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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L15815, doi:10.1029/2007GL030635, 2007
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Meteor smoke particle properties derived from Arecibo incoherent scatter radar observations Irina Strelnikova,1 Markus Rapp,1 Shikha Raizada,2 and Mike Sulzer2 Received 9 May 2007; accepted 20 July 2007; published 15 August 2007.
[1] We present a new algorithm to infer information on the properties of charged meteoric smoke particles (MSPs) from the shape of incoherent scatter radar spectra. We show that in the presence of charged MSPs the spectrum can be approximated as the sum of two Lorentzians. These two distinct spectral lines correspond to two diffusion modes in the D-region plasma, i.e., one due the presence of positive ions and one because of heavy charged MSPs. The widths and amplitudes of these two spectral lines yield information on the radius and number density (the latter only for positively charged particles) of the charged MSPs. We apply this new algorithm to measurements obtained with the 430 MHz ISR at Arecibo and demonstrate that the observed spectra indeed bear the features anticipated in the presence of charged MSPs. Resulting values of retrieved MSP number densities and radii fall well within the range of values expected from models and independent in situ observations. Citation: Strelnikova, I., M. Rapp, S. Raizada, and M. Sulzer (2007), Meteor smoke particle properties derived from Arecibo incoherent scatter radar observations, Geophys. Res. Lett., 34, L15815, doi:10.1029/2007GL030635.
1. Introduction [2] It is now common belief that meteor smoke particles (MSPs) form as a secondary product of meteoroid ablation in the altitude range between 70– 110 km with average number densities of up to several 1000 cm!3 and particle radii in the low nanometer-range [Rosinski and Snow, 1961; Hunten et al., 1980]. Despite their tiny dimensions, it has been suggested that MSPs play a significant role in a host of atmospheric phenomena such as the nucleation of noctilucent clouds, the mesospheric metal atom chemistry, the transport of meteoric material to the ground, and the formation of nitric acid trihydrate-particles in polar stratospheric clouds [e.g., Plane, 2003; Gabrielli et al., 2004; Voigt et al., 2005]. However, due to the above mentioned tiny dimensions, measurements of MSPs have been very difficult to obtain. Hence, until now only a few sounding rocket experiments have succeeded detecting heavy (i.e., much heavier than ordinary molecular and cluster ions) charged constituents in the D-region which were interpreted as charged MSPs [Gelinas et al., 1998; Kelley et al., 1998; Lynch et al., 2005; Rapp et al., 2005]. [3] Very recently, Rapp et al. [2007] considered the question whether these charged MSPs could actually be
detected in the shape of incoherent scatter radar (ISR) spectra following theoretical work by Cho et al. [1998]. Rapp et al. [2007] were able to show that ISR-spectra measured with the 930 MHz EISCAT radar at Tromø, Northern Norway, indeed revealed spectral shapes which were indicative of the presence of charged MSPs. However, the available data quality at that time did not allow these authors to derive quantitative results on the properties of observed MSPs such as number densities and radii. [4] In this manuscript, we describe a new algorithm with which MSP number densities and radii can be inferred from ISR spectra and corresponding autocorrelation functions (ACFs). We then apply this algorithm to data obtained with the Arecibo radar in September 2006 and present first profiles of MSP number densities and radii derived from ground based observations.
2. Meteor Smoke Effects on ISR Spectra [5] In this section we briefly describe the effect of charged MSPs on the shape of ISR spectra and ACFs and how we can use the spectral information to derive information on particle number densities and radii. Figure 1 shows calculations of ISR spectra and corresponding ACFs using the theory described by Cho et al. [1998] and parameters listed in Table 1. Figure 1 illustrates that the presence of charged MSPs leads to the occurrence of a narrow central line on top of the broad Lorentzian background of the spectrum which is due to the presence of highly damped ion acoustic waves [Dougherty and Farley, 1963; Tanenbaum, 1968; Mathews, 1978]. The additional narrow line in the presence of charged MSPs is a consequence of a second diffusion mode in the plasma due to the Coulomb coupling between the electrons and charged MSPs (see Cho et al. [1998] for more details). Figure 1b shows corresponding ACFs which are derived by taking the Fourier transforms of the spectra. In the absence of charged MSPs the Lorentzian spectrum corresponds to an ACF with a magnitude ACF ðtÞ ¼ A0 % expð!t=t 0 Þ
where t is the lag-time at which the ACF is evaluated and t 0 is the decay time of the ACF. The inverse of this decay time is proportional to the spectral width of the Lorentzian spectrum, w0, which is given by w0 ¼
1
Leibniz Institute of Atmospheric Physics, Ku¨hlungsborn, Germany. 2 Arecibo Observatory, National Astronomy and Ionosphere Center, Arecibo, Puerto Rico. Copyright 2007 by the American Geophysical Union. 0094-8276/07/2007GL030635$05.00
ð1Þ
1 32pkB T ¼ pt 0 l2R mi n in
ð2Þ
where kB is Boltzman’s factor, lR is the radar wavelength, T is the temperature (which we assume is identical for ions, electrons, particles, and neutrals), and n in is the ion-neutral
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Figure 1. (a) Calculated ISR spectra and corresponding ACFs without (black lines) and with the presence of charged MSPs (blue lines for particles with rp = 1 nm, red for rp = 2 nm) based on the theory by Cho et al. [1998]. For other parameters see Table 1. (b) The dashed lines indicate the rightmost term in equation 3 for rp = 1 nm (blue) and rp = 2 nm (red), respectively. momentum transfer collision frequency [e.g., Dougherty and Farley, 1963]. [6] In Figure 1, we further illustrate that for the cases with charged MSPs the corresponding ACFs (i.e., their magnitudes) can be approximated as the sum of two exponential decays, i.e., ACF ðtÞ & A0 % expð!t=t 0 Þ þ A1 % expð!t=t 1 Þ
mp n pn
ð3Þ
(equivalently, the spectrum can be approximated as the sum of two Lorentzians; note, however, that the analytical expressions of Cho et al. [1998] can not exactly be separated into the sum of two Lorentzians, i.e., our approach is empirical). In equation 3, the first term is identical to equation 1 and is caused by the interaction of electrons and positive ions whereas the second term accounts for the interaction of electrons and charged MSPs. [7] While the general form of equation 3 correctly describes the shape of the ACF for both positively and negatively charged MSPs, a simple interpretation of the coefficients A0 and A1 is only possible for the case of positive particle charge. In this case, A0 and A1 can be assumed to be proportional to the positive ion number density Ni and particle number density Np, respectively. This means that for t = 0 (the zero-lag), ACF(t = 0) = A0 + A1 is the total received power which is proportional to electron number density [Cho et al., 1998]. Accordingly, the number density of charged MSPs, Np, can be derived from the relation Np/Ne = A1/(A0 + A1). [8] In analogy to equation 2 we further define the decay time t 1 by 1 32pkB T ¼ pt 1 l2R mp n pn
0.5 nm, see Cho et al. [1998]), the product mpvpn in equation 4 is given by
ð4Þ
where mp is the mass of the charged particles and n pn is the particle-neutral momentum transfer collision frequency (see Schunk and Nagy [2000] for details on the derivation of momentum transfer collision frequencies). Using the hard sphere collision model (which is valid for particle radii (
! "2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi"ffi 8mp rp þ rn Nn 2pkB Tmn mp þ mn ! " ¼ mp 3 mp þ mn ! "2 p ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi 8 rp þ rn Nn & 2pkB Tmn 3
ð5Þ
where Nn is the neutral number density, rn = 0.15 nm is the mean radius and mn = 4.8 ) 10!26 kg is the mean mass of a neutral air molecule, and rp is the particle radius. [9] Equations 4 and 5 show that the MSP radius rp can be directly derived from the parameter t 1. In order to determine this parameter from a measured ACF, we first fix t 0 by assuming a positive ion mass of 31 amu in accordance with several previous studies of ISR-spectra in the D-region [e.g., Rapp et al., 2007] and we use temperatures and neutral air number densities from the MSIS-climatology for the relevant altitude and location [Hedin, 1991]. We then subtract A0 % exp(!t/t 0) from the total ACF and fit the difference by A1 % exp(!t/t 1) with A1 and t 1 as the fitting parameters. In this procedure, the major uncertainty is introduced by our choice of the positive ion mass which in reality might take larger values than our assumption of 31 amu, e.g., in the Table 1. Input Parameters for Calculations of ISR Spectraa Parameter
Value
altitude Nn Ne Np Zp T rp mi f
88 km 9.4 ) 1013 cm!3 2600 cm!3 300 cm!3 +1 180 K 2 g/cm3 31 amu 430 MHz
a Nn denotes the neutral air number density, Ne is electron number density, Np is number density of MSPs, Zp is the number of elementary charges on each MSP (ZP = +1 means that the particles are singly positively charged), T is temperature, rp is the particle mass density, mi is the positive ion mass, and f is the radar frequency.
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density of charged particles, which we could have underestimated considerably if our assumption of positive MSP charge were wrong.
3. Arecibo Measurements
Figure 2. Relative error in retrieved MSP radius and number densities for different conceivable positive ion masses assuming a fixed positive ion mass of 31 amu. presence of hydrated proton clusters. In order to determine related errors for particle number densities and particle radii derived with our method, we have simulated this retrieval by calculating ‘true’ ACFs with the full theory of Cho et al. [1998] for different positive ion masses in the range between 31 amu and 100 amu (i.e., rather large cluster ions) and then analyzed these ACFs under the assumption of a fixed positive ion mass of 31 amu. The result of this analysis is presented in Figure 2. This figure shows that even in the case where we underestimated the positive ion mass significantly (i.e., where we assumed 31 amu whereas the ‘real’ positive ion mass was 100 amu) the relative errors for radii and number densities do not exceed 10% and 40%, respectively. In addition, Figure 2 demonstrates that our approach to approximate the ACFs by the sum of two exponential decays (equivalently by approximating the spectrum as the sum of two Lorentzians) yields valid results. Namely, for the correct positive ion mass of 31 amu, our empirical method reproduces the correct values for MSP radius and number density. [10] Note that the simple description above assumes that the particles are positively charged. The problem with the MSP charge is that no conclusive evidence for one or the other charge sign exists. While charging models predict a negative charge in the absence of solar radiation, the literature on the charging of particles in the presence of solar radiation is not conclusive [Havnes et al., 1990; Rapp and Lu¨bken, 1999]. On the experimental side, the few existing rocket soundings comprise four flights where negative particles and three flights where positive particles were reported [see Rapp et al., 2007, Table 1]. Among these flights there is only one case where the sun was above the horizon and illuminated the D-region. This is the flight described by Croskey et al. [2001] showing positively charged particles in the altitude range between 70 and 90 km. Since our Arecibo measurements were conducted around local noon (see below), we hence consider it a sound assumption that photo emission has probably resulted in a positive MSP charge. Note also, that the assumption of particle charge hardly affects the results of our particle size retrieval, i.e., our simulations show that our retrieval gives robust results for the particle radius (within an accuracy of < 50%). This is however different for the number
3.1. Data Collection [11] We now apply the method described in section 2 to measurements obtained with the 430 MHz incoherent scatter radar at the Arecibo Observatory (AO) [e.g., Janches et al., 2006]. On September 10, 2006, we employed the AO 430 MHz transmitter operating at a peak power of 2 MW and using a 52 baud special D-region code (!1 !1 1 !1 !1 !1 !1 !1 !1 1 1 !1 !1 1 !1 1 1 !1 1 !1 1 !1 1 1 1 !1 1 1 !1 !1 1 !1 1 !1 !1 1 !1 !1 !1 1 1 1 1 !1 !1 1 !1 !1 !1 !1 1 1), a baud length of 1 ms (range resolution of 150 m), and an IPP (inter-pulse-period) of 1.04 ms. Data were received from altitudes between 52.5 and 119.7 km using the Gregorian antenna (only the vertical beam was used). Complex time series with a length of 256 samples were used to derive ACFs (from which we also derived spectra), and data from 12:00 to 13:40 LT were averaged to maximize the signal-to-noise ratio at D-region altitudes between 80 –90 km. 3.2. Results [12] In Figure 3 we present a typical example of a measured spectrum and the corresponding ACF, in this case obtained from an altitude of 88.2 km. First of all, we have fitted the observed spectrum and ACF with a single Lorentzian spectrum and an ACF of the form given by equation 1. Clearly, the resulting functions (green lines) only succeed to explain part of the observed features. In the case of the spectrum there is a prominent central feature in the spectrum which can not be explained by a single Lorentzian fit (and hence by classical ISR theory) to the data. [13] As a caveat we need to consider the question whether the observed central narrow spectral feature is truly due to the characteristics of the D-region plasma or whether it could be a consequence of e.g. scatter from the terrain, external interference, or a DC-offset on the data resulting from imperfections in the radar receiver. Fortunately, it turns out that all these points can be excluded as a potential cause of our observations: at AO, scatter from the terrain is restricted to stratospheric ranges and below (i.e., below *50 km) while longer ranges in many directions are blocked by close hills. Scattering from ships is seen at closer ranges and is clearly identifiable because of their fast motion. In any case, no ships were encountered during the measurements presented here. In addition, all potential contamination sources mentioned above would produce a very narrow feature in the frequency domain at zero frequencies. Consequently, when tilting the radar beam, these features would remain at zero frequency whereas the ‘real’ spectrum would be shifted by the Doppler shift owing to a non-zero line-of-sight wind in the mesosphere. In fact, we have considered measurements obtained with tilted beams and could not find any prominent peaks at zero frequency whereas the full spectra were Doppler-shifted as anticipated. [14] Hence, we conclude that the observed spectral features shown in Figure 3 are real and can be used to infer information on MSP properties as outlined in section 2. For
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Figure 3. (a) Observed spectrum and (b) corresponding ACF from Arecibo observations at an altitude of 88.2 km (grey dots). The green lines indicate the best fit to the data if a single Lorentzian is assumed. The red lines show the sum of two Lorentzians (dark and light blue lines) where a positive ion mass of 31 amu has been assumed to determine the spectral width of the broad Lorentzian background (dark blue lines). Note that the measured ACF (t = 0) is the sum of total signal and noise power. Hence, in order to derive the total power from the ACF, we extrapolated the first few lags to zero. the case of the spectrum shown in Figure 3, this results in estimates of the radius and number density of (charged) MSPs of rp = 0.9 nm and Np = 270 cm!3 (with uncertainties of * 10% and 40%, respectively - see discussion of Figure 2 above) at an ambient free electron number density Ne = 3000 cm!3. The latter quantity was derived by calibrating the ISR measurements against ionosonde measurements at the F2 peak. [15] Hunten et al.’s [1980] model predicts that MSPs grow by coagulation resulting in a size distribution rather than just in monodisperse particles as assumed in our analysis above. Hence, we have tried to assess the interpretation of our retrieved radii assuming that the actual MSP size distribution is of the same form as described by Hunten et al. [1980]. Their results can be closely described by a distribution of the form ! " dNp =drp / exp !rp =r0
ð6Þ
where r0 = 0.25 nm is a constant. In order to clarify how the MSP radii retrieved with our simple method are related to such size distributions, corresponding ISR-spectra were calculated using the full theory of Cho et al. [1998] for various values of r0. These spectra were then analyzed using our simple approximation. This results in a near-linear relation between retrieved rp and r0 which can be approximated by rp = 1.55 % r0 + 0.33 within an accuracy of ±10% for r0 < 2.0 nm. Interestingly, rp = 0.9 nm (i.e., the value retrieved from our Arecibo observations) corresponds to r0 & 0.35 nm which is very close to Hunten et al.’s [1980] original model prediction. [16] Finally, we have applied the same analysis to all range gates from which we received sufficiently strong incoherent scatter during our measurements on September 10, 2006. Resulting altitude profiles of MSP radii and number densities are presented in Figure 4. Radii show a near constant value of *0.8 nm, while number densities show a relatively
Figure 4. Altitude profiles of (left) retrieved radii and (right) number densities of charged meteor smoke particles. The solid lines are 3-point running means of the original data. 4 of 5
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steep gradient with only *10 cm!3 at an altitude of 85 km and maximum values of *1000 cm!3 at an altitude of 90 km. Importantly, these retrieved values are all well within the range of values suggested by MSP models and the available data from in situ observations [Hunten et al., 1980; Rapp et al., 2007].
4. Conclusions [17] In the current paper we have presented a new algorithm to infer information on the properties of meteoric smoke particles (MSPs) from incoherent scatter radar (ISR) spectra. The basic idea is that the presence of charged MSPs changes the shape of ISR spectra and corresponding autocorrelation functions (ACFs). In the presence of charged MSPs the spectrum can be approximated as the sum of two Lorentzians, equivalently the corresponding ACFs can be approximated as the sum of two exponential decays. The amplitudes and decay times of these two exponential decays yield information on the radius and number density of the charged MSPs. As a caveat we note that the number density can only be accurately retrieved with our method for the case of positively charged particles. Retrieved particle radii, however, are a robust result independent of particle charge. We have applied this new algorithm to measurements obtained with the 430 MHz ISR at Arecibo and demonstrated that the observed spectra and ACFs bear the features anticipated in the presence of charged MSPs. Finally, we have obtained altitude profiles of number densities and radii with values ranging from *10 cm!3 – *1000 cm!3 and 0.8– 1.0 nm which fall well within the range of values suggested from independent in situ observations and model simulations. [18] For the future, we plan to make systematic D-region observations with the Arecibo radar in order to characterize the seasonal variation of charged MSPs and their relation to atmospheric and ionospheric parameters like the background wind fields and D-region ionization. [19] Acknowledgments. We are grateful to J. Ro¨ttger for helpful discussions. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under cooperative agreement with the National Science Foundation. IS and MR were supported by the Deutsche Forschungsgemeinschaft (DFG) in the frame of the CAWSES priority program under grant RA 1400/2-1.
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Croskey, C., J. Mitchell, M. Friedrich, K. Torkar, U.-P. Hoppe, and R. Goldberg (2001), Electrical structure of PMSE and NLC regions during the DROPPS program, Geophys. Res. Lett., 28, 1427 – 1430. Dougherty, J. P., and D. T. Farley (1963), A theory of incoherent scattering of radio waves by a plasma: 3. Scattering in a partly ionized gas, J. Geophys. Res., 68, 5473 – 5486. Gabrielli, P., et al. (2004), Meteoric smoke fallout over the Holocene epoch revealed by iridium and platinum in Greenland ice, Nature, 432, 1011 – 1014. Gelinas, L. J., K. A. Lynch, M. C. Kelley, S. Collins, S. Baker, Q. Zhou, and J. S. Friedman (1998), First observation of meteoritic charged dust in the tropical mesosphere, Geophys. Res. Lett., 25, 4047 – 4050. Havnes, O., U. de Angelis, R. Bingham, C. K. Goertz, G. E. Morfill, and V. Tsytovich (1990), On the role of dust in the summer mesopause, J. Atmos. Terr. Phys., 52, 637 – 643. Hedin, A. E. (1991), Extension of the MSIS thermosphere model into the middle and lower atmosphere, J. Geophys. Res., 96, 1159 – 1172. Hunten, D. M., R. P. Turco, and O. B. Toon (1980), Smoke and dust particles of meteoric origin in the mesosphere and stratosphere, J. Atmos. Sci., 37, 1342 – 1357. Janches, D., D. C. Fritts, D. M. Riggin, M. P. Sulzer, and S. Gonzalez (2006), Gravity wave and momentum fluxes in the mesosphere and lower thermosphere uding 430 MHz dual-beam measurements at Arecibo: 1. Measurements, methods, and gravity waves, J. Geophys. Res., 111, D18107, doi:10.1029/2005JD006882. Kelley, M. C., C. Alcala, and J. Y. N. Cho (1998), Detection of a meteor contrail and meteoric dust in the Earth’s upper mesosphere, J. Atmos. Sol. Terr. Phys., 60, 359 – 369. Lynch, K. A., L. J. Gelinas, M. C. Kelley, R. L. Collins, M. Widholm, D. Rau, E. MacDonald, Y. Liu, J. Ulwick, and P. Mace (2005), Multiple sounding rocket observations of charged dust in the polar winter mesosphere, J. Geophys. Res., 110, A03302, doi:10.1029/2004JA010502. Mathews, J. D. (1978), The effect of negative ions on collision-dominated Thomson scattering, J. Geophys. Res., 83, 505 – 512. Plane, J. M. C. (2003), Atmospheric chemistry of meteoric metals, Chem. Rev., 103, 4963 – 4984. Rapp, M., and F.-J. Lu¨bken (1999), Modelling of positively charged aerosols in the polar summer mesopause region, Earth Planets Space, 51, 799 – 807. Rapp, M., J. Hedin, I. Strelnikova, M. Friedrich, J. Gumbel, and F.-J. Lu¨bken (2005), Observations of positively charged nanoparticles in the nighttime polar mesosphere, Geophys. Res. Lett., 32, L23821, doi:10.1029/2005GL024676. Rapp, M., I. Strelnikova, and J. Gumbel (2007), Meteoric smoke particles: Evidence from rocket and radar techniques, Adv. Space Res., doi:10.1016/ j.asr.2006.11.021, in press. Rosinski, J., and R. H. Snow (1961), Secondary particulate matter from meteor vapors, J. Met., 18, 736 – 745. Schunk, R. W., and A. F. Nagy (2000), Ionospheres: Physics, Plasma Physics, and Chemistry, Cambridge Univ. Press, Cambridge, U. K. Tanenbaum, B. S. (1968), Continuum theory of Thomson scattering, Phys. Rev., 171, 215 – 221. Voigt, C., et al. (2005), Nitric acid trihydrate (NAT) formation at low NAT supersaturation in polar stratospheric clouds (PSCs), Atmos. Chem. Phys., 5, 1371 – 1380. !!!!!!!!!!!!!!!!!!!!!!
S. Raizada and M. Sulzer, Arecibo Observatory, National Astronomy and Ionosphere Center, HC-03 Box 53995, Arecibo 00612, Puerto Rico. M. Rapp and I. Strelnikova, Leibniz Institute of Atmospheric Physics, Schlossstrasse 6, D-18225 Ku¨hlungsborn, Germany. (
[email protected])
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