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Equatorial mesospheric and F-region airglow emissions observed from latitude 4° south
00324633/89 $3.00+0.00 PergamonPress plc
Plana. Space Sci., Vol. 37, No. 6, pp. 649455, 1989 Printedin Great Britain.
EQUATORIAL MESOSPHERIC AND ®ION AIRGLOW EMISSIONS OBSERVED FROM LATITUDE 4” SOUTH H. TAKAHASHI,
Institute
Y. SAHAI,
de Pesquisas
Espaciais,
B. R. CLEMESHA, D. M. SIMONICH, R. M. LOB0 and A. ERAS
INPE,
N. R. TEIXEIRA,
C.P. 515, 12201, SLo Jose dos Campos,
(Received injinalform
11 November
SP, Brasil
1988)
Abstract-Simultaneous round-based measurements of the atmospheric airglow emissions 015577,630O and 7774 A, NaD 5893 1, OH (9,4) band and O2 atmospheric (0, 1) band at 8645 A have been made at an equatorial station, Fortaleza (3.9”S, 38.4”W, geomag. 2.1”S), Brazil, since November 1986. A microprocessor controlled multichannel tilting filter type photometer was constructed for this purpose. Hydroxyl rotational temperatures in the range 19&210 K were obtained from the OH (9, 4) Q and R
branch measurements. Surprisingly low 015577 A and NaD intensities, about 40% and 25%, respectively, of those observed at low latitude (23”s). were observed. The OH (9.4) band intensities, however. did not show any such difference. Time lagged &cturnal intensity variations ‘ofthe mesospheric emissions observed provide
evidence of dynamical
effects in the equatorial
INTRODUCTION
The need for better airglow observations in the equatorial regions was pointed out by Weill (1967), who reviewed some results from the IGY data, mentioning that there is a difference in airglow intensity and its temporal variation between the equatorial and middle to high latitudes. Weill also emphasized the existence of an equatorial trough of the 01 5577 A emission. The low intensity of the green line along the equatorial region has been confirmed by Donahue et al. (1974) from OGO-6 satellite data, Reed and Chandra (1975) from OGO-4 satellite data, and Cogger et al. (1981) from ISIS-2 satellite data. Wasser and Donahue (1979) analyzed latitudinal variation in the 01 5577 8, emission profile using the OGO-6 data and found a large latitudinal variation in the vertical gradient of the emission rate. Concerning nocturnal intensity variations in the equatorial airglow features, Brenton and Silverman (1970) and Wiens and Weill (1973) summarized their latitudinal dependencies, pointing out that in the equatorial region the mesospheric emissions 01 5577 A, NaD and OH, show in general a monotonic decrease from dusk to dawn or a midnight minimum and slight increase towards dawn. Moreels (1974) explained this as the result of less vertical mixing in equatorial regions resulting in no intensity increase in the OH emission. There have, however, been no systematic observations measuring the various meso-
upper atmosphere.
spheric airglow emissions to investigate the dynamical effects in this region. It is well known that the F-region 01 6300 A emission is directly related to ionospheric dynamics (see, for example, Roach, 1969) and that it is a good indicator of the vertical movement of the F-layer. On the other hand, the 017774 8, emission indicates the peak electron density of the F-layer (Sahai et al., 1983). Simultaneous measurements of the two emissions provide the F-region peak height, hmF2, and F-region peak electron density (Bittencourt et al., 1983). There exist simultaneous data for these emissions from the Appleton Anomaly region adequate to study the Fregion dynamics, but very few observations are available from the equatorial region (Mendillo et al., 1985). After carrying out long-term airglow observations from Cachoeira Paulista (22.7”S, 45.O”W) we have recently started observations at Fortaleza (3.9”S, 38.4”W), a location which is close to both the geographic and geomagnetic equators. Simultaneous measurements from these two stations in the Brazilian sector will be important for studies related to the morphology and dynamics of the upper atmosphere at low and equatorial latitudes. Observations of the mesospheric emissions 01 5577 A, NaD, OH (9, 4) and 0, atmospheric (0, 1) band (hereafter O,A), and the F-region emissions 016300 A and 017774 8, have been carried out on a routine basis since November 1986. The purpose of this work is to present the first results from these observations, showing some exam649
650
H. TAKAHASHI et al. AIRGLOW n
DATA ACQUISiTION 8
PHOTOMETER
CTRL. UNIT INTERFACE r-----------7 I
2
I
UNIT
(INTERFACE
II!
I!OWER
I-
FIG. 1.BLOCK DIAGRAM OF MULTICHANNEL TILTINGFILTERPHOTOMETER.
ples of the observed nocturnal variations, and to compare the results with the data obtained at Cachoeira Paul&a. INSTRUMENTATION
In order to measure several airglow emissions simultaneously, a multichannel tilting filter type photometer was designed and constructed. Each emission line and the corresponding background continuum are measured separately by scanning the wavelength over 80 A from the center wavelength via filter tilting. The photometer unit and the control and data acquisition systems are illustrated in block diagram in Fig. 1. The photometer has a field of view of 2” full angle and an aperture of 50 mm diameter. Five interference filters with 62.5 mm diameter are mounted in the filter wheel, which has its temperature controlled at 35 + OYC. The sixth position of the wheel is used to check the photomultiplier thermal noise level. The filter tilting is carried out by a rack and pinion gear assembly driven by a stepping motor. The data are stored and recorded in a digital cassette recorder and an analog recorder is used as a monitor. The photometer operation and the data acquisition system are controlled by a Z80A microprocessor. One sequence with five filters and the noise level check takes about 3.5 min, which is sufficient to observe airglow intensity variations with periods less than 10 min.
The characteristics of the interference filters and their tilted positions are shown in Table 1. The second position of F2/NaD at 5865 A is used to measure the OH (8, 2) R branch, by which means contamination of the Q, (1) line of the OH (8, 2) band in the NaD measurement is estimated (Takahashi and Batista, 1981). The filter F4/OH (9, 4) is used to measure 01 7774 A and the OH (9, 4) Q and R branches. The intensity of 017774 A can be obtained after the contamination by Q, (3) and Q2(3) of OH (9,4) are subtracted (Sahai et al., 1981). OH rotational temperature hereafter T(OH), are determined from the intensity ratio between the R and Q branches (Takahashi and Batista, 1981), and the OH (9, 4) band intensity is calculated from the Q branch measurement. Rotational transition probabilities given by Mies (1974) were used to calculate the synthetic spectra. The position four at 8660 A of the filter F5/02A is used to obtain the OzA band intensity (Takahashi et al., 1986). The photometer absolute sensitivities at different tilted positions are calibrated by using a laboratory standard light source (Eppley ES-8315) and MgO screen. A calibrated substandard light source (tungsten filament pilot lamp) was used for field observations. Estimated errors in the absolute intensity for 01 5577 A, NaD and 01 6300 A are approximately 15%, and for OH (9, 4) and 0,A the error is around 20% owing to the increased systematic error in cali-
Equatorial
mesospheric
and F-region
airglow
emissions
651
TABLE 1. PHOTOMETERCHARACTERISTICS
Tilting position
Filter
Peak WV
Sens. CPS/R
Meas.
F1/5511
1 2
5577 5540
12.7 15.3
25.3
01 5511 BG*
F2/NaD
1 2 3
5892 5865 5838
13.9 14.7 17.1
20.4
NaD OH (8,2)--R BG
F3/6300
1 2
6300 6275
9.9 12.0
11.1
016300 BG
1 2 3 4
7714 7751 7115 7690
8.4 11.7 15.6 17.2
16.6 11.8 -
011774 OH (9,4)-Q OH (9,4)--R BG
1 2 3 4
8728 8689 8680 8660
11.9 14.8 17.6 14.9
9.0
F4/OH
(9, 4)
F5/02A
* BG : background
continuum.
bration. The main error in calculating the 01 7774 A intensity is in the estimation of contamination by the OH (9,4) Q branch. This contamination is, for example, about 10 R when the OH (9,4) total band intensity is 600 R. The threshold level for 01 7774 A in the present measurement is estimated to be around 2-3 R under clear sky conditions. The instrumental error in determining the rotational temperature T(OH) in this study is + 10 K. OBSERVATIONS
7.8
BG O,A-P O,A-P O,A-P
AND RESULTS
Airglow observations were started on a routine basis at Fortaleza (geographic coordinates 3.9”S, 38.4”W, geomagnetic latitude 2.1’S) in November 1986. At this location a fluxgate magnetometer and an ionospheric polarimeter are also in operation. Ionograms are obtained every 15 min at a nearby site, about 20 km away. Although rain is infrequent and Fortaleza is a dry region, the proximity of the coast leads to frequent low level cloud formation resulting in limited observations with good clear sky conditions.
During the period from November 1986 to August 1987, 30 nights of observations were obtained with good sky conditions, each with more than 6 h of measurement. In Table 2, we show averaged nocturnal mean values of intensity and rotational temperature, and also the minimum and maximum nocturnal mean values observed during the period. Also shown are the same parameters observed from Cachoeira Paulista (22.7”S, 45.O”W) in the same period from November 1986 to August 1987. The 015577E intensities shown in Table 2 were determined by subtracting 20% of measured 016300 A F-region emission from the total measured green line intensity (Silverman, 1970). It should be pointed out that 01 5577E and NaD show very low intensities at Fortaleza, 40% and 25%, respectively, of the Cachoeira Paulista values. The lowest intensities were observed during the DecemberFebruary period. The values shown in Table 2 indicate a large difference in the relative intensities of the oxygen and sodium emissions between the equatorial and low latitude regions. This “equatorial trough” in the atomic
TABLE 2. COMPARMN IN THE NOCTURNAL MEAN VALUES BETWEENTHE AIRGLOW INTENSITY IN RAYLEIGHS AND THE OH ROTATIONAL TEMPERATUREIN KELVINS AT FORTALEZA (FTZ) AND THESE AT CACXOEIRA PAULISTA (CP)
015577E
Mean Min. Max.
NaD
016300
OH (9,4)
WH)
C%A
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
40 20 94
94 48 241
10 2 26
41 10 65
87 45 161
57 24 160
541 327 840
482 340 714
200 188 207
200 176 217
197 120 350
322 180 703
652
H. TAKAHASHI et al.
FORTALEZA 6300
-200 -0 No0
FIG. 2. AVERAGEDNOCTURNALVARIATIONS OFTHEOI ~~~~E,OI~~OO~,O,ATM~SPHERIC(O,~)BANDAT 8645 a, NaD AT 5893 A, OH (9,4) BAND AND THE OH ROTATIONAL TEMPERATUREDURING THE PERIOD
1986 TOAUGUST1987. FROM NOVEMBER
oxygen emission has been reported by Weill (1967) from the ground-based observations and Cogger et al. (1981) from satellite observations, although, as a result of contamination of satellite-borne photometer signals by energetic particle precipitation in the South Atlantic Anomaly, the satellite data for our longitude is very sparse. The remarkable feature of the data shown in Table 2 is, however, the lack of a similar difference in the OH emission, which shows similar levels of intensity at the two stations. This difference in the latitudinal behaviour of the three emissions indicates that the atomic oxygen and hydrogen density profiles, which are responsible for these emissions, are different at the equatorial and low latitude regions. Concerning temporal variations averaged nocturnal intensity variations for the five emissions and the OH rotational temperature variations observed during the present series of measurements are presented in Fig. 2. In the case of the mesospheric emissions, most of the nights show an intensity decrease during the course of night. However, in some cases a small increase after midnight was observed. The Fregion 01 6300 A emission generally shows an enhancement around midnight. The 017774 A intensity levels were very low during this period of measurements, owing to low levels of foF2 during the low solar activity period. In Fig. 3, we show a typical example of the nocturnal intensity variations of the five emissions and the OH rotational temperature, T(OH). The OH (9, 4), NaD and 0,A intensities decreased to about l/3
of their initial values by the end of the night. The OH rotational temperature decreased to about 30 K. The 01 5577E, however, showed an increase towards dawn. Nocturnal intensity variations of this type have also been observed by Wiens and Weill (1973) in the equatorial region in Ethiopia (9”N). The midnight enhancement of the F-region 01 6300 A emission is typical in the equatorial region and is associated to the vertical E x B plasma drifts (Bittencourt and Sahai, 1979). The downward phase propagation of atmospheric
LOCAL TIME.HRS
FIG. 3. OBSERVED NOCTURNAL VARIATIONS OF THE AIRGLOW INTENSITIES,OH (9,4) ROTATIONAL TEMPERATURE AND IONOSPHERICPARAMETERSATFORTALEZAON~~FEBRUARY
1987.
Equatorial
mesospheric
and F-region
waves has been shown to cause time-lagged intensity variations in the atomic oxygen, sodium and hydroxyl emissions (Takahashi et al., 1979, Clemesha et al., 1979, Rodrigo et al., 1985). We have not analyzed the Fortaleza data to see whether this effect is generally present, but on two nights much larger than usual oscillations in the emissions and the rotational temperature show clear evidence of wave propagation. In Fig. 4, we show the observations on the night of 19 August 1987. Also shown are the ionospheric parameters h’Es, h’F and foF2 obtained at Fortaleza. The mesospheric emission 01 5577E, OzA, NaD and OH (9, 4) showed intensity variations with a maximum during the night. A time lag between the different emissions is clearly visible, with the maxima occurring at 0O:OOL.T. for 015577E and OzA, 01:OO L.T. for NaD and 02:OO L.T. for OH (9, 4). T(OH) also showed an increase at around 01:OO L.T. It is interesting to note that during the increasing phase of the 015577E and OIA intensity variations the height of the sporadic-E layer decreases by about 10 km. Dachs (1974) observed a similar behavior for the 01 5577 A emission at Tsumeb (19”s) and suggested that this might be the result of a downward motion of the atmosphere in which the Es layer is embedded. This does not seem to be a very likely explanation, because such large vertical motions of the atmosphere are inconsistent with our knowledge of atmospheric processes at this height. It seems more probable that both the lowering of the Es and the increase in airglow intensity are manifestations of a solar tidal oscillation,
airglow
653
emissions
the E, via the windshear mechanism (Whitehead, 1961) and the airglow as a result of the associated density and temperature changes. The 016300 A emission maintained a high intensity level, around 200 R, up to 22:00 L.T., after which it decreased to about 20 R. Anti-correlation between the movement of h’Fand the 016300 A intensity variation can be clearly seen even in small details. The 01 7774 A emission was about 3.5 R at 22:30 L.T. when foF2 was about 6 MHz. According to Sahai et al. (1983) the 01 7774 A intensity can be related to the F-layer critical frequency as foF2 (MHz)
= [430 x 017774 (R)] “4.
The present results agree well with this expression. Time lagged intensity variations are also seen in the next example of 25 August 1987 in Fig. 5. This was a geomagnetically disturbed night with a Kp index of 6 at 21:00 L.T. The magnetometer installed at the same location showed the maximum AH (main phase) at 19:00 L.T. Large height variations in h’F from 380 to 230 km were observed on the ionograms, when frequency type spread F possibly associated with bottomside sinusoidal irregularities (Valladares et al., 1983) was also observed. The formation of sporadic E was also observed and the height of the layer decreased during the night. The 01 6300 A intensity closely followed the variation of h’F. The 01 7774 A was fairly weak on this night. The nocturnal intensity variations of the mesospheric emissions show that the
EY
F 2
120 IOD E
FORTALEZA AUG. 19, 67
J
3oo
9 !-
2aJ 1-1
F 4’
h’F
120
h’Es ++++++++
E
h’Es
FORTALEZA AUG25. 67
++ ++
++
.__
+I++
IO 220
++++
3
Y a
200
,L
1
120
20
22 LOCAL
4 :IME,
ti&
FIG. 4. SAME AS FIG. 3, BUT FOR 19 AUGUST 1987.
20
i
., ..‘.. :’: ;/1 .:’ ‘.
140
140 -
22 LOCAL
0 TIME,
2
4
HRS
FIG. 5. SAME AS FIG. 3, BUT FOR 25 AUGUST 1987.
H.
654
TAKAHASHI et al.
enhancement in the 015577E precedes 0,A by about 1 h (01 5577E shows a broad enhancement on this night). The NaD and OH (9,4) intensity peaks follow that of 0,A about I.5 h later. Assuming that the 0,A and OH (9, 4) peak emission height are located around 94 and 87 km, respectively, on the basis of rocket measurements (Witt et al., 1979), the estimated downward phase propagation velocity is, in this case, about 5 km h-l. For the first example (19 August), it is about 4 km h-‘. These velocities are comparable with the 6 km h-’ and 4-5 km h-’ values reported by Takahashi et af. (1979) and Rodrigo et al. (1985), respectively, and are consistent with the phase velocities of the semidiurnal tide measured by Batista et al. (1985) at 23”s. CONCLUSIONS
Firstly, results of simultaneous observations of several nightglow emissions from Fortaleza (3.9”S, 38.4”W), close to the geographic and geomagnetic equators, were compared with measurements from Cachoeira Pauhsta (23%) and show lower intensities for the 01 5577E and NaD emissions, confirming Weill’s equatorial trough for the oxygen airglow. However, the OH (9,4) emission does not show such a trend and warrants further investigations. Wave-like intensity enhancements in the mesospheric emissions observed suggest that in the equatorial region dynamical effects are also important, although most of the nights observed showed an intensity decrease during the night, sometimes with a small increase after midnight. The 01 7774 A emission was, in general, very weak during this period of measurement, and the 01 6300 A emission showed a prominent midnight enh~cement associated with the equatorial ionospheric anomaly. We believe that simultaneous measurements of airglow emissions from Fortaleza and Cachoeira Paulista during the coming solar cycle will improve our understanding of behavior of the upper atmosphere in the equatorial and low latitude during quiet and disturbed magnetic regions conditions. Also, simultaneous measurements of the F-region and mesospheric airglow emissions together with the ionospheric parameters should provide an important tool to study thermosphere-mesosphere coupling (Koltsov et al., 1986). dckn~w~edgemen~s-me authors are grateful to R. C. de Oliveira and F. F. T. BiCalho for their work in constructing the multichannel photometer, and to Edmilson M. Forte, Avicena Filho and L, A. Coutinho for operation of the photometer at Fortaleza. Thanks are also due to M. A. Abdu for ionospheric data and comments on the manuscript. This work was partially supported by the Fundo National de
Desenvolvimento Cientifico e Tecnolbgico (FNDCT), Brazil. under contract FINEP-537/CT. and the Conselho Naciohal de Desenvolvimento Cientifico e Tecnologico (CNPq) Brasil (contract 407.437/85-GM).
REFERENCES
Batista, P. P., Clemesha, B. R., Simonich, D. M. and Kirchhoff, V. W. J. H. (1985) Tidal oscillations in the atmospheric sodium layer; J. geophys. Res. 90,3881. Bittencourt. J. A. and Sahai. Y. (1979). Behaviour of the 01 6300 A emission at magnetic equator and its relation to the vertical ExB plasma drift velocity. J. atmos. terr. Phys. 41, 1233. Bittencourt, J. A., Teixeira, N. R., Sahai, Y. and Takahashi, H. (1983) Mapping of ionospheric F-region parameters from atomic oxygen airglow emissions. J. utmos. terr. Phys. 45,697. Brenton, J. G. and Silverman, S. M. (1970) A study of the diurnal variations of the 5577 A [OI] airglow emission at selected IGY stations. Planer. Space Sci. 18,641. Clemesha, B. R., Kirchhoff, V. W. J. H., Simonich, D. M., Takahashi, Hl and Batista, P. P. (1979) Simultaneous observations of sodium densitv and the NaD, OH (8, 3) and 01 5577 8, nightglow emissions. J. geophjs. Rei. &, 6477. Cogger, L. L., Elphinstone, R. D. and Murphree, J. S. (1981) Temporal and latitu~nal 5577 A airglow variation. Can. J. Phys. 59, 1296. Dachs, J. (1974) Oxygen airglow emissions and the dynamics of the upper atmo~phere.Ann. Geophys. 30,85. Donahue. T. M.. Guenther. B. and Thomas, R. J. (1974) Spatial and temporal behavior of atomic oxygen determined by OGO 6 airglow observations. J. geophys. Res. 79, 1959. Koltsov, V. V., Freyzon, I., Gorokhov, Yu. V., Vazhrkin, V. A., Chertok, Yu. I., Potapov, V. P. and Sukhodoyev, V. A. (1986) Relation between wave processes in the F region and the mesosphere. Geomagn. Aeron. 26, 33. Mendillo. M.. Soance. H. and Zalesak, S. T. (1985) Simulation studies bf ionospheric airglow signatures of plasma depletions at the equator. J. utmos. few. Phys. 47,885. Mies, F. H. (1974) Calculated ~brational transition probabilities of OH (Xrrt). J. mole&. Spectrosc. 53, 150. Moreels, G. (1974) Comments on the paper : diurnal, annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the EuropeAfrica Sector. Planet. Space Ski. 22, 344, Reed, E. I. and Chandra, S. (1975) The global characteristics of atmospheric emissions in the lower thermosphere and their aeronomic im~l~~tions. J. geophys. Res. SO, 3053. Roach, F. E. (1969) The nightglow and the F region ionosphere, in Atmospheric Emissions (Edited by McCormac, B. M. and Omholt, A.), p. 439. Van Nostrand Reinhold, New York. Rodrigo, R., Lopez-Moreno, J. J., Lopez-Puertas, M. and Molina. A. f 19851Analvsis of 01557.7 nm, NaD, OH (6, 2) and 6,( ’ bz ) (0, 1) nightglow cm&ions from groundbased observations. J. atmos. terr. Phys. 47, 1099. Sahai, T., Bittencourt, J. A., Teixeira, N. R. and Takahashi, H. (1981) Plasma irregularities in the tropical F-region detected by 01 7714 A and 6300 A nightglow measurements. J. geophys. Res. 86, 3496. Sahai, Y., Bittencourt, J. A., Teixeira, N. R. and Takahashi, H. (1983) Observations of large scale F-region irregu-
Equatorial
mesospheric
and F-region
larities using airglow emissions at 7774 A and 6300 A. Annales Geophys. 1,271. Silverman, S. M. (1970) Night airglow phenomenology. Space Sci. Rev. 11, 341. Takahashi, H. and Batista, P. P. (1981) Simultaneous measurements of OH (9,4), (8, 3), (7, 2), (6, 2) and (5, 1) bands in the airglow. j. gebphys. R& 86, 5632. Takahashi. H.. Batista. P. P.. Clemesha. B. R.. Simonich. D. M. and Sahai, Y. (1979) Correlations between OH; NaD and 01 5577 A emissions in the airglow. Planet. Space Sci. 27, 801. Takahashi, H., Sahai, Y. and Batista, P. P. (1986) Airglow O,(‘Z) atmospheric band at 8645 8, and the rotational temperature observed at 23”s. Planet. Space Sci. 34,301. Valladares, C. E., Hanson, W. B., McClure, J. P. and Cragin, B. L. (1983) Bottomside sinusoidal irregularities in the equatorial F region. J. geophys. Res. 88, 8025.
airglow
emissions
655
Wasser, B. and Donahue, T. M. (1979) Atomic oxygen between 80 and 120 km : evidence for a latitudinal variation in vertical transport near the mesopause. J. geophys. Res. 84, 1297. Weill, Cl. M. (1967) Airglow observations near the equator, in Aurora and Airglow (Edited by McCormac, B. M.), p. 407. Reinhold, New York. Whitehead, J. D. (1961) The formation of the sporadic E layer in the temperate zones. J. atmos. terr. Phys. 20,49. Wiens, R. H. and Weill, G. (1973) Diurnal, annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the Europe-Africa sector. Planet. Space Sci. 21, 1011. Witt, G., Stegman, J., Solheim, B. H. and Llewellyn, E. J. (1979) A measurement of the 0,(6’C,+ -X’Zj) atmospheric band and the 01 (‘S) green line in the nightglow. Planet. Space Sci. 27,341.
Plana. Space Sci., Vol. 37, No. 6, pp. 649455, 1989 Printedin Great Britain.
EQUATORIAL MESOSPHERIC AND ®ION AIRGLOW EMISSIONS OBSERVED FROM LATITUDE 4” SOUTH H. TAKAHASHI,
Institute
Y. SAHAI,
de Pesquisas
Espaciais,
B. R. CLEMESHA, D. M. SIMONICH, R. M. LOB0 and A. ERAS
INPE,
N. R. TEIXEIRA,
C.P. 515, 12201, SLo Jose dos Campos,
(Received injinalform
11 November
SP, Brasil
1988)
Abstract-Simultaneous round-based measurements of the atmospheric airglow emissions 015577,630O and 7774 A, NaD 5893 1, OH (9,4) band and O2 atmospheric (0, 1) band at 8645 A have been made at an equatorial station, Fortaleza (3.9”S, 38.4”W, geomag. 2.1”S), Brazil, since November 1986. A microprocessor controlled multichannel tilting filter type photometer was constructed for this purpose. Hydroxyl rotational temperatures in the range 19&210 K were obtained from the OH (9, 4) Q and R
branch measurements. Surprisingly low 015577 A and NaD intensities, about 40% and 25%, respectively, of those observed at low latitude (23”s). were observed. The OH (9.4) band intensities, however. did not show any such difference. Time lagged &cturnal intensity variations ‘ofthe mesospheric emissions observed provide
evidence of dynamical
effects in the equatorial
INTRODUCTION
The need for better airglow observations in the equatorial regions was pointed out by Weill (1967), who reviewed some results from the IGY data, mentioning that there is a difference in airglow intensity and its temporal variation between the equatorial and middle to high latitudes. Weill also emphasized the existence of an equatorial trough of the 01 5577 A emission. The low intensity of the green line along the equatorial region has been confirmed by Donahue et al. (1974) from OGO-6 satellite data, Reed and Chandra (1975) from OGO-4 satellite data, and Cogger et al. (1981) from ISIS-2 satellite data. Wasser and Donahue (1979) analyzed latitudinal variation in the 01 5577 8, emission profile using the OGO-6 data and found a large latitudinal variation in the vertical gradient of the emission rate. Concerning nocturnal intensity variations in the equatorial airglow features, Brenton and Silverman (1970) and Wiens and Weill (1973) summarized their latitudinal dependencies, pointing out that in the equatorial region the mesospheric emissions 01 5577 A, NaD and OH, show in general a monotonic decrease from dusk to dawn or a midnight minimum and slight increase towards dawn. Moreels (1974) explained this as the result of less vertical mixing in equatorial regions resulting in no intensity increase in the OH emission. There have, however, been no systematic observations measuring the various meso-
upper atmosphere.
spheric airglow emissions to investigate the dynamical effects in this region. It is well known that the F-region 01 6300 A emission is directly related to ionospheric dynamics (see, for example, Roach, 1969) and that it is a good indicator of the vertical movement of the F-layer. On the other hand, the 017774 8, emission indicates the peak electron density of the F-layer (Sahai et al., 1983). Simultaneous measurements of the two emissions provide the F-region peak height, hmF2, and F-region peak electron density (Bittencourt et al., 1983). There exist simultaneous data for these emissions from the Appleton Anomaly region adequate to study the Fregion dynamics, but very few observations are available from the equatorial region (Mendillo et al., 1985). After carrying out long-term airglow observations from Cachoeira Paulista (22.7”S, 45.O”W) we have recently started observations at Fortaleza (3.9”S, 38.4”W), a location which is close to both the geographic and geomagnetic equators. Simultaneous measurements from these two stations in the Brazilian sector will be important for studies related to the morphology and dynamics of the upper atmosphere at low and equatorial latitudes. Observations of the mesospheric emissions 01 5577 A, NaD, OH (9, 4) and 0, atmospheric (0, 1) band (hereafter O,A), and the F-region emissions 016300 A and 017774 8, have been carried out on a routine basis since November 1986. The purpose of this work is to present the first results from these observations, showing some exam649
650
H. TAKAHASHI et al. AIRGLOW n
DATA ACQUISiTION 8
PHOTOMETER
CTRL. UNIT INTERFACE r-----------7 I
2
I
UNIT
(INTERFACE
II!
I!OWER
I-
FIG. 1.BLOCK DIAGRAM OF MULTICHANNEL TILTINGFILTERPHOTOMETER.
ples of the observed nocturnal variations, and to compare the results with the data obtained at Cachoeira Paul&a. INSTRUMENTATION
In order to measure several airglow emissions simultaneously, a multichannel tilting filter type photometer was designed and constructed. Each emission line and the corresponding background continuum are measured separately by scanning the wavelength over 80 A from the center wavelength via filter tilting. The photometer unit and the control and data acquisition systems are illustrated in block diagram in Fig. 1. The photometer has a field of view of 2” full angle and an aperture of 50 mm diameter. Five interference filters with 62.5 mm diameter are mounted in the filter wheel, which has its temperature controlled at 35 + OYC. The sixth position of the wheel is used to check the photomultiplier thermal noise level. The filter tilting is carried out by a rack and pinion gear assembly driven by a stepping motor. The data are stored and recorded in a digital cassette recorder and an analog recorder is used as a monitor. The photometer operation and the data acquisition system are controlled by a Z80A microprocessor. One sequence with five filters and the noise level check takes about 3.5 min, which is sufficient to observe airglow intensity variations with periods less than 10 min.
The characteristics of the interference filters and their tilted positions are shown in Table 1. The second position of F2/NaD at 5865 A is used to measure the OH (8, 2) R branch, by which means contamination of the Q, (1) line of the OH (8, 2) band in the NaD measurement is estimated (Takahashi and Batista, 1981). The filter F4/OH (9, 4) is used to measure 01 7774 A and the OH (9, 4) Q and R branches. The intensity of 017774 A can be obtained after the contamination by Q, (3) and Q2(3) of OH (9,4) are subtracted (Sahai et al., 1981). OH rotational temperature hereafter T(OH), are determined from the intensity ratio between the R and Q branches (Takahashi and Batista, 1981), and the OH (9, 4) band intensity is calculated from the Q branch measurement. Rotational transition probabilities given by Mies (1974) were used to calculate the synthetic spectra. The position four at 8660 A of the filter F5/02A is used to obtain the OzA band intensity (Takahashi et al., 1986). The photometer absolute sensitivities at different tilted positions are calibrated by using a laboratory standard light source (Eppley ES-8315) and MgO screen. A calibrated substandard light source (tungsten filament pilot lamp) was used for field observations. Estimated errors in the absolute intensity for 01 5577 A, NaD and 01 6300 A are approximately 15%, and for OH (9, 4) and 0,A the error is around 20% owing to the increased systematic error in cali-
Equatorial
mesospheric
and F-region
airglow
emissions
651
TABLE 1. PHOTOMETERCHARACTERISTICS
Tilting position
Filter
Peak WV
Sens. CPS/R
Meas.
F1/5511
1 2
5577 5540
12.7 15.3
25.3
01 5511 BG*
F2/NaD
1 2 3
5892 5865 5838
13.9 14.7 17.1
20.4
NaD OH (8,2)--R BG
F3/6300
1 2
6300 6275
9.9 12.0
11.1
016300 BG
1 2 3 4
7714 7751 7115 7690
8.4 11.7 15.6 17.2
16.6 11.8 -
011774 OH (9,4)-Q OH (9,4)--R BG
1 2 3 4
8728 8689 8680 8660
11.9 14.8 17.6 14.9
9.0
F4/OH
(9, 4)
F5/02A
* BG : background
continuum.
bration. The main error in calculating the 01 7774 A intensity is in the estimation of contamination by the OH (9,4) Q branch. This contamination is, for example, about 10 R when the OH (9,4) total band intensity is 600 R. The threshold level for 01 7774 A in the present measurement is estimated to be around 2-3 R under clear sky conditions. The instrumental error in determining the rotational temperature T(OH) in this study is + 10 K. OBSERVATIONS
7.8
BG O,A-P O,A-P O,A-P
AND RESULTS
Airglow observations were started on a routine basis at Fortaleza (geographic coordinates 3.9”S, 38.4”W, geomagnetic latitude 2.1’S) in November 1986. At this location a fluxgate magnetometer and an ionospheric polarimeter are also in operation. Ionograms are obtained every 15 min at a nearby site, about 20 km away. Although rain is infrequent and Fortaleza is a dry region, the proximity of the coast leads to frequent low level cloud formation resulting in limited observations with good clear sky conditions.
During the period from November 1986 to August 1987, 30 nights of observations were obtained with good sky conditions, each with more than 6 h of measurement. In Table 2, we show averaged nocturnal mean values of intensity and rotational temperature, and also the minimum and maximum nocturnal mean values observed during the period. Also shown are the same parameters observed from Cachoeira Paulista (22.7”S, 45.O”W) in the same period from November 1986 to August 1987. The 015577E intensities shown in Table 2 were determined by subtracting 20% of measured 016300 A F-region emission from the total measured green line intensity (Silverman, 1970). It should be pointed out that 01 5577E and NaD show very low intensities at Fortaleza, 40% and 25%, respectively, of the Cachoeira Paulista values. The lowest intensities were observed during the DecemberFebruary period. The values shown in Table 2 indicate a large difference in the relative intensities of the oxygen and sodium emissions between the equatorial and low latitude regions. This “equatorial trough” in the atomic
TABLE 2. COMPARMN IN THE NOCTURNAL MEAN VALUES BETWEENTHE AIRGLOW INTENSITY IN RAYLEIGHS AND THE OH ROTATIONAL TEMPERATUREIN KELVINS AT FORTALEZA (FTZ) AND THESE AT CACXOEIRA PAULISTA (CP)
015577E
Mean Min. Max.
NaD
016300
OH (9,4)
WH)
C%A
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
FTZ
CP
40 20 94
94 48 241
10 2 26
41 10 65
87 45 161
57 24 160
541 327 840
482 340 714
200 188 207
200 176 217
197 120 350
322 180 703
652
H. TAKAHASHI et al.
FORTALEZA 6300
-200 -0 No0
FIG. 2. AVERAGEDNOCTURNALVARIATIONS OFTHEOI ~~~~E,OI~~OO~,O,ATM~SPHERIC(O,~)BANDAT 8645 a, NaD AT 5893 A, OH (9,4) BAND AND THE OH ROTATIONAL TEMPERATUREDURING THE PERIOD
1986 TOAUGUST1987. FROM NOVEMBER
oxygen emission has been reported by Weill (1967) from the ground-based observations and Cogger et al. (1981) from satellite observations, although, as a result of contamination of satellite-borne photometer signals by energetic particle precipitation in the South Atlantic Anomaly, the satellite data for our longitude is very sparse. The remarkable feature of the data shown in Table 2 is, however, the lack of a similar difference in the OH emission, which shows similar levels of intensity at the two stations. This difference in the latitudinal behaviour of the three emissions indicates that the atomic oxygen and hydrogen density profiles, which are responsible for these emissions, are different at the equatorial and low latitude regions. Concerning temporal variations averaged nocturnal intensity variations for the five emissions and the OH rotational temperature variations observed during the present series of measurements are presented in Fig. 2. In the case of the mesospheric emissions, most of the nights show an intensity decrease during the course of night. However, in some cases a small increase after midnight was observed. The Fregion 01 6300 A emission generally shows an enhancement around midnight. The 017774 A intensity levels were very low during this period of measurements, owing to low levels of foF2 during the low solar activity period. In Fig. 3, we show a typical example of the nocturnal intensity variations of the five emissions and the OH rotational temperature, T(OH). The OH (9, 4), NaD and 0,A intensities decreased to about l/3
of their initial values by the end of the night. The OH rotational temperature decreased to about 30 K. The 01 5577E, however, showed an increase towards dawn. Nocturnal intensity variations of this type have also been observed by Wiens and Weill (1973) in the equatorial region in Ethiopia (9”N). The midnight enhancement of the F-region 01 6300 A emission is typical in the equatorial region and is associated to the vertical E x B plasma drifts (Bittencourt and Sahai, 1979). The downward phase propagation of atmospheric
LOCAL TIME.HRS
FIG. 3. OBSERVED NOCTURNAL VARIATIONS OF THE AIRGLOW INTENSITIES,OH (9,4) ROTATIONAL TEMPERATURE AND IONOSPHERICPARAMETERSATFORTALEZAON~~FEBRUARY
1987.
Equatorial
mesospheric
and F-region
waves has been shown to cause time-lagged intensity variations in the atomic oxygen, sodium and hydroxyl emissions (Takahashi et al., 1979, Clemesha et al., 1979, Rodrigo et al., 1985). We have not analyzed the Fortaleza data to see whether this effect is generally present, but on two nights much larger than usual oscillations in the emissions and the rotational temperature show clear evidence of wave propagation. In Fig. 4, we show the observations on the night of 19 August 1987. Also shown are the ionospheric parameters h’Es, h’F and foF2 obtained at Fortaleza. The mesospheric emission 01 5577E, OzA, NaD and OH (9, 4) showed intensity variations with a maximum during the night. A time lag between the different emissions is clearly visible, with the maxima occurring at 0O:OOL.T. for 015577E and OzA, 01:OO L.T. for NaD and 02:OO L.T. for OH (9, 4). T(OH) also showed an increase at around 01:OO L.T. It is interesting to note that during the increasing phase of the 015577E and OIA intensity variations the height of the sporadic-E layer decreases by about 10 km. Dachs (1974) observed a similar behavior for the 01 5577 A emission at Tsumeb (19”s) and suggested that this might be the result of a downward motion of the atmosphere in which the Es layer is embedded. This does not seem to be a very likely explanation, because such large vertical motions of the atmosphere are inconsistent with our knowledge of atmospheric processes at this height. It seems more probable that both the lowering of the Es and the increase in airglow intensity are manifestations of a solar tidal oscillation,
airglow
653
emissions
the E, via the windshear mechanism (Whitehead, 1961) and the airglow as a result of the associated density and temperature changes. The 016300 A emission maintained a high intensity level, around 200 R, up to 22:00 L.T., after which it decreased to about 20 R. Anti-correlation between the movement of h’Fand the 016300 A intensity variation can be clearly seen even in small details. The 01 7774 A emission was about 3.5 R at 22:30 L.T. when foF2 was about 6 MHz. According to Sahai et al. (1983) the 01 7774 A intensity can be related to the F-layer critical frequency as foF2 (MHz)
= [430 x 017774 (R)] “4.
The present results agree well with this expression. Time lagged intensity variations are also seen in the next example of 25 August 1987 in Fig. 5. This was a geomagnetically disturbed night with a Kp index of 6 at 21:00 L.T. The magnetometer installed at the same location showed the maximum AH (main phase) at 19:00 L.T. Large height variations in h’F from 380 to 230 km were observed on the ionograms, when frequency type spread F possibly associated with bottomside sinusoidal irregularities (Valladares et al., 1983) was also observed. The formation of sporadic E was also observed and the height of the layer decreased during the night. The 01 6300 A intensity closely followed the variation of h’F. The 01 7774 A was fairly weak on this night. The nocturnal intensity variations of the mesospheric emissions show that the
EY
F 2
120 IOD E
FORTALEZA AUG. 19, 67
J
3oo
9 !-
2aJ 1-1
F 4’
h’F
120
h’Es ++++++++
E
h’Es
FORTALEZA AUG25. 67
++ ++
++
.__
+I++
IO 220
++++
3
Y a
200
,L
1
120
20
22 LOCAL
4 :IME,
ti&
FIG. 4. SAME AS FIG. 3, BUT FOR 19 AUGUST 1987.
20
i
., ..‘.. :’: ;/1 .:’ ‘.
140
140 -
22 LOCAL
0 TIME,
2
4
HRS
FIG. 5. SAME AS FIG. 3, BUT FOR 25 AUGUST 1987.
H.
654
TAKAHASHI et al.
enhancement in the 015577E precedes 0,A by about 1 h (01 5577E shows a broad enhancement on this night). The NaD and OH (9,4) intensity peaks follow that of 0,A about I.5 h later. Assuming that the 0,A and OH (9, 4) peak emission height are located around 94 and 87 km, respectively, on the basis of rocket measurements (Witt et al., 1979), the estimated downward phase propagation velocity is, in this case, about 5 km h-l. For the first example (19 August), it is about 4 km h-‘. These velocities are comparable with the 6 km h-’ and 4-5 km h-’ values reported by Takahashi et af. (1979) and Rodrigo et al. (1985), respectively, and are consistent with the phase velocities of the semidiurnal tide measured by Batista et al. (1985) at 23”s. CONCLUSIONS
Firstly, results of simultaneous observations of several nightglow emissions from Fortaleza (3.9”S, 38.4”W), close to the geographic and geomagnetic equators, were compared with measurements from Cachoeira Pauhsta (23%) and show lower intensities for the 01 5577E and NaD emissions, confirming Weill’s equatorial trough for the oxygen airglow. However, the OH (9,4) emission does not show such a trend and warrants further investigations. Wave-like intensity enhancements in the mesospheric emissions observed suggest that in the equatorial region dynamical effects are also important, although most of the nights observed showed an intensity decrease during the night, sometimes with a small increase after midnight. The 01 7774 A emission was, in general, very weak during this period of measurement, and the 01 6300 A emission showed a prominent midnight enh~cement associated with the equatorial ionospheric anomaly. We believe that simultaneous measurements of airglow emissions from Fortaleza and Cachoeira Paulista during the coming solar cycle will improve our understanding of behavior of the upper atmosphere in the equatorial and low latitude during quiet and disturbed magnetic regions conditions. Also, simultaneous measurements of the F-region and mesospheric airglow emissions together with the ionospheric parameters should provide an important tool to study thermosphere-mesosphere coupling (Koltsov et al., 1986). dckn~w~edgemen~s-me authors are grateful to R. C. de Oliveira and F. F. T. BiCalho for their work in constructing the multichannel photometer, and to Edmilson M. Forte, Avicena Filho and L, A. Coutinho for operation of the photometer at Fortaleza. Thanks are also due to M. A. Abdu for ionospheric data and comments on the manuscript. This work was partially supported by the Fundo National de
Desenvolvimento Cientifico e Tecnolbgico (FNDCT), Brazil. under contract FINEP-537/CT. and the Conselho Naciohal de Desenvolvimento Cientifico e Tecnologico (CNPq) Brasil (contract 407.437/85-GM).
REFERENCES
Batista, P. P., Clemesha, B. R., Simonich, D. M. and Kirchhoff, V. W. J. H. (1985) Tidal oscillations in the atmospheric sodium layer; J. geophys. Res. 90,3881. Bittencourt. J. A. and Sahai. Y. (1979). Behaviour of the 01 6300 A emission at magnetic equator and its relation to the vertical ExB plasma drift velocity. J. atmos. terr. Phys. 41, 1233. Bittencourt, J. A., Teixeira, N. R., Sahai, Y. and Takahashi, H. (1983) Mapping of ionospheric F-region parameters from atomic oxygen airglow emissions. J. utmos. terr. Phys. 45,697. Brenton, J. G. and Silverman, S. M. (1970) A study of the diurnal variations of the 5577 A [OI] airglow emission at selected IGY stations. Planer. Space Sci. 18,641. Clemesha, B. R., Kirchhoff, V. W. J. H., Simonich, D. M., Takahashi, Hl and Batista, P. P. (1979) Simultaneous observations of sodium densitv and the NaD, OH (8, 3) and 01 5577 8, nightglow emissions. J. geophjs. Rei. &, 6477. Cogger, L. L., Elphinstone, R. D. and Murphree, J. S. (1981) Temporal and latitu~nal 5577 A airglow variation. Can. J. Phys. 59, 1296. Dachs, J. (1974) Oxygen airglow emissions and the dynamics of the upper atmo~phere.Ann. Geophys. 30,85. Donahue. T. M.. Guenther. B. and Thomas, R. J. (1974) Spatial and temporal behavior of atomic oxygen determined by OGO 6 airglow observations. J. geophys. Res. 79, 1959. Koltsov, V. V., Freyzon, I., Gorokhov, Yu. V., Vazhrkin, V. A., Chertok, Yu. I., Potapov, V. P. and Sukhodoyev, V. A. (1986) Relation between wave processes in the F region and the mesosphere. Geomagn. Aeron. 26, 33. Mendillo. M.. Soance. H. and Zalesak, S. T. (1985) Simulation studies bf ionospheric airglow signatures of plasma depletions at the equator. J. utmos. few. Phys. 47,885. Mies, F. H. (1974) Calculated ~brational transition probabilities of OH (Xrrt). J. mole&. Spectrosc. 53, 150. Moreels, G. (1974) Comments on the paper : diurnal, annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the EuropeAfrica Sector. Planet. Space Ski. 22, 344, Reed, E. I. and Chandra, S. (1975) The global characteristics of atmospheric emissions in the lower thermosphere and their aeronomic im~l~~tions. J. geophys. Res. SO, 3053. Roach, F. E. (1969) The nightglow and the F region ionosphere, in Atmospheric Emissions (Edited by McCormac, B. M. and Omholt, A.), p. 439. Van Nostrand Reinhold, New York. Rodrigo, R., Lopez-Moreno, J. J., Lopez-Puertas, M. and Molina. A. f 19851Analvsis of 01557.7 nm, NaD, OH (6, 2) and 6,( ’ bz ) (0, 1) nightglow cm&ions from groundbased observations. J. atmos. terr. Phys. 47, 1099. Sahai, T., Bittencourt, J. A., Teixeira, N. R. and Takahashi, H. (1981) Plasma irregularities in the tropical F-region detected by 01 7714 A and 6300 A nightglow measurements. J. geophys. Res. 86, 3496. Sahai, Y., Bittencourt, J. A., Teixeira, N. R. and Takahashi, H. (1983) Observations of large scale F-region irregu-
Equatorial
mesospheric
and F-region
larities using airglow emissions at 7774 A and 6300 A. Annales Geophys. 1,271. Silverman, S. M. (1970) Night airglow phenomenology. Space Sci. Rev. 11, 341. Takahashi, H. and Batista, P. P. (1981) Simultaneous measurements of OH (9,4), (8, 3), (7, 2), (6, 2) and (5, 1) bands in the airglow. j. gebphys. R& 86, 5632. Takahashi. H.. Batista. P. P.. Clemesha. B. R.. Simonich. D. M. and Sahai, Y. (1979) Correlations between OH; NaD and 01 5577 A emissions in the airglow. Planet. Space Sci. 27, 801. Takahashi, H., Sahai, Y. and Batista, P. P. (1986) Airglow O,(‘Z) atmospheric band at 8645 8, and the rotational temperature observed at 23”s. Planet. Space Sci. 34,301. Valladares, C. E., Hanson, W. B., McClure, J. P. and Cragin, B. L. (1983) Bottomside sinusoidal irregularities in the equatorial F region. J. geophys. Res. 88, 8025.
airglow
emissions
655
Wasser, B. and Donahue, T. M. (1979) Atomic oxygen between 80 and 120 km : evidence for a latitudinal variation in vertical transport near the mesopause. J. geophys. Res. 84, 1297. Weill, Cl. M. (1967) Airglow observations near the equator, in Aurora and Airglow (Edited by McCormac, B. M.), p. 407. Reinhold, New York. Whitehead, J. D. (1961) The formation of the sporadic E layer in the temperate zones. J. atmos. terr. Phys. 20,49. Wiens, R. H. and Weill, G. (1973) Diurnal, annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the Europe-Africa sector. Planet. Space Sci. 21, 1011. Witt, G., Stegman, J., Solheim, B. H. and Llewellyn, E. J. (1979) A measurement of the 0,(6’C,+ -X’Zj) atmospheric band and the 01 (‘S) green line in the nightglow. Planet. Space Sci. 27,341.