AERONOMICA ACTA

Texte intégral

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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 - Avenue Circulaire

B • 1180 BRUXELLES

A E R O N O M I C A A C T A

A - N° 214 - 1980

«s

Nitric oxide cooling in the terrestrial thermosphere 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 I M O M I E

3 - Ringlaan

B • 1180 BRUSSEL

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F O R E W O R D

The paper "Nitric oxide cooling in the terrestrial thermosphere" >s accepted for publication in Geophysical Research Letters, 7, 1980.

A V A N T - P R O P O S

L'article "Nitric oxide cooling in the terrestrial thermosphere" sera publié dans Geophysical Research Letters, 7, 1980.

V O O R W O O R D

Het artikel "Nitric oxide cooling in the terrestrial thermosphere"

zal verschijnen in Geophysical Research Letters, 7, 1980.

V O R W O R T

Die Arbeit "Nitric oxide cooling in the terrestrial thermosphere"

wird in Geophysical Research Letters, 7, 1980 herausgegeben werden.

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NIXBIC OXIDE COOLING IN I H E I E B B E S I B I A L IHEBMOSPHEBE

by

G. KOCKABIS

Abstract

A simple formulation of the nitric oxide cooling by the emission of the 5.341m fundamental band in the terrestrial thermosphere indicates that this process is an important mechanism which has been omitted previously in all thermospheric models. I h e cooling effect if highly variable as a consequence of its strong temperature dependence and of the observed variability of the nitric oxide concentration. I h e infrared emission is never in local thermodynamic equilibrium above 120 km altitude and the predominant cooling probably occurs during NO auroral enhancements.

Bésumé

Une expression simple du refroidissement de la thermosphère

terrestre par l'émission à 5.3 M™

d e l a

bande fondamentale de l'oxyde

d'azote indique que ce processus est un mécanisme important dont on a

jamais tenu compte dans les modèles thermosphériques antérieurs. La

forte dépendence avec la température et les variations observées de la

concentration en oxyde d'azote rendent l'effet du refroidissement extrê-

mement variable. L'émission infrarouge n'est jamais en équilibre thermo-

dynamique local au-dessus de 120 km d'altitude et le refroidissement le

plus important s'effectue probablement durant les accroissements

auroraux de l'oxyde d'azote.

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Samenvatting

Een eenvoudige u i t d r u k k i n g van de afkoeling van de aardthermo- sfeer door emissie bij 5.3 pm van de fundamentale band van NO toont aan dat dit verschijnsel een, belangrijk mecanisme is waarmee vroeger nooit rekening gehouden werd bij alle vroegere uitgewerkte thermo- sferische modellen. Het afkoelingseffekt is zeer wisselvallig ten gevolge van haar zeer sterke temperatuur afhankelijkheid en van de NO concen- t r a t i e . De I . R . emissie is nergens in thermodynamisch evenwicht boven 120 km hoogte en de belangrijkste afkoeling gebeurd waarschijnlijk tijdens de toename van NO in het poolgebied.

Zusammenfassung

Ein einfacher Ausdruck f ü r die Stickstoffoxydabkühlung durch die 5 . 3 um Ausstrahlung der Hauptbande in der erdischen Thermosphäre zeigt, dass dieser Prozess ein wichtiges Mechanismus ist. Dieses Mechanismus war in keinen thermosphärischen Model einbegriffen. Die Abkühlung ist sehr veränderlich, weil sie streng von der Temperatur und von der beobachteten Variabilität in der NO Dichte abhängt. Ober- halb 120 km ist die infrarote Ausstrahlung nicht in lokales thermo- dynamisches Gleichgewicht und die grösste Abkühlung findet wahr- scheinlich während NO Erhöhungen in den polaren Zonen statt.

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INTRODUCTION

Although many physical mechanisms involved in the energy balance of the terrestrial thermosphere have been identified, there is still a significant uncertainty in their quantitative evaluation. This is particu_

larly true for time variations of the solar extreme ultraviolet heating and for time and space variations of energy inputs such as Joule heating,

v . .

particle precipitation and gravity waves dissipation. Less attention has been paid to possible energy loss mechanisms by infrared radiation. The possibility of an infrared loss at 63 pm due to the transition O ^ P . , ) 0 (

3

P

2

) was first demonstrated by Bates (1951). Such a mechanism is, however, not very efficient, since below 150 km the thermo- sphere becomes optically thick at 63 pm and a radiative transfer calcu- lation (Kockarts and Peetermans, 1970) has shown that the volume emission rate decreases below 150 km compared to the optically thin case and becomes negligible around 100 km altitude. Furthermore, measurements by Grossmann and Offermann (1978) suggest that the 63 pm line of atomic oxygen is not in local thermodynamic equilibrium. As a consequence a further reduction of the infrared cooling occurs above 120 km altitude. In absence of any efficient loss mechanism very large temperature gradients would be permanently present around .120 km (Banks and Kockarts, 1973; Kockarts, 1975), and theoretical thermo- spheric models would lead to thermopause temperatures much higher than usually observed.

Satellite observations of molecular radiation of the upper atmosphere (Markov, 1969) indicated a maximum intensity in the 4.5 - 8.5 pm region more than ten years ago, and it was established by Gordiets and Markov (1977) that the infrared radiation above 120'km resulting from the 5.3 M ™ band of nitric oxide should play a role in the thermosphere heat balance.

Strong enhancements of the 5.3 pm band have been observed by

Stair et al. (1975) during high auroral activity. In a detailed analysis of

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infrared excitation processes, Ogawa (197.6) has shown that collisional 2

excitation to the vibrational level v = 1 of the ground state X 11 of nitric oxide becomes more important than resonant absorption of solar and terrestrial radiation above 120 km altitude.

N I T R I C OXIDE COOLING R A T E

Since a search for an efficient cooling process is the present objective, only excitation mechanisms which extract kinetic energy from the atmosphere will be considered. The major collisional excitation mechanisms are vibrational exchange with other molecules and a process of oxygen atom exchange between nitric oxide and atomic oxygen as sug- gested by Dalgarno (1963). Considering excitation and deactivation of NO by collisions with an atmospheric constituent M and spontaneous emission, the population of the v = 1 level is determined by the following processes

NO

V = 0

+ M NO

v = 1

+ M ( k

Q 1

) (1)

N 0 V

= 1

+ M N O

v = 0

+ M ( k

1 0

) ( 2 )

N O

v=1 *

N O

v = 0

+ h v ( A

1 0

} ( 3 )

-1 -13 where a photon with energy hv = 1876 cm = 3.726 x 10 erg is emitted in process (3) with a transition probability A

1 Q

( s ). Through thermo- dynamic considerations of detailed balancing the rate coefficients k

Q 1

and

k

1 Q

(cm

3

s~

1

) are related by

k

Q1

= ( g ^ ) k

1 Q

exp ( - hv/.kT) (4)

where k is Boltzmann's constant, T is the kinetic temperature and g^ and

g

Q

are the statistical weights for the levels v = 1 and v = 0, respec-

tively. Neglecting stimulated emission and induced absorption, a steady

state condition for processes (1) to (3) corresponds to a concentration

ratio

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n ( N O

v = 1

) / . n ( N O

v = 0

) = k ^ n ( M ) / [ k

1 0

n ( M ) + A ^ ] (5)

or, taking (4) into account

n ( N O

v = 1

) / n ( N O

v = 0

) = ( g . , / ^ ) k

1 0

n ( M ) e x p ( - h v / k T ) / . [ k

1 0

n ( M ) + A

1 0

]

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where n ( N O

v = 1

) , n ( N O

v

_

Q

) are the concentrations of nitric oxide in the vibrational levels v = 1 and v = 0, and n ( M ) is the concentration of constituent M. Expression (6) shows that a dilution factor ID representing a departure from a Boltzmann distribution is given by

lu = k

1 Q

n(M)/.[k

1 0

n ( M ) + A

1 ( J

] ( / )

When the deactivation rate coefficient is sufficiently large so that k

1 Q

n ( M ) » A

1 0

, the population of the level v = 1 is given by a Boltzmann distribution, since u> = 1 in this case. Numerical calculations show that this condition is never satisfied for the 5.3 pm emission above 120 km altitude. As a consequence the nitric oxide fundamental band is not in local thermodynamic equilibrium and the cooling rate l_

N O

for an optically thin medium is given by

L

n q

= hv n ( N O

v = 1

) A

1 0

= hv n ( N O

y = 0

) u> A ^ C g ^ ) exp ( - h v / k T ) (8)

The integrated intensity of the NO fundamental band was found to

be 135 atm"

1

cm"

2

at S . T . P . by King and Crawford (1972). With g

1

= g

Q

,

such a value leads to a transition probability A

1 Q

= 13.3 s . Quenching

of vibrationally excited nitric oxide by molecular oxygen and nitrogen has

been measured by Murphy et al. (1975) who obtained k

1 Q

( 0

2

) =

2.4 x 10"

1 4

cm

3

s "

1

and k

1 Q

( M

2

) = 1.7 x 10"

1 6

cm

3

s "

1

. Isotopic

exchange of oxygen atoms with NO measured at 300 K by Herron

and Klein (1964) leads to k

1 Q

( 0 ) =1.8 x 1 0

- 1 2

cm

3

s '

1

. Correcting the

exchange rate for the isotopic substitution ratio, Klein and Herron (1964)

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suggest a rate coefficient k

1 Q

( 0 ) = 3.3 x 10*

1 2

cm

3

s "

1

at 300 K.

Glanzer and Troe (1975) measured a rate coefficient k

1 Q

( 0 ) = 3.6 x 10 cm

3

s "

1

at 2700 K while Fernando and Smith (1979) obtained k

1 Q

( 0 ) = 6.5 x 10"

1 1

cm

3

s~

1

at 300 K. The highest value is adopted in the present work. It is clear .that k

1 Q

( N

2

) < k

1 Q

( 0

2

) « k

1 Q

( 0 ) and above 120 km only atomic oxygen is considered as an efficient collisional partner in processes (1) and (2).

The energy rate t -

N O

/ n ( N O

v ; : : 0

) computed with equation (8) is shown in Figure 1 as a function of temperature for various atomic oxygen con- centrations which can be present between 120 km and 250 km altitude.

The nitric oxide energy rate is strongly temperature dependent whereas the corresponding quantity for the 63 pm emission of atomic oxygen varies

-1Q -19 -1

slowly between 5.8 x 10 and 8.8 x 10 erg s for temperatures ranging from 300 K to 2000 K (Kockarts and Peetermans, 1970). Further- more, the energy rate for the 5.3 pm emission of NO is always larger than for the 63 um O emission. The dilution factor u) ranges from

o 1 "1 ~ 3 9 3.3 x 10"

1

to 4.9 x 10 when n ( O ) decreases from 10 cm to 10

cm"

3

. The 5.3 pm emission is, therefore, never in local thermodynamic equilibrium.

EFFECT 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

A simple estimate of the effect of nitric oxide cooling in the terrestrial thermosphere is obtained by solving the steady state heat con- duction equation

dE/dz = P

E U V

- L

0

- L

n o

(9)

where E = - X dT/dz is the downward heat conduction flux, P

E U V

is the

extreme ultraviolet heat production rate, L

Q

and L

N Q

are the atomic

oxygen and nitric oxide cooling rates, respectively. The heat conduction

coefficient X is taken from Banks and Kockarts (1973). Numerical inte-

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Fig. 1 . - E n e r g y rate l _

N O

/ n ( N O ) as a function of tempera- ture for various atomic o x y g e n concentrations.

k

i n

( 0 ) = 6.5 x 1 0 "

1 1

cm

3

s '

1

.

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gration of equation (9) requires a knowledge of lower boundary conditions at 120 km, EUV fluxes with their heating efficiency, absorption c r o s s sections and a nitric oxide distribution. In order to consider a realistic case we adopt a nitric oxide profile measured simultaneously by a

rocket-borne ultraviolet photometer and by a mass spectrometer ( T r i n k s et al., 1978) on June 29, 1974 at 1514 local time. At 120 km the

-j

_ o 7

nitric oxide concentration is 4.6 x 10 cm with a maximum of 6.2 x 10 c m "

3

at 130 km- At the time and location of these measurements, the semi-empirical model DTM (Drag-Temperature-Model) of Barlier et al (1978) leads to the following conditions at 120 km : T = 380 K, n ( O ) = 5.69 x 10

1 0

c m "

3

, n ( 0

2

) = 4.78 x 10

1 0

c m "

3

and n ( N ^ ) = 3.34 x 10

1 1

c m "

3

for a solar decimetric flux of 86 x 10 Wm Hz , a mean flux of 87.2 x 1 0 "

2 2

Wm"

2

H z "

1

and a geomagnetic index K

p

- 2.66.

For a solar decimetric flux of 95 x 1 0

- 2 2

Wm"

2

H z '

1

Schmidtke (1976) measured a total EUV flux of 3.37 erg cm"

2

s "

1

which is used in equation ( 9 ) with a heating efficiency of 0.3, a unique absorption cross section of 1 x 1 0 "

1 7

cm

2

and a solar zenith angle x = 43.6°.

The temperature resulting from the integration of the steady state heat conduction equation (9) is shown in Figure 2. Without any loss process except downward heat conduction ( c u r v e P

E U V

) / the thermopause temperature reaches a value of 1350 K. The introduction of nitric oxide cooling reduces the thermopause temperature to 910 K and the addition of the 63 pm O cooling produces a further reduction of 35 K leading to a steady state temperature of 875 K ( c u r v e P

E ( J V

- L

Q

- L

N Q

) . The 63 m™

cooling effect ( c u r v e P

E U V

- L

Q

) is computed for an optically thick

atmosphere where the reduction factor introduced by Kockarts and

Peetermans (1970) is approximated by the empirical expression given by

Glenar et al. (1978). It is clear from Figure 2 that nitric oxide cooling is

a v e r y efficient process. At 120 km altitude the temperature gradient

decreases from 20K/km with P

E U V

alone to 6 K/.km when nitric oxide and

atomic o x y g e n are introduced.

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500

400

E

JC

UJ

§ 300

^

P

EUV

_ L

0 ~

L

NO (2)

p E

U V ~

l

O "

l

n o

O )

P

EUV"

L

0 r w

200

100

with n(NO)/lO

500 1000

T E M P E R A T U R E ( K )

1500

Fig. 2 . - V e r t i c a l d i s t r i b u t i o n of thermospheric tempera- t u r e . Numbers indicate which heating and loss processes are taken into account. C u r v e ( 2 ) indicates the e f f e c t of a r a d u c t i o n by a f a c t o r of 10 of the n i t r i c oxide d i s t r i b u t i o n used to compute c u r v e ( 1 ) .

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F i g u r e 2 shows also t h e t e m p e r a t u r e p r o f i l e ( c u r v e ( 2 ) ) obtained when t h e n i t r i c o x i d e c o n c e n t r a t i o n of T r i n k s et al. (1978) is a r b i t r a r y reduced b y a f a c t o r of 10.

T h e p r o d u c t i o n and loss terms of t h e e n e r g y equation are shown in F i g u r e 3 where i t appears t h a t ' L

N Q

is more i m p o r t a n t t h a n L

Q

and can even exceed P

E U V

- Since t h e e n e r g y rate l _

N O

/ n ( N O ) (see f i g u r e 1) is an i n c r e a s i n g f u n c t i o n w i t h h e i g h t , t h e volume e n e r g y loss rate l_

N Q

is c h a r a c t e r i z e d b y a maximum w h i c h - i s at 140 km in t h e p r e s e n t case.

DISCUSSION

In a calculation of t h e 5.3 |jm e x c i t a t i o n Ogawa (1976) uses t h e low value k

1 Q

( 0 ) of H e r r o n and Klein (1964) up to 120 km, whereas Gordiets et al.

(1978) use between 90 and 250 km t h e h i g h value at 2700 K obtained b y Glanzer and T r o e (1975). T h e h i g h value of Fernando and Smith (1979) : _7 - 3 - i

leads to a maximum emission of 1.1 x 10 e r g cm s at 140 km w h i l e -7 -3 -1

Gordiets et al. (1978) f i n d 2 x 10 e r g cm s . O b s e r v e d 5.3 pm emission of NO can be h i g h e r t h a t values deduced from / i g u r e 3 since resonant e x c i t a t i o n b y solar a n d / o r e a r t h ' s r a d i a t i o n c o n t r i b u t e s to t h e 5.3 |jm emission b u t is not i n t r o d u c e d in t h e t h e r m o s p h e r i c heat b u d g e t . I n t e g r a t i o n of l _

N 0

g i v e n in F i g u r e 3 leads to an a p p a r e n t emission of 1 . 2 MR.

T h e o r e t i c a l calculations ( O r a n et a l . , 1975; Strobel et a l . , 1976;

Ogawa and Kondo, 1977) c l e a r l y show large d i u r n a l v a r i a t i o n s of n i t r i c o x i d e . These r e s u l t s combined w i t h t h e s t r o n g t e m p e r a t u r e dependence of t h e n i t r i c oxide cooling indicate t h a t t h e decrease of t h e o r d e r of 500 K in t h e t e m p e r a t u r e obtained in F i g u r e 2 should be considered as i n d i - c a t i v e . T h e actual e f f e c t must be analyzed f o r v a r i o u s c o n d i t i o n s of the t h e r m o s p h e r e . Nevertheless a r e d u c t i o n b y a f a c t o r of 10 of the NO d i s t r i b u t i o n measured b y T r i n k s et al. (1978) s t i l l leads to a s i g n i f i c a n t e f f e c t (see c u r v e ( 2 ) in F i g u r e 2 ) .

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250

? 200

«»rf»

O LU 3

«J <

ENERGY PRODUCTION AND LOSS (erg crrrV 1 )

Fig. 3.- Height distribution of the energy production and loss rates involved in the computation of Fig. 2 using the NO distribution of T r i n k s et al. (19Z8).

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High-latitude nitric oxide measurements (Gerard and Barth, 1977) show that auroral activity may lead to an NO density increase by a factor of eight. Rees and Roble (1979) predict NO increases during auroral electron precipitations. Under such disturbed conditions it is suggested that nitric oxide cooling plays a regulating rôle above 120 km in order to avoid unrealistic temperature increases in the thermospherè.

Since carbon dioxide seems to be the major thermospheric cooling agent below 120 km (Kockarts 1975; Alcaydé et al. 1979) and since nitric oxide cooling appears as a major effect above 120 km, it is not excluded that the initially assumed atomic oxygen cooling is actually a minor process in many cases. A systematic introduction of nitric oxide cooling is, however, not straightforward since the nitric oxide distribution depends on the thermal structure and on the composition of the thermo- sphere. Such a situation can be compared to thé case of the homosphere where the thermal structure is also strongly influenced by trace constituents.

ACKNOWLEDGMENTS

I am grateful to Emile Falise for his valuable programming help.

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T r i n k s , H., U. von Zahn, C . A . Barth, and K . K . Kelly, A joint nitric oxide measurement by rocket-borne ultraviolet photometer and mass spectrometer in the lower thermosphere, J. Geophys. Res., 83, 203-206, 1978.

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