A model for 5577Å and 8446Å atomic oxygen dayglow emission at midaltitudes

A&. Space Res. Vol. 20, No. 6, pp. 11451148,1997 81997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1’177197 517.00 + 0.00

Pergamon

PII: SO273-1177(97)0076&6

A MODEL FOR 5577s4 AND 84&i& ATOMIC OXYGEN DAYGLOW E~SSION AT M~~TrTUDES Vir Singh and Satish Tyagi Depatirnent of Physics, University ofRoorkee, Roorkee-247667, India

ABSTRACT In this paper we present the results of volume emission rate profiles of 5577A and 8446A dayglow emission at midlatitudes. A model is developed by taking into account the variation in solar EUV flux as a function of solar activity and uses recent cross section data. The thermospheric peak emission rate ratio V(5577)/V(8446) is studied as a function of local time at midlatitudes. This ratio is found ~~~~xi~~~u~~~ at local noon time. The ratio is c~mparetively larger in the afternoon sector than the forenoon sector. In addition, the sensitivity of emission rate (for both emission) to atomic oxygen composition is studied by varying the density of atomic oxygen in the model. At local noon time, when the atomic oxygen density is reduced to half in the model, the ratio V(5577)/V(8446) is about 6.0 aud this ratio approaches to about 2.0 if the atomic oxygen density is made double. The photoelectron flux is found more sensitive to atomic oxygen density. 01997 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION The 5577A and 8446A emissions arising from the atomic oxygen are very important for studies of thermosphere. In recent years a great deal of aeronomic research has focussed on 5577A airglow emission (Torr et.aL(1993)). This emission provides valuable information about the chemical and dynamical processes which control the state of the upper mesosphere and lower thermosphere. On the otherhand, very few results have been reported in the literature for 8446A airglow emission. The 8446A is mainly produced above 150 Km in the sunlit atmosphere. In general, the thermospheric peaks of 5577A and 8446A are produced in the same altitude region. These emissions are sensitive to solar radiation and the abundance of atomic oxygen in thermosphere. Photoelectron impact excitation of atomic oxygen is the main source ofthese emissions above 150 km in the thermosphere. Consequently, the study of peak volume emission rate ratio V(5577)N(8446) would provide valuable information about the variation in atomic oxygen concentration in thermosphere. In this paper we present the results of peak volume emission rate ratio V(5577)/V(8446) as a function The sensitivity of this ratio to changes in the abundance of atomic of local time at midlatitudes. oxygen density is presented for midlatitude conditions. The latitudinal variation of this ratio is also preseuted between 30* N and 50* N latitudes. 1145

V. Singh and S.Tyagi

1146

MODEL The following reactions have been identified as the main sources of green line emission in the dayglow: 0(3P ) + el,b+ 0(‘S ) + eph

(Photoelectron

O+, + elk

(Dissociative recombination)

(2)

(Energy transfer from N, fA3ZII))

(3)

(Photo dissociation

(4)

-

N, (A3CU) + O(3P) *

0, + hv(hC13325A)

0(‘S ) + O(3P) N, + 0(‘S) +

0+0(‘S)

impact excitation)

of OJ

(1)

iu reactions ( 1) and (2) ep,,and etb represent photoelectrons and thermal electrons respectively. These reactions contribute primarily at altitudes above 100 km. Below 100 km, the following three body recombination of atomic oxygen is a significant source. o(JP) + O(3P) f O(‘P)

--)

0, + 0(‘S)

(5)

The 8446A emission is mainly produced by the following processes

O(-‘P)+ 0,

epb

--3 O(~P) + eph

+ ePb --+ O(3p) + 0(3P) + eph

:

(Photoelectron

impact excitation of 0)

(6)

(Photoelectron

impact dissociation of 02)

(7)

Detailed description of the procedures for calculating the photoelectron fIux is described by Singh et al. (1996) and is not discussed here. This model is further modified by taking into account the effect of solar flux variation on photoelectrons as given by Richards et al, (1994). The production rates of 0(‘S) due to above sources are computed by using all parameters from the paper of Singh et al. (1996). The production rate of 844611 emission due to reactions (6) and (7) is calculated by using the parameters from the paper of Singh (1992). For the present work the neutral atmosphere is taken from MSIS-90 (Hediu, 1991) and the densities ofthermal electrons, O+, and NO+ are obtaiued from the International Reference Ionosphere (IRI) (Bilitza, 1986).

RESULT

AND DISCUSSI[ON

In the present work we have choosen the day of 27, September,1992. The Wind Imaging Interferometer (WINDII) has made measurements of 5577A emission (Shepherd et al. 1993) on this day. These measurements are used to test the present model. The Figure 1 shows the comparison between the measured and modelled emission rate for 5577A emission. It is evident from Figure 1 that the model is in good agreement with the measurements. As such no meas~ements are available to compare emission rate of 8446A emission in day glow. However, the integrated emission rate of 8446A emission obtained from the present model is in good agreement with the ground based twilight measurements of Bahsoun-Hamade et al. (1989). Figure 2 shows the volume emission rate profiles of 8446A emission as obtained from the present model for 27, September,1992 at 35*N, 94OE. Figure 3 shows the thermospheric peak emission rate ratio V(5577)/V(8446) as a function of fractional change in

5511~

and 8446A

1147

Atomic Oxygen Emission I

250

230 220 -

35N,9LE LT=10.50

I

I

I

I

BdL6A

230

.-

220

27 Sep,92 210 -

I

35N, 9LE LT - 10.50

240

2LO -

210

-

MODEL l . WIND11

200 .

7

I

I

-

190 -

Y

180 -

w n

170 -

I>

c

160.

I-1

a

0

150 -

i

IhO .

0

130 120 0

100 200 300 RATE

EMISSION Fig.

I 1 LOO 500 600700 (ph

cm-3

800

10 50 60 70 80 90 100 110 120 130 1LO EMlSSlON RATE (ph cm-3 s-11

s-11

Fig. 2. profile

Comparison

I.

twasurcd

bctwccn c;~lculntcd (model) and volume emission rate proftlcs for omission.

(WINDII)

5577A dqglow

I

I

35N. 9LE 27 sept.92

Calculalod (mo$l) volu~nc emission for 8446 A dayglow emission.

r/ *+***LT -6.30 +*+++LT =lO.OO l e*** LT =12.00 SCX;YJCLT =lL.OO AaAaaLT =16.00 II:PXXLT =17..40

Oxygen

O.L

1

0.8

FRACTIONAL

CHANGE

IN ATOMIC

1.2 OXYGEN

i.T

density

1

I

~12.00

hrs reduced

by

o factor

‘0.5

hrs hrs hrs hrs hrs hrs

Oxygen I

1

27 Sep.1992 LONG =9L*E

rate

1.00

DENSITY

Vnrialion of tltcrmosphcric peak emission rate Pig. 3. ralio V(5577)N(8446)3s a function of fraclional ch:mGc itt atomic okygcn density.

1.6

l-

30

density

increased

by-a

factor

2.0 I

1

I

35

LATITU:!

(deg.N)

I

I

45

50

Fig. J. Latitudinal variation of lhcrmosphcric peak emission rak ratio V(5577)N(8446) for diffcrcnl atomic o.qgcn densilks.

V. Singh and S. Tyagi

1148

atomic oxygen concentration at different local times. The sensitivity of these emissions to the atomic oxygen is determined by changing the density in the MSIS-90 neutral atmosphere model. Figure 3 shows that the ratio V(5577)/\/(8446) ismaximum at local noon time. This ratio is comparetively larger iu the afceruoon sector than the forenoon sector. At local uoon time, when the atomic oxygen density is reduced to halfin the model, the ratio V~5577~/V(8446) is 6.0 and this ratio approaches to 2.0 if the atomic oxygen density is made double. Due to changes in atomic oxygen density the photoelectron fhtx is redistributed in thermosphere. Since the photoelectron impact excitation of atomic oxygen is one of the major source in thermosphere therefore volume emission rate profiles are changed quite significantly for these emissions. In Figure 4 we show latitudinal variation of ratio V( 5577)/V( 8446) for difIerent atomic oxygen densities. A closer examination ofFigure 4 reveals that the ratio V(5577)/V8446) shows latitudinal dependence as the atomic oxygen density decreases in the model. However, a very weak latitudinal dependence is found when the atomic oxygen density increases in the model. These results indicate that the photoelectron impact excitation of atomic oxygen is very sensitive to the abundance of atomic oxygen in the thermosphere.

CONCLUSIONS Volume emission rate profiles of 5577A aud 8446A dayglow emissions are obtained at midlatitudes. The thermospheric peak emission rate ratio V(5577)/V(8446) has been found very sensitive to the abundance of atomic oxygen in thermosphere. The ratio V(5577)/V(8446) shows latitudiual dependence as the atomic oxygen density decreases in the model. The photoelectron impact excitation of atomic oxygen is found quite sensitive to the variation in atomic oxygen density.

ACKNOWLEDGEMENTS Satish Tyagi gratefully acknowledges

fiuancial support

from UGC, India.

REFERENCES Bahsoun - Hamade, F., R.H. Wiens, and G.G. Shepherd, The 01844.6 nm emission in evening twilight, Geoyhys. lies. Lett., l6, 1449 (1989). Bilitza, D., IRI: Recent Developments, Radio Sci., 21, 343 (1986). Hedin, A.E., Extension of the MSIS thermosphere Model into the Middle and Lower Atmospbere, J. Geophys. Res., 96, 1159 (1991). Richards, P.G., J.A. Fennelly, and D.G. Torr, EUVAC : A solar EUV flux model for aerouomic calculations, J. Geophys. Res., 99, 8981 (1994). Shepherd, G.G., G. Thuillier, W.A. Gault, B.H. Solheim, C. Hersom et al., WIND11 the wind imaging interferometer on the Upper Atmosphere Research Satellite, f. Geophys. Res. 98, 10725 (1993). Singh, Vir, Model Calculations of the 01 844.6 nm emission in evening twilight, Ind. J. Radio & Space Phys., 21, 370 (1992). Singh, Vir, I.C. McDade, G.G. shepherd, B.H. Solheim, and W.E. Ward, The 0(‘S ) dayglow emission as observed by the WIND Imaging Interferometer on UARS, Ann. Gephys., 14, 637 (1996). Torr, D.G., Marsha R. Torr, and P.G. Richards, Thermospherio Airglow emissions : A Comparison of measurements from ATLAS-l and theory, Geophys, Res. Left., 20, 519 (1993).