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Solar activity effects on the equatorial thermosphere temperature profile
Adv. Space Res. Vol. 20, No. 6, pp. 1191-l 195,1997 81997 COSPAR. Publishedby Elsevier ScienceLtd. All ri&ts reserved Printedin &eat Britain 0273-1177197 $17.00 + 0.00 AI: s~73-11~(~~771-0
SOLAR ACTIVITY EFFEXZTSON THE EQUATURIAL, THE~OSP~~ ~MPE~~~ PROFILE C. Arduini, G. Laneve and L. Nobile ~n~verszt~di Rma “La Sapienm”, Centro di Ricerca Progretto San Marco, Via SalariaTW, 00138 Ropna, Italy
ABSTlUCT In this paper we present the effects ofsolar activity on the temperature profiles of the equatorial thermosphere as derived &om the neutral density data collected by the San Marco 5 (SMS) satellite. This satellite flew during the increasing part of the solar cycle 22 (198X). It had a quasi-equatorial orbit, with ~cl~ation lower than 3 O.The range of rn~~~en~, from April to December, allows the inference of seasonal and diurnal effects on the temperature profiles. The density data are collected every second along arcs of orbit lasting up to 50 minutes. The ana1ysi.sof these densities has been already partialIy presented and provided evidence for several interesting features, in particular the vertical structure of the diurnal harmonic content and its seasonal variations. The temperatures derived Tom the same data set provide a useful complement to this picture. The SM5 satellite carried on board 5 instruments for studying the equatorial ionosphere and thermosphere, among them, the Drag Balance Instrument (DBI) for measuring the neutraI density and the Ion Drift Meter and Potential Retarding Analyzer (IVI) that allow the evaluation of ions ~oncen~ation, velocity and Tommie. It is possible, therefore, to compare the neutral temperature derived from the neutral density data with the ion temperature given by the IVI. Q1997 COSPAR. published by Elsevier Science Ltd.
~TRO~U~TIO~ One of the most important structural parameters ofthe thermosphere is the neutral temperature. It is also one of the more difficult parameters to measure. Knowledge of the temperature profiles in the ~e~osphere is irn~~t since this parameter is directly correlated with the energy input. Thus its ,variation is of great importance in ~derst~ding the energetics and dynamics ofthe thermosphere. Any variation on this energy input is reflected in the temperature profiles. On the other hand, although the solar EWAJV radiation is the primary source of the ~e~osphere and, hence, drives most of the processes in this region, our ~owledge of the absolute values of the photon fluxes and its variability is still lacking, (G. Schmidtke et al., 1989). In this paper we present some results on the molecular temperatures as deduced Corn S. Marco 5 neutral density data. The region of interest, from 200 to 400 km, is the site of large XUV, EUV radiative absorption. Measurement of F10.7 is not an exact measure of these radiations in this region (the solar 10.7 cm radiation is generated within the lower corona, in a layer situated somewhat higher than the sources of EUV emission (chronospheric region above the sunspot areas) (II. Votland, 1988)). DATA ANALYSIS This
paper is based on the San Marco 5 (SMS) neutraI density data. The technique to obtain temperature profile using density data is not original (L. I&o&o, 1969) but probably due to the duration of some passes (half orbit 50 minutes) and the latitude constance this is the first thne profiles of the temperate in the ~e~osphe~ without any statistical analysis but in an orbit-to-orbit fashion have been obtained. The neutral thermospheric density was measured with the SMS Drag Balance Instrument (DBI) t?om Aprii to December 1988. The measurement is continuous over orbit sections lasting up to 50 minutes. A coIlection of several hundred passes giving density versus time has been produced include the co~spon~g g~~aphi~ ~oord~~s deduced from the NORAD orbital elements and the orbital propagator as a function of time. An overall accuracy of 3% has been estimated for the relative variations (Arduini et al., 1992). The resolution in space and time can be evaluated on the basis of the integration time of the numerical filter, equal to the spin period, that is 10 set = 80 km along the path and < I km on average in altitude. The analysis presented in this paper starts ftom a collection of the orbital passes described above and spans the entire spacecraft lifetime. 1191
Our original data are time histories ofthe neutral density. The density variation with the Universal Time can be expressed in terms of the solar local time and satellite altitude variations as: j ?!?= ~l?~z+dp.dz at az at dr at
where T is the orbital period+Ai, & and Oi ~p~~d~~ variation with local time and
phases and frequencies of the harmonic terms m~resent~g the diurnal density
where p is the semilatum rectum, e the orbit eccentricity, p the gravitational constant and @the true anomaly. Ass~in~ @pieally Az = 40 km the variation of p is aronnd the 20%, of raround 1.5 [hours] and of @around 20 [deg], at the orbit perigee where we have the worst conditions. This method allows the local time correction to be performed without using any model not strictiy based on our data base. In fact, the diurnal density variation with solar local time represented by a fourth order Fourier series has been previously obtained using DBI data with a 2 km resolution (C. Ardumi et al., 1996). The problem of the solar local time effect on the neutral density is less spout during the night, when the density variation with solar local time is at lowest. However, as is evident, in the following, the night-side sector is also the most interesting from the point of view of highiy unpredictable density variations. The values of I$, obtained moving upward with a 1 km step are fitted with a mean least square procedure. The pressure scale height q is then deduced from H,using the basic equations defining the atmospheric structure (perfect gas law and hydrostatic equilibria). When the pressure scale height value is known, t~~osphe~c tern~a~e versus altitude is found using the scale height defnitlon H, = $QJ$@ where R is the un&ersal gas constant, g(z) is the ~vitatio~a1 a~~ele~t~on and assuming as knom the molecular weight M(z) (we actually used the MSIS86 model for this value). A comparison between t~pem~s deduced fkom density data and those obtained by IVI, that is, an event also flown on SM5 satellite, has been done. This ins~ent consists ofa Retarding Potential Analyzer (RPA) and an Ion Drift Meter (IDM) provi~ng every 10 seconds me~~ement ofthe total ion concentration and ion temperature. These data have been kindly provided by the University of Texas, Dallas. A lMODEL FROM SM5 DATA A simple empirical model (SI-IDM) has been constructed by fitting the SM5 data. The model assumes an exponential behavior of the pressure scale height with altitude: $$ = H,
ffdao RI
where H0 is the scale height value at z, = f 80 km, whereas iio= I24 km is a constant consistent with the reference ahitude G, The dens& variation with aiti~de becomes:
This rel~t~o~hip has been used for fitting the density curves &om SM5 assuming as free parameter H0 and correlating p0 with H0 by using the following relationship:
where H& and phn are mean constant valuesoffi, and p* In this way all the effects of solar flux, geomagnetic activity, local solar time, etc. are accounted for by the I$ variations driven by these quantities. ~VA~UATIUN OF THE SOLAR HEAT INPUT Using the temperature profiles given by the SHDM model one can estimate the solar heat input (in W/m3) by applying the equation of vertical heat conduction:
Equatorial Thermosphere Temperature Profile
1193
k is the coefficient of molecular heat condu~~~~. Neglecting the variation of this parameter with height (I-I. Volland, 1988) we can write:
where
q =
-kaZT= _d.( az2
!$)[l+z_f$l”‘-2
and for the variation of q with altitude:
acr=
(8)
a2
We define the term s = I/q+/&
and we find: s
=
_-L( I:n_,).[,+~l_l
(9)
As can be seen this quantity decreases as the solar flnx increases, in accordance with the term representing the inverse of the scale height of the solar energy released (H. Volland, 1988). RESULTS AND DISCUSSION Figs. 1 and 2 show the temperatures obtained using the SHDM model with the corresponding values (ion temperatures) measured by the IV1 instrument on board of SM5 satellite and the instantaneous temperature profiles deduced thorn the DBI density data. During the night, when photoioni~tion ceases, thermal equilib~~ is restored, and ion and neutral temperatures are quite similar. Fig. 3 shows model and IVI temperatures for different geophysical conditions (different F10.7 mean and daily and solar local time, both passes are nocturnal). We can see that the model reproduces the solar flux effects on the temperature profiles well. Now, comparing “instantaneous density (and/or temperature) profiles” with the model it is possible to see some short period (day to day and/or orbit to orbit) effects not accounted for using the usual geophysical parameters F10.7, Ap, etc. This phenomenon was first identified through comp~isons between the vitreous btrlge profiles (density v~ation with iocal solar time) with the statistical 24 hour density variation obtained by means ofthe perigee drift due to the orbital perturbations with -50 days period, (G. Laneve et al., 1996). The observed fluctuations are reflecting a dependence on Universal Time and Longitude different from the simple dependence on the combination corresponding to the local time. We could postulate a longitude dependence (through orography?) and/or a dependence on a relatively short period variation of the heat source, not reflected by F10.7. Very noticeable are the discrepancies from the averaged profiles in the zone of the nocturnal min~um. Here the fluct~tion appears to be larger than elsewhere, with strong secondary maxima (Midnight Density Maximum, MDMJ occurring at various times. Temperature gradients with altitude larger than normal are cleariy observable in the plots of figs. 4 and 5. Possibly, this result could be evidence of the fact that fast, highly altitude dependent dynamics, dominate the behavior of the thermosphere in the night sector. A reason for this could be the absence, during the night, of the strong solar energy input that is the main driving source of the a~ospheri~ diurnal characteristics. During the night, dynamics and dissipative effects, geomagnetical effects and inputs ffom other sources are of comparable magnitude. Further, EUV variations at periods shorter than a solar day could excite the higher harmonics of the diurnal density variation (3” and 4*) and we showed elsewhere (G. Laneve et al., 1996 and C. Arduini, F. A. Herrero et al., 1996) that the 3’d and 4%”harmonics must be accounted for in order to completely characterize the MDM. Thus, the EUV variations could act mainly on these harmonics that influence especially the density vaiues during the night. Even the molecular dissipation {viscosity and thermal conduction) have been shown to act mainly on these two harmonics and especially on the 4&,,(G.Laneve et al., 1995). Figs. 6 shows the inverse of the Solar Energy Flux Height Scale (s) versus the altitude for conditions at two different solar times. It is clear that this parameter decreases as the solar flux increases. If we evaluate the ratio q/p we find a value around IO [W/Kg] that is a value plausible in the light of those usually given in the available literature which is to be found between 1 and IO [W/Kg], (H. Volland, 1988). If we assume the relationship (4) as being applicable even to the night side, that is, we see q simply as a derivative with altitude of the temperature, we find, for the same ratio, values around IO0 [W/Kg]. CONCLUSlONS A mode1 (SHDM), using the S. Marco 5 neutral density data, has been developed, based on a hypothesis of exponential variation of the pressure scale height with altitude. This model reproduces quite well thermospheric temperature profiles in different conditions of solar flux, geomagnetic activity and solar local time. The model has been compared to the instantaneous neutral temperatures and to the ion temperatnre data from the IV1 ins~ent also present on board of the SM5 satellite. The ag~ement is acceptable, when the different equilibrium conditions between neutrals and ions in the dayside is taken into account. Departure of the instantaneous temperature profiles from the model are an indication of short period variability, more often present in the nightside, particularly in the form of anomalous temperature gradients. The observed phenomena have been explained postulating
1194
Fig. 1 - Comparison between modelled, deduced by DBI and measured by IV1 (ions) temperatures. Dayside conditions.
Fig. 2 - Comparison between modelled, deduced by DBI and measured by IV1 (ions) temperatures. Night side conditions.
Fig. 3 - Comparison between temperatures given by SHDM model and measured bv IV1 for different
Fig. 4 ” Variation of the inverse of the solar energy released scale height with altitude and solar flux F10.7.
Fig. 5 - Temperature profiles for pass TB280gRl. High gradients are clearly present (night side conditions).
Fig. 6 - Temperature profiles for pass TB2848Rl. High gradients are clearly present (night side conditions).
Equatorial ThermosphereTemperatureProfile
1195
a short period variation of the EUV radiations that could excite the higher harmonics of the diurnal neutral density variation with solar local time. Although, different interpretations of the above shown phenomena could be given, no doubt that an effort must be done to carry out future experiments dedicated to measure both the thermospberic parameters (neutral and plasma density and temperature, etc.) and the input of solar energy. Some profiles of the scale height: of the energy released in the atmosphere, as computed from the temperature profiles, have also been shown. REFERENCES Arduini C., U. Ponzi, A. Agneni, G. Laneve, and D. Mortari, S. Marco V Data Reduction Process and Accuracy Assessment, Cl&f Inrerptal Document, N 502 ( 1992) Arduini C., L. Broglio, U. Ponzi, and G. Laneve, S. Marco V Drag Balance Neutral Density Compared to the Models, A&. Space Rex, 18,351 ( 1996). Arduini C.. L. Broglio, and U. Ponzi, Drag Balance Measurements in the San Marco D/L Mission, A& Space Res., 13, 185, (1993). Arduini C., G. Laneve, and U. Ponzi, The Midnight Density Maximum in the S. Marco V and the S. Marco II1 Equatorial Density Data Sets. Adv. &mm Res., 18,361, (1996a). Arduini C., F. A. Herrero, and G. Laneve, Local Time and Altitude Variation of Equatorial Thermosphere Midnight Density Maximum (MDM): San Marco Drag Balance Measurements, Geophys.Res. Letf., (1996b). Arduini C., G. Laneve. and U. Ponzi, Tidal analysis ofSan Marco V and S. Marco III density data in equatorial orbit. JournaZ o~Atrnosp~~~~ and Terrestrial Physics, ( 1997) Brogho L., Equatorial Atmospheric Density obtained from S. Marco II satellite between 200 and 3.50km. Space Research, 9,547, North Holland Publishing Comp., (1969) Forbes J. M., and H. B. Garrett, Theoretical studies of atmospheric tides. Rev. Geophys. S’pucePlays.. 17, 1951, (1979) Harris 1.. and H. G. Mayr, Diurnal variation in the The~osphere: I. Theoretical fo~u~ation, .I Geopltys. Res., 80,2X, (1985). Hedin A. E.. MSIS-86 Thermospheric Model. J. Geophys. Res, 92,4649, (1987) Herrero F. A., and N. W. Spencer, On the Horizontal Dis~ibution of the Equatorial Thermospheric Midnight Temperature Maximum and its Seasonal Variation. Geophys. Res. Lett., 9, 1179, (1982). HerreroF. A.. H. G. Mayr and N. W. Spencer, Latitudinal (seasonal) variations in the Thermospheric Midnight Temperature Maximum: A Tidal Analysis. J. Geophys. Res., 88, 7225, (1983). Herrero F. A., N. W. Spencer, and I-1.G. Mayr, Thermosphere and F-region Plasma Dynamics in the Equatorial Region. A$v.SpclceRes., 13, 1, 1201. (1992). Laneve G.. F. A. Herrero, and C. Arduini, Seasonal Variations in the Altitude Dependence of the Equatorial Midnight Density Maximum: San Marco 5 Satellite Data. American Geophys. Union Fall Meeting. Abstract on EOS, (1995). Mayr H. G.. A. E. Hedin, C. A. Reber, and G. R. Cabin, GIobal Characteristics in the Diurnal Variations of the Thermospheric Temperature and Composition. .I Geophys. Res., 79,619, (1974). Volland H, Afmospheric Tidai and Plurzetay waves, Kluwer Academic Publishers, (1988). Schmidtke G., Doll H. and Wita C., The variation of Solar EUVlw of the steeply rising Solar cycle 22 during the San Marco 5 mission. (1989) Mayr H. G.. Harris I. and Spencer N. W., Thermospheric Temperatures. J ofGeo&. l&s., 79,292l. (1974). Meriwether J. W., et al., Evidence for Orographic Wave Heating in the ~quatori~ ~e~osphe~ at Solar Minims. Geoph~s. Res. Let.. (I 995) Newton G. P.. Pelz D. T., Neutral Thermosphere Temperatures from Density Scale Height Measurements. J&u& Geophy. Res, ‘78,n. 4, 725, (1973)
SOLAR ACTIVITY EFFEXZTSON THE EQUATURIAL, THE~OSP~~ ~MPE~~~ PROFILE C. Arduini, G. Laneve and L. Nobile ~n~verszt~di Rma “La Sapienm”, Centro di Ricerca Progretto San Marco, Via SalariaTW, 00138 Ropna, Italy
ABSTlUCT In this paper we present the effects ofsolar activity on the temperature profiles of the equatorial thermosphere as derived &om the neutral density data collected by the San Marco 5 (SMS) satellite. This satellite flew during the increasing part of the solar cycle 22 (198X). It had a quasi-equatorial orbit, with ~cl~ation lower than 3 O.The range of rn~~~en~, from April to December, allows the inference of seasonal and diurnal effects on the temperature profiles. The density data are collected every second along arcs of orbit lasting up to 50 minutes. The ana1ysi.sof these densities has been already partialIy presented and provided evidence for several interesting features, in particular the vertical structure of the diurnal harmonic content and its seasonal variations. The temperatures derived Tom the same data set provide a useful complement to this picture. The SM5 satellite carried on board 5 instruments for studying the equatorial ionosphere and thermosphere, among them, the Drag Balance Instrument (DBI) for measuring the neutraI density and the Ion Drift Meter and Potential Retarding Analyzer (IVI) that allow the evaluation of ions ~oncen~ation, velocity and Tommie. It is possible, therefore, to compare the neutral temperature derived from the neutral density data with the ion temperature given by the IVI. Q1997 COSPAR. published by Elsevier Science Ltd.
~TRO~U~TIO~ One of the most important structural parameters ofthe thermosphere is the neutral temperature. It is also one of the more difficult parameters to measure. Knowledge of the temperature profiles in the ~e~osphere is irn~~t since this parameter is directly correlated with the energy input. Thus its ,variation is of great importance in ~derst~ding the energetics and dynamics ofthe thermosphere. Any variation on this energy input is reflected in the temperature profiles. On the other hand, although the solar EWAJV radiation is the primary source of the ~e~osphere and, hence, drives most of the processes in this region, our ~owledge of the absolute values of the photon fluxes and its variability is still lacking, (G. Schmidtke et al., 1989). In this paper we present some results on the molecular temperatures as deduced Corn S. Marco 5 neutral density data. The region of interest, from 200 to 400 km, is the site of large XUV, EUV radiative absorption. Measurement of F10.7 is not an exact measure of these radiations in this region (the solar 10.7 cm radiation is generated within the lower corona, in a layer situated somewhat higher than the sources of EUV emission (chronospheric region above the sunspot areas) (II. Votland, 1988)). DATA ANALYSIS This
paper is based on the San Marco 5 (SMS) neutraI density data. The technique to obtain temperature profile using density data is not original (L. I&o&o, 1969) but probably due to the duration of some passes (half orbit 50 minutes) and the latitude constance this is the first thne profiles of the temperate in the ~e~osphe~ without any statistical analysis but in an orbit-to-orbit fashion have been obtained. The neutral thermospheric density was measured with the SMS Drag Balance Instrument (DBI) t?om Aprii to December 1988. The measurement is continuous over orbit sections lasting up to 50 minutes. A coIlection of several hundred passes giving density versus time has been produced include the co~spon~g g~~aphi~ ~oord~~s deduced from the NORAD orbital elements and the orbital propagator as a function of time. An overall accuracy of 3% has been estimated for the relative variations (Arduini et al., 1992). The resolution in space and time can be evaluated on the basis of the integration time of the numerical filter, equal to the spin period, that is 10 set = 80 km along the path and < I km on average in altitude. The analysis presented in this paper starts ftom a collection of the orbital passes described above and spans the entire spacecraft lifetime. 1191
Our original data are time histories ofthe neutral density. The density variation with the Universal Time can be expressed in terms of the solar local time and satellite altitude variations as: j ?!?= ~l?~z+dp.dz at az at dr at
where T is the orbital period+Ai, & and Oi ~p~~d~~ variation with local time and
phases and frequencies of the harmonic terms m~resent~g the diurnal density
where p is the semilatum rectum, e the orbit eccentricity, p the gravitational constant and @the true anomaly. Ass~in~ @pieally Az = 40 km the variation of p is aronnd the 20%, of raround 1.5 [hours] and of @around 20 [deg], at the orbit perigee where we have the worst conditions. This method allows the local time correction to be performed without using any model not strictiy based on our data base. In fact, the diurnal density variation with solar local time represented by a fourth order Fourier series has been previously obtained using DBI data with a 2 km resolution (C. Ardumi et al., 1996). The problem of the solar local time effect on the neutral density is less spout during the night, when the density variation with solar local time is at lowest. However, as is evident, in the following, the night-side sector is also the most interesting from the point of view of highiy unpredictable density variations. The values of I$, obtained moving upward with a 1 km step are fitted with a mean least square procedure. The pressure scale height q is then deduced from H,using the basic equations defining the atmospheric structure (perfect gas law and hydrostatic equilibria). When the pressure scale height value is known, t~~osphe~c tern~a~e versus altitude is found using the scale height defnitlon H, = $QJ$@ where R is the un&ersal gas constant, g(z) is the ~vitatio~a1 a~~ele~t~on and assuming as knom the molecular weight M(z) (we actually used the MSIS86 model for this value). A comparison between t~pem~s deduced fkom density data and those obtained by IVI, that is, an event also flown on SM5 satellite, has been done. This ins~ent consists ofa Retarding Potential Analyzer (RPA) and an Ion Drift Meter (IDM) provi~ng every 10 seconds me~~ement ofthe total ion concentration and ion temperature. These data have been kindly provided by the University of Texas, Dallas. A lMODEL FROM SM5 DATA A simple empirical model (SI-IDM) has been constructed by fitting the SM5 data. The model assumes an exponential behavior of the pressure scale height with altitude: $$ = H,
ffdao RI
where H0 is the scale height value at z, = f 80 km, whereas iio= I24 km is a constant consistent with the reference ahitude G, The dens& variation with aiti~de becomes:
This rel~t~o~hip has been used for fitting the density curves &om SM5 assuming as free parameter H0 and correlating p0 with H0 by using the following relationship:
where H& and phn are mean constant valuesoffi, and p* In this way all the effects of solar flux, geomagnetic activity, local solar time, etc. are accounted for by the I$ variations driven by these quantities. ~VA~UATIUN OF THE SOLAR HEAT INPUT Using the temperature profiles given by the SHDM model one can estimate the solar heat input (in W/m3) by applying the equation of vertical heat conduction:
Equatorial Thermosphere Temperature Profile
1193
k is the coefficient of molecular heat condu~~~~. Neglecting the variation of this parameter with height (I-I. Volland, 1988) we can write:
where
q =
-kaZT= _d.( az2
!$)[l+z_f$l”‘-2
and for the variation of q with altitude:
acr=
(8)
a2
We define the term s = I/q+/&
and we find: s
=
_-L( I:n_,).[,+~l_l
(9)
As can be seen this quantity decreases as the solar flnx increases, in accordance with the term representing the inverse of the scale height of the solar energy released (H. Volland, 1988). RESULTS AND DISCUSSION Figs. 1 and 2 show the temperatures obtained using the SHDM model with the corresponding values (ion temperatures) measured by the IV1 instrument on board of SM5 satellite and the instantaneous temperature profiles deduced thorn the DBI density data. During the night, when photoioni~tion ceases, thermal equilib~~ is restored, and ion and neutral temperatures are quite similar. Fig. 3 shows model and IVI temperatures for different geophysical conditions (different F10.7 mean and daily and solar local time, both passes are nocturnal). We can see that the model reproduces the solar flux effects on the temperature profiles well. Now, comparing “instantaneous density (and/or temperature) profiles” with the model it is possible to see some short period (day to day and/or orbit to orbit) effects not accounted for using the usual geophysical parameters F10.7, Ap, etc. This phenomenon was first identified through comp~isons between the vitreous btrlge profiles (density v~ation with iocal solar time) with the statistical 24 hour density variation obtained by means ofthe perigee drift due to the orbital perturbations with -50 days period, (G. Laneve et al., 1996). The observed fluctuations are reflecting a dependence on Universal Time and Longitude different from the simple dependence on the combination corresponding to the local time. We could postulate a longitude dependence (through orography?) and/or a dependence on a relatively short period variation of the heat source, not reflected by F10.7. Very noticeable are the discrepancies from the averaged profiles in the zone of the nocturnal min~um. Here the fluct~tion appears to be larger than elsewhere, with strong secondary maxima (Midnight Density Maximum, MDMJ occurring at various times. Temperature gradients with altitude larger than normal are cleariy observable in the plots of figs. 4 and 5. Possibly, this result could be evidence of the fact that fast, highly altitude dependent dynamics, dominate the behavior of the thermosphere in the night sector. A reason for this could be the absence, during the night, of the strong solar energy input that is the main driving source of the a~ospheri~ diurnal characteristics. During the night, dynamics and dissipative effects, geomagnetical effects and inputs ffom other sources are of comparable magnitude. Further, EUV variations at periods shorter than a solar day could excite the higher harmonics of the diurnal density variation (3” and 4*) and we showed elsewhere (G. Laneve et al., 1996 and C. Arduini, F. A. Herrero et al., 1996) that the 3’d and 4%”harmonics must be accounted for in order to completely characterize the MDM. Thus, the EUV variations could act mainly on these harmonics that influence especially the density vaiues during the night. Even the molecular dissipation {viscosity and thermal conduction) have been shown to act mainly on these two harmonics and especially on the 4&,,(G.Laneve et al., 1995). Figs. 6 shows the inverse of the Solar Energy Flux Height Scale (s) versus the altitude for conditions at two different solar times. It is clear that this parameter decreases as the solar flux increases. If we evaluate the ratio q/p we find a value around IO [W/Kg] that is a value plausible in the light of those usually given in the available literature which is to be found between 1 and IO [W/Kg], (H. Volland, 1988). If we assume the relationship (4) as being applicable even to the night side, that is, we see q simply as a derivative with altitude of the temperature, we find, for the same ratio, values around IO0 [W/Kg]. CONCLUSlONS A mode1 (SHDM), using the S. Marco 5 neutral density data, has been developed, based on a hypothesis of exponential variation of the pressure scale height with altitude. This model reproduces quite well thermospheric temperature profiles in different conditions of solar flux, geomagnetic activity and solar local time. The model has been compared to the instantaneous neutral temperatures and to the ion temperatnre data from the IV1 ins~ent also present on board of the SM5 satellite. The ag~ement is acceptable, when the different equilibrium conditions between neutrals and ions in the dayside is taken into account. Departure of the instantaneous temperature profiles from the model are an indication of short period variability, more often present in the nightside, particularly in the form of anomalous temperature gradients. The observed phenomena have been explained postulating
1194
Fig. 1 - Comparison between modelled, deduced by DBI and measured by IV1 (ions) temperatures. Dayside conditions.
Fig. 2 - Comparison between modelled, deduced by DBI and measured by IV1 (ions) temperatures. Night side conditions.
Fig. 3 - Comparison between temperatures given by SHDM model and measured bv IV1 for different
Fig. 4 ” Variation of the inverse of the solar energy released scale height with altitude and solar flux F10.7.
Fig. 5 - Temperature profiles for pass TB280gRl. High gradients are clearly present (night side conditions).
Fig. 6 - Temperature profiles for pass TB2848Rl. High gradients are clearly present (night side conditions).
Equatorial ThermosphereTemperatureProfile
1195
a short period variation of the EUV radiations that could excite the higher harmonics of the diurnal neutral density variation with solar local time. Although, different interpretations of the above shown phenomena could be given, no doubt that an effort must be done to carry out future experiments dedicated to measure both the thermospberic parameters (neutral and plasma density and temperature, etc.) and the input of solar energy. Some profiles of the scale height: of the energy released in the atmosphere, as computed from the temperature profiles, have also been shown. REFERENCES Arduini C., U. Ponzi, A. Agneni, G. Laneve, and D. Mortari, S. Marco V Data Reduction Process and Accuracy Assessment, Cl&f Inrerptal Document, N 502 ( 1992) Arduini C., L. Broglio, U. Ponzi, and G. Laneve, S. Marco V Drag Balance Neutral Density Compared to the Models, A&. Space Rex, 18,351 ( 1996). Arduini C.. L. Broglio, and U. Ponzi, Drag Balance Measurements in the San Marco D/L Mission, A& Space Res., 13, 185, (1993). Arduini C., G. Laneve, and U. Ponzi, The Midnight Density Maximum in the S. Marco V and the S. Marco II1 Equatorial Density Data Sets. Adv. &mm Res., 18,361, (1996a). Arduini C., F. A. Herrero, and G. Laneve, Local Time and Altitude Variation of Equatorial Thermosphere Midnight Density Maximum (MDM): San Marco Drag Balance Measurements, Geophys.Res. Letf., (1996b). Arduini C., G. Laneve. and U. Ponzi, Tidal analysis ofSan Marco V and S. Marco III density data in equatorial orbit. JournaZ o~Atrnosp~~~~ and Terrestrial Physics, ( 1997) Brogho L., Equatorial Atmospheric Density obtained from S. Marco II satellite between 200 and 3.50km. Space Research, 9,547, North Holland Publishing Comp., (1969) Forbes J. M., and H. B. Garrett, Theoretical studies of atmospheric tides. Rev. Geophys. S’pucePlays.. 17, 1951, (1979) Harris 1.. and H. G. Mayr, Diurnal variation in the The~osphere: I. Theoretical fo~u~ation, .I Geopltys. Res., 80,2X, (1985). Hedin A. E.. MSIS-86 Thermospheric Model. J. Geophys. Res, 92,4649, (1987) Herrero F. A., and N. W. Spencer, On the Horizontal Dis~ibution of the Equatorial Thermospheric Midnight Temperature Maximum and its Seasonal Variation. Geophys. Res. Lett., 9, 1179, (1982). HerreroF. A.. H. G. Mayr and N. W. Spencer, Latitudinal (seasonal) variations in the Thermospheric Midnight Temperature Maximum: A Tidal Analysis. J. Geophys. Res., 88, 7225, (1983). Herrero F. A., N. W. Spencer, and I-1.G. Mayr, Thermosphere and F-region Plasma Dynamics in the Equatorial Region. A$v.SpclceRes., 13, 1, 1201. (1992). Laneve G.. F. A. Herrero, and C. Arduini, Seasonal Variations in the Altitude Dependence of the Equatorial Midnight Density Maximum: San Marco 5 Satellite Data. American Geophys. Union Fall Meeting. Abstract on EOS, (1995). Mayr H. G.. A. E. Hedin, C. A. Reber, and G. R. Cabin, GIobal Characteristics in the Diurnal Variations of the Thermospheric Temperature and Composition. .I Geophys. Res., 79,619, (1974). Volland H, Afmospheric Tidai and Plurzetay waves, Kluwer Academic Publishers, (1988). Schmidtke G., Doll H. and Wita C., The variation of Solar EUVlw of the steeply rising Solar cycle 22 during the San Marco 5 mission. (1989) Mayr H. G.. Harris I. and Spencer N. W., Thermospheric Temperatures. J ofGeo&. l&s., 79,292l. (1974). Meriwether J. W., et al., Evidence for Orographic Wave Heating in the ~quatori~ ~e~osphe~ at Solar Minims. Geoph~s. Res. Let.. (I 995) Newton G. P.. Pelz D. T., Neutral Thermosphere Temperatures from Density Scale Height Measurements. J&u& Geophy. Res, ‘78,n. 4, 725, (1973)