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On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—I. Observations of tidal variability
On tidal variabili~ and the existence of planetary wave-like oscillations in the upper thermospher~r. Observations of tidal variability
Laboratorio Ionosf6rico de la Armada de la Reptiblica Argentina, Se.N.I.D. and C.O.N.I.C.E.T..
Av. de1Libertador 327, 1638Vicente Lopez, Argentina
Abstract-The
evidence for the existence of tidal variability as observed in the meridional thermospheric wind fapprox. 300 km height) is presented for a set of eight ionosonde locations (three in the northern hemisphere and five in the southern hemisphere). The data set corresponds to a full year (1984) of hourly values. The detected friability can be seen in the tidal components of the meridional wind. The diurnal and semidiurnal components are spectrally analysed. The quarterly spectra show that the tidal amplitudes oscillate with periods between 2 and 60 days. The more important oscillations have periods from f 5 to 3 days. No direct link between solar and magnetic activity indices was detected. Possible reasons for the observed tidal va~ability are discussed in the light of the current theory developed for tbe mesosphere and lower thermosphere.
1. lNTRODUCTlON It is currentiy thought that the mean beh~viour of the
upper thermospheric dynamics is fairly we11 understood, since the results obtained by the TGCMs (Thermospheric General circulation Models ; DICKINSONet al.. 1984 ; REESet al., 1987; FE~EN et ad., 1986,199 la, I 99i b and references therein) are in good agreement with experime~ta~ observations obtained by various techniques, at diRerent locations. This would imply that all major physical processes have been included in the models, and the remaining task is to ‘fine tune’ them. In consequence, attention is now being focused on understanding the so-called the~osphe~~ ‘weather’, i.e. the day-to-day focal and global variabiiity. This variability has many sources, such as magnetic storms, particle precipitation events in aurora1 regions, changes in the solar energy flux, etc. Even tropospheric storms can have effects on the thermospheric processesl as shown by W~L~&K and J~NFS (1987). Waves can propagate from lower regions of the atmosphere or be locally generated. These perturbations introduce modifications in the composition and dynamics of the upper thermosphere. They can be detected in the appearance of high frequency oscillations in the ionospheric plasma content or the wind. These oscillations are identified as travefling ionospheric disturbances (TIDs) associated with gravity waves. Since in this paper, frequent use shall be made of the terms ‘tide’ and ‘tidal variability’, it is convenient,
before proceeding further, to clarify the meaning given to the term ‘tide’. As clearly pointed out by VIAL ef al. (199 I ) and FESENel at. {I99 I a). a careful approach to the term ‘tide’ is necessary. It can be interpreted in one of two ways. VIAL et ad. (1991) do not consider it truly appropriate to speak of ‘tidal variabiIity’ given the definition of tides as processes resulting from a precise asciiiatory foreing such as the sofar thermat forcing, with we&defined periods, Hough mode structures, etc. Given this point of view, FESENef &. (1991a) refer to a broader definition of tides, generally imposed by usage, whereby these are considered oscillations of atmospheric variables with 24, 12 or 8 h periods. This is effectively what one observes when working with Fourier decomposed data. In consequence, given the characteristics of the data set, we shall refer to this latter interpretation and continue using the term ‘tidal variability’, in the broader sense given above. Recentiy, C~NzIhiir et ill. (f982, from now on Paper I) and G~RA~.DEZand CANZIANI (1992, from now on Paper II), analysed, for the year 1984, the mean monthly behaviour of the thermospheric meridional wind in the 250-400 km height range. as a first step in understanding global scale dynamic medium- and long-term processes. This wind component was derived from data obtained by a network of ionosondes located approximately at 35% and 35”N. Seasonal trends were observed both in the daily evolution and the tidal decomposition. An important feature was the magnetic latitude control of the diurnal tidal
902
P. 0.
amplitude, but not of the semidiurnal one. The results also showed for the first time the existence of slowly changing longitudinal global patterns for the diurnal and semidiurnal mean monthly amplitudes. The results in both papers were in good agreement with other observational studies. The day-to-day variability, as observed in the data once the high frequency filtering is done to remove the gravity waves, remains as an important feature. It can be observed as amplitude and. to a lesser extent, phase changes of the tidal components. For example, CANZIANI and GIRALDEZ (1989), in a spectral analysis of the height of the F2-layer, hmF2, detected slowly varying amplitudes for the solar diurnal and semidiurnal components during equinox. While analysing the daily wind estimates studied in Papers I and II, it was effectively observed that the gravity waves, superimposed on the daily trend, were not the only source of day-to-day variability. Important differences could be seen which could not be attributed to these high frequency oscillations. Figures 1 to 4 show the cvolution of the daily meridional winds at four ionosonde locations (two for each hemisphere), for different seasons. Each sample is 60 days long. Note the relatively simple daily trend of the meridional upper thermospheric wind. which basically blows poleward dur-
n
Fig. I. Daily evolution
of the meridional
CANZIANI
ing daytime and equatorward during nighttime. The day-to-day changes are evident in all figures. Gravity waves, even if somewhat smoothed during the generation of the three-dimensional graphs, are clearly visible, particularly during the night hours. They have a variable occurrence, which points to their random origin. However, an important feature in all these figures is the existence of long period modulations in the wind amplitude. This implies that the day-to-day changes are not purely random from one day to the next. but that there exists some kind of process, be it local or global, that is responsible for the existence of the modulation spanning many days at a time. This cannot be considered an artifact of the wind estimation method (cf. Paper I and references therein), since each hourly wind estimate is independent from the rest. The two sites corresponding to summer and (Wallops Island, U.S.A., and early autumn Hermanus, South Africa) show a slow modulation, while the other two (TokyoKokubunji, Japan, and Mundaring, Australia) with winter and early spring data, yield very rapid fluctuations spanning only a few days. WALTERSCHEID(I 98 I) proposed that there exists an important interaction between the semidiurnal tide and the gravity waves, which leads to local modi-
Tok
yo-
Kok ubun
J 1
thermospheric wind over Tokyo-Kokubunji February 1984.
during
January
903
Fig. 2. Same as Fig.
1 for Hermanus
during
March-April
Wal
Fig. 3. Same as Fig.
lops
1 for Wallops Island during June-July
1984.
Is.
1984.
904
CANZIAP~~
P. 0.
Mundar
-Qi.x^
“2
Fig. 4. Same as Fig. I for ~undaring
fications of this tidal component. This would be due to the modulation, by the forced semidiurnal tide, of the gravity wave momentum deposition on the mean flow. Such a process would generate secondary waves with periods very close to the semidiurnal tide. GIRALDEZ et al. (1989) studied the non-linear coupling between gravity waves and the tidal components. They proposed that there exists a permanent energy exchange between the tidal modes and the local gravity waves. This exchange appears to depend, both in direction and intensity, on the local time. A good agreement between thetmospheric observations and model results was obtained. The aim of this paper and its sequel is to analyse the variability as observed in the tidal components of the meridional thermospheric winds, determine its nature, and to detect the possible existence of low frequency oscillations (with periods between 2 and 25 days), with characteristics similar to those of planetary waves. Planetary waves have been detected and anaiysed in the lower and middle atmosphere (SALBY, 1984, and references therein), but very littIe has been done regarding their possible existence in the upper mesosphere and the thermosphere. Planetary wavelike structures have been reported by CAVALIERI (1976) and FRASER (1977) in the E-region of the ionosphere (approx. 100-120 km height). Above this height
Ing
crc;&
f
during August--September 1984.
there are no reports in the open literature regarding the existence of low frequency oscillations or global scale wave structures other than tides. As for the data samples to be considered, Table 1 shows the ionosonde network. The year 1984 corresponds to the descending phase of solar cycle 2 I. According to the HO.7 solar index the level of solar activity was medium to low, throughout that year. For further details refer to Paper I. 2. ON TIDAL VARlABlLlTY
(i) Obsewntional widewe Figures 5-7 show the diurnal and semidiurnal tide amplitudes and phases for the year 1984 at three sites. Each point represents the mean amplitude and phase (maximum southward wind), obtained by leastsquares fit of a 48-h sample. To obtain this time series the sampling window was displaced in 6 h steps. Thus the effects of gravity waves and their interactions with tides are smoothed out in the process. The dispersion of the amplitude and phase is practically the same at all sites. The phase tends to show, in certain cases, as for example during the first part of the year over Buenos Aires (Fig. 5), an important dispersion. This is due to the fact that in those cases the amplitude was practically null and this resulted in
905
Tidal variability--l Table 1. Ionosonde Geographic latitude
Location
network
Geographic longitude
Geomagnetic latitude
Magnetic dip
Buenos Aires Hermanus Mundaring Canberra Norfolk Is.
34.55 S 34.42’S 32.00 S 35.30’S 29.00 S
301.3wE 19.22-E 116.20 E 149.00 E 168.00’ E
23.21 33.30 43.45 43.99 34.15
‘S s s s s
- 32.2 -65.8“ -66.6 -65.9 - 56.4
Wallops Is. Gibilmanna Kokubunji (Tokyo)
37.90 N 37.60 N 35.67 N
284.50 E 14.00 E 139.55 E
49.31 N 39.10 N 25.41 N
69.9 53.0 4X.8
fits of dubious
stability. with the consequent undefined phase behaviour. Close scrutiny of these figures reveals that, besides the seasonal trends, the data follow other patterns, i.e. within the ‘dispersion band’ the distribution of amplitudes is not purely random, but rather appears to follow a sequence. They seem to have an oscillatory behaviour. This is particularly evident for the semidiurnal amplitude over Canberra and Buenos Aires. No well-defined trend can be observed for the phases. In order to better observe these oscillations, the hourly value of the wind for each of the first four tidal harmonics was plotted. Figures S-11 show some of these plots for different stations and months. This is
equivalent to evaluating the inverse Fourier transform on a narrow frequency band about each harmonic. Each figure includes the time evolution of the correlation coefficient r between the raw wind data and the sum of the four tidal components. In general the correlation is good, except during certain periods over Buenos Aires. The outstanding feature of all these figures is the well-defined oscillatory nature of the tidal amplitudes. In some cases the oscillations are rapid, spanning only a few days, in other cases the process can last for more than a week. This is in agreement with the behaviour observed in Figs 14. Sometimes an overlapping of oscillations with different periods can be observed too.
68 - 2411
1211
III
III 1
98
180
27%
I
368
I
I
98
I
I
I
188
I
I
I
I
I
270
I
368
DAYOF YEAR Fig. 5. Evolution,
throughout
1984, of the amplitude and phase of the diurnal meridional wind over Buenos Aires.
and semidiurnal
tides of the
906
P. 0. C.~NZIANI
1
90
180
278
368
96
DAYOF YEAR Fig. 6. Same as Fig. 5, for Canberra.
188
270
360
Tidal variability-l
50
8h
31
1 Fig. 8. Hourly Norfolk
evolution of the meridional Island during January-March.
1
DAYOF YENI
61
91
thermospheric wind, separated into tidal components, over The top row shows the correlation coefficient of the fit.
Terdiurnal and higher harmonics show a very rapid variability. The first preliminary results on such modulations in the upper thermosphere were presented by CANZIANI and GIRALDEZ (1990). The amplitude modulation appears to be highly variable. Under some circumstances the tidal amplitude practically disappears, while in others, it seems PZ
907
to be a mere ripple about the average amplitude. Furthermore, the variability of the diurnal and semidiurnal components at a given site and period is not the same, i.e. there does not appear to be any evident or direct link between the modulation of one tidal component and the other. It is not possible to further compare the behaviour of higher harmonics given that
III,~I,II~~~IIIIII,,IIII~.,,~,,II.I.~~~II~~~II~I~IIII~~,II,,I.~I~~~
‘6h
BA
iz_~~~~ 92
122
DAYOF YEAR
Fig. 9. Same as Fig. 8, for Buenos Aires, during
152 April-June.
182
908
P. 0.
“,
.-._
,,
”
CANZIANI
.”
-50 50 0 -50
-Llllllllllll
122
152
they are not mainly tidal in nature (TEITELBAUMef ul., 1989), i.e. they result from a combination of tidal and wave-wave interactions. It is not possible at this stage, with the current data set to state that at a given region or time of the year, the oscillations are more or less important. In order to reach a better understanding of these modulations, the quarterly power spectra for the diur-
305
182
DAY OF YENi
Fig. IO. Same as Fig. 8. for Gihilmanna,
215
LLJ
II,~-I.l_u~u.L~_~~u
during
212
MaypJuly.
nal and semidiurnal amplitudes were evaluated, using MESA (Maximum Entropy Spectral Analysis). The stability of the spectral lines obtained with MESA was closely monitored to avoid spurious spectra. There is a good agreement between the spectra thus obtained and the results obtained by FFT. Quarterly power spectra for the eight stations are shown in Figs 12-15. Most of the spectral power appears to be concentrated
DAYOF YEAR
Fig. Il. Same as Fig. 8, for Hermanus,
during
335
October-December.
909
Tidal variability-l 24
h. Ampl.
-
1st Quarter
24 h. Ampl. -
Iilrd Quarter
3200 WI 2800
Cl
2400
KK
& 2000 3 2
KK
BA
1600 HE
HE
MU
MU
CA
CA
NI
NI
1200 800 400 0 9.0’
i3
Period
1.1
(d)
Fig. 12. Power spectra of the diurnal tide amplitude during the first quarter of 1984, at all locations. The units of power are arbitrary. Each spectrum has been displaced by 400 units with respect to the previous one.
in the lower frequencies i.e. in oscillations with periods greater than 5 or 6 days. The main spectral lines occur with periods between 60 and 3 days. In many cases the most prominent spectral lines appear between the 16 and 6 day periods. When comparing the different quarters there does not appear to be any seasonal behaviour or trend. In some cases, it must be noted, important levels of power appear for 60 day oscillations. Using the possibility of determining, with MESA, the existence of spectral lines corresponding to oscillations with periods greater than the length of the data samples it was possible to observe that these strong 60 or more days oscillations showed up when important seasonal changes could be detected in the tidal amplitude. The two vertical short-dashed lines in all these figures correspond to 30 and 13.84 day oscillations, which are the closest possible harmonics to the solar; rotation period (approx. 27.8 days) and its first har-’ manic. Figure 16 shows the normalized power spectrum of the F10.7 solar activity index for the year 1984. The spectrum yields a number of peaks, the most prominent of which occurs at a period of 122 days.
Period
(d)
Fig. 13. Same as Fig. 12, for the third quarter of 1984
The solar rotation is clearly present with a maximum at 28.1 days. The first harmonic is not well defined with maxima at 15.25 and 12.6 days. Important peaks occur in the vicinity of these periods, but few coincide with the solar rotation periods. Furthermore, whereas the solar index power spectrum has an abrupt cutoff after the 12.6 day oscillation the tidal amplitude oscillations do show strong spectral lines with periods up to 3-2 days. If the solar thermal forcing were a dominant cause for the tidal variability prominent spectral lines should appear in all cases, coincident with the major solar spectral lines. This is not the case. On the other hand, it may be possible that mechanisms exist by which the energy input modulated by the solar rotation is redistributed by the atmospheric processes to other frequencies in its vicinity. In some cases, for a given station, during the same quarter, the diurnal and semidiurnal power spectra have similar spectral structures, even if the particular features, i.e. maxima and minima, do not fall exactly on the same periods. This coincides with the observation that the modulations of the tides, in Figs 8-11, are not the same. However, the fact that the general structure of the spectra can be similar could imply that there exist modulation frequency bands, to which
910
P. 0. 12 h. Ampl. -----
12 h. AmpI. - Wth
Ist Quarter
1
CANZ~ANI
auarter
1
3200
2400 KK eh HE MU CA
CA
400 NI
NI
0 e.0
43
Period Fig. 14. Power spectra during the first quarter
(d)
of the semidiurnal tide amplitude, of 1984. The comments are as for Fig. 12.
Period
(d)
Fig. 15. Same as Fig. 14, for the fourth ql .wter.
modulation has shorter characteristic times than those required by these modes to reach equilibrium. Thus both tidal amplitudes respond. Regarding this, a partides would never reach such a state. The modulation ticular feature can be seen in Figs 13 and 14 (third would be the visible feature of such continuous changes. According to FORBES (1984) the consequarter) where, both in the diurnal and semidiurnal tide, a spectral line corresponding to a period of 12.8 quences of such a process with respect to the distortion of the shapes of normal Hough modes remains to be days can be detected over M undaring, Hermanus and determined. FELLOUSet al. (1985) after comparing Buenos Aires. During the first quarter a similar behaviour can be observed. the results obtained in a joint campaign with various European radars, concluded that the observed modu(ii) Prel~rn~~~rydiscussion lations tended to be similar at all locations. Thus they were not of local origin, but at least existed on a GLASS et al. (1978) FELLOUSet al. (1985) and regional scale. In agreement with these results from CEVOLANI(1987) using wind measurements between the lower thermosphere, CANZIANIet crf. (1990) also 80 and 100 km, from meteor wind radars located in observed the regional extent of the modulations in different places in Europe, have reported similar tidal the upper thermosphere, when comparing meridional modulations both for meridional and zonal winds. winds over Tucuman (26.9’S, 65.4 W), Buenos Aires This height range corresponds to the E-layer of the (34.5’S 58.5 W) and Puerto Argentino (51.7’S, ionosphere. Thus, the oscillation of the tidal ampli57YW). tudes, particularly the semidiurnal one, also appears Various mechanisms have been proposed to explain as an important feature of the tidal variability in the meteor region. FELLOUSet al. (1975) and BERNARD the tidal variability in the lower and middle atmos(1981) carried out detailed studies of the amplitude, phere. For example it could be due, at least in tropical phase and vertical wavelength of the data presented regions, to changes in the tropospheric water vapour content and insolation absorption. According to by GLASSet al. (1978). BERNARD(198 1) suggested that such a variability could be generated by changes in LINDZEN(1968) in the particular case of the diurnal tide, the tidal variability could be due to local instathe Hough mode composition, even if the observed
Tidal variability--I
911
proposed by BERNARD(1981). They propose that the oscillations observed in the upper mesosphere/lower thermosphere observations could be a consequence of beats between the tidal modes and transient waves near tidal periods. The source of variability was changes in the water vapour content, though they do 0.75 not discard the possible effects of changes in ozone distribution. Their setup times (the time required by a tidal mode in order to reach the steady state) differ from those obtained by BERNARD(1981). During the setup time the tidal modes oscillate around the mean steady state with periods similar to those obtained in the above quarterly spectra, i.e. 2-l 5 days. Should the behaviour of the tidal modes in the upper thermosphere be similar to those in the vicinity of the mesopause, then the spectral results here obtained point to the existence of such a mechanism in this region. VIAL et al. (1991) do not preclude the existence of other mechanisms which, acting together with their transience theory could lead to the results obtained in different regions of the middle and upper atmosphere. Since the phenomena considered here are subject to coupled electrodynamics with the Earth’s magnetic 0.00 36.6 18.3 12.2 9.2 7.3 field, which is not the case for most of the middle atmosphere. it could be argued that these oscillations PERIOD (days) are essentially generated by the magnetic storms or Fig. 16. Normalized power spectrum of the solar index F10.7 perturbations. The poor correlations obtained for the year 1984. The letter A denotes the maximum with a between the oscillations of the diurnal and semiperiod of 28.1 days, in the vicinity of the solar rotation diurnal amplitudes, at these latitudes, with the daily period. Kp magnetic activity (sum of the eight 3-h indices), does not appear to confirm this hypothesis. In Paper 1 it was noted that the impact of magnetic storms was bilities in the tidal modes. TEITELBAUM and BLAM~NT mainly observed in an increase of the gravity wave (1975) propose the existence of non-linear interactivity and changes of the mean wind, and at most actions, basically of local origin. However, FORBES after 24 h, the quiet condition flow is re-established. (1984) points out that such local phenomena could What is more, in most cases there was practically no not explain at least the regional scale so far observed. difference in the observed variability of the daily winds Due to the large phase speed of the diurnal tide, with for quiet and perturbed days, when these were binned respect to the zonal wind, the possibility that these according to the level of daily magnetic activity. It mechanisms could justify the observed variability is must be noted that various authors (FORBES,1988 for minimal. In the case of the semidiurnal tide it does example) have shown that the atmospheric response not seem to be possible that changes in the distribution to magnetic perturbations is strongly dependent on and insolation absorption of the ozone layer could magnetic latitude. as well as on the local time of the influence to such an extent its behaviour in the mesosonset of the perturbation. Hence, this would imply phere and lower thermosphere. According to the curthat the modulations are linked to the neutral dynamrent tidal theories, at mid- and high-latitudes the semiics of the thermospheric fluid. diurnal heating rate, and consequently the forcing of Finally, as mentioned above, it could be argued the semidiurnal modes, depends to first-order on the that the solar variability can cause these amplitude solar input, and only to second-order on ozone variafluctuations. A spectral analysis of the F10.7 solar bility (FORBESand GARRETT, 1978). At any rate, the index, yields a well-defined spectral line with periods of approximately 28 days and two maxima in the true role of the ozone and its variability in the genervicinity of its first harmonic. Yet there are no strong, ation of tides is still being discussed. permanent lines in’the quarterly amplitude spectra at VIAL et ul. (1991) have revised the mechanisms 1.00
912
P. 0.
CANZIA~~~
these frequencies. If the main source of these oscillations were the Sun, these spectral lines ought to be
a major Feature all year round, at all locations. It is not the case in the samples here analysed. This does not preclude the role of the solar forcing as a source of thermospheric dynamic variability, but rather points to possibly more complex energy exchanges. with other forcings competing with the Sun. The observational evidence so far presented shows that the tidal variability in the upper thermosphere, on the medium and long term. is of oscillatory nature, with periods between 2-3 and 60 days. There does not appear to be a direct link between these results and the variability of the solar forcing or magnetic activity as one could have expected N priori. The extension of other mechanisms into the upper thermosphere, as originally proposed by BERNARD (1981), and revised by VIAL et al. (1991), is an interesting option. Keeping in mind that the semidiurnal tide in this region is partially due to propagating modes from the middle atmosphere, it is possible that the modulation observed in this component could effectively have
sources in the mechanisms discussed. On the other hand, this does not apply to the diurnal tide which is basically of local origin, and to the other part of the semidiurnal tide, also mostly of local origin. The spatial limitations, both in the vertical and the horizontal, of the current data set preclude any further inferences on the applicability of the mechanisms suggestcd in the fast two referenced papers. It could be of great value if the model runs could be extended well into the thermosphere. As yet, the major modelfing efforts of the upper the~osphere have not taken into consideration the issue of tidal variability. on the time scales considered here. In CANZIANI (this issue), another mechanism is considered, notwithstanding the possible and non-trivial extension of middle atmosphere models to the upper atmosphere. The mechanism to be considered is the non-linear wave-wave interaction. This mechanism requires the existence of low frequency oscillations or waves in the region. These have been effectively detected with non-negligible power levels in the spectra of the meridional wind (GIRALOEZand CANZIANI,1992).
REFERENCES BERNARDR. CAIXZIANI P. 0. and GIRALDEZA. E. CANZIANIP. 0. and GIRALIXZ A. E. CAXZIANIP. O., GIRAL~EZA. E. and PLJIGL. CAVALIERID. J. CEVOLANG. DICKINSON R. E., RIDLEY E. C. and ROBLER. G. FELLOUSJ. L., BERNARDR., MASSEBOEUF M.. GLASSM. and SPIZZICHINO A. FELLOUSJ. L., CEVOLANIG., KINGSLEYS. P. and MC’LLEKH. G. FESENC. G., DICKINSON R. E. and ROULER. G. FESENC. G., RUBLE R. G. and RIIXEY E. C. FESENC. G., ROBLER. G. and RIPLEY E. C. FONES J. M. F~KBESJ. M. FORBESJ. M. and GARRETTH. B. FRASERG. J., CODRESCUM. and HALL T. J. GIRALDEZA. E., BOLDESU. and COLMANJ. GIRALDEZA. E. and CANZIANIP. 0. GLASS M., BERNARDR.. FELLOUSJ. L. and MASSEBOEUF M. LINVZENR. S. REES D., FULLER-R•WELLT. J., QUEFANS., MOFFETTR. J. and BAILEYG. J. SALBII.M. L. TEITELBAUM H. and BLAMONT J. E. TE~TIZLBAC’M H., VIAL F.. MANSONA. H., GIKALDEZA. E. and MASSEBOEUF M. VIAL F., FORBESJ. M. and MIYAHARAS. WALDOCKJ. A. and JONEST. B. WALTERSCHEID R. L.
1981 1989 1990 1992 1976 I987 1984 1975
1. uimos. tcrr, Phys. 43, 663. J. geophys. Rex 94, 17,009. Ann. Geophys. 8, 549. Ann. G~ophvs. 10, 86 I. J. utmos. terr. Phys. 38, 965. MAP Hundhook 25, I2 I. J. atmos.Sci. 41, 205. J. utrmu. tcrr. Phy.u. 37, I5 I I
1985
J. LII~IOS.terr. Phys. 47, 233.
1986 199Ia l99lb 1984 1988 1978 1977 1989 1992 1978
J. geophvs. lies. 91,447l. J. y&&,ss. Rrs. 96, 3647. J. ueonhu. RET. 96. 3663. d. Y&&+. icrr. Phy.;. 46, 1049. G-eoph?:y.Re.r. Lett. 15, 249. Geopi~,~~s.Re.r. Lett. 5, 1016. J. utmos. trrr. Phys. 39, i21. Lat. Am. Appl. Res. 19, 125. Ann. Geoph,vs. 10, 874. J. rrtmos. terr. Php. 40, I33 I.
1968 1987
Proc. R. Sot. A303,299. Ann. Geopltw. 6A, 303.
1984 1975 1989
Rer>.Geophys. Space Phys. 22, 209. J. atmos. terr. Ph.ys. 37, 697. J. atmos. fur. Phw 51, 627.
1991 1987 1981
J. geoph~s. Rex %, 12 15. J. atmos. terr. Phvs. 49. 105 J. geophw Res. 86,9648.
Laboratorio Ionosf6rico de la Armada de la Reptiblica Argentina, Se.N.I.D. and C.O.N.I.C.E.T..
Av. de1Libertador 327, 1638Vicente Lopez, Argentina
Abstract-The
evidence for the existence of tidal variability as observed in the meridional thermospheric wind fapprox. 300 km height) is presented for a set of eight ionosonde locations (three in the northern hemisphere and five in the southern hemisphere). The data set corresponds to a full year (1984) of hourly values. The detected friability can be seen in the tidal components of the meridional wind. The diurnal and semidiurnal components are spectrally analysed. The quarterly spectra show that the tidal amplitudes oscillate with periods between 2 and 60 days. The more important oscillations have periods from f 5 to 3 days. No direct link between solar and magnetic activity indices was detected. Possible reasons for the observed tidal va~ability are discussed in the light of the current theory developed for tbe mesosphere and lower thermosphere.
1. lNTRODUCTlON It is currentiy thought that the mean beh~viour of the
upper thermospheric dynamics is fairly we11 understood, since the results obtained by the TGCMs (Thermospheric General circulation Models ; DICKINSONet al.. 1984 ; REESet al., 1987; FE~EN et ad., 1986,199 la, I 99i b and references therein) are in good agreement with experime~ta~ observations obtained by various techniques, at diRerent locations. This would imply that all major physical processes have been included in the models, and the remaining task is to ‘fine tune’ them. In consequence, attention is now being focused on understanding the so-called the~osphe~~ ‘weather’, i.e. the day-to-day focal and global variabiiity. This variability has many sources, such as magnetic storms, particle precipitation events in aurora1 regions, changes in the solar energy flux, etc. Even tropospheric storms can have effects on the thermospheric processesl as shown by W~L~&K and J~NFS (1987). Waves can propagate from lower regions of the atmosphere or be locally generated. These perturbations introduce modifications in the composition and dynamics of the upper thermosphere. They can be detected in the appearance of high frequency oscillations in the ionospheric plasma content or the wind. These oscillations are identified as travefling ionospheric disturbances (TIDs) associated with gravity waves. Since in this paper, frequent use shall be made of the terms ‘tide’ and ‘tidal variability’, it is convenient,
before proceeding further, to clarify the meaning given to the term ‘tide’. As clearly pointed out by VIAL ef al. (199 I ) and FESENel at. {I99 I a). a careful approach to the term ‘tide’ is necessary. It can be interpreted in one of two ways. VIAL et ad. (1991) do not consider it truly appropriate to speak of ‘tidal variabiIity’ given the definition of tides as processes resulting from a precise asciiiatory foreing such as the sofar thermat forcing, with we&defined periods, Hough mode structures, etc. Given this point of view, FESENef &. (1991a) refer to a broader definition of tides, generally imposed by usage, whereby these are considered oscillations of atmospheric variables with 24, 12 or 8 h periods. This is effectively what one observes when working with Fourier decomposed data. In consequence, given the characteristics of the data set, we shall refer to this latter interpretation and continue using the term ‘tidal variability’, in the broader sense given above. Recentiy, C~NzIhiir et ill. (f982, from now on Paper I) and G~RA~.DEZand CANZIANI (1992, from now on Paper II), analysed, for the year 1984, the mean monthly behaviour of the thermospheric meridional wind in the 250-400 km height range. as a first step in understanding global scale dynamic medium- and long-term processes. This wind component was derived from data obtained by a network of ionosondes located approximately at 35% and 35”N. Seasonal trends were observed both in the daily evolution and the tidal decomposition. An important feature was the magnetic latitude control of the diurnal tidal
902
P. 0.
amplitude, but not of the semidiurnal one. The results also showed for the first time the existence of slowly changing longitudinal global patterns for the diurnal and semidiurnal mean monthly amplitudes. The results in both papers were in good agreement with other observational studies. The day-to-day variability, as observed in the data once the high frequency filtering is done to remove the gravity waves, remains as an important feature. It can be observed as amplitude and. to a lesser extent, phase changes of the tidal components. For example, CANZIANI and GIRALDEZ (1989), in a spectral analysis of the height of the F2-layer, hmF2, detected slowly varying amplitudes for the solar diurnal and semidiurnal components during equinox. While analysing the daily wind estimates studied in Papers I and II, it was effectively observed that the gravity waves, superimposed on the daily trend, were not the only source of day-to-day variability. Important differences could be seen which could not be attributed to these high frequency oscillations. Figures 1 to 4 show the cvolution of the daily meridional winds at four ionosonde locations (two for each hemisphere), for different seasons. Each sample is 60 days long. Note the relatively simple daily trend of the meridional upper thermospheric wind. which basically blows poleward dur-
n
Fig. I. Daily evolution
of the meridional
CANZIANI
ing daytime and equatorward during nighttime. The day-to-day changes are evident in all figures. Gravity waves, even if somewhat smoothed during the generation of the three-dimensional graphs, are clearly visible, particularly during the night hours. They have a variable occurrence, which points to their random origin. However, an important feature in all these figures is the existence of long period modulations in the wind amplitude. This implies that the day-to-day changes are not purely random from one day to the next. but that there exists some kind of process, be it local or global, that is responsible for the existence of the modulation spanning many days at a time. This cannot be considered an artifact of the wind estimation method (cf. Paper I and references therein), since each hourly wind estimate is independent from the rest. The two sites corresponding to summer and (Wallops Island, U.S.A., and early autumn Hermanus, South Africa) show a slow modulation, while the other two (TokyoKokubunji, Japan, and Mundaring, Australia) with winter and early spring data, yield very rapid fluctuations spanning only a few days. WALTERSCHEID(I 98 I) proposed that there exists an important interaction between the semidiurnal tide and the gravity waves, which leads to local modi-
Tok
yo-
Kok ubun
J 1
thermospheric wind over Tokyo-Kokubunji February 1984.
during
January
903
Fig. 2. Same as Fig.
1 for Hermanus
during
March-April
Wal
Fig. 3. Same as Fig.
lops
1 for Wallops Island during June-July
1984.
Is.
1984.
904
CANZIAP~~
P. 0.
Mundar
-Qi.x^
“2
Fig. 4. Same as Fig. I for ~undaring
fications of this tidal component. This would be due to the modulation, by the forced semidiurnal tide, of the gravity wave momentum deposition on the mean flow. Such a process would generate secondary waves with periods very close to the semidiurnal tide. GIRALDEZ et al. (1989) studied the non-linear coupling between gravity waves and the tidal components. They proposed that there exists a permanent energy exchange between the tidal modes and the local gravity waves. This exchange appears to depend, both in direction and intensity, on the local time. A good agreement between thetmospheric observations and model results was obtained. The aim of this paper and its sequel is to analyse the variability as observed in the tidal components of the meridional thermospheric winds, determine its nature, and to detect the possible existence of low frequency oscillations (with periods between 2 and 25 days), with characteristics similar to those of planetary waves. Planetary waves have been detected and anaiysed in the lower and middle atmosphere (SALBY, 1984, and references therein), but very littIe has been done regarding their possible existence in the upper mesosphere and the thermosphere. Planetary wavelike structures have been reported by CAVALIERI (1976) and FRASER (1977) in the E-region of the ionosphere (approx. 100-120 km height). Above this height
Ing
crc;&
f
during August--September 1984.
there are no reports in the open literature regarding the existence of low frequency oscillations or global scale wave structures other than tides. As for the data samples to be considered, Table 1 shows the ionosonde network. The year 1984 corresponds to the descending phase of solar cycle 2 I. According to the HO.7 solar index the level of solar activity was medium to low, throughout that year. For further details refer to Paper I. 2. ON TIDAL VARlABlLlTY
(i) Obsewntional widewe Figures 5-7 show the diurnal and semidiurnal tide amplitudes and phases for the year 1984 at three sites. Each point represents the mean amplitude and phase (maximum southward wind), obtained by leastsquares fit of a 48-h sample. To obtain this time series the sampling window was displaced in 6 h steps. Thus the effects of gravity waves and their interactions with tides are smoothed out in the process. The dispersion of the amplitude and phase is practically the same at all sites. The phase tends to show, in certain cases, as for example during the first part of the year over Buenos Aires (Fig. 5), an important dispersion. This is due to the fact that in those cases the amplitude was practically null and this resulted in
905
Tidal variability--l Table 1. Ionosonde Geographic latitude
Location
network
Geographic longitude
Geomagnetic latitude
Magnetic dip
Buenos Aires Hermanus Mundaring Canberra Norfolk Is.
34.55 S 34.42’S 32.00 S 35.30’S 29.00 S
301.3wE 19.22-E 116.20 E 149.00 E 168.00’ E
23.21 33.30 43.45 43.99 34.15
‘S s s s s
- 32.2 -65.8“ -66.6 -65.9 - 56.4
Wallops Is. Gibilmanna Kokubunji (Tokyo)
37.90 N 37.60 N 35.67 N
284.50 E 14.00 E 139.55 E
49.31 N 39.10 N 25.41 N
69.9 53.0 4X.8
fits of dubious
stability. with the consequent undefined phase behaviour. Close scrutiny of these figures reveals that, besides the seasonal trends, the data follow other patterns, i.e. within the ‘dispersion band’ the distribution of amplitudes is not purely random, but rather appears to follow a sequence. They seem to have an oscillatory behaviour. This is particularly evident for the semidiurnal amplitude over Canberra and Buenos Aires. No well-defined trend can be observed for the phases. In order to better observe these oscillations, the hourly value of the wind for each of the first four tidal harmonics was plotted. Figures S-11 show some of these plots for different stations and months. This is
equivalent to evaluating the inverse Fourier transform on a narrow frequency band about each harmonic. Each figure includes the time evolution of the correlation coefficient r between the raw wind data and the sum of the four tidal components. In general the correlation is good, except during certain periods over Buenos Aires. The outstanding feature of all these figures is the well-defined oscillatory nature of the tidal amplitudes. In some cases the oscillations are rapid, spanning only a few days, in other cases the process can last for more than a week. This is in agreement with the behaviour observed in Figs 14. Sometimes an overlapping of oscillations with different periods can be observed too.
68 - 2411
1211
III
III 1
98
180
27%
I
368
I
I
98
I
I
I
188
I
I
I
I
I
270
I
368
DAYOF YEAR Fig. 5. Evolution,
throughout
1984, of the amplitude and phase of the diurnal meridional wind over Buenos Aires.
and semidiurnal
tides of the
906
P. 0. C.~NZIANI
1
90
180
278
368
96
DAYOF YEAR Fig. 6. Same as Fig. 5, for Canberra.
188
270
360
Tidal variability-l
50
8h
31
1 Fig. 8. Hourly Norfolk
evolution of the meridional Island during January-March.
1
DAYOF YENI
61
91
thermospheric wind, separated into tidal components, over The top row shows the correlation coefficient of the fit.
Terdiurnal and higher harmonics show a very rapid variability. The first preliminary results on such modulations in the upper thermosphere were presented by CANZIANI and GIRALDEZ (1990). The amplitude modulation appears to be highly variable. Under some circumstances the tidal amplitude practically disappears, while in others, it seems PZ
907
to be a mere ripple about the average amplitude. Furthermore, the variability of the diurnal and semidiurnal components at a given site and period is not the same, i.e. there does not appear to be any evident or direct link between the modulation of one tidal component and the other. It is not possible to further compare the behaviour of higher harmonics given that
III,~I,II~~~IIIIII,,IIII~.,,~,,II.I.~~~II~~~II~I~IIII~~,II,,I.~I~~~
‘6h
BA
iz_~~~~ 92
122
DAYOF YEAR
Fig. 9. Same as Fig. 8, for Buenos Aires, during
152 April-June.
182
908
P. 0.
“,
.-._
,,
”
CANZIANI
.”
-50 50 0 -50
-Llllllllllll
122
152
they are not mainly tidal in nature (TEITELBAUMef ul., 1989), i.e. they result from a combination of tidal and wave-wave interactions. It is not possible at this stage, with the current data set to state that at a given region or time of the year, the oscillations are more or less important. In order to reach a better understanding of these modulations, the quarterly power spectra for the diur-
305
182
DAY OF YENi
Fig. IO. Same as Fig. 8. for Gihilmanna,
215
LLJ
II,~-I.l_u~u.L~_~~u
during
212
MaypJuly.
nal and semidiurnal amplitudes were evaluated, using MESA (Maximum Entropy Spectral Analysis). The stability of the spectral lines obtained with MESA was closely monitored to avoid spurious spectra. There is a good agreement between the spectra thus obtained and the results obtained by FFT. Quarterly power spectra for the eight stations are shown in Figs 12-15. Most of the spectral power appears to be concentrated
DAYOF YEAR
Fig. Il. Same as Fig. 8, for Hermanus,
during
335
October-December.
909
Tidal variability-l 24
h. Ampl.
-
1st Quarter
24 h. Ampl. -
Iilrd Quarter
3200 WI 2800
Cl
2400
KK
& 2000 3 2
KK
BA
1600 HE
HE
MU
MU
CA
CA
NI
NI
1200 800 400 0 9.0’
i3
Period
1.1
(d)
Fig. 12. Power spectra of the diurnal tide amplitude during the first quarter of 1984, at all locations. The units of power are arbitrary. Each spectrum has been displaced by 400 units with respect to the previous one.
in the lower frequencies i.e. in oscillations with periods greater than 5 or 6 days. The main spectral lines occur with periods between 60 and 3 days. In many cases the most prominent spectral lines appear between the 16 and 6 day periods. When comparing the different quarters there does not appear to be any seasonal behaviour or trend. In some cases, it must be noted, important levels of power appear for 60 day oscillations. Using the possibility of determining, with MESA, the existence of spectral lines corresponding to oscillations with periods greater than the length of the data samples it was possible to observe that these strong 60 or more days oscillations showed up when important seasonal changes could be detected in the tidal amplitude. The two vertical short-dashed lines in all these figures correspond to 30 and 13.84 day oscillations, which are the closest possible harmonics to the solar; rotation period (approx. 27.8 days) and its first har-’ manic. Figure 16 shows the normalized power spectrum of the F10.7 solar activity index for the year 1984. The spectrum yields a number of peaks, the most prominent of which occurs at a period of 122 days.
Period
(d)
Fig. 13. Same as Fig. 12, for the third quarter of 1984
The solar rotation is clearly present with a maximum at 28.1 days. The first harmonic is not well defined with maxima at 15.25 and 12.6 days. Important peaks occur in the vicinity of these periods, but few coincide with the solar rotation periods. Furthermore, whereas the solar index power spectrum has an abrupt cutoff after the 12.6 day oscillation the tidal amplitude oscillations do show strong spectral lines with periods up to 3-2 days. If the solar thermal forcing were a dominant cause for the tidal variability prominent spectral lines should appear in all cases, coincident with the major solar spectral lines. This is not the case. On the other hand, it may be possible that mechanisms exist by which the energy input modulated by the solar rotation is redistributed by the atmospheric processes to other frequencies in its vicinity. In some cases, for a given station, during the same quarter, the diurnal and semidiurnal power spectra have similar spectral structures, even if the particular features, i.e. maxima and minima, do not fall exactly on the same periods. This coincides with the observation that the modulations of the tides, in Figs 8-11, are not the same. However, the fact that the general structure of the spectra can be similar could imply that there exist modulation frequency bands, to which
910
P. 0. 12 h. Ampl. -----
12 h. AmpI. - Wth
Ist Quarter
1
CANZ~ANI
auarter
1
3200
2400 KK eh HE MU CA
CA
400 NI
NI
0 e.0
43
Period Fig. 14. Power spectra during the first quarter
(d)
of the semidiurnal tide amplitude, of 1984. The comments are as for Fig. 12.
Period
(d)
Fig. 15. Same as Fig. 14, for the fourth ql .wter.
modulation has shorter characteristic times than those required by these modes to reach equilibrium. Thus both tidal amplitudes respond. Regarding this, a partides would never reach such a state. The modulation ticular feature can be seen in Figs 13 and 14 (third would be the visible feature of such continuous changes. According to FORBES (1984) the consequarter) where, both in the diurnal and semidiurnal tide, a spectral line corresponding to a period of 12.8 quences of such a process with respect to the distortion of the shapes of normal Hough modes remains to be days can be detected over M undaring, Hermanus and determined. FELLOUSet al. (1985) after comparing Buenos Aires. During the first quarter a similar behaviour can be observed. the results obtained in a joint campaign with various European radars, concluded that the observed modu(ii) Prel~rn~~~rydiscussion lations tended to be similar at all locations. Thus they were not of local origin, but at least existed on a GLASS et al. (1978) FELLOUSet al. (1985) and regional scale. In agreement with these results from CEVOLANI(1987) using wind measurements between the lower thermosphere, CANZIANIet crf. (1990) also 80 and 100 km, from meteor wind radars located in observed the regional extent of the modulations in different places in Europe, have reported similar tidal the upper thermosphere, when comparing meridional modulations both for meridional and zonal winds. winds over Tucuman (26.9’S, 65.4 W), Buenos Aires This height range corresponds to the E-layer of the (34.5’S 58.5 W) and Puerto Argentino (51.7’S, ionosphere. Thus, the oscillation of the tidal ampli57YW). tudes, particularly the semidiurnal one, also appears Various mechanisms have been proposed to explain as an important feature of the tidal variability in the meteor region. FELLOUSet al. (1975) and BERNARD the tidal variability in the lower and middle atmos(1981) carried out detailed studies of the amplitude, phere. For example it could be due, at least in tropical phase and vertical wavelength of the data presented regions, to changes in the tropospheric water vapour content and insolation absorption. According to by GLASSet al. (1978). BERNARD(198 1) suggested that such a variability could be generated by changes in LINDZEN(1968) in the particular case of the diurnal tide, the tidal variability could be due to local instathe Hough mode composition, even if the observed
Tidal variability--I
911
proposed by BERNARD(1981). They propose that the oscillations observed in the upper mesosphere/lower thermosphere observations could be a consequence of beats between the tidal modes and transient waves near tidal periods. The source of variability was changes in the water vapour content, though they do 0.75 not discard the possible effects of changes in ozone distribution. Their setup times (the time required by a tidal mode in order to reach the steady state) differ from those obtained by BERNARD(1981). During the setup time the tidal modes oscillate around the mean steady state with periods similar to those obtained in the above quarterly spectra, i.e. 2-l 5 days. Should the behaviour of the tidal modes in the upper thermosphere be similar to those in the vicinity of the mesopause, then the spectral results here obtained point to the existence of such a mechanism in this region. VIAL et al. (1991) do not preclude the existence of other mechanisms which, acting together with their transience theory could lead to the results obtained in different regions of the middle and upper atmosphere. Since the phenomena considered here are subject to coupled electrodynamics with the Earth’s magnetic 0.00 36.6 18.3 12.2 9.2 7.3 field, which is not the case for most of the middle atmosphere. it could be argued that these oscillations PERIOD (days) are essentially generated by the magnetic storms or Fig. 16. Normalized power spectrum of the solar index F10.7 perturbations. The poor correlations obtained for the year 1984. The letter A denotes the maximum with a between the oscillations of the diurnal and semiperiod of 28.1 days, in the vicinity of the solar rotation diurnal amplitudes, at these latitudes, with the daily period. Kp magnetic activity (sum of the eight 3-h indices), does not appear to confirm this hypothesis. In Paper 1 it was noted that the impact of magnetic storms was bilities in the tidal modes. TEITELBAUM and BLAM~NT mainly observed in an increase of the gravity wave (1975) propose the existence of non-linear interactivity and changes of the mean wind, and at most actions, basically of local origin. However, FORBES after 24 h, the quiet condition flow is re-established. (1984) points out that such local phenomena could What is more, in most cases there was practically no not explain at least the regional scale so far observed. difference in the observed variability of the daily winds Due to the large phase speed of the diurnal tide, with for quiet and perturbed days, when these were binned respect to the zonal wind, the possibility that these according to the level of daily magnetic activity. It mechanisms could justify the observed variability is must be noted that various authors (FORBES,1988 for minimal. In the case of the semidiurnal tide it does example) have shown that the atmospheric response not seem to be possible that changes in the distribution to magnetic perturbations is strongly dependent on and insolation absorption of the ozone layer could magnetic latitude. as well as on the local time of the influence to such an extent its behaviour in the mesosonset of the perturbation. Hence, this would imply phere and lower thermosphere. According to the curthat the modulations are linked to the neutral dynamrent tidal theories, at mid- and high-latitudes the semiics of the thermospheric fluid. diurnal heating rate, and consequently the forcing of Finally, as mentioned above, it could be argued the semidiurnal modes, depends to first-order on the that the solar variability can cause these amplitude solar input, and only to second-order on ozone variafluctuations. A spectral analysis of the F10.7 solar bility (FORBESand GARRETT, 1978). At any rate, the index, yields a well-defined spectral line with periods of approximately 28 days and two maxima in the true role of the ozone and its variability in the genervicinity of its first harmonic. Yet there are no strong, ation of tides is still being discussed. permanent lines in’the quarterly amplitude spectra at VIAL et ul. (1991) have revised the mechanisms 1.00
912
P. 0.
CANZIA~~~
these frequencies. If the main source of these oscillations were the Sun, these spectral lines ought to be
a major Feature all year round, at all locations. It is not the case in the samples here analysed. This does not preclude the role of the solar forcing as a source of thermospheric dynamic variability, but rather points to possibly more complex energy exchanges. with other forcings competing with the Sun. The observational evidence so far presented shows that the tidal variability in the upper thermosphere, on the medium and long term. is of oscillatory nature, with periods between 2-3 and 60 days. There does not appear to be a direct link between these results and the variability of the solar forcing or magnetic activity as one could have expected N priori. The extension of other mechanisms into the upper thermosphere, as originally proposed by BERNARD (1981), and revised by VIAL et al. (1991), is an interesting option. Keeping in mind that the semidiurnal tide in this region is partially due to propagating modes from the middle atmosphere, it is possible that the modulation observed in this component could effectively have
sources in the mechanisms discussed. On the other hand, this does not apply to the diurnal tide which is basically of local origin, and to the other part of the semidiurnal tide, also mostly of local origin. The spatial limitations, both in the vertical and the horizontal, of the current data set preclude any further inferences on the applicability of the mechanisms suggestcd in the fast two referenced papers. It could be of great value if the model runs could be extended well into the thermosphere. As yet, the major modelfing efforts of the upper the~osphere have not taken into consideration the issue of tidal variability. on the time scales considered here. In CANZIANI (this issue), another mechanism is considered, notwithstanding the possible and non-trivial extension of middle atmosphere models to the upper atmosphere. The mechanism to be considered is the non-linear wave-wave interaction. This mechanism requires the existence of low frequency oscillations or waves in the region. These have been effectively detected with non-negligible power levels in the spectra of the meridional wind (GIRALOEZand CANZIANI,1992).
REFERENCES BERNARDR. CAIXZIANI P. 0. and GIRALDEZA. E. CANZIANIP. 0. and GIRALIXZ A. E. CAXZIANIP. O., GIRAL~EZA. E. and PLJIGL. CAVALIERID. J. CEVOLANG. DICKINSON R. E., RIDLEY E. C. and ROBLER. G. FELLOUSJ. L., BERNARDR., MASSEBOEUF M.. GLASSM. and SPIZZICHINO A. FELLOUSJ. L., CEVOLANIG., KINGSLEYS. P. and MC’LLEKH. G. FESENC. G., DICKINSON R. E. and ROULER. G. FESENC. G., RUBLE R. G. and RIIXEY E. C. FESENC. G., ROBLER. G. and RIPLEY E. C. FONES J. M. F~KBESJ. M. FORBESJ. M. and GARRETTH. B. FRASERG. J., CODRESCUM. and HALL T. J. GIRALDEZA. E., BOLDESU. and COLMANJ. GIRALDEZA. E. and CANZIANIP. 0. GLASS M., BERNARDR.. FELLOUSJ. L. and MASSEBOEUF M. LINVZENR. S. REES D., FULLER-R•WELLT. J., QUEFANS., MOFFETTR. J. and BAILEYG. J. SALBII.M. L. TEITELBAUM H. and BLAMONT J. E. TE~TIZLBAC’M H., VIAL F.. MANSONA. H., GIKALDEZA. E. and MASSEBOEUF M. VIAL F., FORBESJ. M. and MIYAHARAS. WALDOCKJ. A. and JONEST. B. WALTERSCHEID R. L.
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