I N S T I T U T D ' A E R O N O M I E S P A T I A L E D E B E L G I Q U E
3 - A v e n u e C i r c u l a i r e B • 1 1 8 0 B R U X E L L E S
A E R O N O M I C A A C T A
A - N° 229 - 1981
Effects of solar variations on the upper atmosphere
by
G. KOCKARTS
B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M I E 3 - Ringlaan
B - 1180 BRUSSEL
V O O R W O O R D
T h e paper " E f f e c t s öf solar v a r i a t i o n s on the u p p e r atmosphere"
has been p r e s e n t e d at the 14th E S L A B S y m p o s i u m in S c h e v e n i n g e n (16-19 September 1980). It will be p u b l i s h e d in Solar P h y s i c s , 1981.
A V A N T - P R O P O S
L ' A r t i c l e " E f f e c t s of solar variations on the u p p e r atmosphere" a été p r é s e n t é au c o u r s du 14ème S y m p o s i u m E S L A B à S c h e v e n i n g e n ( I 6 - I 9 September 1980). Il sera publié d a n s Solar P h y s i c s , 1981.
V O O R W O O R D
De t e k s t " E f f e c t s of solar variations on the u p p e r atmosphere"
werd v o o r g e d r a g e n op het 14de E S L A B S y m p o s i u m die p l a a t s v o n d in S c h e v e n i n g e n (16 - 19 september 1980). Deze tekst zal in Solar P h y s i c s 1981 v e r s c h i j n e n .
V O R W O R T
Die A r b e i t " E f f e c t s of solar v a r i a t i o n s on the u p p e r atmosphere"
w u r d e zum 14. E S L A B S y m p o s i u m in S c h e v e n i n g e n ( 1 6 . - 1 9 . September 1980) v o r g e s t e l l t . Sie wird in Solar P h y s i c s 1981 h e r a u s g e g e b e n werden.
E F F E C T S OF S O L A R V A R I A T I O N S O N T H E U P P E R A T M O S P H E R E
b y
G. K O C K A R T S
Institut d ' A é r o n o m i e Spatiale 3 A v e n u e C i r c u l a i r e B - 1 1 8 0 B r u x e l l e s , Belgium
A b s t r a c t
S e v e r a l semi-empirical models of the terrestrial u p p e r atmosphere are p r e s e n t l y available. T h e s e models take into account solar activity effects b y u s i n g the solar decimetric flux as an i n d e x . S u c h a proce- d u r e is a c o n s e q u e n c e of the lack of c o n t i n u o u s determinations of the solar s p e c t r u m directly r e s p o n s i b l e for the physical s t r u c t u r e of the u p p e r atmosphere. Variations of the thermopause temperature are d i s c u s s e d . U s i n g five sets of solar i r r a d i a n c e s measured in the u l t r a - violet and in the extreme ultraviolet, the penetration of solar radiation is analyzed as a function of solar activity. Several examples of a b s o r p t i o n profiles and ion p r o d u c t i o n rates are d i s c u s s e d for variable conditions. V a r i o u s energetic effects are also d e s c r i b e d . All computa- tions are made for p h y s i c a l conditions above S c h e v e n i n g e n ( 5 2 . 0 8 ° N ) where the 14th E S L A B s y m p o s i u m was held.
Résumé
Il e x i s t e a c t u e l l e m e n t p l u s i e u r s modèles s e m i - e m p i r i q u e s de l ' a t m o - s p h è r e s u p é r i e u r e de la t e r r e . Ces modèles t i e n n e n t compte dés e f f e t s de l ' a c t i v i t é s o l a i r e en u t i l i s a n t le f l u x s o l a i r e d é c i m é t r i q u e comme i n d e x . C e t t e p r o c é d u r e e s t u n e c o n s é q u e n c e de l ' a b s e n c e de d é t e r m i - n a t i o n c o n t i n u e d u s p e c t r e s o l a i r e d i r e c t e m e n t r e s p o n s a b l e de la s t r u c t u r e p h y s i q u e de l ' a t m o s p h è r e s u p é r i e u r e . Les v a r i a t i o n s de la t e m p é r a t u r e à la t h e r m o p a u s e s o n t d i s c u t é e s . La p é n é t r a t i o n d u
r a y o n n e m e n t s o l a i r e e s t a n a l y s é e en f o n c t i o n de l ' a c t i v i t é s o l a i r e en u t i l i s a n t c i n q s é r i e s de f l u x s o l a i r e s m e s u r é s dans l ' u l t r a v i o l e t et dans l ' e x t r ê m e u l t r a v i o l e t . P l u s i e u r s exemples de p r o f i l s d ' a b s o r p t i o n et de t a u x de p r o d u c t i o n s i o n i q u e s s o n t d i s c u t é s p o u r des c o n d i t i o n s v a r i a b l e s . D i v e r s e f f e t s é n e r g é t i q u e s s o n t d i s c u t é s . T o u s les c a l c u l s c o r r e s p o n d e n t a des c o n d i t i o n s p h y s i q u e s a u - d e s s u s de S c h e v e n i n g e n ( 5 2 . 0 8 ° N ) où e u t lieu le 14ème s y m p o s i u m E S L A B .
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Samenvatting
V e r s c h i l l e n d e semi-empirische modellen v a n de a a r d s e hogere atmo- sfeer zijn v o o r het o g e n b l i k b e s c h i k b a a r . Deze modellen h o u d e n r e k e n i n g met zonne-activiteitseffecten door g e b r u i k te maken v a n de decimetrische z o n n e f l u x als i n d e x . Een dergelijke methode is het g e v o l g v a n een g e b r e k aan continue b e p a l i n g e n v a n het z o n n e s p e c t r u m dat r e c h t s t r e e k s verantwoordelijk is v o o r de f y s i s c h e s t r u c t u u r v a n de h o g e r e atmosfeer. V e r a n d e r i n g e n in de temperatuur v a n de thermopauze worden b e s p r o k e n . De penetratie v a n de z o n n e s t r a l i n g in functie v a n de zonne-activiteit werd g e a n a l y s e e r d door te s t e u n e n op vijf r e e k s e n v a n z o n n e - b e s t r a l i n g s s t e r k t e n gemeten in het ultraviolet en in het uiterst ultraviolet.. V e r s c h i l l e n d e voorbeelden v a n absurptleproflêlên en ionen- productie v e r h o u d i n g e n worden b e s p r o k e n voor v e r a n d e r l i j k e v o o r - w a a r d e n . V e r s c h e i d e n e e n e r g e t i s s c h e effecten werden eveneens be- s c h r e v e n . Al de b e r e k e n i n g e n werden gemaakt voor f y s i s c h e v o o r - waarden boven S c h e v e n i n g e n (52°08N) waar het 14e E S L A B s y m p o s i u m werd g e h o u d e n .
Z u s a m m e n f a s s u n g
V e r s c h i e d e n e s e m i - e m p i r i s c h e Modellen d e r e r d i s c h e n h ö h e r e n A t m o s p h ä r e s i n d z u r z e i t d i s p o n i b e l . Diese Modellen f ü h r e n R e c h n u n g ü b e r d i e S o n n e n a k t i v i t ä t mit H i l f e d e r S o n n e n d e c i m e t r i s c h e s t r a h l u n g als
I n d e x . Solche M e t h o d e f o l g t aus dem Mangel an d u r c h l a u f e n d e n M e s s u n g e n des S o n n e n s p e k t r u m v o r a n t w ö r t l i c h f ü r die p h y s i k a l i s c h e S t r u k t u r d e r h ö h e r e n A t m o s p h ä r e . V a r i a t i o n e n d e r T e m p e r a t u r z u r T h e r m o p a u s e s i n d d i s k u t i e r t . F ü n f M e s s u n g e n d e r UV u n d EUV S o n n e n - s t r a h l u n g w e r d e n g e b r a u c h t um d i e D u r c h s c h l a g s k r a f t d e r S o n n e n - s t r a h l u n g f ü r v e r s c h i e d e n S o n n e n a k t i v i t ä t e n zu a n a l y s i e r e n . B e i s p i e l e d e r A b s o r p t i o n u n d d e r l o n e n p r o d u k t i o n w e r d e n f ü r v e r ä n d e r l i c h e n B e d i n g u n g e n v o r g e s t e l l t . V e r s c h i e d e n e e n e r g e t i s c h e E f f e k t e n s i n d auch b e s c h r i e b e n . Die R e c h n u n g e n s i n d g ü l t i g f ü r p h y s i k a l i s c h e n Be- d i n g u n g e n ü b e r S c h e v e n i n g e n ( 5 2 ° . 0 8 N ) wo das 14. ESLAB Symposium s t a t t g e f u n d e n h a t .
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I N T R O D U C T I O N
S o o n after 1900 it became p r o g r e s s i v e l y clear that the p r o p a g a t i o n of wireless w a v e s is influenced b y ionized particles in the u p p e r atmo- s p h e r e . A r o u n d 1925, the ionizing agent was tentatively attributed to solar ultraviolet radiation. T h e a b s e n c e of direct measurements of this radiation, the lack of laboratory data on a b s o r p t i o n c r o s s sections and recombination coefficients and the poor knowledge of the u p p e r atmo- s p h e r e composition lead to several c o n t r a d i c t o r y e x p l a n a t i o n s . T h e early i o n o s p h e r i c i n v e s t i g a t i o n s d e s c r i b e d b y Bates (1973), Waynick (1975) and Ratcliffe (1978) are, h o w e v e r , a f i r s t milestone for our urider_
s t a n d i n g of the u p p e r atmosphere. G r o u n d based radio p r o p a g a t i o n experiments remained d u n i q u e technique until the a d v e n t of space vehicles.
T h e f i r s t ultraviolet solar s p e c t r u m down to 240 nm was recorded at 55 km altitude on October 10, 1946 u s i n g a V - 2 rocket ( B a u m et al.
1946) and J o h n s o n et al. (1952) o b s e r v e d s h o r t e r w a v e l e n g t h s above 100 km in 1949. T h e s e experiments opened a new area for u p p e r atmo- s p h e r e r e s e a r c h , since they p r o v e d the existence and the measurement feasability of the s h o r t w a v e l e n g t h part of the solar spectrum. Since that time, s i g n i f i c a n t p r o g r e s s has been achieved in the m e a s u r i n g t e c h n i q u e s which will not be d i s c u s s e d here. The p r i m a r y objective of this paper is to s t r e s s the importance of reliable solar ultraviolet data for a quantitative u n d e r s t a n d i n g of the physical phenomena o c c u r i n g in the u p p e r atmosphere. S u c h an objective is sometimes in conflict with the p r i m a r y interest of solar p h y s i c i s t s since they may c o n s i d e r the E a r t h ' s atmosphere as an obstacle. It is, h o w e v e r , suitable to know the s t r u c t u r e and the behavior of an obstacle if one wants to avoid it.
A s a c o n s e q u e n c e the b e h a v i o r of the u p p e r atmosphere as a function of solar activity is d i s c u s s e d in the framework of the limited amount of solar ultraviolet data p r e s e n t l y available.
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2. S O L A R A C T I V I T Y A N D T E M P E R A T U R E OF T H E T H E R M O S P H E R E
A f t e r the l a u n c h i n g of the f i r s t artificial satellites, Jacchia ( 1 9 5 9 ) , G r o v e s (1959), Priester (1959), and K i n g - H e l e and Walker (1959) showed almost simultaneously that the a p p a r e n t l y erratic fluctuations of the orbital periods are a direct c o n s e q u e n c e of total d e n s i t y variations in the u p p e r atmosphere. Satellite d r a g data actually lead to a determination of the atmospheric total d e n s i t y in the v i c i n i t y of the satellite perigee. Variations of the total d e n s i t y at a g i v e n h e i g h t may r e s u l t from composition c h a n g e s and/or temperature modifications. A b o v e the mesopause, near 85 km altitude, molecular o x y g e n is photo- dissociated b y solar radiation with w a v e l e n g t h s s h o r t e r than 175 nm and atomic o x y g e n becomes p r o g r e s s i v e l y more a b u n d a n t than O ^ and Ng as a c o n s e q u e n c e of d i f f u s i v e separation in the E a r t h ' s gravitational field.
F u r t h e r m o r e , d i f f u s i v e t r a n s p o r t is s u c h that two minor c o n s t i t u e n t s at the mesopause, namely helium and atomic h y d r o g e n , become s u c c e s s i v e l y the most a b u n d a n t components above 500 km altitude. In s u c h an o v e r - simplified p i c t u r e , the terrestrial t h e r m o s p h e r e is characterized by four belts in which molecular n i t r o g e n , atomic o x y g e n , helium and atomic h y d r o g e n are s u c c e s s i v e l y the most a b u n d a n t c o n s t i t u e n t s ( K o c k a r t s and Nicolet, 1962; 1963). For a g i v e n temperature profile, the vertical variation of the total d e n s i t y is, h o w e v e r , insufficient to explain the o b s e r v e d d r a g data o v e r a long period of time. It is actually n e c e s s a r y to introduce temperature v a r i a t i o n s which also affect the atmospheric composition. T e m p e r a t u r e v a r i a t i o n s can be induced by particle t r a n s p o r t p r o c e s s e s , by day to n i g h t v a r i a t i o n s and b y c h a n g e s of the external heat s o u r c e s . T h e importance of molecular heat conduction and of external heat s o u r c e s were indicated b y Lowan (1955) and J o h n s o n (1956) in several attempts to c o n s t r u c t t h e r m o s p h e r i c models. Detailed calculations of theoretical models are extremely difficult, since they r e q u i r e a simultaneous solution of c o n t i n u i t y , momentum and e n e r g y c o n s e r v a t i o n equations. Even with an a p p r o p r i a t e mathematical t e c h n i q u e , it is still n e c e s s a r y to make use of experimental r e s u l t s for
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u p p e r a n d l o w e r b o u n d a r y c o n d i t i o n s . Solar u l t r a v i o l e t f l u x e s below 175 nm a r e among t h e most i m p o r t a n t b o u n d a r y c o n d i t i o n s and t h e s e d a t a s h o u l d be a v a i l a b l e f o r all solar a c t i v i t y c o n d i t i o n s .
P r e s e n t t h e r m o s p h e r i c models a r e , h o w e v e r , based on an e m p i r i c a l a p p r o a c h p a r t i a l l y j u s t i f i e d b y t h e lack of c o n t i n u o u s solar u l t r a v i o l e t f l u x m e a s u r e m e n t s a n d b y t h e mathematical d i f f i c u l t i e s t o solve t h e c o u p l e d c o n s e r v a t i o n e q u a t i o n s . S e v e r a l t y p e s of v e r t i c a l t e m p e r a t u r e p r o f i l e s a r e commonly u s e d in t h e r m o s p h e r i c m o d e l l i n g . A p r a t i c a l p r o - f i l e w h i c h allows an a n a l y t i c a l e x p r e s s i o n f o r t h e v e r t i c a l d i s t r i b u t i o n s of t h e a t m o s p h e r i c c o n s t i t u e n t s was i n t r o d u c e d b y Bates ( 1 9 5 9 ) . I t s h o u l d be r e a l i z e d t h a t t h i s p r o f i l e is not a s o l u t i o n of t h e heat c o n - d u c t i o n e q u a t i o n . T h e r e f o r e , i t can o n l y be c o n s i d e r e d as an a p p r o x i - m a t i o n . S u c h a p r o f i l e d e p e n d s on t h e t h e r m o p a u s e t e m p e r a t u r e , on t h e l o w e r b o u n d a r y t e m p e r a t u r e at 120 km a n d on a shape p a r a m e t e r w h i c h r e l a t e s t h e s e t w o q u a n t i t i e s t o t h e t e m p e r a t u r e g r a d i e n t at t h e lower b o u n d a r y a l t i t u d e . With g i v e n shape p a r a m e t e r and lower b o u n d a r y tem- p e r a t u r e i t is p o s s i b l e t o e s t a b l i s h an a n a l y t i c a l r e l a t i o n p ( z ) = f C T ^ z ) b e t w e e n t h e t o t a l d e n s i t y p ( z ) at h e i g h t z and t h e t h e r m o p a u s e t e m p e r a t u r e T . D e t e r m i n i n g T ^ remains t h e n t h e f u n d a m e n t a l p r o b l e m . C o l l e c t i n g all n i g h t t i m e t o t a l d e n s i t i e s n e a r 4 t o 5 h o u r s local solar time a n d s e a r c h i n g to w h i c h t h e r m o p a u s e t e m p e r a t u r e t h e y c o r r e s p o n d , one can e s t a b l i s h an e m p i r i c a l r e l a t i o n b e t w e e n t h e t h e r m o p a u s e t e m p e r a t u r e a n d a p a r a m e t e r r e f l e c t i n g t h e solar a c t i v i t y l e v e l . In all s e m i - e m p i r i c a l t h e r m o s p h e r i c m o d e l s , solar a c t i v i t y is r e p r e s e n t e d b y t h e 10.7 cm
r a d i o e l e c t r i c f l u x m e a s u r e d r e g u l a r l y at O t t a w a . T h i s d e c i m e t r i c f l u x has no i n f l u e n c e on t h e t h e r m a l s t r u c t u r e of t h e u p p e r a t m o s p h e r e . When t h e s o l a r d e c i m e t r i c f l u x is a v e r a g e d o v e r s e v e r a l solar r o t a t i o n s , u s u a l l y t h r e e , t h e long t e r m v a r i a t i o n o f t h e t h e r m o p a u s e t e m p e r a t u r e T ( 1 ) is g i v e n b y
T ^ U ) = a + b F (1)
- 2 2 - 2 - 1
w h e r e F is t h e d a i l y solar d e c i m e t r i c f l u x in u n i t s of 10 Wm Hz
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averaged over three solar rotations and a and b are model dependent numerical coefficients.
t
The 27 days variation is taken into account by correcting expression (1) in a way such that
1J2) = 1 J I )
+ c(F - F) (2)
where F is the daily 10.7 cm solar flux taken one day before the day to which T (2) corresponds. A third approximation is obtained by correcting T (2) for a geomagnetic effect associated with particle preci- pitations and or Joule heating. In this case,
T
»
( 3 ) =T . ( 2 ) + AT (3)
where AT is essentially a function of the geomagnetic index in the first thermospheric models (Jacchia, 1965). The temperature increase AT is
not associated with solar ultraviolet flux variations. Roemer (1971) has shown that the geomagnetic effect depends on the latitude. A fourth correction intends to represent the semi-annual variation observed in the total density such that the thermopause temperature is given by
T.C4) = T J 3 ) + f ( t ) (4)
where f ( t ) is a function of the day count in the year. Such a proce- dure was used by Jacchia (1965) to obtain a nighttime minimum temperature and the diurnal variation was then estimated from a one- dimensional theoretical calculation of Harris and Priester (1962).
A functional relation like expression (4) combined with constant boundary conditions at 120 km and the diffusion equilibrium hypothesis, implies that any temperature variation induces instantaneously a modi- fication of the total density. In situ measurements have shown that
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composition c h a n g e s are not n e c e s s a r i l y associated with temperature c h a n g e s . A s an example, Champion (1967) has shown that a seasonal- latitudinal variation of the total d e n s i t y e x i s t s in the lower thermo- s p h e r e below 200 km and Keating and Prior (1968) o b s e r v e d a s t r o n g increase of helium concentration above the winter pole as compared to the local summer value. With s u c h c o n s i d e r a t i o n s a new set of models (Jacchia, 1971) was c o n s t r u c t e d and adopted as the C O S P A R International Reference A t m o s p h e r e ( C I R A , 1972). Lower b o u n d a r y conditions are now taken at 90 km; a d e p a r t u r e from d i f f u s i v e equilibrium is introduced for atomic a n d molecular o x y g e n between 90 km and 120 km; the geomagnetic effect is latitude dependent and associated simultaneously with a temperature variation and a total d e n s i t y v a r i a t i o n ; the semi-annual effect is r e p r e s e n t e d as a correction to the total d e n s i t y ; a n d , the diurnal variation of the thermopause temperature is g i v e n b y an empirical e x p r e s s i o n .
Two important aspects are, h o w e v e r , not c o v e r e d b y these im- p r o v e m e n t s . F i r s t of all, temperature determinations from incoherent scatter o b s e r v a t i o n s ( C a r r u et al. 1967; N i s b e t , 1967) clearly indicate that the thermopause temperature peaks later than the 14 h o u r s local solar time maximum of the total d e n s i t y . T h i s important c o n t r i b u t i o n is only taken into account in v e r y recent models and specific models are now available for S a i n t - S a n t i n ( 4 4 . 6 ° N ) ( B a u e r et al. 1970) and for Millstone Hill ( 4 2 . 6 ° N ) ( S a l a h et al. 1976). F u r t h e r m o r e , O G O - 6 mass spectrometer o b s e r v a t i o n s ( M a y r et al. 1974) have shown that the partial d e n s i t y of each constituent reaches a maximum value at a local time which d e p e n d s on the altitude and on the constituent.
Since the description of the terrestrial t h e r m o s p h e r e is becoming more complex as it a p p r o a c h e s physical reality, Jacchia (1977) developed a new set of models c o n s i s t i n g of two p a r t s : static models and a set of empirical e x p r e s s i o n s to compute the thermopause temperature and the expected deviations from the static models r e s u l t i n g from all recognized t h e r m o s p h e r i c v a r i a t i o n s . T h e formulation is, h o w e v e r , more com-
p l i c a t e d , since any new refinement must not d e s t r o y the p r e v i o u s achievements.
A completely d i f f e r e n t a p p r o a c h has been made b y Hedin et a I.
(1974) who used s p h e r i c a l harmonics f o r a global r e p r e s e n t a t i o n of m a s s - s p e c t r o m e t r i c data. In a v e r y general case, one writes
T . = T . Gj (5)
T120 = f1 2 0 G2 ( 6 )
s = s G3 (7)
n.(120) = n (120) exp (G. - 1) (8)
where Tw is the thermopause t e m p e r a t u r e , T1 2 Q is the temperature at 120 km, s is the shape parameter in the analytical temperature p r o f i l e g i v e n b y Bates (1959) and n. (120) is the c o n c e n t r a t i o n of c o n s t i t u e n t i at 120 km a l t i t u d e . G1, G2, G3 and G. are s p h e r i c a l harmonics e x p a n s i o n s w h i c h d e p e n d on latitude, local solar time, geomagnetic index K or A , d a i l y solar decimetric f l u x F, solar decimetric f l u x F
P P
a v e r a g e d o v e r t h r e e solar rotations and day count in the y e a r . T^, T ( 1 2 0 ) , s and n.(120) are a v e r a g e d values of the r e s p e c t i v e q u a n t i t i e s , when the c o r r e s p o n d i n g G f u n c t i o n is one. Formal e x p r e s s i o n s G^ to G.
s l i g h t l y v a r y from one aouthor to a n o t h e r . Several semi-empirical models based on s p h e r i c a l harmonics are p r e s e n t l y available ( H e d i n et al.
1977a, 1977b; von Zahn et al. 1977; B a r l i e r et al. 1978; K o h n l e i n , 1980). A l l these models allow a computation of the thermopause tempera- t u r e at any place over the globe as a f u n c t i o n of solar a c t i v i t y , geo- magnetic a c t i v i t y , local solar time and day count in the y e a r . Com- p a r i s o n s between v a r i o u s models have been made b y B a r l i e r et al.
(1977), by J a c c h i a (1979) and b y K o c k a r t s (1981). For v e r y low (F = 70 x 1 0 "2 2 Wm"2 H z "1) or v e r y h i g h (F = 200 x 10~22 Wm"2 H z "1) the
- 1 0 -
r e s u l t i n g d a i l y a v e r a g e d t h e r m o p a u s e t e m p e r a t u r e can v a r y b y a p p r o x i - m a t e l y 100 K d e p e n d i n g on t h e a d o p t e d s e m i - e m p i r i c a l model.
F i g u r e s 1a and 1b g i v e t h e t h e r m o p a u s e t e m p e r a t u r e above S c h e v e n i n g e n ( 5 2 . 0 8 ° N ) b e t w e e n 1958 a n d 1980. C o m p u t a t i o n s are made at 17 h o u r s LST ( l o c a l solar t i m e ) on t h e f i r s t of each m o n t h . T h e a d o p t e d LST a p p r o x i m a t e l y c o r r e s p o n d s t o time of t h e d a i l y t e m p e r a t u r e maximum. V e r t i c a l d o t t e d lines show t h e t e m p e r a t u r e i n c r e a s e r e s u l t i n g f r o m g e o m a g n e t i c a c t i v i t y . T h e f u l l line in t h e lower p a r t i n d i c a t e d t h e 10.7 cm solar f l u x F a v e r a g e d o v e r t h r e e solar r o t a t i o n s w h e r e a s t h e d a s h e d line g i v e s t h e d a i l y solar d e c i m e t r i c f l u x F t a k e n t h e d a y b e f o r e t h e f i r s t of each m o n t h as r e q u i r e d b y t h e model of B a r l i e r et al.
( 1 9 7 8 ) . T h i s m o d e l , called DTM ( D r a g T e m p e r a t u r e Mnrtel) is based on s a t e l l i t e d r a g data and on a t e m p e r a t u r e model c o n s t r u c t e d b y T h u i l l i e r et a l . (1977) f r o m F a b r y - P e r o t i n t e r f e r o m e t e r measurements of s p e c t r a l p r o f i l e s of t h e 630 nm a i r g l o w line as o b s e r v e d on O G O - 6 b y Blamont and L u t o n ( 1 9 7 2 ) . I n c o h e r e n t s c a t t e r data are also i n c l u d e d in t h e t e m p e r a t u r e model. T e m p e r a t u r e d a t a w e r e o b t a i n e d f r o m 8 J u n e 1969 to 16 A u g u s t 1970 as i n d i c a t e d b y t h e h o r i z o n t a l line in F i g u r e 1b. T h e D T M model is a d o p t e d h e r e s i n c e it is t h e sole s e m i - e m p i r i c a l model w h i c h is c o n s t r u c t e d w i t h o p t i c a l l y m e a s u r e d t e m p e r a t u r e s . In all o t h e r s e m i - e m p i r i c a l model t h e r m o p a u s e t e m p e r a t u r e s are q u a n t i t i e s d e d u c e d in a w a y as to f i t m e a s u r e d c o n c e n t r a t i o n s or t o t a l d e n s i t i e s . O p t i c a l d e t e r m i n a t i o n s are p r e s e n t l y t h e sole t e c h n i q u e u s e d to d e t e r m i n e d i r e c t l y g l o b a l t e m p e r a t u r e c o v e r a g e , s i n c e t h e i n c o h e r e n t s c a t t e r t e c h n i q u e is u n f o r t u n a t e l y r e s t r i c t e d b y t h e l i m i t e d amount of i n - c o h e r e n t s c a t t e r i n g s t a t i o n s . F u r t h e r m o r e , F a b r y - P e r o t i n t e r f e r o m e t e r m e a s u r e m e n t s a r e l i m i t e d u p to now t o a v e r y s h o r t time p e r i o d as it can be seem in F i g u r e 1 b . From an a b s o l u t e p o i n t of v i e w , all t e m p e r a - t u r e s i n F i g u r e s 1a a n d 1b b e f o r e J u n e 1969 a n d a f t e r A u g u s t 1970 are e x t r a p o l a t i o n s ! H o w e v e r , t h e s e t e m p e r a t u r e s lead to c o n c e n t r a t i o n s and t o t a l d e n s i t i e s in reasonable a g r e e m e n t w i t h mass s p e c t r o m e t e r data and s a t e l l i t e d r a g d a t a .
F i g . la : Thermopause temperature at 17 hours LST above Scheveningen (52.08°N) on the first of each month from 1958 to 1969. Vertical dashed line in- dicate temperature increases resulting from geomagnetic a c t i v i t y . Dashed curve in the lower part gives the daily 10.7 cm solar flux F ^ ^
- 2 2 - 2 - 1 -
in 10 Wm Hz ; full curve represents the averaged value F 7 over three solar rotations.
Y E A R S
F i g . lb : Continuation of F i g . la from 1969 to 1980. Horizontal line labeled 0G0-6 indicate the time period over which temperature data were measured with a Fabry-Perot interferometer.
S e v e r a l i n t e r e s t i n g f e a t u r e s can be seen in F i g u r e s 1a a n d 1 b . T h e v a r i a t i o n o f t h e t h e r m o p a u s e t e m p e r a t u r e w i t h t h e 11 y e a r s solar c y c l e is c l e a r l y a p p a r e n t . F u r t h e r m o r e , t w o c o n s e c u t i v e solar c y c l e s a r e not n e c e s s a r i l y i d e n t i c a l . As an e x a m p l e , t h e t h e r m o p a u s e t e m p e r a t u r e was m u c h h i g h e r d u r i n g t h e 1958 maximum t h a n d u r i n g t h e 1969 m a x i m u m , w h i c h seems t o be l o w e r t h a n t h e 1980 maximum. A n a n n u a l v a r i a t i o n a p p e a r s in t h e t e m p e r a t u r e a n d i t is not c o r r e l a t e d w i t h t h e solar d e c i m e t r i c f l u x . T h e r e f o r e , a h i g h e r solar d e c i m e t r i c f l u x can c o r r e s p o n d to a lower t e m p e r a t u r e , d e p e n d i n g on t h e d a y in t h e y e a r . Peaks in t h e d a i l y f l u x F do n o t n e c e s s a r i l y lead t o peaks in t h e thermo- pause t e m p e r a t u r e w h i c h c a n n o t be c o n s i d e r e d as d i r e c t l y p r o p o r t i o n a l t o F o r F o r a c o m b i n a t i o n of b o t h .
3 . , G L O B A L T E M P E R A T U R E A N D T E R R E S T R I A L I N S O L A T I O N
In t h e DTM model of B a r l i e r et al. (1978) t h e s p h e r i c a l h a r m o n i c s e x p a n s i o n s u s e d in e x p r e s s i o n s ( 5 ) to ( 8 ) a r e f o r m a l l y g i v e n b y
oo °°
G = 1 + F + M + I a P ° ( 0 ) + p 1 b° P ° ( 8 ) cos [p Q(d - 6 ) ]
1 q = i q q P= i p p p
+ p ? I { Cm pm ( 6 ) cos (mwt) + dm Pm(6) s i n (muit)) (9) n = l m=l n q n n
w h e r e P™(6) a r e associated L e g e n d r e f u n c t i o n s of t h e c o l a t i t u d e 8 ,
- 1 - 1
fi = 2ti/365 ( d a y ) and uj = 2n/24 ( h o u r ) . T h e solar d e c i m e t r i c f l u x d e p e n d e n c e and t h e g e o m a g n e t i c e f f e c t are r e s p e c t i v e l y g i v e n b y F^ and M. C o e f f i c i e n t 0 is 1 + F ^ , d is t h e d a y c o u n t in t h e y e a r and t is t h e local s o l a r t i m e . T h e d e v e l o p m e n t ( 9 ) is l i m i t e d s u c h t h a t 35 u n k n o w n c o e f f i c i e n t s m u s t be d e t e r m i n e d b y a least s q u a r e t e c h n i q u e . It s h o u l d be n o t e d t h a t t h e f o r m of some t e r m s in e x p r e s s i o n ( 9 ) is chosen a p r i o r i b e f o r e a least s q u a r e is u s e d t o d e t e r m i n e t h e u n k n o w n c o e f f i - c i e n t s . T h i s is t h e case f o r t h e solar a c t i v i t y e f f e c t g i v e n b y
F = A4 (F - F) + A5( F - F )2 + A6( F - 150) (10)
- 1 4 -
where the decimetric fluxes F and F are measured in units of 10
- 2 - 1
Wm Hz and for the geomagnetic activity effect given by
- 2 2
- 1
M = (A? + A8 2 g P°(0)) K P (11)
where Kp is the three-hourly planetary index taken 3 hours before the considered local solar time.
When F = F = 150 and K = O expressions (5) and (9) lead to a P
global temperature distribution which only depends on annual, semi- annual and diurnal variations. The last effect can also be eliminated by integration over the 24 hours day period. A global diurnally averaged temperature distribution can be computed as shown in Figure 2 where thermopause temperature isopleths are given as a function of geographic latitude and day count in the year. Such a map shows a pronounced seasonal variation in both hemispheres. T h i s variation is independent of a modification in the decimetric flux (F = F = 150 = constant through the year) or in K^ kept always equal to zero. Such a hypothetical case can be used to make a comparison with the daily averaged insolation which should be at least partially responsible for a seasonal variation.
According to Milankovitch (1930), the daily averaged insolation T on a horizontal surface at latitude q> for a solar declination 6 is given by
— - 1 2
I = I 71 (a/r) (4>q sin <p sin 6 + sin cos cp cos 6 ) (12)
where I is the solar flux at 1 AU = a, r is the actual Sun-Earth a ' distance and i|<o = - tgcptg 6. For regions with permanent sunlight t|/Q =
71 and during polar night = O. Application of expression (12) leads to the isopleths shown in Figure 3 computed for an arbitrary incident flux I = 1 . Values of I are given on each curve. Solar declination is a a
indicated by the dashed line and limits of polar nights are shown by dotted-dashed lines. Figure 3 can be applied to any solar ultraviolet
-15-
Fig. 2 : Isopleths of daily averaged thermopause temperature for F ^ ^ - F 150 x 10~2° Win^Hz"*1 and K = 0.
P
Fig. 3 : Isopleths of daily averaged insolation for an arbitrary solar ir- radiance I = 1 at 1 A.U. Solar declination is given by the dashed line
a
and permanent polar nights are indicated by dotted-dashed lines.
f l u x r e a c h i n g a h o r i z o n t a l s u r f a c e at t h e top of the atmosphere b e f o r e a b s o r p t i o n processes become e f f e c t i v e . Daily averaged values are o b t a i n e d b y m u l t i p l y i n g solar u l t r a v i o l e t f l u x e s b y the n u m b e r s on each i s o p l e t h . F i g u r e s 2 and 3 have some o b v i o u s similarities i n d i c a t i n g that t h e g e o m e t r y of the insolation plays a s i g n i f i c a n t role in the global s t r u c t u r e of t h e thermopause t e m p e r a t u r e . F u r t h e r m o r e , both F i g u r e s show a d e f i n i t e N o r t h - S o u t h a s y m m e t r y : d u r i n g local summer t h e t e m p e r a t u r e is h i g h e r in t h e s o u t h e r n hemisphere w h i c h receives also more solar e n e r g y t h a n t h e n o r t h e r n h e m i s p h e r e . Such asymmetries e x i s t also in t h e t h e r m o s p h e r i c composition ( B a r l i e r et al. 1974), b u t no q u a n t i t a t i v e e x p l a n a t i o n has been g i v e n .
F i g u r e s 2 and 3 are superimposed in F i g u r e 4 in o r d e r to chow t h e relation between insolation and thermopause t e m p e r a t u r e . In the s o u t h e r n hemisphere t h e local summer maximum of the thermopause t e m p e r a t u r e c o r r e s p o n d s r a t h e r well w i t h the maximum insolation. How- e v e r , in t h e n o r t h e r n hemisphere the maximum t e m p e r a t u r e o c c u r s before maximum i n - s o l a t i o n . T h e p a r t i a l s i m i l a r i t y between b o t h t y p e s of isopleths in F i g u r e 4 is, n e v e r t h e l e s s , an i n d i c a t i o n to j u s t i f y another semi-empirical r e p r e s e n t a t i o n as g i v e n b y e x p r e s s i o n ( 9 ) . Since t h e i n - stantaneous as well as t h e d a i l y a v e r a g e d illumination are known a n a l y - t i c a l l y it could be r e w a r d i n g to i n t r o d u c e them in e x p r e s s i o n ( 9 ) f o r a b e t t e r r e p r e s e n t a t i o n of the e f f e c t of solar u l t r a v i o l e t r a d i a t i o n .
When t h e d a i l y mean t e m p e r a t u r e is a v e r a g e d in each hemisphere one o b t a i n s t h e r e s u l t shown in F i g u r e 5 w h i c h g i v e s also the daily mean insolation as a f u n c t i o n of the day c o u n t in t h e y e a r . The N o r t h - South asymmetry appears v e r y c l e a r l y in the mean hemispheric tempera- t u r e . H o w e v e r , it should be noted t h a t the annual mean t e m p e r a t u r e as well as t h e annual mean insolation are identical in b o t h hemispheres.
No u l t r a v i o l e t solar f l u x data are i n c l u d e d in any semi-empirical model, b u t t h i s does not imply t h a t much b e t t e r models can be con- s t r u c t e d w i t h o u t such d a t a . The 10.7 cm decimetric f l u x is commonly
DAY C O U N T
Fig. 4 : Superposition of Figs. 2 and 3 in order tc show a correlation between insolation (dashed lines) and temperature (full lines).
1200I | i UJ cr
z>
!5 25 1100 Q.
LLI
<
w 1000 _ s
I • I 1 I 1 I 1 1 '
^
' I "F = F = 150x10 Wm"2Hz^
Kp = 0 "
North /
/ _ /
/ / /
V S o u t h s /
<
o
900)- 3 0.4
0.3
s
%
o o _i CO z
- 0.2
z <
UJ 2
s - \ A
\
h- \
North
South i I
20 60 100 140 180 220 260 300 340 DAY COUNT
. 5 : Daily mean thermopause temperature averaged over northern (full curve) and southern (dashed curve) hemisphere. Relative daily mean insolation is given in the lower part.
- 2 0 -
u s e d , s i n c e no b e t t e r i n d e x is p r e s e n t l y a v a i l a b l e to r e p r e s e n t v a r i a - t i o n s of s o l a r u l t r a v i o l e t f l u x e s r e s p o n s i b l e for the h i g h l y v a r i a b l e state of t h e t e r r e s t r i a l u p p e r a t m o s p h e r e .
4. P E N E T R A T I O N O F S O L A R R A D I A T I O N
B e f o r e e s t i m a t i n g e f f e c t s of s o l a r u l t r a v i o l e t r a d i a t i o n on the t e r r e s t r i a l a t m o s p h e r e , it is n e c e s s a r y to k n o w how d e e p l y a n y solar
r a d i a t i o n can p e n e t r a t e into the a t m o s p h e r e . T h e b e s t q u a n t i t y to make s u c h an estimation is the optical d e p t h d e f i n e d b y
t ( A ,X, z ) = I a. (A) n. H. Ch (X fz ) ( 1 3 )
1 L 1 1
w h e r e x is the s o l a r z e n i t h d i s t a n c e , a . ( A ) is the a b s o r p t i o n c r o s s s e c t i o n at w a v e l e n g t h A f o r the i - t y p e a t m o s p h e r i c c o n s t i t u e n t with c o n c e n t r a t i o n n. a n d scale h e i g h t H. at a l t i t u d e z. C h ( x , z ) is s i m p l y the i i s e c a n t of the s o l a r d i s t a n c e w h e n X is smaller t h a n 75°: F o r l a r g e r d i s t a n c e s C h a p m a n ( 1 9 3 1 ) h a s s h o w n t h a t the E a r t h ' s c u r v a t u r e m u s t be t a k e n into a c c o u n t a n d v a r i o u s analytical a p p r o x i m a t i o n s are available f o r t h e C h a p m a n f u n c t i o n C h ( x , z ) ( S w i d e r , 1964; S w i d e r a n d G a r d n e r , 1 9 6 9 ) .
A s a c o n s e q u e n c e of the e x p o n e n t i a l d e c r e a s e of the a t m o s p h e r i c c o n s t i t u e n t s , maximum of a b s o r p t i o n a p p r o x i m a t e l y o c c u r s at the altitude- w h e r e x ( A , x , z ) = 1- U s i n g the s e m i - e m p i r i c a l model D T M of B a r l i e r et al. ( 1 9 7 8 ) a n d the a b s o r p t i o n c r o s s s e c t i o n s t a b u l a t e d for 5 nm w a v e l e n g t h i n t e r v a l s b y T o r r et al. ( 1 9 7 9 ) b e t w e e n 5 nm a n d 105 nm, o n e o b t a i n s in F i g u r e 6 the a l t i t u d e s w h e r e u n i t optical d e p t h is r e a c h e d as a f u n c t i o n of w a v e l e n g t h . C o m p u t a t i o n s a r e made for 12 h o u r s L S T a b o v e S c h e v e n i n g e n for s p e c i f i c d a y s w h e n solar u l t r a v i o l e t f l u x e s are a v a i l a b l e in t a b u l a r f o r m . A b o v e 105 nm molecular o x y g e n a b s o r p t i o n c r o s s s e c t i o n s are a d o p t e d as a v e r a g e d v a l u e s of the detailed m e a s u r e m e n t s of O g a w a a n d O g a w a ( 1 9 7 5 ) . T h e s e v a l u e s are in r e a s o n - able a g r e e m e n t with the a v e r a g e d c r o s s s e c t i o n s s u g g e s t e d b y A c k e r m a n
- 2 1 -
250
200
UJ o
z>
150
100 80.
—i—i—i—|—i—i—i—i—[7771—rjn—1—|—1—1—1—1—|—1— T—1 1 | 1 1 —1—1—|—1—1—1 <
eCTTf -
Unit optical depth Latitude 52.08* N
LST 12 h
sec x
I.IB7 July 18 . 1976 --- 1.301 April 23 . 1974
- • - " * " - - - 2.252 February 19. 1979 3.226 January 22 . 1979 3.931 Oecember K . 1978
1 Lya
I , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 iL,l J i > 1 i i > i
25 50 75 !00
WAVELENGTH(nm)
125 150 175
Fig. 6 : Altitude for unit optical depth as a function of wavelength. Computations are made at 12 hours LST above Scheveningen for five days.
- 2 2 -
i
( 1 9 7 1 ) . Since semi-empirical models u s u a l l y do not e x t e n d below 120 km a l t i t u d e , the U . S . S t a n d a r d A t m o s p h e r e f o r 45°N was used below that h e i g h t w i t h some a r b i t r a r y ajustment to match t h e c o n c e n t r a t i o n s g i v e n b y DTM at 120 km. It can be seen in F i g u r e 6 t h a t t h e h e i g h t f o r unit optical d e p t h s t r o n g l y depends on the w a v e l e n g t h , on the solar zenith distance and on solar a c t i v i t y . A l t h o u g h no solar f l u x value e n t e r s in the computation of the optical d e p t h , the n e u t r a l atmosphere above 120 km is s u f f i c i e n t l y v a r i a b l e w i t h solar a c t i v i t y to m o d i f y the h e i g h t were maximum a b s o r p t i o n o c c u r s . Solar a c t i v i t y v a r i a t i o n s induce s t r o n g modifications in the composition and t e m p e r a t u r e of the u p p e r atmo- s p h e r e and it is completely u n r e a l i s t i c to adopt a u n i q u e c u r v e f o r the a l t i t u d e of u n i t optical d e p t h . F u r t h e r m o r e , seasonal and d i u r n a l v a r i a t i o n s alco a f f c c t the p e n e t r a t i o n of suldr r a d i a t i o n .
Solar u l t r a v i o l e t f l u x e s c o r r e s p o n d i n g to the dates i n d i c a t e d in F i g u r e 6 were measured on board of A t m o s p h e r e E x p l o r e r E and are t a b u l a t e d b y T o r r et al. (1979) below 105 nm. F i g u r e 7 shows the extreme values c o r r e s p o n d i n g to h i g h solar a c t i v i t y (19 F e b r u a r y 1979) and to low solar a c t i v i t y (23 A p r i l 1974). Dots and crosses c o r r e s p o n d to solar emission lines whose i n t e n s i t i e s are i n c l u d e d in the f l u x values a v e r a g e d o v e r 5 nm i n t e r v a l s . Above 105 nm the h e i g h t of u n i t optical d e p t h is o n l y g i v e n f o r 23 A p r i l 1974 since t a b u l a t e d f l u x values are only available f o r this date ( H e r o u x and H i n t e r e g g e r , 1978) at the p r e s e n t time. H o p e f u l l y more data will become available v e r y soon, since the lower t h e r m o s p h e r e between 85 km and 120 km is still p o o r l y k n o w n . Good solar i r r a d i a n c e data in the S c h u m a n n - R u n g e c o n t i n u u m below 175 nm are of paramount i m p o r t a n c e , since molecular o x y g e n is s t r o n g l y photodissociated in t h i s w a v e l e n g t h r a n g e in the lower t h e r m o - sphere and leads to atomic o x y g e n w h i c h becomes a major atmospheric c o n s t i t u e n t above 150 km a l t i t u d e . Comparisons between atomic o x y g e n c o n c e n t r a t i o n s at 120 km from semi-empirical models and f r o m optical measurements reveal discrepancies w h i c h may reach a f a c t o r of f o u r ( K o c k a r t s , 1981).
1 1 ] 1 1 1 1 r — | i 1 1 1 1 1 r
19 February 1979 23 April 1974
Hell
Si*XI
OY X
He I
Mg IX
X
0 IV
Hell,SiX
M g X
0 IV
i i Olli,
Ne VII Fe XV. x
'x
1 Ne VIII
*NIV
c m x HI
' ' x i i
-J L I I L. L___1__J I 1 I 1 L.
10 20 30 40 50 60 70 80 90 100 110 WAVELENGTH (nm)
. 7 : Extreme values of solar irradiances between 5 run and 105 nm corresponding to minimum (23 April 1974) and to maximum (19 February) solar activity conditions. Dots and crosses correspond to solar emission lines included in the 5 nm intervals averaged values.
Whereas semi-empirical atmospheric models use only two solar activity indices ( F and F ) , solar ultraviolet f l u x e s measurements deal with more than 1000 individual w a v e l e n g t h s . H i n t e r e g g e r (1981) d i s c u s s e d v a r i o u s representations of solar f l u x e s for aeronomical applications. T h e r e is actually no u n i v e r s a l solution, since the adopted w a v e l e n g t h intervals d e p e n d on the objective which has to be tackled.
O n one h a n d the solar spectrum has its own c h a r a c t e r i s t i c s and on the other h a n d the atmospheric species have their individual a b s o r p t i o n s p e c t r a . T h e c r o s s sections tabulated by T o r r et al. (1979) at 5 nm w a v e l g n t h intervals are weighted v a l u e s computed from a compilation by K i r b y et al. (1979) at all w a v e l e n g t h s of the solar lines and continua g i v e n in a solar reference s p e c t r u m of H i n t e r e g g e r (1976). Molecular o x y g e n and molecular n i t r o g e n have complicated a b s o r p t i o n and ioniza- tion c r o s s sections (see B a n k s and K o c k a r t s , 1973a; L o f t h u s and
K r u p e n i e , 1977) which are not always perfectly known.
A s stated p r e v i o u s l y the optical d e p t h s used in F i g u r e 6 are computed with 5 nm interval a v e r a g e d molecular o x y g e n a b s o r p t i o n c r o s s sections between 105 nm and 175 nm. M e a s u r e d a b s o r p t i o n c r o s s sections ( O g a w a and O g a w a , 1975) are s h o w n in F i g u r e 8 for the
3 _
g r o u n d state 0?( X I ). T h e s t r o n g band s t r u c t u r e below 130 nm clearly indicates that a detailed a n a l y s i s of the penetration of solar radiation would r e q u i r e a much h i g h e r resolution of the solar s p e c t r u m . I n c i d e n - tally, the small a b s o r p t i o n c r o s s section of O ^ at L y m a n - a indicates w h y this radiation can penetrate in the terrestrial atmosphere down to meso- s p h e r i c levels. L y m a n - a line at 121.567 nm is not included in the unit optical depth computation of F i g u r e 6.
A n o t h e r interesting example is g i v e n by the h i g h l y variable a b s o r p t i o n c r o s s section of O^ in the S c h u m a n n - R u n g e b a n d s . U s i n g the a b s o r p t i o n c r o s s sections of A c k e r m a n et al. (1970) it is possible to estimate the s t r u c t u r e of the solar spectra which could be o b s e r v e d at 85 km altitude for 90° solar zenith distance. T h e r e s u l t s of s u c h a computation is s h o w n in F i g u r e 9 for the 10-0 b a n d . T h e incident solar
10 I I' ' ' I I I I I | I I I I I I | I I | I I I I I I I I | | I | I I I I I I I I I I I I ! I I I I | I I ! |
ro
CT>
10 ~
10
o 2
<ƒ) in
>/) g J9L
u 10 2 0
t— 01
a o l/l <n
<
.701
10
} Lyc
110 ' ' I' l l I I I I I I I I I I I I I I I I I H I I I I i | | | | ! I ! ! I I I I I I I 130 U0
W A V E L E N G T H ( n m )
150 160
Fig. 8 Absorption cross section of ground state molecular oxygen between 109 nm and 165 nm.
i r r a d i a n c e , s h o w n in the u p p e r part of F i g u r e 9, is taken from Samain and Simon (1976). It a p p e a r s that the incident s p e c t r u m is s t r o n g l y modified and the r e s u l t i n g s p e c t r u m is even temperature dependent since the a b s o r p t i o n c r o s s section in the S c h u m a n n - R u n g e b a n d s is influenced b y the atmospheric temperature. S u c h an experiment could p r o v i d e informations not only on the solar s p e c t r u m , but also on the a b s o r p t i o n s p e c t r u m of molecular o x y g e n . A f i r s t a p p r o a c h along this line has been made b y Longmire et al. (1979) d u r i n g r e - e n t r y of a rocket launched from the White S a n d s , New Mexico, missile r a n g e . O t h e r examples s h o w i n g how the optical depth v a r i e s in several b a n d s of the S c h u m a n n - R u n g e system are g i v e n b y K o c k a r t s (1971). In photo- chemical models dealing with n u m e r o u s chemical reactions and t r a n s p o r t phenomena, it is, h o w e v e r , difficult to introduce the detailed a b s o r p t i o n in the S c h u m a n n - R u n g e b a n d s . T h e r e f o r e , several simple approximations h a v e been established (see K o c k a r t s , 1976) in o r d e r to p r o v i d e a c o n - v e n i e n t t e c h n i q u e for the computation of solar penetration in the S c h u m a n n - R u n g e b a n d s of molecular o x y g e n .
5. E N E R G E T I C E F F E C T S O F S O L A R R A D I A T I O N
A n y consideration of the effect of solar radiation on the terrestrial atmosphere s h o u l d deal with three aspects : the intensity of the solar radiation, the capability for atmospheric species to a b s o r b this radiation and the a b u n d a n c e of the a b s o r b i n g species. With, the tabulated v a l u e s of T o r r et al. (1979) for solar i r r a d i a n c e s between 5 nm and 105 nm and the v a l u e s of H e r o u x and H i n t e r e g g e r (1978) between 105 nm and 175 nm, the atmospheric model of B a r l i e r et al. (1978) leads to the a b s o r b e d e n e r g y rates above S c h e v e n i n g e n s h o w n in F i g u r e 10 for 12 h o u r s L S T on the d a y when the solar f l u x e s were measured. T h e U . S . S t a n d a r d A t m o s p h e r e was u s e d below 120 km altitude. T h e full c u r v e g i v e s the total e n e r g y a b s o r b e d between 5 nm and 175 nm. T h e d a s h e d , d o t t e d - d a s h e d a n d dotted c u r v e s are the c o n t r i b u t i o n s of the indicated w a v e l e n g t h r a n g e s to the total a b s o r p t i o n , t y m a n - a is not included in the 105-135 nm interval, since this radiation is o n l y a b s o r b e d below
-27-
Fig. 9 : Penetration of solar radiation in the 10-0 band of the Schumann-Runge system of Grazing incidence (X = 90°) is assumed at 85 km altitude where two extreme temperatures are adopted. Incident solar flux is shown in the upper part.
- 2 8 -
>
Fig. 10 : Absorbed energy rate as a function of height at 12 hours LST on 23 April 1974 above Scheveningen.
Energy distribution in various wavelength ranges is shown by dashed line, dotted line and dotted dashed line.
-29-
100 km as a consequence of the low absorption cross section of O^ (see Figure 8) at 121.567 nm. It can be seen that the extreme ultraviolet wavelength range ( M 5-105 nm) is mainly absorbed above 150 km, whereas absorption in the Schumann-Runge continuum (AA. 135-175 nm) peaks around 100 km and Lyman-a has its maximum absorption in the mesosphere below 85 km altitude. T h e wavelength range KX 105 - 135 nm corresponds to a highly variable absorption cross section for O^ (see Figure 8) but the absorbed energy rate has been computed with averaged cross sections. Crosses in Figure 10 indicate total absorbed energy rates obtained when the neutral model of Hedin et al. (1977a, b) is used instead of DTM.
In order to emphasize the high variability of solar absorption in the extreme ultraviolet, Figure 11 shows the absorbed energy rate for five days between 1974 and 1979, i.e. from minimum (curve 1) to maximum (curve 5) solar activity conditions. Extreme ultraviolet irra- diances ( T o r r et al. 1979) increase during that period, but composition and temperature in the neutral model are simultaneously modified.
Furthermore, at 12 hours L S T the solar zenith angle x is not constant with seasons. As a result, there is no simple relation between the absorbed energy rate and the variations of solar extreme ultraviolet fluxes. Figure 11 indicates that around 150 km for the same local solar time, the absorbed energy rate can be much smaller for maximum solar activity than for minimum solar activity conditions. The vertical integral of the absorbed energy is, however, greater for maximum solar activity.
Since the first ionization potentials of O
2, O, N
2and He are at
102.7 nm, 91.0 nm, 79.6 nm and 50.4 nm respectively, a significant
part of the absorbed energy rate below 105 nm is consumed in ionization
processes. Figures 12 and 13 show ion production rates at 12 hours
L S T on 18 july 1976 and 14 December 1978, respectively. These two
days have been chosen since they correspond to the extreme solar
zenith distances used previously. In Figure 12 the major ion production
ABSORBED ENERGY RATE ( e r g c m J s J )
Fig. 11 : Absorbed energy rates above 120 km for the five days considered in Fig.
6. Solar zenith distances X are given for each.day.
350
CO I ro I
300
eS 250 E UJ O 3
< 200
150 120
I M I I 1 I 111 1 1 I \ I I 11| 1
^ v * 11111 1—I—I I II II) 1—I—I I I I III
o S \
N \ Total Latitude 52.08 N
18 Duly 1976 12 h L ST
X = 31.0°
J I I I I I III J L.
10" _1 10
AX5-105nm \ \ \
\ \
x 1\ I 1 \ I / Kl- /
_l |-T I I ) ll i ~"I L-J^U-t I I l i n n
10 10 10
PRODUCTION RATE (cm~3s~1)
F i g . 12 : Total and partial ion production rates on 18 July 1976 at 12 hours LST in the extreme ultraviolet. Low solar activity conditions.
PRODUCTION RATE (crrr
3s
J)
Fig. 13 : Same as ditions.
Fig. 12 but for 14 December 1978. High solar activity con- H e+ ion production is apparent.
+
results from N2 whereas the N2 production rate is never the most im- portant in Figure 13. Furthermore the He+ production rate is completely negligible below 350 km in Figure 12. Actually the total ion production rates in Figures 12 and 13 follow closely the corresponding absorbed energy rates in Figure 11.
For a given extreme ultraviolet irradiance, ion production rates v a r y strongly d u r i n g the day as it can be seen on Figures 14 and 15 where diurnal variations of and 0+ are shown at 150 km and 200 km altitude. Such an effect is a combination of the time variation of the solar zenith angle, and the diurnal variation of the neutral atmosphere.
Both Figures indicate that ionic production is more important at 150 km than at 200 km for s p r i n g and summer conditions. During winter the opposite situation occurs and no simple dependence on solar activity can be deduced. Furthermore, a good knowledge of solar irradiances is re- quired when specific applications are attempted. As an example, Taieb et al. (1978) explained an observed daytime valley in the ionospheric F^
layer by introducing a downward ionization d r i f t . These calculations
- 2 2
were made by using Hinteregger's (1976) fluxes (F1 f ) 7 = 73 x 10
- 2 - 1
Wm Hz ) and measurements on board of A E R O S - A (Schmidtke, 1976)
- 2 2 - 2 - 1
when F1 q 7 = 95 x 10 Wm Hz . Schmidtke's values were closer to the solar activity conditions at the time when the valley was observed and only these data lead to a good fit with the incoherent scatter results.
A fundamental parameter for the determination of the thermal s t r u c t u r e of the upper atmosphere is the heating efficiency which is the fraction of the absorbed energy rate (see Figure 10) transfered into heat. Some of the absorbed energy is used for photodissociation, mainly in the Schumann-Runge continuum and below 102.6 nm some energy is consumed for ionization processes. T h e problem is, however, compli- cated by the fact that products of photodissociation such as excited atomic oxygen 0 ( D) can undergo collisional deactivation and can be transported a lower heights. Moreover, after the production of electron-
- 3 4 -
Fig. 14 : Daily variation of N2 + production rates at 150 km and 200 km. Solar activity increases from 1974 to 1979.
LOCAL SOLAR T I M E ( h r s )
Fig. 15 : Daily variation of 0+ production rates for same conditions as in Fig.
14.
ion p a i r s the photoelectrons can excite neutral species leading to a i r - glow phenomena and/or to atmospheric heating. B a n k s and K o c k a r t s ( 1 9 7 3 b ) s u g g e s t e d that the heating efficiency s h o u l d d e p e n d on the solar cycle because the relative a b u n d a n c e s of the different ions, atoms and molecules are v a r i a b l e . R e c e n t l y , T o r r et a I. (1980) f o u n d that the heating efficiency is a function of altitude a n d solar cycle. It p e a k s actually near the h e i g h t of maximum extreme ultraviolet a b s o r p t i o n (see F i g u r e 11) at a value of approximately 50%. A t p r e s e n t time, it is extremely difficult to c o n s t r u c t theoretical models which could g i v e r e s u l t s comparable to those obtained in F i g u r e s 1a and 1b from semi- empirical models, since there are not e n o u g h solar ultraviolet i r - radiances available for all solar activity c o n d i t i o n s . B u t , there are also h i g h latitude heat s o u r c e s which must be included in three-dimensional models. T h e s e h i g h latitude heat s o u r c e s result from particle p r e c i p i t a - tions and from Joule dissipation b y electric c u r r e n t s (see B a n k s , 1977) a n d the whole t h e r m o s p h e r e is influenced b y dynamical interactions between the atmosphere, the ionosphere a n d the m a g n e t o s p h e r e (see M a y r et al. 1978). H i g h latitude heat s o u r c e s may reach v a l u e s as h i g h
- 2 - 1
as 100 e r g cm s ( B a n k s , 1977). B u t solar extreme ultraviolet is tha most important heat s o u r c e since it influences permanently 50% of the whole t h e r m o s p h e r e .
U s i n g the v a l u e s of T o r r et al. (1979) for the extreme ultraviolet, the v a l u e s of H e r o u x and H i n t e r e g g e r (1978) in the S c h u m a n n - R u n g e continuum a n d their v a r i a t i o n s quoted b y H i n t e r e g g e r (1981) a n d the L y m a n - a v a r i a t i o n s of V i d a l - M a d j a r (1975), the energetic v a r i a t i o n s of solar i r r a d i a n c e s between minimum and maximum solar activity can be summarized as follows :
- 2 - 1
- from 5 nm to 105 nm, E = (5 ± 3) e r g cm s
- 2 - 1
- from 105 nm to 175 nm, E = (9 ± 3) e r g cm s
- 2 - 1
- at L y m a n - a , E = (6 ± 2) e r g cm s
When these i r r a d i a n c e s are compared to the solar c o n s t a n t value of
6 - 2 - 1
1.367 x 10 e r g cm s (Willson, 1978), it a p p e a r s that o b s e r v e d v a r i a t i o n s in the solar ultraviolet f l u x e s only r e p r e s e n t p a r t s per million of the total electromagnetic emission of the S U n . It could seem s u r - p r i s i n g how s u c h a small fraction can induce the o b s e r v e d large v a r i a t i o n s in the u p p e r atmosphere. T h e quantitative importance of solar f l u x v a r i a t i o n s can be u n d e r s t o o d b y comparing the total kinetic e n e r g y in a vertical atmospheric column to the solar e n e r g y which can be a b s o r b e d in s u c h a column. Table I g i v e s a r o u g h comparison between these quantities. T h e available solar e n e r g y is estimated b y a s s u m i n g a 24 h o u r s permanent illumination. It is, t h e r e f o r e , an u p p e r limit. It is seen that the extreme ultraviolet e n e r g y (AA 5-105 nm) is comparable to the kinetic e n e r g y p r e s e n t in a column above 100 km, S i n c e this solar e n e r g y is actually a b s o r b e d in t h i s column, it can s t r o n g l y modify the t h e r m o s p h e r i c s t r u c t u r e . T h e S c h u m a n n - R u n g e continuum i n c l u d i n g L y m a n - a is comparable to the atmospheric kinetic e n e r g y down to the mesopause level. E v e n when the available solar e n e r g y is e x t e n d e d up to 200 nm or 300 nm it becomes less important compared to the e x i s t i n g kinetic e n e r g y in the s t r a t o s p h e r e . A s an example the e n e r g y available in the 5 nm-200 nm r a n g e r e p r e s e n t s o n l y 1% of the kinetic e n e r g y at the s t r a t o p a u s e . Between 200 nm and 300 nm a b s o r p t i o n by ozone is clearly important in the s t r a t o s p h e r e . B u t above 200 nm there is a p - p a r e n t l y not e n o u g h variation in the solar i r r a d i a n c e ( C o o k et al. 1980;
T h u i l l i e r and Simon, 1981) to induce c h a n g e s in the a b s o r b e d e n e r g y comparable to the atmospheric kinetic e n e r g y .
6. C O N C L U S I O N
Some solar ultraviolet i r r a d i a n c e s below 175 nm are now available for aeronomical applications. O n e solar cycle, h o w e v e r , can be v e r y different from the next one ( H i n t e r e g g e r , 1979) and up to now no s i n g l e measurement has ever been performed for v e r y h i g h solar activity s u c h as in 1958. C o n s i d e r i n g the period from 1 J a n u a r y 1958 to 30 A p r i l 1980, i.e. 8156 d a y s , it is possible to compute the probability
- 3 8 -
TABLE I : Kinetic energy column content and available solar energy for 24 hours illumination.
XX(nm) Solar energy (erg cm 2)
5-300 5-200 5-175 5-105
1.4 x 10 (7 ± 0.8) x 10(
(1.7±0.7) x 10 (4.3±2.6) x 10"
6 Altitude
(km)
Kinetic energy (erg cm
15 50 85
1 0 0 ,
7.0- x 10 7.1 x 10 2.5 x 10*
2.5 x 10f
10 8
- 3 9 -
for f i n d i n g solar activity conditions approximately identical to the d e c i - metric f l u x e s F a n d F e x i s t i n g on the d a y s when the five sets of i r r a - diances u s e d in this paper were m e a s u r e d . F i g u r e 16 s h o w s this p r o - bability as a function of simultaneous v a r i a t i o n s in the daily 10.7 cm f l u x and in the three solar rotation a v e r a g e d v a l u e s . S u c h a probability is seen to be v e r y low, especially for h i g h solar activity. In semi- empirical models a variation AF = AF = 10 leads to a t h e r m o p a u s e temperature variation of the o r d e r of 50 K and a quantitative d e t e r - mination of the variation of solar ultraviolet i r r a d i a n c e s is far from being available f o r all solar activity c o n d i t i o n s .
I n d e p e n d e n t l y of the complicated interaction between solar radiation a n d the E a r t h ' s atmosphere, it s h o u l d be clear that solar i r r a d i a n c e s constitute the most important u p p e r b o u n d a r y condition which s h o u l d be determined with the h i g h e s t possible a c c u r a c y . S i n c e this u p p e r b o u n d a r y condition is c h a n g i n g o v e r s h o r t and long time p e r i o d s , many measurements are r e q u i r e d before a definite picture of the variability can be e s t a b l i s h e d .
A C K N O W L E D G E M E N T S
T h e p r o g r a m m i n g help of E. Falise and the careful d r a w i n g s of A . Simon were g r e a t l y appreciated.
-40-
10
10'
5 1 0 -2
CD <
CD
o
oc Q.
10 _3
10 _4
- F F
« 73 86.2
o 68 71.3
X 203 174.3
A 234 194
+ 243 196.9
- O
- o
o
•
o •
—o
•
X A A
x +
x
A A A + +
A A •
A + A
Total number of days = 8156
_l I L. J I U
5 10 AF and AF (10" W m "22 2H zJ)
1
3 i ]
n ]
J
i 1
15 LJ
Fig. 16 : Probability to encounter solar flux conditions shown in the upper left corner as a function of simultaneous variations in the daily decimetric flux F and in the averaged values over three sola:c rotations. F = 73, on 23 April 1974; F = 68 on 13 July 1976; F = 203 on 14 December 1978; F = 234 on 22 January 1979; F = 243 on 19 February 1979.
Probability of occurence has been computed over 8156 days extending from 1958 to 1980.
REFERENCES
A c k e r m a n , M. : 1971, i n G. Fiocco ( e d . ) / M e s o s p h e r i c Models a n d
Related E x p e r i m e n t s , D . Reidel P u b l . C o . , D o r d r e c h t , H o l l a n d , p . 149.
A c k e r m a n , M . , B i a u m e , F. a n d K o c k a r t s , G. : 1970, P l a n e t . Space S e i . , 18, 1639.
B a n k s , P . M . : 1977, J . A t m o s . T e r r . P h y s . , 39, 179.
B a n k s , P . M . a n d K o c k a r t s , G. 1973a, A e r o n o m y P a r t A , Academic P r e s s , New Y o r k .
B a n k s , P . M . a n d K o c k a r t s , G. : 1973b, A e r o n o m y P a r t B , Academic P r e s s , New Y o r k .
B a r l i e r , F . , B a u e r , P . , J a e c k , C . , T h u i l l i e r , G. a n d K o c k a r t s , G.
1974, J . G e o p h y s . R e s . , 79, 5273.
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