be simulated by an exospheric distribution with T = 103°K and n = 103 atom.cm -3 at the exobase level. However, there are less hot atoms between 2000 and 4000 km than predicted by an ionospheric source. A byproduct of the analysis is a determination of a very high solar Lyman et flux of 7.6 × 10" photons (cm2 sec/~)-~ at line center (1 AU) in December 1978.

INTRODUCTION

Hydrogen atoms present in the upper atmosphere of Venus are emitting Lyman ot (Lot) radiation through resonance scattering of solar Lot photons. Practically all space missions to Venus from 1967 to 1978 have carried instruments capable of measuring the scattered Lot radiation (photometers, spectrophotometers, or spectrometers). In1Paper presented at "An International Conference on the Venus Environment," Palo Alto, California, November 1-6, 1981.

terest for H atoms in the upper atmosphere of Venus arises from several factors. They are produced through aeronomic reactions from H-bearing molecules ( H 2 0 , HCI, HF, H2), which play a very important role in the photochemistry of stratosphere, mesosphere, and thermosphere (Kumar, 1982). Venus is severely deficient in water compared to Earth, and H escape processes are of great interest for the history of H20 content since the formation of Venus. Production of H is restricted to the dayside, but day-to-night thermospheric winds of the 221 0019-1035/82/110221-24502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

222

BERTAUX ET AL.

major gas CO2 bring H to the nightside, where it recombines at lower altitudes (Hartle et al., 1978). Therefore H on the dayside is controlled by both photochemistry and dynamics (wind advection, eddy diffusion, escape), whereas on the nightside it can be considered as a tracer of thermospheric dynamics. The scale height of H in the exosphere is related to the exospheric temperature, providing in principle a simple way to derive this important parameter of the upper thermosphere from L a observations at the limb. However, measurements of Mariner 5 (Barth et al., 1967) indicated a distribution with two different scale heights, one being roughly double the other. In addition to a "normal" population of H atoms, the presence of H2, or deuterium atoms, was suggested (Barth, 1968) and later refuted. It is now clear that the large scale height has to be attributed to nonthermal " h o t " H atoms, after a number of theoretical works and some observations [for a review on this subject, see, for instance, Kumar et al. (1978) and Kumar (1982)]. Preliminary results obtained by two E.U.V. spectrophotometers flown near Venus in 1978 have already been published (Bertaux et al., 1981). This paper presents an analysis of the L a measurements obtained during these missions and a determination of H quantity in the thermosphere, the exospheric temperature, and the structure of the nonthermal H atom distribution. OBSERVATIONS

The multichannel extreme ultraviolet spectrophotometers placed on board Venera 11 and Venera 12 have been previously described in some detail (Bertaux et al., 1981). It uses a mechanical collimator, a holographic objective grating, and 10 detectors placed in the focal surface of the grating, at the focusing positions of 10 wavelengths of interest. The field of view is 0.33 (at half-maximum) by 1.1 °, with the long side perpendicular to the ecliptic plane (Fig. 1). The detector No. 7 is placed at L a

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and picks up photons in a bandwidth of 2 nm. Photoelectrons are counted each second and it is assumed that they all come from L a photons (except for a small contribution of "dark counts", 0.76 and 1.33 counts.sec -I, respectively, for Venera 11 and Venera 12). The sensitivity a of the instrument has been estimated to be 0.026 counts.sec -I at La for an intensity of one Rayleigh (1 Rayleigh = 106/(4"rr)photons.cm -2 sec -1 ster-~). However, as will be discussed later, the determination of absolute concentration of H in the atmosphere of Venus will not depend on the exact value of the sensitivity of the instrument, which presumably is not known precisely. When flying by the planet Venus on its hyperbolic orbit on the dayside, the spacecraft and the line of sight of the instrument both kept a fixed celestial orientation. The orbital drift of the spacecraft at a velocity of =6 km.sec -1 provided a scan of the line of sight across the disk and the extended H corona of Venus, the cythereocorona, at about the same velocity. The track of the line of sight is displayed in Fig. 1, where the disk of Venus has been represented with the south pole at the top for convenience.

VENUS OBSERVATIONS OF HYDROGEN Projected on the planet, the full size of the field of view was 770 × 220 km, from the pericenter distance of 40 × 103 km (Fig. 1). Each 30 sec the region of the cythereocorona observed through the FOV is displaced by -~180 km, approximately the width of the FOV. Geometrical parameters relevant to the geometry of observations are summarized in Table I and displayed in Fig. 2, such as local time and latitude of the point of the line of sight intersecting a sphere of 6200 km. The bright limb crossing, on the afternoon side, was made at a rather high (south) latitude (respectively 79 and 59 ° for Venera 11 and Venera 12). The orientation of the FOV was not parallel to the limbs, and it covered a range of altitude of a few hundred kilometers.

TABLE I GEOMETRICAL PARAMETERS Parameter Time of encounter (pericenter) (UT) Pericenter distance (km) Angle of line of sight with V e n u s ~ u n line (o) Minimum impact parameter of the line of sight (km) Maximum impact parameter (kin) Time of dark limb crossing (UT) Time of bright limb crossing (UT) Phase angle (Sun-Venus-S/C at pericenter (°)) Solar zenith angle at bright limb (°) Latitude at bright limb (°) Local time at bright limb (hr) Solar zenith angle at dark limb Latitude at dark limb Local time at dark limb (hr) Vertical extent of the projected FOV (km): - - a t bright limb - - a t dark limb

Venera I 1

Venera 12

03 hr 23 rain 58 sec Dec. 25, 1978 40,412

03 hr 26 min 42 sec Dec. 21, 1978

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After automatic elimination of some spurious data points (telemetry errors), 30 successive points were averaged together, and the dispersion tr was calculated. The "error" bar is ---tr/~/3-O. The Lot results are displayed on a logarithmic scale for Venera ! 1 and Venera 12 in Figs. 3a and b respectively. Each vertical bar is centered on the average, its length being 2cr/V'3-O. The time is flowing from left to right, as in Fig. 1. Different regions of interest are clearly identified on the Lot signal of Fig. 3a. The interplanetary background is measured far from the disk, with a growing contribution of the corona when the line of sight approaches the planet. Assuming that the upper atmosphere is rotating in the same direction as the uv marks of the cloud top, a local time system can be defined. The line of sight crossed the dark limb of the planet,

BERTAUX ET AL.

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Fio. 3(a) The counting rate of the Lyman a channel of the Venera 11 EUV spectrophotometer is plotted as a function of time. Each point contains an error bar of ___~r/x/3-6.The interplanetary background is measured at the end of the measurements. The H exosphere of Venus is seen on both sides of the planet, morning and afternoon. The bright limb crossing is made in the polar region. (b) Same as (a) for the Venera 12 EUV spectrophotometer.

and the m o r n i n g terminator slightly afterwards. T h e illuminated disk w a s s w e p t a c r o s s until the bright limb w a s r e a c h e d . T h e phase angle w a s o f the order o f 19 ° at pericenter. T h e n , as the i m p a c t p a r a m e t e r o f the line o f sight i n c r e a s e d (up to 50 × 103 k m for V e n e r a 11), the Lot contribution o f the c y t h e r e a n H e x o s p h e r e d e c r e a s e d and

the pure Lot interplanetary b a c k g r o u n d w a s reached• Out o f the planetary disk the afternoon side o b s e r v a t i o n s o f the l o w e r e x o s p h e r e are easier to interpret for t w o r e a s o n s . First, the terminator is not s e e n on the afternoon side, and all a t o m s on the line o f sight are directly illuminated by the solar

VENUS OBSERVATIONS OF HYDROGEN Let. Second, since there is a large asymmetry of the temperature distribution between day and night (Keating et al., 1980), one can also expect an asymmetry in the H distribution, with a complex situation around the terminator, observed on the morning side. In order to obtain the pure H corona Let signal, the " d a r k " counting rate Md was subtracted from all measurements, and the counting rate Mb corresponding to the interplanetary background was subtracted from all measurements with lines of sight not intersecting the disk of Venus. The asymptotic value Ma reached by the counting rate at the end of the measurements provides the sum Mb + Md. Md was estimated to be 0.76 and 1.33 counts.sec -l, yielding, by subtraction from Ma, an interplanetary background of 21.2 and 24.7 counts.sec -1, respectively, for Venera I 1 and Venera 12. After subtraction of Mb and Md, the remaining counting rate was divided by the assumed sensitivity of the instrument of 0.026 counts.sec -1 per Rayleigh to obtain " p u r e l y " cytherean Let apparent emission rates, displayed in Figs. 4a and b as a function of the impact parameter p of the line of sight. On the disk, the variation of Let intensity is very smooth and rather flat. The counting rates of the two instruments are nearly identical, at positions on the disk for which a model predicts the same intensity. The Venera 12 counting rate is = I% larger than that of Venera 1I. This difference can reflect either a I% difference in the sensitivities of the two instruments (which were assumed to be equal for Figs. 3a and b) or the same variation of the solar flux in 4 days, or even a combination of both sensitivity difference and solar flux change. Out of the disk, there is clearly a change of slope around the radial distance of 9000 km (or -~3000 km of altitude). This feature was already observed during the flight of Mariner 5 in 1967 (Barth et al., 1967) and also by Mariner 10 in 1974 (Takacs et al., 1980). It is widely accepted now that this

225

double slope distribution of L a is the signature of a " h o t " component of the exospheric hydrogen distribution superimposed to a cooler exospheric distribution with the temperature of exosphere, which could be called the "thermal" component (Kumar and Hunten, 1974; Anderson, 1976). Indeed, Let line profile measurements performed with an absorption hydrogen cell with Venera 9 indicated that the temperature was increasing rather sharply above =3000 km of altitude (Bertaux et al., 1978). Three regions can therefore be distinguished in the observations: (i) the disk, (ii) the lower exosphere, where the "thermal" component dominates, and (iii) the outer exosphere, where the " h o t " component dominates (Table II). Each region is seen on both sides of the planet (morning and afternoon). THE MODEL What is aimed at is to derive the density distribution n(~) of neutral H in the atmosphere of Venus from a set of Let intensity observations I(p), where ~ is a point of the atmosphere at radial distance r and p is the impact parameter of a line of sight. In the outer regions of the exosphere, which are optically thin, the observed hydrogen emission can be written as 4~rl (Rayleigh) ,ire 2 = 10-6 goN =- mecf.Fs.N, lO-6,

(1)

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FIG. 4. (a) After subtraction of "dark counts" and interplanetary background, the "pure" cytherean La emission is plotted in kilorayleighs as a function of the impact parameter p of the line of sight of Venera 11. p is increasing on both sides of the minimum value Pminof p, found on the disk. The two different slopes of the inner exosphere and outer exosphere are clearly seen on both sides of the planet, corresponding, respectively, to the thermal component of H (which has the same temperature as the bulk of the atmosphere) and the nonthermal component of H. (b) Same as (a) for Venera 12.

10-6 ~re 2 M = -4~r me c f ' a F ~ ' N = e ~ a F s N ,

(2)

w h e r e a is a c o n s t a n t . This f o r m u l a s h o w s that in order to d e r i v e the integrated density N from the m e a s u r e m e n t M , it is not n e c e s s a r y to k n o w i n d e p e n d e n t l y the e x a c t v a l u e o f the solar flux Fs and the s e n s i t i v i t y

a o f the i n s t r u m e n t . O n l y the v a l u e o f the product aFs has to be k n o w n • In this c a s e o f optically thin region, N is strictly proportional to M. If, in addition, the h y d r o g e n distribution has the spherical s y m m e t r y , n u m e r i c a l m e t h o d s o f integral i n v e r s i o n can be u s e d to d e r i v e directly the distribution n ( r ) from a set o f m e a s u r e m e n t s M ( p ) .

VENUS

OBSERVATIONS

227

OF HYDROGEN

T A B L E II

HYDROGEN VERTICAL DISTRIBUTION ( a t o m s . c m -3) IN THE VENUS EXOSPHERE Altitude z (km)

110 120 130 150 200 250a 350 500 750 1,O00 1,300 l, 800 2,500 3,500 4,400 4,500 5,000 7,000 10,000 14,000 20,000 27,000 35,000

A T h ermal component nl = 4 × 104 cm -3 T = 300°K

B Nonthermal c o m p o n e n t n2 = 103 cm -3 Tz = 103°K

3.2 × 105 2.4 × 105 1.96 x 105 1.48 x 105 8 x 104 4 × lO4 2.9 x 104 1.8 x 104 8.7 x lO3 4.4 × 103 2.1 x 103 680 175 35 10.9 5.5 0.9 0.13 2 × 10 -2 4 x 10 -3 1 × 10 -3 3.5 × 10 -4

0 0 0 0 0 103 897 775 617 498 393 271 175 103 68

C Truncated hot component

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

51.5 26 12 5.5 2.35 1.1 0.6

51.5 26 12 5.5 2.3 1.1 0.6

C o m p o s i t e mode l Nontruncated A+B

3.2 x 105 2.4 × 105 1.96 x 105 1.48 × 105 8 x l04 4.1 × 104 2.99 × 104 1.9 × 104 9.4 x 104 4.9 × 103 2.5 × 103 951 350 138 78.9 75 58 27 12.2 5.5 2.35 1.1 0.6

Truncated A+C

3.2 × 105 2.4 × 105 1.96 x 105 1.48 × 105 8 x 104 4 × 104 2.9 × 104 1.8 x 104 8.7 × 103 4.4 × 103 2.1 × lO3 680 175 35 10.9 75 58 27 12.2 5.5 2.35 1.1 0.6

a The exobase is a s s u m e d to be at 250 km.

If the optical medium is not thin, which is the case of the internal region of the cytherean exosphere, the role of multiple scattering is important and cannot be neglected. Even in the case of spherical symmetry for n(r), there is no way to derive directly the distribution n(r) from measurements M(p). Their relationship is of the form

M = aaFsfIn(r)],

(3)

w h e r e f i s a function that cannot be written explicitly and is no longer linear. Still, only the product aFs appears, and a and Fs have no discriminable role. An indirect method can be used to retrieve n(r). For an arbitrary starting distribution nl(r), the radiative transfer of L~t can be computed numerically, and intensities IO(p) are computed for the geometrical

conditions of observations and compared to the measured intensities Ira(P). Then a new density distribution n2(r) is constructed, in which the ratio n2/n~ at distance r is related to the ratio lm/Ic I for p = r in some nonlinear fashion, with a method devised by Bertaux (1974). With only a few iterations (two or three), one finally finds the distribution n(r) which gives a computed intensity distribution in agreement with the observed intensity distribution. The radiative transfer equation was solved with the formula developed by Thomas (1963) and a matrix numerical method (Bertaux, 1974). Complete frequency distribution is assumed for the multiple scattering of Lot photons by H atoms, in a spherical isothermal atomic hydrogen atmosphere with CO2 as a pure absorber,

228

BERTAUX ET AL.

with a sharp boundary at the altitude of 110 km. The cross section for pure absorption of Lc~ by CO2 is 7.3 × 10-2° cm 2 (Sun and Weissler, 1955). The vertical distribution of CO2 was measured by the Bus Neutral Mass Spectrometer on the Bus of Pioneer Venus (Von Zahn et al., 1980). With this vertical distribution, it is found that the number density of CO2 is 5.26 × 1013 cm -3 at 110 kin, and that the optical thickness of CO2 at L a is -r = 1.56 above 110 km, decreasing very fast with altitude (the scale height of CO2 is 4.1 km in this region). For radiative transfer purposes it was assumed that all H atoms are at the same temperature, even those of a hot component. This assumption should be valid, since this component dominates only at high altitudes, in the optically thin regions. An illuminated column density of 2.9 × 1012 atoms.cm -2 at 300°K would give an optical thickness of 1 and an intensity of =25 kR, found at an altitude of ---1000 km. In the present analysis, we have not attempted to use the iterative method briefly described above to determine n(r) from observations Im(p) because it requires the exact knowledge of aFs. Rather, the radiative transfer equation was solved for a large number (-60) of distributions n(r), and the intensity I~(p) (proportional to Fs) which would have been seen with the geometrical conditions of Venera was computed and compared to the measured intensities Im(p) (proportional to the sensitivity a). The distributions n(r) were constructed as follows. Below the exobase (from 110 to 250 km), theoretical "thermospheric" profiles taken from McElroy and Hunten (1969), Liu and Donahue (1975), Kumar et al. (1978), and Chen and Nagy (1978) were used, with a multiplicative factor adjusted to take care of continuity at 250 km with the exospheric part of the distribution. The shapes of the various thermospheric profiles are somewhat different, and for the same density at 250 km they have different integrated column densities Nt (from 110 to 250 km). These vertical distributions, how-

ever, present a difficulty for radiative transfer calculations, because they have a very large vertical gradient below the homopause level. The source function has to be computed on a tight grid, implying quite large computer expenses. Other ad hoc vertical distributions were built with a smoother gradient, requiring less computing time. It was verified for a few cases that the emerging Le~ intensity was the same for "realistic" distributions and ad hoc distribution, provided that the integrated number density was identical in the distributions. Above the exobase, classical exospheric distributions are taken (Chamberlain, 1963), which depend only on two parameters: the temperature Tc and the density nc at the exobase level, taken at 250 km. No satellite particles are included. For the " h o t " component, a second classical exospheric distribution is defined by nh and Th, modified in some models by suppression of atoms below a certain altitude, as will be seen later. As could be verified by intensity computations on numerous models, the intensity distributions in the three observed regions (disk, inner exosphere, outer exosphere) are not equally sensitive to the three parts of the density distributions (thermospheric, "thermal," and " h o t " exospheric components). The outer exosphere is only sensitive to the hot exospheric component. The inner exosphere is virtually only sensitive to the thermal component. On the disk, the intensity is sensitive both to the thermospheric distribution and to the "thermal" component. However, the disk intensity is not sensitive to the details of the thermospheric altitude distribution, but only to its total content Nt. Therefore, we are unable to retrieve the detailed shape of the thermospheric vertical profile of H distribution from the observations. We can only determine the total content Nt. In the external part of the exosphere, which is optically thin, the shape of the intensity l(p) depends only on the shape of

VENUS OBSERVATIONS OF HYDROGEN the distribution n(r), and not on the absolute value of the density. Doubling n(r) would double I(p). Therefore, if the constant aF~ is not known, one can only derive the shape of n(r) and not its absolute value. On the contrary, for the disk and the inner exosphere, the shape of I(p) depends both on the shape of n(r) and on the absolute value of n(r). The shape of the distribution of l(p) across the disk and in the inner exosphere depends on the distribution of the optical thickness (Stewart et al., 1979). This is particularly clear at the limb. If the hydrogen optical thickness is small, there is a peak of intensity when the line of sight is tangent to the limb. This peak disappears when the H optical thickness is large. But optical thickness effects are also quite important to determine the shape of I(p) in the inner part of the exosphere, say, up to =1000 km of altitude. C O M P A R I S O N O F D A T A TO M O D E L

We first examine observations of the disk and of the inner exosphere performed around the bright limb, which correspond to the afternoon/polar side of the planet (Fig. 5). All atoms along the line of sight are illuminated, and the impact point (the nearest point to the planet along the line of sight) is on the dayside of the planet. Since the maximum of emissivity is usually at this point, these observations are more representative of the dayside situation of Venus than the morning side observations (Fig. 5). Assuming that the value of aFs is unknown, we can only compare the shape of profiles of measured and computed intensity distributions. But because of optical thickness effects, the three parameters Nt, no, Tc defining the distribution n(r) can be determined from this comparison, together with the exact value of aft. The Lot observations present only a very small bump at the bright limb, whereas various models do show a peak. The relative peak height decreases when the total hydrogen content increases in the model. However, since the orientation of the FOV

229

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FIG. 5. T h e bright limb (afternoon/pole) intensity m e a s u r e m e n t s of V e n e r a 11 are c o m p a r e d to various exospheric models for which the radiative transfer equation has been c o m p u t e d . All model distributions are classical exospheric distributions defined by the exobase density nc = 5 × 104 a t o m s . c m -3 at 250 k m of altitude, and various values of the temperature To. The computed intensity took into a c c o u n t the finite size of the FOV, which s m o o t h e s out a peak at the bright limb. The slope of the model is quite sensitive to the temperature. The n o n t h e r m a l c o m p o n e n t begins to appear around 2000 k m o f altitude. T h e g e o m e t r y of observations is s c h e m a t i z e d at b o t t o m left. Measurements were taken near the s o u t h pole. With the a s s u m e d sensitivity of the i n s t r u m e n t a = 0.026 c o u n t s . s e c Rayleigh -1, a solar flux F~ = 15.6 x 10" photons.(cm 2 s e c / ~ ) - t has to be selected in order to fit the data and model levels.

is highly inclined with respect to the bright limb (Fig. 1), the limb peak is smoothed by the finite size of the FOV. In order to perform a correct comparison between model and data, each " m o d e l " intensity was in fact a weighted average of the intensities of nine points within the FOV. As can be seen in Figs. 5 and 6 in the regions from 103 to 2 × 103 km of altitude, where the medium is reasonably thin and the " h o t " component still does not dominate, the slope of the intensity profile de-

230

BERTAUX ET AL. 0

A~ITUDE (km) 1000 2000

--

•

sured and c o m p u t e d intensities can be matched by selecting the right value of the constant a f t . If the estimated value for the instrument sensitivity is a s s u m e d to be correct (a = 0.026 counts.sec -I Rayleigh-1), it requires a solar flux F~ = 15.6 x 10 II photons (cm 2/~ sec -I) at the center of the solar line, at the distance of Venus, or 7.6 x 101~ photons.(cm 2 ~ sec) -1 at 1 AU. This value of F~ was selected for the models displayed in all the figures of this paper. The same exercise was made for V e n e r a 12, yielding rather similar results. Since there is some subjectivity in a visual fit of the curves shape, we have also used a least-squares method to find the set of parameters best fitting the data. A residue Q is defined as

3000

OATA

I

30-

- -

MODEL

20 -

\ , _ _

I

6000

I

7000

J

i

8000

I

_ _

9000

IMPACT PARAMETER p (kin)

FIG. 6. Measurements are the same as in Fig. 5. Models are for Tc = 300°K and various values of nc from 1 to 10 × 104 at cm-L The slope of the model between 1000 and 2000 km does not depend on the density no, whereas the shape of the curve near the limb depends strongly on no. pends mostly on the t e m p e r a t u r e T~ and not on the density no. The intensity measurements of Venera 11 in this region are best fitted with Tc = 300°K. In Fig. 5 the data and models c o m p u t e d for various values of T~ and a fixed value of n¢ are c o m p a r e d whereas in Fig. 6 the temperature is kept fixed at 300°K and various values of nc are compared. It can also be observed that the intensity ratio, for instance, I(500 km)/I(2000 km), is very sensitive to the t e m p e r a t u r e To, and less sensitive to the exobase density no. Finally, a global best fit of the shape is found visually for the bright data for the following set of parameters: Nt = 2 x 1012 a t o m s . c m -2 b e t w e e n 110 and 250 km, n~ = 5 × I04 a t o m s . c m -3 at 250 km, T~ = 300°K. Once the shape of the intensity curves has been fitted, the absolute values of mea-

1

(log 1 n

lm i --

~ (log Mi

log Ic') 2 log a - log It9 2,

(4)

i=l

where n is the n u m b e r of m e a s u r e m e n t points, Imi = Mi/a is the measured intensity at point i, and I j is the intensity for the same point c o m p u t e d with the reference value of Fs = 15.6 x l0 II photons. (cm 2 A sec) -1. For a given model distribution, the value of the sensitivity a which will minimize Q is found with the condition a Q/ o(log a) = 0, which yields 1 log a = n ~i=J (log I j - log Mi)

(5)

and the minimum value Qm of Q is calculated with this value of log a. Qm is then computed for various models combining n~, To, and Art to search for a minimum minim o r u m value Q0. This exercise was done to fit the bright limb data of Venera 11 and Venera 12, selecting 14 m e a s u r e m e n t points around the bright limb (both on and out of the disk). With models in which Nt was varying proportionally to n~, a value of Q0 = 0.17 × 10 -3 was found for the following set of parameters:

VENUS OBSERVATIONS OF HYDROGEN

T~ nc Nt

Venera 11 Venera 12 300 275 4 × 10 4 5.8 × 10 4 1.7 x 10~2 2.6 x 10 ~2

(°K) (atoms.cm -3) (atoms.cm -2)

It means that the average value of the relative deviation (Im - Ic)/Im between the model and the data points is V ~ 0 = 1.3 x 10-2.

The value of Qm increases rapidly when the parameters are different from the best set. Figure 7 represents, in coordinates of T~ and he, a line joining the " p o i n t s " (each point for a set of n~ and T~) where Qm is equal to 1.5 Q0. The shape of this zone allows one to determine the uncertainty bars on the determination of Tc and n¢ quoted above. It is clear that there is a coupling between nc and T~; a slight increase of Tc can be " co mp en s at ed" by a slight decrease of n¢ to yield an equivalent good fit. Now we turn our attention to the observations across the disk. In Fig. 8 the model best fitting the afternoon/polar side (no = 4 × 10 4, Tc = 300°K) has been represented together with the data of Venera I 1 across the disk. Though they both decrease when going from the afternoon side to the morning side, the data decrease less than the model, indicating a larger quantity of hy-

~Tc (gK)

.

.

.

.

.

.

.

.

325

300

275

n c (at0m. [m'3l

FIG. 7. A least-squares fit analysis of the bright limb data compared to various models for Venera 11 and Venera 12 allows one to determine the two parameters n¢ and To. The stars represent the place of best models in the plane (n¢, To) for Venera 11 and Venera 12, where the value of the residue Q reaches a minimum Q0. The contours represent models where Q = 1.5 Q0, respectively, for Venera 11 and Venera 12. There is a "coupling" between nc and T¢. Venera 11 has a slightly higher temperature and a lower density.

231

drogen on the morning side. No model with a spherical symmetry distribution can well represent the data on both the afternoon/ polar and the morning sides. However, it can be assumed that a spherical model fitting the data well in a particular observed region will have a density distribution representing well the density distribution in this region. As can be seen in Fig. 9, a model with nc = 105 and T = 300°K does represent rather well the morning side data, at least on the disk and in the inner exosphere up to an altitude of 500 km. Above, the measured intensity falls below the computed intensity. The L a measurements from the disk indicate that the vertical integrated number density is slowly increasing from the morning side to the afternoon side, with a factor of =2 of variation. This increase can affect the H quantity below the exobase and/or above the exobase. Figure 9 represents the data and several models for the limb of the morning side. Of course in this case the constant aFs was fixed at the value found with the analysis of the afternoon/polar side. The geometry of observations is illustrated in Fig. 9. For lines of sight intersecting the illuminated disk, the observed part of the exosphere is completely on the dayside, whereas when it is out of the disk, the line of sight contains parts of the exosphere which are on the dayside and others which are on the nightside (though directly illuminated by the Sun). Since there is a large variation of exospheric temperature at the terminator as measured by Pioneer Venus (Keating et al., 1980), a scale height analysis is less reliable. The fact that the measured intensity falls faster than the computed intensity with altitude (Fig. 9) probably means that the decrease of exospheric temperature at the terminator is the controlling factor for the observed region of the exosphere, more than the increase of H density at the exobase level. At any rate, the asymmetry which is indicated in the exospheric part of the Venera data most likely reflects a day-to-night

232

BERTAUX ET AL. ALTLTUOE (km) 3000 ' '

100

ALTITUDE (km)

2000 1000 ' ' ' "

0 .

DARK LIHB [MORNING)

~

0 .

.

.

.

1000

2000

.

'

3000 '

tO0

105

• _

~

BRIGHTLIHB(AF'i~NOQN/I:~

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IMPACT PARAMETER p (km)

FzG. 8. The data of Venera 11 across the disk are compared to models with Tc 300°K and various values of no. The quantity Nt of H below the exobase was taken proportionnal to n~. The shape of the models depends strongly on the values of nc and Nt. The shape of the data does not coincide with any value of the model, indicating that the total content of H is changing from one side of the planet to the other, by a factor of =2.5. It may be a latitude or a local time effect. =

asymmetry, whereas disk data may indicate an asymmetry between morning and afternoon (Fig. 8) or a latitude effect, since bright limb crossing occurred at high latitudes. The hydrogen " h o t " component is clearly seen above an altitude of 3 x 103 km (Figs. 4a,b) for both sides of the planet and both instruments. Composite density distributions were constructed to fit the data by adding to the " t h e r m a l " c o m p o n e n t nl = 4 × liP and TI = 300°K determined earlier a second component with a classical distribution fixed by the density n2 and the temperature T2 at the exobase level. A reasonable fit was found above =6000 km with n2 = 103 atoms.cm -3 and T2 = 1000°K, as indicated in Fig. 10. H o w e v e r , the fact that the composite model yields a higher intensity in the region around 3000 km suggested that a better fit could be obtained by suppressing H atoms in the " h o t " component below a certain level. Figure 10 shows the result of this exercise, where the " h o t " component is totally suppressed below 2.5 x 10 3 km. The physics behind such a model will be discussed in the next section.

DISCUSSION

The Let Solar Flux

A by-product of our analysis of the bright limb data (afternoon/polar side) is the absolute determination of the constant aFs. If the estimated value of the instrument sensitivity a = 0.026 counts.sec -1 Rayleigh -1 is valid, it implies a solar flux Fs at 1 AU of 7.6 x 1011 photons.(cm 2 ,~, sec) -l at the center of the line. This is higher than previously reported for the 1969 solar maximum, where the flux reached values of 4.6 x l0 II photons.(cm 2 ,~ sec)-l (Vidal-Madjar, 1975). It is not very easy to imagine how the instrument could be more sensitive than estimated. What happens generally is that the instruments are, during flight, less sensitive than measured at the laboratory, estimated, planned, or hoped. Therefore, we may have some confidence in our value of F~. Hinteregger (1981) reported for February 19, 1979, a solar flux of 6.4 × l011 photons.cm -2 sec -t (corresponding roughly to 6.4 x lO II photons.cm -2 ,~-1 sec-i at line center), a value larger than any value previously reported. In contrast, Mount et al.

VENUS OBSERVATIONS OF HYDROGEN ALTITUDE (kin) 2000 1000

3000

100

=

0

,

,

SO • -

--~

DATA MODEL

m . ~

S ?o

E5 3

nc lO'clO&

7 QO ° i

I

i

9000

i

t

8000

i

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i

7000

I

I

6000

IMPACT PARAMETER p (kin)

FIG. 9. The dark limb data of Venera 11 (on the morning side of the planet) are compared to models with Tc = 300°K and various values of the density no. The data are well above the model best-fitting bright limb data (no = 4 x 104). Near 500 km of altitude, the exobase density required would be 105atoms.cm -3. At higher altitudes, the data fall below this model, an indication of a complex situation near the terminator. The geometry of observations is schematized at bottom right. (1980) reported only 4.36 × 10 I1 photons.cm -2 sec -m on June 5, 1979; B o s s y and Nicolet (1981) have reinterpreted the long series of H i n t e r e g g e r ' s and Vidal-Madjar's m e a s u r e m e n t s and p r o p o s e d one argument to criticize the calibration of Hinteregger and to decrease his values, by postulating that the statistical relationship between Let solar flux and solar activity should be perfectly stable with time, even on a long time scale (12 years). The present study seems to confirm the original Hinteregger m e a s u r e m e n t s . Other indications that the solar flux has been very high in 1978-1979 are the Let measurements of UVS on V o y a g e r (Broadfoot et al., 1979), both on Jupiter and in the interplanetary background. This ultraviolet s p e c t r o m e t e r has been rather well calibrated in flight, thanks to a careful analysis

233

of O- and B-type star observations (Holberg et al., 1982). Therefore, it indicates that the statistical relationship of Let and F10.7 is not the same around the 1979 solar m a x i m u m and around the 1968 solar m a x i m u m . In our first analysis of Venera results (Bertaux et al., 1981) we derived an e v e n larger solar flux (twice as high) by comparing the measuring Let of the interplanetary background to a model of interplanetary hydrogen. H o w e v e r , the model was assuming that H a t o m s have straight-line trajectories, a situation which occurs when Fs = 3.32 x 10 '1 p h o t o n s . ( c m 2 s e c / i ) - ' , because the radiation pressure is equal to the gravity. Since m o s t likely Fs was m u c h larger in 1978, this "straight-line" model was not pertinent to 1978 data. When an adequate model is available the determination of aFs from Venus observations will allow one to determine from Venera 11 and 12 Let interplanetary background m e a s u r e m e n t s the absolute n u m b e r density of H a t o m s in the nearby interstellar medium.

The Thermal Component o f Hydrogen In Table III we have listed the determinations of H density and t e m p e r a t u r e in the a t m o s p h e r e of Venus derived from various space missions. Venera 4 (Kurt et al., 1968) observed the nightside of Venus in 1967, and comparisons with our dayside observations are not relevant. Venera 9 measured in 1975, at a time of solar minimum of 1.5 x 10 4 a t o m s . c m -3 and T = 500°K on the dayside (Bertaux et al., 1978), and T smaller than 200°K on the nightside (Bertaux et al., 1979). F r o m Lot m e a s u r e m e n t s of Mariner 5 in 1967, Anderson (1976) derived a temperature of 275 _+ 50°K and a density of 2 -- 1 × 105 a t o m s . c m -3 at 250 km, significantly larger than the present Venera 11 and 12 v a l u e o f = 5 × 10 4 a t o m s . c m -3. He used a method very similar to ours, including multiple scattering. In the same fashion, he also determined a new calibration factor for the Let p h o t o m e t e r of Mariner 5. H o w e v e r , the determination of the density at the e x o b a s e

234

BERTAUX ET AL. ALTITUDE (10~ kml I00

1

Z

3

t.

S

6

7

8

9

10

100

.z.

MARINER I0"

IMPACT PARAMETER (103 kin|

FIG. 10. The exospheric data points (solid circles) collected by Venera 11 on the bright limb of Venus are compared to model predictions of the emission rate (solid lines). The "full hot model" is the addition of a thermal distribution defined by nc = 4 x 104cm 3and Tc = 300°Kand a nonthermal model defined by nc = 103cm-3, Tc -- 103°K.The data points fall below this model between 2,000 and 4,500 km of altitude. The curve marked MARINER 10 is the non thermal model best fitting the MARINER l0 data, multiplied by a factor of two for normalization to VENERA results on the disc. The non thermal population is about twice more abundant in 1978 than in 1974, probably as a result of solar activity.

level may have been hampered by the fact that all measurements below -~700 km of altitude were probably contaminated by CO1 Rayleigh scattering and eliminated. The bright limb observations of Mariner 10, in 1974, were analyzed by Takacs et al. (1980). Since they did not use a model including multiple-scattering effects, they tried to fit the data only above 2000 km of altitude, finding T~ = 275 --- 50°K and nj = 1.5 × 105 cm -3 extrapolated down to 250 km at the exobase level. More recently, the Pioneer Venus uv spectrometer L a measurements yielded 1 to 2 × 105 cm -3 at the exobase level and T 300°K (Stewart et al., 1979), also using a technique of curve shape fitting. A completely different method of hydrogen determination was used by Brinton et al. (1980) from Pioneer Venus in situ measurements of n(H+), n(O+), n(O), and n(CO2), under the assumption of chemical equilibrium. The concentration n(H) in the chemically controlled thermosphere is given by

.. n(H +) n(H) = a ~ n(O) derived from the consideration of the fast charge exchange reaction H + O+,~__~H+ + O. Here we neglect the role of CO2 for simplification, [it was taken into account in Brinton et al. (1980)]. They were able in this fashion to derive the diurnal variation of H near 196 km of altitude on 25 orbits of Pioneer Venus spacecraft between D e c e m b e r 1978 and July 1979 (Fig. 11). They discovered a very large bulge of H at night, with a density at 165 km of ---2 × 10 7 cm -3 at 04.00 and 4 × 104 cm -3 at 15.00 local time, which can be compared to our data on the bright limb. Three corrections have to be applied before a comparison with our value determined at 250 kin. First, a factor -~0.64 should be applied to yield the density at 250 km, assuming a diffusive equilibrium profile with a temperature T = 275°K. Second, all neutral

300 at 850 km

1020 ± 100 1.3 x 103 1500 _+ 200 1.0 x 103 0

275 ± 50 (2 -+ 1) × 105 150 - 50 (2 ± 1) x 105

1967

1967

T > 1000b

50

60

500 1.5 × 104 T < 200

1975

Venera 9 (Bertaux et al., 1978, 1979)

1250 ± 100 ( 5 ± 1) x 102

275 ± 50 1.5 × 105 150 ± 25 (1 +-- 0.5) x 105

1974

Mariner 10 (Takacs et al., 1980

a Referred at the exobase level assumed at 250 km, unless otherwise specified. b Measured spectroscopically with a hydrogen absorption cell.

Dayside temperature (°K) Dayside densitya (cm -3) Nightside temperature (°K) Nightside densitya (cm -3) Solar zenith angle (dayside) (°)

Nonthermal component

Dayside temperature (°K) Dayside densitya (cm -3) Nightside temperature (°K) Nightside density~ (cm -3)

Thermal component

Year of mission:

Mariner 5 (Anderson, 1976)

Venera 4 (Kurt et al., 1968)

HYDROGEN DISTRIBUTION IN VENUS EXOSPHERE FROM Ltx OBSERVATION

TABLE III

70/85

1000 103

2300 1 to 2 × 105

300 ± 25

1978

Pioneer/Venus (Stewart et al., 1979)

4_2 +3 × 104

1978

Venera 1 1 / 1 2 (present work)

tO t~ t~

Z

0

©

Z r~

< > ,-q

©

0'2

<

236

BERTAUX ET AL. 108

I

I

I

1

I

I

I

I

I

I

I

..

•

107 n(H)

•

( c m -3 )

• •

10 6

• • •

•

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i 4

i 6

I 8

•

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i 12

HOUR

•

•

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i 14

I 16

J 18

I 20

I 22

24

ANGLE

FIG. 11, D i u r n a l v a r i a t i o n o f a t o m i c h y d r o g e n c o n c e n t r a t i o n in V e n u s t h e r m o s p h e r e , d e r i v e d f r o m in situ m e a s u r e m e n t s of ion a n d n e u t r a l c o m p o s i t i o n . D a t a w e r e o b t a i n e d n e a r 165 k m a l t i t u d e o n 25 o r b i t s o f P i o n e e r V e n u s s p a c e c r a f t b e t w e e n D e c e m b e r 1978 a n d J u l y 1979 [after B r i n t o n et al. (1980)]. In a d d i t i o n to the l a r g e n i g h t b u l g e , t h e r e is a s l o w b u t c o n s t a n t i n c r e a s e f r o m m o r n i n g t o a f t e r n o o n , as in the V e n e r a data.

mass spectrometer density data from ONMS should be multiplied by = 1.6, a corrective factor necessary to reconcile orbiter drag data and in situ ONMS measurements (Niemann, 1981). Third, a new measurement of the coefficient K by W. T. Huntress yielded a value increased by 20%. These three corrections imply a density of 4.9 x 104 cm -3 at 250 km, to be compared to our bright limb value. The excellent agreement between the two determinations, obtained at about the same time (December 1978) by two completely different techniques, makes us confident in both sets of measurements. They are a factor of 2 - 4 lower than values derived from L a observations by Mariner 5, Mariner I0, and uv spectrometer of Pioneer Venus. For Mariner 5 and Mariner 10, differences may be attributed to variations of solar activity and the response of the Venus atmosphere (as usual when two measurements taken at different times do not agree!). For uv spectrometer of Pioneer Venus, taken at the same time, the derived value was quoted from a very preliminary

analysis, and a refined analysis could yield different results. It is quite remarkable that this exobase density for Venus of 5 x 104 cm -3 is exactly the same as the average exobase density found on Earth (Bertaux, 1975), though the two planets are totally different in many aspects relevant to H distribution: the Hbearing molecules content, the composition of the atmosphere, the structure and temperature of the thermosphere, and the escape mechanisms. Concerning the temperature, the agreement is also excellent with Pioneer Venus determinations, either from the Bus Neutral Mass Spectrometer (BNMS) [275°K; Von Zahn et al. (1980)], from the Orbiter Neutral Mass Spectrometer (ONMS) (Niemann et al., 1980), or from Orbiter Atmospheric Drag measurements (Keating et al., 1980), which found 285°K as the mean days•de temperature. What is specific to Venera measurements is their very high latitude (=79°S), whereas ONMS measurements were made around 19°N, and BNMS measurements were

VENUS OBSERVATIONS OF HYDROGEN made at 38°S. The temperature of the polar exosphere is found to be the same as the mid-latitude exosphere. It probably means that the rotation of the atmosphere (zonal circulation) is a less important factor than the solar zenith angle in controlling the exospheric temperature distribution. From measurements on the disk, we also have a determination of the vertical column density down to 110 km, the level of penetration of L a photons, something which cannot of course be achieved by the technique of the chemical equilibrium. One-dimensional aeronomic models have been used to predict the H vertical profile in the upper atmosphere of Venus. The H profile depends on a number of parameters, including eddy diffusion coefficient, nonthermal escape flux, and mixing ratio of H2, H20, and HC1. Two examples are shown in Fig. 12, taken from Liu and Donahue (1975) and from K u m a r e t al. (1981). The Pioneer Venus in s i t u data points are also shown, without correction. They were taken around

11

V12

237

08.00 of local time. The Venera 11 and 12 bright limb data are displayed at the exobase level (250 km), together with the altitude profiles which were taken in the bestfit models. As was said before, the shape of our models has no physical meaning, and was chosen because its nearly constant vertical gradient allows one to save a significant amount of computer time. H o w e v e r , if the shape of the vertical profile is rather arbitrary, the total content Nt (between 110 and 250 km) is determined from the Let measurements. We found an average (between Venera ll and Venera 12) value of Nt = 2.1 × 1012 atoms.cm -2, whereas the model of Liu and Donahue predicts 5.7 x I012, and the model of K u m a r e l al. predicts 1.2 x I013. Clearly, our measurements indicate a lower content of H in the thermosphere than predicted by aeronomical models: a factor of 3 to 6 in the column density. This is probably because the mixing ratios of Hbearing molecules have been overestimated in the models. In particular, H2 has not

EXOBASE LEVEL

/ Z20 -

~-.,ll--

B

2'00 -

RINTON ET AL., 1980

~80 160 1,0-

~

KUMAR ET AL..IgBl

VII

100

Lo co, ABSORPTION LEVEL

m 10z•

I

~/

~ ,~,) ....

I I I05

t

~"2~:~

L,UANO BONAHU ,9 d

I0 t I I ~ I I i 1 6 H DENSITy107 cm-3 108

I

] , 109

i

1010

FIG. 12. Vertical distribution of H density in the upper atmosphere of Venus for various models and measurements. For Venera 11 and 12, the density was determined at the exobase level, as well as the total columndensity Nt of H between 250 and 1l0 km (the level of CO2 absorption of La). The shape of Venera profiles has no pretention to represent reality since the emerging intensity does not depend on it. It was built with a smoothly varying vertical gradient to alleviate some problems of radiative transfer computations. The total column density of H, however, is realistic. Aeronomic models from Liu and Donahue (1975) and Kumar et al. (1981) have much more H than Venera measurements indicate. The data of Brinton et al. (1980) are indicated• They were taken during the morning, whereas Venera measurements are relevant to afternoon/pole conditions.

238

BERTAUX ET AL.

been measured directly. Kumar et al. (1981) have derived an estimated mixing ratio of 10 ppm of H2 below 140 km, from mass 2 in situ measurements of PVO Ion Mass spectrometer attributed to H2÷, and from a simple photochemical model. Our lower measurements of thermospheric H imply that aeronomical models should be revised. There is a possible restriction to this conclusion: Hartle et al. (1978) have produced a model of the global circulation and distribution of hydrogen in the thermosphere of Venus, which indicates that global circulation is also an important factor controlling the vertical distribution of hydrogen. Not only the density at the exobase level is changing from the afternoon limb to the morning limb, but the column density, which can be estimated from Let measurements on the disk, is also changing by a factor of -~2. In the case of Venera 11 and 12, it could either be an effect of local time or latitude. We note, however, in Fig. 12 that Pioneer Venus H concentration derived by Brinton et al. (1980) show a decrease by a factor of =2 also. However, Pioneer Venus measurements were made over a long period of time, the local time of pericenter changing slowly with time. Therefore it was not possible to discriminate a " t i m e " variation from a "local time" variation. The present Venera data do demonstrate a clear variation of total H column density along the track of the line of sight. Latitude or local time effect? All measurements of Pioneer Venus were taken at the same latitude (-~ 19°N), implying rather a local time effect. This diurnal variation, with an afternooon-morning asymmetry, should be explained by global circulation models. Indeed, predictions made by Mayr et al. (1980) with such a model (including the rotation of the atmosphere, and adjusted to an empirical distribution of CO2, He, and H) show an asymmetry between morning and afternoon, with about twice more H at 08.00 than at 16.00. The global circulation

model includes the rotation of the atmosphere, and parameters are adjusted to fit CO2, He, and H "empirical distributions." These empirical distributions are, in their turn, best fit to Pioneer Venus measurements with the first three harmonics (Niemann et al., 1980; Brinton et al., 1979). What is rather remarkable is that both hydrogen data and predicted H results from the global circulation model show this diurnal variation, though the "empirical model" for H, which is supposed to represent the data, shows barely any diurnal variation between 08.00 and 16.00 hr local time [Fig. I of Mayr et al. (1980)]. It means that the H diurnal variation shown by the circulation model is not forced by the H data, but is a consequence of CO2 circulation. Since Venera measurements cover high-latitude regions, it may be the indication that the 4day rotation extends also at high altitudes in the upper atmosphere. The Nonthermal Component o f Exospheric Hydrogen

The large scale height component of Lyman a airglow has been observed in the Venus corona by several space missions, and is now generally attributed to a nonthermal population of H atoms produced by a number of reactions in the upper atmosphere of Venus. The radial distribution of this nonthermal component can be simulated by a " h o t " thermal distribution of the Chamberlain type (classical exospheric distribution), determined by the density n2 and temperature T2 at the exobase level. The results of this exercise are summarized in Table III for Mariner 5 (Anderson, 1976), Mariner 10 (Takacs et al., 1980), and the present study for Venera 11 and Venera 12. There is some variability both in temperature T2 and density n2, around a typical value of =103°K and 103 at cm -3. Several different mechanisms have been proposed to explain the presence of the hot component. Chamberlain (1977) first proposed charge exchange of the thermal component with a hot plasma, deriving a radial

VENUS OBSERVATIONS OF HYDROGEN distribution. Kumar et al. (1978) published an inventory of various possible sources of nonthermal hydrogen, concluding that the O+-H2 reaction was the most important: (1)

O + + H2---) OH + + H (E < 0.6 eV),

(2)

OH + + e ~ O

+ H (E -~ 8.28 eV).

The energy E left to the H atoms is such that OH + + e is the major source of escaping atoms (about 3.3 x 1026 sec -l for the whole planet), whereas O ÷ + H2 is contributing more to the population of the hot corona. Charge exchange reactions of H ÷ with H and O have also been investigated by numerical Monte Carlo simulations (Hodges and Tinsley, 1981), but found to be less important than reaction (I) (Cravens et al., 1980; Kumar, 1982). Measurements of the Lyman o~ linewidth with a hydrogen absorption cell on Venera 9 orbiter (Bertaux et al., 1978) were rather puzzling, because the linewidth was found to increase with altitude sharply above =3000 km of altitude. This did not happen only once, but was a systematic feature of all six orbits where data was collected on the dayside. In the case of a pure addition of a cold and a hot component, the linewidth would have been increasing slowly from 1000 to =5000 km. Therefore there was the possible indication in the linewidth measurements that nonthermal atoms were present only above a certain altitude, rather than being generated at the exobase level as in the case of chemical and ionic reactions. In an attempt to explain this feature, it was suggested (Bertaux et al., 1978) that charge exchange with solar wind protons could produce a significant number of fast atoms. Most of them would escape readily on hyperbolic trajectories. Only those solar wind protons which would have been decelerated, after bow shock crossing, below the escape velocity would produce fast atoms fed into

239

bound orbits. Atoms with satellite orbits could contribute much more to the local density than atoms on trajectories crossing the exobase, since they could accumulate with time. This explanation of local accumulation of fast satellite atoms just below the bow shock has been criticized (Hunten, private communication) because elastic collisions of these atoms would let them spread downward. In fact, after a few collisions with H atoms of the thermal component, they would have velocities of the thermal component and would not be distinguishable from linewidth measurements, which reflect velocity distributions. In this context, one has to remind that the distribution of hot atoms is the result of the equilibrium between production and destruction processes, and that a local enhancement of density does not necessarily imply a locally larger production, but only a local balance more biased toward production processes. Hot oxygen atoms have been observed by the Pioneer Venus Orbiter Ultraviolet Spectrometer (Nagy et al., 1981) with an altitude dependence above the exobase corresponding to model calculations of various source mechanisms for nonthermal oxygen, but the observed densities are lower than predictions by a factor of 4 to 5. By analogy, one may think that the efficiency of aeronomic reactions producing hot H atoms might have been also overestimated. Kumar (1982) concluded that observed nonthermal H atoms may be produced by a combination of the ionosphere O+-H2 source, the ionosphere H+-H charge exchange, and the solar wind charge exchange. In principle, the Lot determination of the concentration radial profile ne(r) of nonthermal atoms in the outer exosphere of Venus could be used to discriminate between the various source mechanisms which have been proposed. Cravens et al. (1980) have computed the energy distribution function f ( E ) of hot atoms at the exobase level for O+-H2 reactions and charge

240

BERTAUX ET AL.

exchange reactions. With the help of Liouville's theorem, one can find the corresponding distribution ne(r) of H. The distribution f u n c t i o n f ( E ) is not exactly maxwellian in shape, and one could expect departures of ne(r) from classical exospheric distributions which have been used to simulate the observed nonthermal distributions. In fact, the agreement between ne(r) and the one deduced by Anderson (1976) for dayside conditions is within -~20% up to r = 20 × 103 km of radial distance (Cravens et al., 1980). One would have to compare densities at a larger distance r --- 40 x 103 km, because only atoms having energy E > 0.44 eV can reach this altitude and this is the energy above which the distribution function f ( E ) departs seriously from a maxwellian distribution, as seen in Fig. 2 of Cravens et al. (1980). Unfortunately, the L a signal at these radial distances is rather low (Fig. 4a) and density determinations are not very accurate. We have attempted to detect departures between a classical distribution no(r) and the observed distribution n(r) at a lower altitude (around =2500 km) in the region where the nonthermal component becomes more important than the thermal and cold component. As was said in the previous section, a better fit to the data is obtained when all nonthermal particles below a certain level are suppressed, a distribution which is supposed to simulate the case of production of hot H atoms in the exosphere by charge exchange with the solar wind, as suggested by the Venera 9 linewidth measurements. Therefore, the density profiles observed by Venera 11 and Venera 12 give some support to the hypothesis of local creation. In fact, such an effect is also seen in Mariner 10 data [Fig. 6 in Takacs et al. (1980)]. At - 3 0 0 0 km altitude, a kink, or sharp change in the scale height, is present and the observed L a intensity is lower by a factor of = 2 than the addition of the two classical components nl and n2 simulating the thermal and nonthermal components. It was suggested (Hunten, private com-

munication) that such a departure in this region is inherent to our data analysis procedure, in which a best fit is found separately for n~ and n2 in two separate regions of altitude. Perhaps another good fit to the data could be obtained by using the full " h o t " component and slightly decreasing the temperature T1 of the cold component. One would have to find simultaneously a set of five parameters (nl, Tj, n2, T2, aFt) minimizing the departure between model and data, and then check if an even better fit could be obtained with a truncated hot component. Such an exercise is beyond the scope of the present analysis and will be deferred to the future. At any rate, the density of our composite model where the nonthermal component has been truncated gives a better representation of the Ltx observed intensities than the full hot model (Fig. 10) and is certainly nearer the actual density distribution, whatever the mechanism for nonthermal H production. The distribution is listed in Table II and displayed in Fig. 13, where the contribution of the two components has been distinguished. In addition, the distribution calculated for nightside conditions by Cravens et al. (1980) is shown; the agreement is striking, since our observations are more relevant to the dayside. Between 2500 and 4500 km of altitude, the intensity data are slightly larger than the truncated model. The real distribution must lie somewhere between the truncated model and the full hot model. Some irregularities in the Lot profile are observed in Venera I 1 and 12 data (Fig. 10) at an altitude of 7000 km and one is clearly seen as a bump at 21-24 x 103 km radial distance in Mariner 10 data (Fig. 14). It would be most interesting to see if they also exist in the PVO UVS data. If these bumps do not propagate radially, they would probably indicate a local production of hot atoms, whereas if they propagate outward, they could be the signature of a time variation of the ionospheric source. The Venus ionosphere is highly variable, particularly

VENUS OBSERVATIONS OF HYDROGEN ALTITUDE ( I03

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This increase of the nonthermal population between the time of Mariner 10 (1974) and 1978 is probably a result of increased solar activity. Whether this correlation favors the ionosphere or the solar wind interaction as the source of nonthermal H atoms is not straightforward to determine. However, since HE is the major ionospheric source according to model caculations (Kumar et al., 1978), and since we find that Hbearing molecules may have been largely overestimated in the past, there is a hint that the abundance of hot atoms produced in the ionosphere may also have been overestimated.

I

CONCLUSIONS L

L

,

,

30 R A D I A L DISTANCE

i

t

J

20

,

,

I0 (103kin)

FIG. 13. The radial distribution of H is shown for several models. The solid line is the truncated model in which atoms below 4500 km of altitude are suppressed from the hot distribution (n2 = l03 atoms.cm 3, T2 = 10~°K). Dashed portions represent hot and cold models alone. The dotted portion represents the addition of the full hot model and the cold model ( T = 300°K, nc = 4 x 104 cm-3). A model best fitting the data would lie between the solid line and the dotted line. For comparison, model calculations of ionospheric source from Cravens et al. (1980) are represented as open circles. However, their calculation is for the nightside, whereas Venera measurements are more relevant to afternoon/pole conditions.

on the nightside. In this respect, one should remember that hot atoms produced in the nightside corona can populate the dayside corona if their energy is sufficiently high. Finally, the absolute number density of the Venera I1 hot model is significantly larger (by a factor of 2) than the Mariner 10 hot model (5 × 102 atoms.cm -2 at exobase). This can also be seen in Fig. 10, where the intensity of the Mariner 10 hot model is plotted, after multiplication by a factor of 2 to take into account the variation of solar flux and/or calibration differences. This factor of 2 is obtained by normalizing the intensities found on the disk near the bright limb, of the two experiments.

From Lot measurements across the disk and the whole exosphere of Venus, and from comparison with spherically symmetric H models for which the radiative transfer equation was solved, we derived a number of parameters describing the H distribution in the whole exosphere. (1) The dayside exospheric temperature was measured for the first time in the polar regions, with T¢ = 300 _+ 25°K for Venera 11 (79°S) and Tc = 275 -+ 25°K (59%) for Venera 12. At high latitudes, the exospheric temperature of Venus is the same, or perhaps slightly larger than that measured by Pioneer Venus at medium latitudes. This is in contrast with the Earth, where auroral activity increases significantly the exospheric temperature in the polar regions. (2) The dayside density at the exobase level (250 km) is 4_2 +3 × 104 atoms.cm -3, and the integrated number density between 110 and 250 km is 2.1 x 1012 atoms.cm -2. The exobase level density is quite coherent with in situ measurements of Pioneer Venus. The integrated number density is 3 to 6 times lower than that predicted by aeronomical models, which should be revised, perhaps in their content of H-bearing molecules in the atmosphere, and/or with inclusion of dynamic effects. (3) The integrated number density de-

242

BERTAUX ET AL.

FAR HI

"1-

~

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POST

ENCOUNTER

DRIFT

1216

•

no'

NO BockgrOund Correction

• =

Removed

o= IPBG I

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0

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t/)

n-SxlO

=E bJ

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2

~

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•

•

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•

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•

•

•

•

x

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I

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I

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I

I

3

6

9

12

15

18

21

24

27

30

Rp x I0 a (km)

FIG. 14. Mariner 10 Let measurements [after Takacs et al. (1980)]. The solid curves represent the thermal model at 275°K and the hot model at 1250°K, simulating the distribution of nonthermal atoms. Data points near the limb have been suppressed owing to pointing uncertainties affecting the steep slope on the limb. Note the bump around 24 x l03 km of radial distance.

creases smoothly and up to a factor of 2 along the track of the line of sight on the dayside, which could be either an effect of local time or latitude. If it is an effect of local time, it indicates an asymmetry around noon between the morning and the afternoon sides. Such an effect is present in PVO in situ measurements. (4) The nonthermal component of H is clearly identified, populating the outer exosphere up to ---40,000 km of altitude. Its distribution can be simulated above 4000 km by an exospheric distribution having n2 = 103 atoms.cm -3 at 250 km and T2 = 103°K, and the density is higher than the Mariner l0 density by a factor of 2, probably as a result of increased solar activity. A better fit to the data is obtained by suppressing atoms of the nonthermal distribution below - 4 0 0 0 km of altitude, indicating the possible existence of a source of hot atoms above this altitude. (5) If the sensitivity of the E U V instruments of Venera l l and 12 is not higher than estimated, then the Let solar flux at line center was 7.6 x l0 ll photons.(cm 2 sec ~)-1 at l A U in December 1978, confirming

the very high values found by Hinteregger around the same time. ACKNOWLEDGMENTS This experiment was a cooperative effort between the Laboratory of UV Astronomy at the Institute of Cosmic Research (IKI) of the Academy of Science of USSR and Service d'A6ronomie du CNRS in France. Data reduction was performed under the responsibility of V. M. Pokrass at the Computer Center of the Institute for Space Research, Moscow, and M. Monchy at the Division Math6matique du C.S.T., CNES Toulouse. We wish to thank particularly J. C. Lebrun, mathematical engineer at the Service d'A6ronomie, for managing data comparison with radiative transfer calculations, and Shaleindra Kumar for useful discussions. This work was supported by CNES under Contract CNES 81-201. REFERENCES ANDERSON, D. E. JR. (1976). The Mariner 5 ultraviolet photometer experiment: Analysis of hydrogen Lyman alpha data. J. Geophys. Res. 81, 1213. BARTH, C. A., J. B. PEARCE, K . K . KELLY, L . WALLACE, AND W. G . FASTIE (1967). U l t r a v i o l e t e m i s sions observed near Venus from Mariner 5. Science 1511, 1675. BARTH, C. A. (1968). Interpretation of the Mariner 5 Lyman alpha measurements. J. Atmos. Sci. 25,564. BERTAUX, J. L. (1974). L'hydrogdne atomique dans l'exosphdre terrestre: mesures d'intensit# et de lar-

VENUS OBSERVATIONS geur de raie de l'(mission Lyman alpha d~board du satellite OGO-5 et interprdtation. Thb.se d'Etat, Universit~ de Paris. BERTAUX, J. L. (1975). Observed variations of the exospheric hydrogen density with exospheric temperature. J. Geophys. Res. 80, 639-642. BERTAUX, J. L., J. BLAMONT, M. MARCELIN, V. G. KURT, N. N. ROMANOVA, AND A. S. SMIRNOV (1978). Lyman alpha observations of Venera 9 and 10. 1. The non thermal hydrogen population in the exosphere of Venus. Planet. Space Sci. 26, 817-831. BERTAUX, J. L., J. E. BLAMONT, M. MARCELIN, V. G. KURT, AND A. S. SMIRNOV (1979). Consequences of the day-to-night variation of the exospheric temperature of Venus. Nature 277, 546-548. BERTAUX, J. L., J. E. BLAMONT, V. M. LEPINE, V. G. KURT, N. N. ROMANOVA, AND A. S. SMIRNOV (1981). Venera 11 and Venera 12 observations of EUV emissions from the upper atmosphere of Venus. Planet. Space Sci. 29, 149-166. BossY, L., AND M. NICOLET (1981). On the variability of Lyman alpha with solar activity. Planet. Space Sci. 29, 907-914. BRINTON, H. C., H. A. TAYLOR, JR., H. B. NIEMANN, AND H. G. MAYR (1979). Venus night time hydrogen bulge. Bull. Amer. Astron. Soc. 11, 538. BRINTON, H. C., H. A. TAYLOR, JR., H. B. NIEMANN, H. G. MAYR,A. F. NAGY, T. E. CRAVENS,AND D. F. STROBEL (1980), Venus night-time hydrogen bulge. Geophys. Res. Lett. 7, 865-868. BROADFOOT, A. L., M. J. S. BELTON, P. Z. TAKACS, B. R. SANDEL, D. E. SHEMANSKY,J. B. HOLBERG, J. M. AJELLO, S. K. ATREYA,T. M. DONAHUE, H. W. Moos, J. L. BERTAUX, J. E. BLAMONT, D. F. STROBEL, J. C. MCCONNELL, A. DALGARNO, R. GOODY, AND M. B. MCELROY (1979). Extreme ultraviolet observations from Voyager 1 encounter with Jupiter. Science 204, 979-982. CHAMBERLAIN, J. W. (1963). Planetary coronae and atmospheric evaporation. Planet. Space Sci. 11, 901. CHAMI~ERLAIN, J. W. (1977). Charge exchange in a planetary corona: Its effect on the distribution and escape of hydrogen. J. Geophys. Res. 82, 1-9. CHEN, R. H., AND A. F. NAGY (1978). A comprehensive model of the Venus ionosphere. J. Geophys. Res. 83, A3, 1133-1140. CRAVENS, T. E., T. I. GOMBOm, AND A. F. NAGY (1980). Hot hydrogen in the exosphere of Venus. Nature 283, 178-180. HARTLE, R. E., H. G. MAYR, AND S. J. BAUER (1978). Global circulation and distribution of hydrogen in the thermosphere of Venus. Geophys. Res. Lett, 5, 719-722. HINTEREGGER, H. E. (1981). Representations of solar EUV fluxes for aeronomical applications. Advances in Space Res. l, 39-52.

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TAKACS, P. Z., A. L. BROADFOOT, G. R. SMITH, AND S. KUMAR (1980). Mariner 10 observations of hydrogen Lyman alpha emission from the Venus exosphere: Evidence of complex structure. Planet. Space Sci. 28, 687-701. THOMAS, G. E. (1963). Lyman alpha scattering in the Earth's hydrogen geocorona, I. J. Geophys. Res. 68, 2639.

VIDAL-MADJAR, A. (1975). Evolution of the solar Lyman alpha flux during four consecutive years. Solar Phys. 40, 69-86. VON ZAHN, U., K. H. FRICKE, D. M. HUNTEN, D. KRANKOWSKY, K. MAUERSBERGER, AND A. O. NIER (1980). The upper atmosphere of Venus daring morning conditions. J. Geophys. Res. 85, 7829-7840.