GEOPHYSICAL RESEARCH LETTERS, VOL. 7, NO. 2, PAGES 137-140, FEBRUARY 1980
NITRIC OXIDE COOLING IN THE TERRESTRIAL THERMOSPHERE
G. Kockarts
Institut d'A•ronomie Spatiale
3 Avenue Circulaire B-1180 Bruxelles, Belgium
Abstract. A simple formulation of the nitric collisional excitation to th• vibrational level
oxide cooling by the emission of the 5.3 •m v - 1 of the ground state X-H of nitric oxide fundamental band in the terrestrial thermosphere becomes more important than resonant absorption indicates that this process is an important of solar and terrestrial radiation above 120 km mechanism which has been omitted previously in altitude.
all thermospheric models. The cooling effect is
highly variable as a consequence of its strong Nitric Oxide Cooling Rate temperature dependence and of the observed varia-
bility of the nitric oxide concentration. The Since a search for an efficient cooling infrared emission is never in local thermodynamic process is the present objective, only excitation equilibrium above 120 km altitude and the pre- mechanisms which extract kinetic energy from the dominant cooling probably occurs during NO atmosphere will be considered. The major col-
auroral enhancements. lisional excitation mechanisms are vibrational exchange with other molecules and a process of
Introduction oxygen atom exchange between nitric oxide and atomic oxygen as suggested by Dalgarn 0 (1963).
Although many physical mechanisms involved Considering excitation and deactivation of NO by in the energy balance of the terrestrial thermo- collisions with an atmospheric constituent M and
sphere have been identified, there is still a spontaneous emission, the population of the v = 1
significant uncertainty in their quantitative level is determined by the following processes
evaluation. This is particularly true for time
variations of the solar extreme ultraviolet NO + M + NO + M (ko•) (1)
heating and for time and space variations of NO v=O NO v=l
energy inputs such as Joule heating ' particle NO v=l + M + + NO v=0 + M (k•j) (2) (A{•) (3) precipitation and gravity waves dissipation. Less v=l v=O + hv
attention has been paid to possible energy loss where a p•ton with energy hv - 1876 cm = -1
mechanisms by infrared radiation. The possibility 3.726 x 10 erg is emitted • process (3) with
of an iõfrared l•ss at 63 •m due to the transi-
tion O(Pt) + O(Po) was first demonstrated by a transition probability Alo(s ). Through
Bates (19S1). Such•a mechanism is, however, not thermodynamic consideratibhs of detailed ba- and
lancing •he rate coefficients k01
very sphere becomes optically efficient, since below thick 150 at 63 •m and a km the thermo- k10(cm• s- ) are related by
radiative transfer calculation (Kockarts k 0 - (gl/g0) k exp (- hv/kT) (4)
and Peetermans, 1970) has shown that the volume 1 10
emission rate decreases below 150 km compared to where k is Boltzmann's constant, T is the kinetic the optically thin case and becomes negligible temperature and g. and are the statistical
around 100 km altitude. Furthermore, measurements weights for the l•vels v g• 1 and v = 0, respec-
by Grossmann and Offermann (1978) suggest that tively. Neglecting stimulated emission and in-
the 63 •m line of atomic oxygen is not in local duced absorption, a steady state condition for thermodynamic equilibrium. As a consequence a processes (1) to (3) corresponds to a concen- further reduction of the infrared cooling occurs tration ratio
above 120 km altitude. In absence of any ef-
ficient loss mechanism very large temperature n(NOv= )/n(NO v 0 ) - k01n(M)/[k10n(M) + AlO] (5)
gradients would be permanently present around 1 -
120 km (Banks and Kockarts, 1973; Kockarts, or, taking (4) into account
1975), and theoretical thermospheric models would
lead to thermopause temperatures much higher than n(NO v 1)/n(NOv 0) -
usually observed. = -
Satellite observations of molecular radia- (gl/go) k10n(M)exp(-hv/kT)/[k10n(M)+A10 ] (6)
tion of the upper atmosphere (Markov, 1969)
indicated a maximum intensity in the 4.5-8.5 •m where n•O•=x•d % n(NO__0) are the concentrations
region more than ten years ago, and it was of nitr in tVh• vibrational levels v- 1
established by Gordiets and Markov (1977) that and v = 0, and n(M) is the concentration of the infrared radiation above 120 km resulting constituent M. Expression (6) shows that a di- from the 5.3 •m band of nitric oxide should play lution factor w representing a departure from a a role in the thermosphere heat balance. Strong Boltzmann distribution is given by
enhancements of the 5.3 •m band have been ob-
served by Stair et al. (1975) during high auroral w n(M)/ n(M) + ] (7)
•activity. In a detailed analysis of infrared - k10 [k10 A10
• is
excitation processes Ogawa (1976) has shown that When the deactivation rate coefficient k10 the
sufficiently large so that kl•n(• >> A10 ,
Copyright 1980 by the American Geophysical Union. population of the level v = ' given by a Paper number 9L1661.
0094-82761801009L-1661501. O0
137
138 Kockarts' Nitric Oxide Cooling
10 -13
m 0_15
cm 1
(• 10 -16
Z
10 -17
n(O)(cm-•
NO 5.3 •m :
..
,.
10_1 I , I . I , I . I , I , I 400 600 800 1000 1200 1400 1600
atomic oxygen lS considered as an efficient collisional partner in processes (1) and (2).
The energy rate LNo/n(NOv_ 0) computed with
equation (8) is shown in Figur• 1 as a function of temperat.•ure 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 ?•
atomic oxyge_nl•varies •1owly between 5.8 x 10 -•- and 8.8 x 10 -*w erg s -l for temperatures ranging
from 300 K to 2000 K (Kockarts and Peetermans, 1970). Furthermore, the energy rate for the 5.3 pm emission of NO is always larger than for
the 63 pm O emissi.o_•. The diluti_o• factor w
ranges f•om 3.3 w 10 to 4•99x 103• when n(O)
decreas.e's from 10 ql cm -3 to 0 cm- . The 5.3 pm
emission is, therefore, never in local thermo- dynamic equilibrium.
Effect on the Thermopause Temperature 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 - PE]JV - LO - LNO (9)
where E = - k dT/dz is the downward heat con-
TEMPERATURE (K) duction flux, P_. is the extreme ultraviolet
Fig. 1. Energy rate LN•/n(NO } as a function of heat production r•a•e, L^ u and L.^ are NU the atomic
temperature for variousVatomic oxygen oxygen and nitric oxide cooling rates, respecti-
concentrations. rely. The heat conduction coefficient k is taken
k10(o}=6.5 x 10 -11 5 -1 cm s . from Banks and Kockarts (1973). Numerical inte- gration of equation (9) requires a knowledge of lower boundary conditions at 120 km, EUV fluxes Boltzmann distribution, since w = 1 in this case. with their heating efficiency, absorption cross Numerical calculations show that this condition sections and a nitric oxide distribution. In is never satisfied for the 5.3 •m emission above order to consider a realistic case we adopt a 120 km altitude. As a consequence the nitric nitric oxide profile measured simultaneously by a oxide fundamental band is not in local thermo- rocket-borne ultraviolet photometer and by a mass
dynamic equilibrium and the cooling rate LNO for spectrometer (Trinks et el., 1978) on June 29,
an optically thin medium is given by
LNO hv n(NOv=_l) A10 -
= hv n(NOv= O) to Alo(gl/g o) exp (-hv/kT) (8)
The integrated intensity of the_•O (•da-
mental band was found to be 135 atm cm at
S.T.P. by King and Crawford (1972). With g. = g , ß 1
such a valu•l leads to a transition probability
A 1 = 13.3 s . Quenching of vibrationally ex- cited nitric oxide by molecular oxygen and ni- trogen has been measured by Murp•_hg et el. _q1975) who obtained k.^(O2] = 2•4 x,}6q• c m3 s and
Iu -16 3 -•
k]a (N 9) = 1.7 x 10 cm s . Isotopic exchange
of - oxygen atoms with NO measured at 300 K by
Herron and •lein• •964) leads to kl,(O)
=1•8 x 10 •12 - cm • s . Correcting the exchange
rate for the isotopic substitution ratio, Klein and Herron (1964) s•ages[ a •rate coef-
ficient klD(O) = 3.3 x 10 -•[ cm 3 s -• at 300 K.
Gl•nzer an•Troe (1975) me•%ure• a •rate coef- ficient k•m(O) = 3.6 x 10 -• cm • s -• at 2700 K
while Fernando •od Smith (1979) obtained k•^(O) = 6.5 x 10 -11 cm 3 s -1 at 300 K. The highes•Uvalue
' I ' ' ' ' I ' , I, : ,
- I I
. I
(1) (2) (3)1
&00 - (1) PLrUV ' LO ' LNO I I - (2) PE'UV' LO -LNO with n(NO)/10 I .
'•' (3) PEUV- LO
ß • I I
•' (z,) PL, uv
• 300 -
"; ////
/
200-
.
100 i I , , , , I i i i ,
500 1000 1500
TtMPtR•TURt (K)
Fig. 2. ¾ertical distribution of the•mospheric temperature. Numbers indicate which heating and loss processes are taken into account. Curve (2) indicates the effect of a reduction by a factor is adopted in the present work. It is clear that of 10 of the nitric oxide distribution used to
k10(.N2) < k10(O2! << k10(O) and above 120 km only compute curve (1}.
Kockarts: Nitric Oxide Cooling 139
1974 at 1514 local time. At 12•km •e nitric
oxide concentratil• • is_34.6 x 10' cm-• with a
maximum of 6.2 x cm 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 condition• a• 120 km : T = 380 • •) =
5.69 x 10 'O cm •1 n(02• = 4.78 x 10 •v cm • and
n(N•) flu• of = 3•• x 10.• x 10• -z _W• cm-i •z Hz for -1, a mean m solar decimetric flux of
87.2 x 10- Wm-- Hz TM and a geomagnetic index 2 6 F• a • solar decimetric flux of
9• x 1•- Wm -• Hz -1 Schmidtke (1•76) measured a
total EUV flux of 3.37 erg c• •2 s -• which is used
in equation (9) with a heating-efficiency of 0z•?
a •nique absorption cross section of 1 x 10
cm and'a solar zenith angle X = 43.6 ø .
The temperature resulting from the inte- gration of the steady state heat conduction equation (9) is shown in Figure 2. Without any loss process except downward heat conduction
curve value PERU,), Bt 1350 the K. thermopause The introduction temperature of reaches 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 (curve
P EUV O- •NO - L - ) The 63 pm . cooling effect (curve PEUV -.Lo) •s computed for an optically thick
atmosphere where the reduction factor introduced by Kockarts and Peetermans (1970) is approximated by the empirical expression given by
200 L. O xxxxPEu v
150
100 10 '9 1 0' 8 10 -?
ENERGY PRODUCTION AND LOSS(erg crn-3s J)
Fig. 3. Height distribution of the energy production and loss rates involved in the computation of Fig. 2 using the NO distribution of Trinks et al. (1978).
Glenar et al. (1978). It is clear from Figure 2 Strobel et al., 1976; Ogawa and Kondo, 1977) that nitric oxide cooling is a very efficient clearly show large diurnal variations of nitric process. At 120 km altitude the temperature oxide. These results combined with the strong
gradient decreases from 20K/kmwith PEUV alone to temperature dependence of the nitric oxide
6 K/km when nitric oxide and atomic oxygen are cooling indicate that the decrease of the order
introduced. of 500 K in the temperature obtained in Figure 2
Figure 2 shows also the temperature profile should be considered as indicative. The actual (curve (2)) obtained when the nitric oxide con- effect must be analyzed for various conditions of centration of Trinks et al. (1978) is arbitrary the thermosphere. Nevertheless a reduction by a reduced by a factor of 10. factor of 10 of the NO distribution measured by
The production and loss terms of the energy Trinks et al. (1978) still leads to a significant equation are shown in Figure 3 where it appears effect (see curve (2) in Figure 2).
that L. is more important than Lmand can even High-latitude nitric oxide measurements
exceeded. . Since the energy rate'•/n(NO) (see (G•rard and Barth, 1977) show that auroral acti- figure l•Vis an increasing function•'•ith height, vity may lead to an NO density increase by a
the volume energy loss rate L.^ is characterized
by a maximum which is at 14•m in the present factor increases during auroral electron precipitations. of eight. Rees and Roble (1979) predict NO
case.
Discussion
In a calculation of the 5.3 pm excitation Ogawa
(1976) uses the low value k•(00)kmOf Herron
and Klein (1964) up to , whereas Gordiets et al. (1978) use between 90 and 250 km the high value at 2700 K obtained by Gl•nzer and Troe (1975). The high value of
Under such disturbed conditions it is suggested that nitric oxide cooling plays a regulating role above 120 km in order to avoid unrealistic tem- perature increases in the thermosphere.
Since carbon dioxide seems to be the major thermospheric cooling agent below 120 km (Kockarts 1975; Alcayd& et al. 1979) and since nitric oxide cooling appears as a major effect above 120 km, it is not excluded that the ini- tially assumed atomic oxygen c•eling is actually a minor process in many ca•'es. A systematic
Fernando and Smith (1•79) lead• t•l a maximum
e-•i• of •7• • 10- erg cm- s at _•40 km introduction not straightforward of nitric since oxide cooling the nitric is, however, oxide
whi•e Gordiets cm- s-1. Observed 5.3 pm emission of NO can be et al. (1978) find 2 x 10 erg distribution depends on the thermal structure and higher that values deduced from Figure 3 since on the composition of the thermosphere. Such a resonant excitation by solar and/or earth's situation can be compared to the case of the radiation contributes to the 5.3 pm emission but homosphere where the thermal structure is also is not introduced in the thermospheric heat strongly influenced by trace constituents.
budget. Integration of LNO given in Figure 3
leads to an apparent emission of 1.2 MR. Acknowledgments. I am grateful to Emile Falise Theoretical calculations (Oran et al., 1975; for his valuable programming help.
140 Kockarts' Nitric Oxide Cooling
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