Based on CACTUS (Capteur Accélérométrique Capacitif Triaxial Ultra Sensible) microaccelerometer measurements it has been demonstrated that — after taking into account all effects included in the MSIS'86=CIRA'86 (COSPAR, 1988) model - there are residual fluctuations in the density of the upper atmosphere much larger than that the accuracy of the measurements can account for. These fluctuations are attributed to some kind of wave activity (Illés-Almár, 1993, Illés-Almár et al. 1996a). The average deviations from a model are considered as a measure of the amplitude of the waves in the atmosphere and are analysed as a function of geomagnetic coordinates, altitude and local solar time, in order to identify possible wave sources either in the lower lying atmosphere or in the thermosphere/ionosphere system. As a first step, the present investigation intends to make a map of the wave pattern by this method

Almár, Iván

Bencze, P.

Illés-Almár, Erzsébet

Repository of the Academy's Library

Pergamon Adv. Space Res. Vol. 20, No. 6, pp. 1273-1276.1997 01997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/97 $17.00 + 0.00 PE SO273-1177(97~0785-0 ~VE~TIGATION OF THE DENSITY WAVE ACTIVITY IN THE THERMOSPHERE ABOVE 220 KM E. Ilk%-Aim&r*, I. Alm&* and P. Bencze** *Konkoly Observatory, H-1525 Budapest, P.O.B. 67, Hungary **Geodetic and Geophysical Research Institute, Hungarian Academy of Sciences, H-9401 Sopron, P.O.B. 5, Hungary ABSTRACT Based on CACTUS (Capteur Accelerometrique Capacitif Triaxial Ultra Sensible) microaccelerometer measure- ments it has been demonstrated that - after taking into account all effects included in the MSIS%=CIRA’86 &XW’AR. 1988) model - there are residual fluctuations in the density of the upper atmosphere much larger than that the accuracy of the measurements can account for. These fluctuations are attributed to some kind of wave activity fIll&Almdr, 1993, IlIPs-Alm& et al. 1996a). The average deviations from a model are considered as a measure of the amplitude of the waves in the atmosphere and are anaiysed as a function of geomagnetic coordinates. altitude and tocal solar time, in order to identify possible wave sources either in the lower lying atmosphere or in the thermosphere/ionosphere system. As a first step, the present investigation intends to make a map of the wave pattern by this method. 01997 COSPAR. Published by Elsevier Science Ltd. MEASUREMENTS AND METHOD Thermospheric density data (p) were derived from the CACTUS microaccelerometer measurements (Bar/&r et al. 1975). The time interval of the measurements was 1975-1979 (fow and rising solar activity), the height interval 220-700 km, the geomagnetic latitude (pPgeom) varied between -t-40” and -40” as the inclination of the orbit was only 30”. The model used was our new upper atmospheric model (d&ISIS, IKs-Alm& et ai. 1996bf: P dMSlS = p MStS’86 + p MSfs86 [ b(h)DsT + c(h)] where b(h)=A,+Bb~h+Cb~h2=0.00719138-4.88831~10-5*h~5.12381~10-8’hZ c(h) = A, + 8, l h = -0.21 + 0.000355 l h Dst is the axisymmetric component of the geomagnetic disturbance field the height (h) is measured from the surface of the Earth in km. The average deviation was defined by: 6 Cjlnf-lnfl = n where f = (pcAms I pmode’) f 1 and has been considered as a measure of the amplitude of the waves, especially of internal gravity waves in the atmosphere. In different heights the 6 values were calculated in appropriate altitude intervals to assure a more or less similar number of points for averaging. Quiet and storm time data were selected on the basis of the Dst curve (daily mean of hourly Dst values as a function of time), Quiet time data contain 29 time intervals of 100 days total length 1273 1274 EL ~l~s-Alrn~ et aL where the Dst curve remained constant in the vicinity of the maximum value at least for several days. Storm time data contain the observations of the 96 hours (4 days) following Dst minima of 2 1 quick geomagnetic disturbances where the descending branch of the Dst curve was steep enough to reach the Dst minimum in less than two days. The 8 average values have been investigated as function of different parameters: - geomagnetic activity level - altitude - local solar time (LEST) - latitude (rp) in So intervals - geoma~etic latitude - season. RESULTS As can be seen on Figure 1 and 2, the height dependence of 6 is an almost monotone increasing function of altitude that is generally steeper above 550-600 km. As regards the dependence on geomagnetic activity, the 8 values are dominantly larger on disturbed days under 550-600 km than on quiet days, but the tendency is reversed over 550- 600 km (except in spring and in autumn). The dependence of 6 on latitude is demonstrated in Figure 1 for disturbed and quiet days separately, There is no dependence on latitude on quiet days below 500 km while the situation is different on disturbed days. The 6 values separated according to season (taking into ac- count measurements 15” away from the equator only) are plotted on Figure 2 as a function of height. Below 550-600 km the average deviation is always larger on disturbed days white above 550-600 km the average deviation is larger on quiet days in the winter and summer periods but smaller in spring and autumn. In Figure 3 there is a plot of 6 as a function of LST for different heights for disturbed and quiet days separately. There is a definite dependence on LST with a maximum in the earIy hours and with a minimum in the daytime both for quiet and for disturbed days. On disturbed days, however, 6 values are targer than on quiet days except in the midnight hours at higher altitudes. DISCUSSION AND CONCLUSIONS wage deviations versus height epamed accodby 0 l8tiade A 191 6 25-300 0.1 I zil-250 0.1 . s-20* a.1 . 10-150 a.1 . 5-100 a.1 _ O-50 I total a: : Il. * ~ 200 300 height [km) 600 700 Fig.1. The height variation of the wave amplitude for different latitudes on quiet and disturbed days respectiveiy. Error bars f 1 ci are indicated. The existence and parameters of internal atmospheric gravity waves in the neutral upper atmosphere at heights above 200 km have only sporadicai~y been studied. For the study of the density deviations it was reasonable to follow the idea of H&s ~196~), according to which the fluctuations of the wind speed, called the irregular component was jnvestigated assuming that it is related to a~ospheric gravity waves. Considering the inte~retation of the CACTUS data, it is necessary to take into account the sources of these internal gravity waves. It has been found that internal gravity waves have sources not only in the troposphere, but also in the upper atmosphere (‘for example Gossard, 197.5). It has also been revealed by Booker (2979) that gravity waves could play a role in the generation of irregularities in the F-region of the ionosphere (spead F). Recently, investigations at low latitudes have indicated that F-region irreguIarjties generally occur when irregularities are observed at the altitude of the electrojet fBlunc et al., 1996). Other studies have shown that at low latitudes there is a strong ion-neutral drag due to the plasma drift affecting the motion of the neutral atmosphere in the F-region during geomagneticai~y Density Wave Activity in the Thermosphere disturbed periods (Fagundes et al., 1996). Moreover, during geomagnetic storms the zonal temperature gradients are varying and not well expressed possibly because of the occurence of gravity waves. It is to be noted that between 300 and 550 km the seasonal variation of the magnitude of the fluctuations indicates a summer maximum in geomagnetically quiet times and a winter maximum in geomagnetically disturbed times (Figure 2). The fact that the average deviation is generally larger on disturbed days hints at the conclusion that the increasing wave amplitude must be somehow connected to the geomagnetic activity effect 6 0.1 I 0.1 0.1 0 0.1 0.1 0.1 C a ; ii . verage deviations versus height eparated according to seasons :aking into account measurements fCp2W) - quiet ---storm - at least below 550 km. 6 0.1 1 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0’ .I 0.’ 0.’ average deviations versus LST separatad according to height - quiet --- etorm IJ height (km) 650 km 600 km 550 km 500 km ,450 km ’ 400 km .375 km .350 km .325 km -300 km .290 km -280 km - 270 km ‘260 km ‘250 km 0 6 12 18 24 6 LST (hours) 24 Fig. 3. The diurnal variation of the wave ampli- tude for different altitudes on quiet and disturbed days respectively. The O-24 hours interval is repeated twice to recog- nize the night time behaviour more clearly. Error bars flo are indicated. f 200 300 height (km) 600 1275 winter autumn spring total I Fig. 2. The height variation of the average deviations for different seasons on quiet and distur- bed days respectively. Error bars +lo are indicated. Gravity waves can be generated in the upper atmosphere above 100 km by wave-wave interaction as a result of the nonlinearity of atmospheric tidal waves. Further source of gravity waves at low latitudes can be the fluctuations of the equatorial electrojet (Knudsen, 1969, Chimonas, 1970). Our results indicate that in the height range between 300- 550 km there may be two different sources of the density fluctuations. One of the sources is limited to equatorial latitudes < 15”, the other occurs at latitudes >20” (Figure 1). The equatorial sources can be fluctuations of the intensity of the equatorial electrojet. Namely, the waves generated by the intensity fluctuations of the equatorial electrojet with periods from 40 to 50 minutes propagate both upwards and horizontally away from the source (Knudsen, 1969, Chimonas, 1970). 1276 E, Ill&-Alis es al. The source appearing in the latitudinal range 20” - 30’ might be attributed to wave-wave interaction due to the diurnal tide becoming nonlinear, or the residuals am small scale and small amplitude TIDs. As it is known, small scale TIDs as gravity waves can be ettributed to phenomena in the troposphere (meteorological fronts, jet streams etc.), but the propagation of which is affected by the wind system and attenuation in the stratosphere. Thus, the appearance of small scale TIDs does not depend on geomagnetic activity, they are always present. The same argument can be brought up in connection with the wave-wave internetion related to the nonline~i~ of the diurnal tide. The probability of both wave-wave interaction and small scale TIDs show diurnal changes. There are signatures of both of these sources, as the amplitude of the residuals indicates diurnal type variations (Figure 3). The amplitude of the fluctuations is, however, larger in geomagnetically disturbed periods and the disturbed daytime values are also larger than the corresponding quiet time values. It seems that the residuals attributed to gravity waves are not related to atmospheric disturbances originating in the aurora1 zones, since their magnitude does not increase with increasing geomagnetic latitudes above 30”. The fact, however, that the amplitude of the ~uctuations increases to 30°, hints at a possible connection rather with the equatorial ionospheric anomaly. In this case, namely, the fluctuations can be related to the descending branch of the plasma stream in a narrow latitudinal range at about 20° on both sides of the equator and in the time interval from 06 to 20 h. The development of the equatorial anomaly depends on the eastward zonal electric field, which in turn is related to geomagnetic activity through the z component of the interplanetary magnetic field (Fejer, 1986). The authors are indebted to F. Barlier for the CACTUS data and to P. Decsy, A. Hall, M. Nagy and J. Nuspl for their able help in preparation of the material of this paper. Barlier, F., J. Bouttes, M. Delattre, A. Olivero, P. Contensou, Experimentation on vol sur satellite d’un accelerometre de tres haute sensibilite. Compt. Rend. Acud. SC. Paris, 281 B, 145 (1975). Blanc, E., B. Mercandalli, E. Houngninou, Kilometric irregularities in the E and F regions of the daytime equatorial ionosphere observed by a high resolution HF radar. Geo@ys_ Res. Letters, 23, 645-648 (1996). Booker, H. G,, The role of acoustic gravity waves in the generation of spread F and ionospheric scintillation. J. ~~~~s. tear. Piatys., 41, Sol-515 (1979). Chimonas, G., Infrasonic waves generated by aurora1 currents. Planet. space Sci., l&591-598 (1970). COSPAR International Reference Atmosphere: 1986, Part I: Thermosphere models, ed.: D. Rees, Adv. Space Res. 8, No. 5-6 (1988) Fagundes, P. R., J. A. Bittencourt, Y. Sahai, H. Takahashi, N. R. Teixeira, Thermospheric-ionospheric coupling during geomagnetic storms at low latitudes. Paper Cl .I-0023,3 1st Scientific Assembly of COSPAR, Birmingham, July 15-20 (1996). Fejer, B: G., in: SoEar Wind - ~ag~etasp~ere Coupling, p. 5 19., eds.: Y. Kamide and J. A. Slavin, Terra Sci. Publ, Co., Tokyo (I 986). Gossard, E. E., W. H. Hooke, Waves in the Atmosphere. pp. 418-420, Elsevier, Amsterdam (1975)” Hines, C. O., Internal atmospheric gravity waves at ionospheric heights. Canad. J. Phys., 38, 1441-1481 (1960). Ill&+Alm&r, E., Separation of the atmospheric geomagnetic effect of aurora1 and ring current origin on the basis of their diurnal course II. 7th Scientific Assembly, Aug. 8-20. 1993, Buenos Aires, IAGA Bulletin No 55, Part C: Abstracts, p. 23 (1993). Ill&+Almdr, E., I. Almar, P. Bencze, Observational results hinting at the coupling of the thermosphere with the ionosphere/magnetosphere system and with the middle atmosphere, A& space Res. 18, (3)45-(3)48 (1996a). Ill&+Almiu, E., I. Alml, P. Bencze, CIRA’86 supplemented by a Dst dependent term to improve the modelling of the geomagnetic effect in the equatorial region. Paper D 0.7-0023, XxX1. General Assembly of COSPAR, Birmingham, July 15-20 (1996b). Knudsen, W. C., Neutral atmosphere wave generation by the equatorial electrojet. J. Geophyx Res., 74, 4191-4192 (1969). 

Investigation of the density wave activity in the thermosphere above 220 km

Abstract

Similar works

Full text

Available Versions

Repository of the Academy's Library