Comparison of atmospheric parameters derived from

Transcription

1 Radio Science, Volume 35, Number 3, Pages , May-June 2000 Comparison of atmospheric parameters derived from meteor observations with CIRA Manuel A. Cervera x and Iain M. Reid Atmospheric Radar Systems Pry. Ltd., Thebarton, South Australia, Australia Abstract. In this paper we compare monthly averages of the atmospheric parameter T/v (where T is temperature and p is pressure) derived from the decay of underdense meteor echoes with the CIRA (1986) atmospheric model. The meteor data were collected with the Buckland Park VHF radar situated - 40 km north of Adelaide, Australia. We examine the overall agreement between the meteor observations and CIRA as well as seasonal differences between the two. Comparison is made with the results of Hocking et al. [1997]. Our results are complimentary to those of Hocking et al.; our data were obtained in the Southern Hemisphere as opposed to the Northern Hemisphere. A discussion on the effect of the geomagnetic field on the diffusion of meteor trails and its effect on the measurement of atmospheric parameters is also included. We note that the geomagnetic field is a very important consideration when using meteors for the derivation of atmospheric temperatures and pressures above heights of around km. This effect is required to be taken into account above these heights as failure to do so leads to errors in the interpretation of the data. Recent researchers have avoided this problem by restricting their data to below 90 km. 1. Introduction When a particle from interplanetary space (a meteoroid) enters the Earth's atmosphere, it loses kinetic energy through collisions with the atmospheric molecules. The lost kinetic energy goes into heating the meteoroid and radiation. If the meteoroid is large enough for a given speed, density, and composition, the meteoroid will eventually reach its boiling point and start to ablate. The ablating material collides with atmospheric molecules and produces ionized gas. A trail of ionization, referred to as a meteor, is thus formed by the meteoroid, and Also at Surveillance Systems Division, Defence Science and Technology Organisation, PO Box 1, Salisbury, South Australia, Australia. 2Also at Department of Physics and Mathematical Physics, University of Adelaide, Adelaide, South Australia, Australia. Copyright 2000 by the American Geophysical Union. Paper number 1999RS /00/1999RS this may be detected by a backscatter radar. The trail, once formed, diffuses through ambipolar diffusion, and the rate of diffusion is dependent on both the atmospheric temperature and pressure as well as the species of ions present. If the electron line density of a meteor trail is low, the meteor is referred to as being underdense, and for this case the incident radio waves are scattered by individual electrons, and secondary absorptive effects may be ignored. The meteor echo will be attenuated due to the radiation scattered from the back of the trail being delayed in phase compared to that from the front. Thus, as the underdense trail diffuses, the returned echo decays, and as the meteor radius approaches A/4, the attenuation becomes severe. If the length of the trail is greater than a Fresnel zone length, Fresnel scattering applies, and it may be shown that the decay of the echo is exponential. Measurement of the rate of decay in this case yields the ambipolar diffusion coefficient upon the application of the scattering theory. Meteoroids which are massiv enough and/or have a high enough velocity can produce meteor trails with large electron line densities, and these are termed 833

2 834 CERVERA AND REID- COMPARISON OF METEOR OBSERVATIONS WITH CIRA overdense. The meteor in this case acts as a metallic cylinder with respect to the scattering of the incident radio waves, and therefore the echo does not decay. However, once the electron density has fallen low enough through diffusion, the meteor becomes underdense and, the echo then begins to decay. Conventionally, an electron line density of 10 TM electrons m - is used to classify meteors into the two regimes. This value is arrived at when the transition from underdense to overdense is defined as occur- ring when the evanescent wave amplitude is reduced to e - at the trail axis [McKinley, 1961]. However, the work of Poulter and Baggaley [1977, 1978] shows that there is not a sharp boundary between the two regimes and meteors do not become fully underdense until the electron line density falls below at least 10 3 electrons m -. Likewise, a meteor does not become fully overdense until an electron line density of at least 10 Selectronsm - is exceeded. Meteors with electron line densities between the above limits are neither fully underdense nor overdense, and the scattering process is much more complex than that greater than a Fresnel zone length, i.e., where Fresdescribed for the underdense and overdense cases. nel diffraction theory applies. First, considering the A full wave analysis using complex reflection coefficients is required. A description of this is beyond the case for no diffusion, Fresnel diffraction theory gives [McKinley, 1961] scope of this paper; the reader is referred to Poulter and Baggaley [1977, 1978]. The study of meteor decay times dates back to 2, (1) the 1950s [e.g., Huxley, 1952; Kaiser, 1953; Green- where PR is the received echo power, PT is the transhow and Neufeld, 1955]. More recent work includes mitted power, GT and GR are the transmission and that of Tsutsumi et al. [19941, Chilson et al. [1996], reception antenna gains, q is the electron line density and Hocking et al. [1007]. Hocking et al. [p. 2977] state "... recent theory [e.g., Jones and Jones, 1990; Jones, 1995] has shown that D is proportional to of the trail, ) is the wavelength, R0 is the perpendicular distance to the trail, and C and $ are the Fresnel integrals from diffraction theory. T 2/p...", where D is ambipolar diffusion coefficient, If one assumes the radial density of electrons in T is the temperature, and p is pressure. However, this relation appears in the work of McDaniel and Mason [1973] and has, in fact, been known since the 1950s when L. G. H. Huxley first pointed it out (W. the trail to be Gaussian, then one may show that the total echo power from a thin slice of trail of width ds, is given by [McKinley, 1961] G. Elford, private communication, 1995). Hocking et al. [1997] take an interesting approach in that they do not deal with D but rather T/v/, which they compare to the CIRA (1986) empirical where Da is the ambipolar diffusion coefficient, r0 is the initial radius, and dp is the echo power for case model along with lidar and satellite observations. of no diffusion and the electrons concentrated at the They found reasonable agreement between the me- axis (r0 = 0). Integration along the trail (and applyteor and CIRA comparisons with systematic differ- ences in certain months. However, we point out that the overall agreement is highly dependent on the value of the zero field reduced mobility factor of the ion species in the trail which is required to calcu- late T/v from the ambipolar diffusion coefficient, and care must be taken in the interpretation of the results. This will be discussed in the following sections. Our approach in this paper is similar to that of Hocking et al. with our data from the Southern Hemisphere (latitude of-35 ø ) complimenting Hocking et al.'s Northern Hemisphere (latitude of 43 ø) data. It is perhaps better to deal with T2/p as this quantity is proportional to Da, but we use instead T/v so that our results may be more easily compared with those of Hocking et al. [1997]. The meteor data presented here covers a period of 18 months from September 1993 to February 1995 collected by the University of Adelaide's VHF radar at Buckland Park (located some 40 km north of Adelaide). 2. Theory of Meteor Echo Decay for Underdense Trails This section will be concerned with the theory of meteor echo decay for underdense trails which are P -2.SxlO-32pTGTGR 3C2+$2 dp - dp e-(s 2 o /X2)e(-32 2z> t/x2)ds, (2) ing appropriate phase shifts) yields the echo due to the entire trail. When this is done, it is important to include the attenuation of the meteor echo power due to the finite velocity of formation effect [Peregudov, 1958; Thomas et al., 1988; Steel and Elford, 1991].

3 CERVERA AND REID: COMPARISON OF METEOR OBSERVATIONS WITH CIRA 835 The first exponential term in equation (2) causes an immediate attenuation of the received echo power owing to the finite initial radius of the trail. The second exponential term is what is of interest to us and is the time varyi'ng attenuation factor of the echo power due to the diffusion of the trail once formed. The echo decay time constant r is defined as the time taken for the echo power to decay by a factor of e -, and it is given by 2 r = 32 r2d a. (3) We rearrange this to yield the ambipolar diffusion coefficient as a function of the echo decay time, i.e., Da- 32 r2r, (4) and r is measured from the observed meteor echo. Jones [1995] has shown the radial distribution of electrons in a meteor trail is, in fact, not Gaussian but more correctly described by dense narrow core and a more diffuse central region. However, this does not affect the "decay part" of equation (2). Thus we may use equation (4) with no further consideration of the cross-sectional distribution meteor trail. of electrons in the We are now required to relate the ambipolar diffusion coefficient to the temperature T and pressure p of the atmosphere. Now, the Einstein relation for the diffusion coefficient Di of a collection of ions in a neutral gas is =, (S) where k is Boltzmann's constant, e is the electronic charge, and K is the zero field mobility factor of the ion species in the neutral gas. Although K varies with temperature, Ceplecha et al. [1998] state that this is < 20% for a change in the ambient temperature of 100 ø K. Now, the value of K is inversely proportional to the molecular concentration of the ambient gas. Thus, from the ideal gas equation, K is proportional to T/p, and we may conveniently write K in terms of a "reduced mobility" Ko defined at standard temperature and pressure: x 105 T p (6) Mason and McDaniel [1988] show that the ambipo- lar diffusion coefficient for electrons in a meteor trail when the effect of the geomagnetic field is small may be written as D = Di(I + Te/Ti), (7) where Te and Ti are the electron and ionic temperatures, respectively. Once the meteor trail forms, the electrons and ions rapidly come into thermal equilibrium with the atmosphere, and D = 2Di. Substituting this into the Einstein relation and using the zero field reduced mobility factor, the following expression for the arebipolar diffusion coefficient of me- teor trails is obtained: T 2 D, x Ko. (8) Clearly, it is important to use a value of Ko in the above expression which is representative of meteor trails. Thomas et al. [1988] use a value for Ko of 2.2 x 10-4m 2s- V-. They derived this figure from laboratory measurements of the diffusion of various ionic species (found in meteor trails) in N2 summarized by McDaniel and Mason [1973]. Jones and Jones [1990] compared values of K derived from Massey's [1971] mobility formula with laboratory measurements at standard temperature and pressure (STP) [from Mitchell and Ridlet, 1934] and found excellent agreement between the two. Chilson et al. [1996] use Massey's formula (they only refer to Jones and Jones) over the mass range of ions that are likely to be found in meteor trails (Na + - Fe +) to derive a value for Ko of 2.5 x 10-4 m 2 s - V - for meteor trails. This value is also used by Hocking et al. [ Steel and Elford [1991] were able to detect meteors up to 140 km in altitude with a radar operating at 2 MHz. They were able to do this because the echo decay times in addition to the attenuation of the echoes due to the initial trail radius and finite velocity effect (not discussed here) are greatly reduced at these low operating frequencies. Elford et al. [1997] have shown that in order for a meteoroid to ablate at these heights it must have a much lower boiling point than the typical stony/metallic meteoroids ablating at lower altitudes. In fact, the results of Elford et al. suggest that the material ablating at these altitudes must be "tarry" in nature. Thus the species of ions produced by these particles ablating will be very different from the usual meteoroids, and therefore a different zero field mobility factor will need to P

4 836 CERVERA AND REID: COMPARISON OF METEOR OBSERVATIONS WITH CIRA Diffusion Coefficient (m2s-t) Figure 1. Diffusion coefficients plotted as a function of height. Dr applies to diffusion in the plane containing the trail and the Earth's magnetic field. Dy applies to diffusion in the orthogonal direction, and its value depends on the angle between the trail and the direction of the field, as indicated on the five curves for Dy. Data supplied by W.G. Elford (private communication, 1999). be used. Researchers will need to keep this in mind when analyzing meteors observed with MF radars [e.g., Tsutsumi et at., 1999]. The last point we wish to make here is that the effect of the Earth's magnetic field on the meteor trail is a very important consideration. This problem has been analyzed in detail by Jones [1991] and is summarized by Ceptecha et at. [1998]. The diffusion of the trail in the plane containing trail and the magnetic field line Dx is not affected if the angle between the trail and field is > 20-3 ø. However, the diffusion orthogonal to the plane containing the trail and the field line Dy is greatly suppressed at altitudes above 95 km. Below this height the effect is small and is negligible below - 93 km. Thus meteor trails forming above 95 km are generally elliptical in cross section, and if the geometry is such that these trails are observed from a direction other than parallel to their major axis, a smaller value for the diffusion coefficienthan expected from equation (8) will be measured. The degree of suppression of Dy decreases from a maximum when the angle of the trail to the field is 90 ø to a minimum when the trail is aligned at - 5 ø to the field. For cases where the trail is aligned within - 2 ø of the field, both Dx and Dy will be suppressed above 95 km. Figure I shows D and Dy (at various angles from orthogonality to the plane containing the meteor trail and the magnetic field line) plotted versus height [from Ceptecha et at., 1998]. One can see immediately that at large altitudes, the trail lifetime can be enhanced by a factor > 100 if the radar is looking in the appropriate direction. 3. Meteor Detection and Processing of Data The Buckland Park VHF radar used to observe the meteors has been described by Vincent et at. [1987], and the techniques used to detect meteors has been described by Cervera and Reid [1995]. A brief overview is given here. The VHF radar which operates at a frequency of 54MHz, uses a coaxialcollinear array of 32 rows of antennas with 48 elements per row. This yields a narrow pencil beam with a half-power halfwidth of ø when tilted 30 ø off zenith (east and west on alternate minutes) for meteor detection. The radar was operated with a pulse length of 13.3tts (equivalent range 2 km), a pulse repetition frequency (PRF) of 1024 Hz, and an RMS pulse power of - 30 kw. The raw receiver output was sampled at 2 km range intervals from 80 km to 128 km (height from 70 km to 110 km). Meteor echoes were detected in these range bins using the technique described by Cervera and Reid [1995]. On average, - meteor echoes were detected per day with subsequent data processing rejecting about half of these. Figure 2 displays a typical strong underdense meteor echo (peak signal-to-noise ratio 22 db) detected by the Buckland Park VHF radar showing the amplitude (Figure 2a) and phase (Figure 2b). The amplitude information shows the characteristic Fresnel diffraction pattern of a long (i.e., greater than a Fresnel zone length) underdense meteor trail decaying exponentially. The phase information of the meteor echo is not required for our purposes here, but it may be used to derive wind speeds by measuring the Doppler shift of the echo caused by the drift of the trail once formed [e.g., $tubbs, 1973; Cervera and Reid, 1995]. The speed of the ablating particle can also be calculated from the examination of the rapid change of phase produced during the formation of the trail [Etford et at., 1995; Cervera et at., 1997]. These will not be expanded upon here. The portion of the echo amplitude displaying the exponential decay is what is of interest for our pur-

5 CERVERA AND REID: COMPARISON OF METEOR OBSERVATIONS WITH CIRA 837 2OOO o 45O Time (radar pulses) - '/2 o Time (radar pulses) 75O Figure 2. Typical underdense meteor echobserved by the Buckland Park VHF radar showing the (a) amplitude and (b) phase. poses here. However, only the well behaved under- complication above 95 km is the effect of the geodense echoes are retained for analysis. Overdense magnetic field suppressing the trail diffusion which meteor echoes are generally difficult to treat and leads to enhanced decay times. are thus rejected. These echoes are long-lived and The echo decay times r were calculated by peras such can be distorted by the background wind forming least squares fits to the log of the echo power in addition to saturating the receivers. Long dura- of the meteors over the region of interest. This region tion underdensechoes occurring at low altitudes can was taken to be 0 16 ms after the peak amplitude (to also be distorted by the background wind and these allow any large Fresnel effects to die down) to where are rejected. Other "nonclassical" meteor echoes the amplitude had dropped to 10% of the peak. Once such as head echoes, the "moving ball" diffraction r had been measured, the diffusion coefficents of the echoes produced by very short trails at high alti- meteor trails were calculated using equation (4) and tudes, and echoes which exhibit "beating effects" then T/vffi from equation (8). Comparison of the meproduced by meteoroid fragmentation or severe trail teor values of T/vffi with CIRA (1986) required that distortion generating multiple reflection points are monthly averages be calculated. However, before this also rejected. Cerver and Reid [1995] discuss the could be done, further processing of the meteor data recognition and rejection of these echoes in greater was necessary. This is described next. detail. The remaining underdense echoes (some 50% Previous researchers have shown that log 0(Da ) of the echoes) cover a height range of 80-95km varies roughly linearly with height over the height with too few meteors being detected below 80 km to range of -, km [e.g., Greenhow and Neufeld, make useful comparisons with CIRA and the meteors 1955; Tsutsumi et al., 1994]. Indeed, the following above 95km decaying too rapidly and their echoes approximate relation has been derived from previous departing from the decaying Fresnel diffraction pat- work by Thomas et al. [1988]: tern to yield reliablecho decays and hence diffusion coefficients. As previously discussed, an additional log10(da ) = h- 6.51, (9)

6 . 838 CERVERA AND REID- COMPARISON OF METEOR OBSERVATIONS WITH CIRA 11o log o(d ) Figure 3. Scatterplot of height versus the log of the arebipolar diffusion coefficient of the observed meteors. where h is the height in km and the usual units for Da are used (m 2 S--1). Thus it is natural to plot height versus log10(da). This has been done and is shown in Figure 3. The scatter shown in Figure 3 is typical for this type of scatterplot [e.g., Tsutsumi et al., 1994; Chilson et al., 1996; Hocking et al., 1997]. The horizontal rastering effect that may be observed in Figure 3 is due to the meteors being detected in 2 km range bins. The spread is due to variations in temperature, pressure, and the ionic species found in the meteor trails which have different values of mobility K. However, these factors alone cannot explain all of the spread, which is a common feature in D - h studies. Another cause of the spread is instrumental and is due to - 10% of meteors being detected in the sidelobes of the main beam. This figure was calculated from an analysis of the meteor radar response function [Elford, 1964; Thomas et al., 1988]. Furthermore, the response function analysis indicates that - 80% of the sidelobe meteors are de- tected in the sidelobe farther from zenith rather than nearer, and these meteors are misinterpreted as occurring - 6 km higher than they actually do. The meteors detected in the sidelobe closer to the zenith are interpreted as occurring 5 km lower. Using equation(9), it is possible to show that meteors which are misinterpreted as ablating 6 km higher than in actuality yield values for the ambipolar diffusion coeffcient, which are too small by a factor of 0.35 for that height. Likewise, the meteors which are misinterpreted as occurring 5 km lower yield diffusion coeffcients which are greater by factor of Using these numbers together with their percentage occurrence rates, we may correct our results to remove the effects of the sidelobes on the monthly mean values of T/V/. Before the monthly averages of the data were calculated, the outliers were removed. Failure to do so would have biased the results because there are upper and lower height cutoffs with the meteor data. This means that the results at lower altitudes would be biased high due to low-altitude cutoff and vise versa at high altitudes. The clean up of the data was performed in the following manner. First, the meteors with diffusion coefficients < 0.1 and > 100 were rejected. The scatterplot was then divided into 200 equal width vertical strips. The meteors in each strip were binned into 2km height bins, and a Gaussian fit was performed to the resulting histogram. The upper and lower 5% of the meteors were rejected. The remaining data were then divided horizontally into 2 km wide strips, and the meteors in each strip were binned. Again, Gaussian fits to the resulting histograms were performed, and the outer 5% of meteors were rejected. The result of this procedure is shown in Figure 4, where only the main band of meteors now remain. 4. Results and Discussion Figure5 compares the monthly mean values of T/rip calculated from the measured ambipolar diffusion coefficient using equation (8) (circles) with that derived from the CIRA (1986) model (diamonds) for several heights from 81.4km to 93.5km. The data were folded into one representative year and were averaged over all hours and days of each month. The error bars for the meteor observations were calcu- lated from the error in the least squares fit to the log-amplitude information. The missing meteor data point for November in the plot at 83.1km is due 110 loo log,o(d a) Figure 4. Scatterplot of height versus the log of the arebipolar diffusion coefficient of the observed meteors after the cleaning process has been applied (see text for details).

7 , CERVERA AND REID: COMPARISON OF METEOR OBSERVATIONS WITH CIRA 839 i i i i i i i i i i i i i i i i i i i i i i i _ 81.4 km 83.1 krn 6OO 200 -,, i,,,, i t I, 200 ', t ' Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov _ i i i!! i I i i i I i i i i i i i i i i i i 84.9 km 86.6 km I I I I I I I I I I I I' - I I I I I I I I I I I Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Figure 5. ly mean T/v/ values calculated from the measured ambipolax diffusion coefficient (circles) and the CIRA (1986) empirical model (diamonds) for heights from 81 km to 94km. The number in each plot refers to the height of the center of the 2 km range bin in which the meteors were detected. 200 to the radar not being operational for most of the month, and data rates were low. The CIRA data were interpolated to the height bin in which the meteors were observed. A value of 2.5 x 10-4 m 2 s -1V -1 was used for the zero field reduced mobility factor, which is typical for the type of ions found in meteor trails. We found that this value produced results for T/vr which agreed best with the CIRA (1986) model, and this value also agrees with that used by Hocking et al. [1997]. Chilson et al. [1996] found from a comparison between lidar and meteor data that a value for K0 of 1.5 x 10-4 m 2 s -1 V -1 was appropriate, which is much lower than normal. They suggest the reason for this could be due to the meteor trails containing ionized atmospheric molecules such as 02 + and N2 + (spectral studies [e.g., Bronshten, 1983] show this), and these ions have lower values of K0. However, we believe that this is not the case for the following reasons: (1) the concentration of these species is low in comparison to the usual ionic species and the overall average value of K0 for the meteor trails will reflect this. (2) The values of K0 for these ions are not low enough. Chilson et al., in re- ferring to McDaniel and Mason [1973], state that N2 + in N2 and 02 + in O2 have values for K0 of 1.87 x 10-4 and 2.24 x 10-4 m 2 s -1 V -1 at STP, respectively (Chilson et al. have these values reversed). In fact, K0 for 02 + in N2 should really be used which makes the hypothesis of Chilson et al. [1996] even more unlikely as it is relatively simple to show from Massey's [1971] formula that this value is larger at 2.32 x 10-am 2 S --1 V -1. (3) No other meteor research has shown any evidence for such low values of K0. We also rule out the geomagnetic field causing smaller values of Da to be measured which would

8 840 CERVERA AND REID' COMPARISON OF METEOR OBSERVATIONS WITH CIRA _ i i i i i I i i i i i i _ 7OO i i i i i i i i i i _ 88.3 km 90.1 km 5OO I I I I I I I I I I l 2OO ' I I I I I I I I I, I I Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov 7OO 5OO _ i i i I i i i i i i i i km - - i i i i i i i i i i 93.5 km 3OO 2OO ' I I I I I I I I I I I Jan Mar May Jul Sep Nov Jan Figure 5. (continued) I I I I I I I I I I Mar May Jul Sep Nov lead to a smaller value than usual for K0 as most of Chilson et al.'s meteor data is below 90 km. In any case, we do not believe that their meteor data is the cause of the difference. From equation (9) we calculate that meteors with a measured value for D, of 1.0m 2 s -1 generally occur at a height of 85.8 km. This agrees with Chilson et al.'s meteor data as well as ours and that of Hocking et al. [1997]. Thus we conclude that the lidar data of Chilson et al. [1996] may be in error. This conclusion is supported by the results of Hocking et al., who found that their lidar and meteor data were comparable. From Figure 5 we observe good overall agreement between the meteor and CIRA values at all heights shown. However, it appears that the seasonal variation in the CIRA values is not reflected in the meteor values. In fact, above 88 km the meteor values tend to be larger than CIRA during the Southern Hemisphere summer months. Although this difference be- tween the meteor and CIRA values is within the er- ror bars, it is consistent and, indeed, increases with height. The exception is at 93.5 km. Here the meteor observations of T/v/ appear to be suppressed with respecto CIRA (1986); best agreement is found during the summer months, while in winter the meteor values are smaller than CIRA (1986). We shall return to this later. Systematic differences are also reported by Hocking et al. [1997], again with the meteor values tending to be larger during the summer months. Figure6 shows more clearly the differences between the CIRA (1986) and meteor seasonal variations in T/v. In Figure 6 the percentage deviation from the annual mean at each height is displayed against month for CIRA (1986) (Figure6, top) and the meteors (Figure6, bottom). The horizontal pattern displayed by the meteor data in Figure 6 is an artifact of a combination of the 2 km binning of the me-

9 CERVERA AND REID: COMPARISON OF METEOR OBSERVATIONS WITH CIRA o C!RA (i 986) Model 6 '4., Jan Mar May Jul Sep Nov Meteors " ' L : a...:. 8.4 '" -a -4 " Jan Mar May... C::::':'... : a Jul Sep Nov Figure 6. Percentage deviation from the annual mean of T/xi at each height for (top) CIRA (1986) and (bottom) meteors. teor data and the contouring process and should be ignored. The meteoresult show two distinct bands at the equinoctal months extending over all heights. The vernal equinox displays values of T/x/,.., 4% greater than the annual mean, while valuesome 6% greater are observed at the autumnal equinox. In contrast, the CIRA (1986) values of T/x/ show little variation from the annual mean during the equinoxes and are N 6% greater during winter and N 6% lower during summer. This indicates that either the CIRA model is not accurately reflecting the actual seasonal variations in temperature and pressure or the diffusion of the meteor trails is being affected in some way during the equinoctal months.

10 842 CERVERA AND REID' COMPARISON OF METEOR OBSERVATIONS WITH CIRA with that derived from the CIRA (1986) empirical model. Excellent overall agreement was found for heights below 93 km when a zero field mobility factor of 2.5 x 10-4m 's-1v -1 was used. Above a 5OO height of 93 km we observed that Da was less than expected. This was probably due to the effect of the geomagnetic field in suppressing the ambipolar diffusion of the meteor trail. However, analysis of the meteor trajectory angles with respecto the ge- omagnetic field direction will be required to verify 95.3 km this. We found that when the Thomas et al. [1988] 200 ' I I I I I I I I I I I value of 2.2 x 10-4 m ' s -1 V -1 for K0 was used, the Jan Mar May Jul Sep Nov meteor observations agreed better with the CIRA values - 1.0km higher. Given the uncertainty in Figure 7. As for Figure 5, except for a height of 95.3 km. an appropriate value to use for K0 (values over the CIRA (1986) values of T/V at a height of 94.7 km are range 2.2x 10-4 to 2.7x 10-4 m ' s -1V -1 being valid) also shown (squares). Note the meteor data agree bet- due to variation in the composition of meteoroids enter with these values, indicating the geomagnetic field is tering the Earth's atmosphere, there is some uncerprobably affecting meteo results at this altitude (see text for details). tainty in the comparison between meteor-derived and CIRA (1986) atmospheric parameters. However, we can conclude that the CIRA (1986) empirical model gives a good overall picture of the atmosphere to We also repeated the comparison for the case of the meteor values of T/v being calculated using the Thomas et al. [1988] value for the zero field mobility factor (i.e., 2.2 x 10-4 m ' s -1 V-l). In this case the within a height limit of :k I km. While good overall agreement was found between the meteor and CIRA values, systematic differences were observed on a seasonal basis with the metemeteor T/v/ observations were consistently larger ors tending to be larger during the summer months. than the CIRA (1986) values. However, good agree- These differences were also found to increase with ment was found between the meteor observations and CIRA (1986) some 1.0 km above the actual meteor height bins. Thus, if we accept the Thomas et al. value for the zero field mobility factor, we interpret this result as a systematic height offset in the CIRA (1986) model. As mentioned previously, the meteor results at 93.5 km appear to be suppressed with respect to the CIRA (1986) values. Consider Figure 7, which compares the meteor and CIRA (1986) T/x/ values at a height of 95.3km. In addition, CIRA (1986) valheight. This result was also observed by Hocking et al. [1997], whose experiment is complimentary to ours with their data having been obtained from the Northern Hemisphere as opposed to the Southern Hemisphere. A more detailed analysis showed that the monthly variation in T/x/ for the meteors was very different from that of CIRA (1986). The CIRA (1986) values showed their greatest positive excursion from the annual mean at a given height during the winter months and the greatest negative excursion during the summer months. In contrast, ues m lower at 94.7km are displayed (squares). the meteor values of T/x/ had their greatest devi- Clearly, the suppression of the meteor observations is much worse, and we believe the reason for this is that the diffusion of the meteor trails is starting to be suppressed by the geomagnetic field. Thus larger decay times are observed, which result in smaller values for T/x/ being calculated. ation from the annual mean during the equinoctal months. There are two possible explanations for the differences between the meteor and CIRA (1986) seasonal variations of T/x/ : (1) The CIRA (1986) empirical model of the atmosphere does not correctly describe the seasonal variation in atmospheric temperature 5. Conclusion and pressure and (2) the diffusion of the meteor trails are being affected by an atmospheric phenomenon in We have compared the atmospheric parameter a currently unknown way. Of the two, we feel that T/v/ calculated from the decay of meteor echoes that first is more likely; however, more long-term ob-

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