J. Geomag. Geoelectr., 36, 305-316, 1984 Production of the Upper Atmospheric Sodium from Impinging Meteors Nobuo MARUYAMA The 1st Research Center of Technical Research and Development Institute, Defense Agency, Nakameguro, Meguro-ku, Tokyo, Japan (Received January 29, 1983; Revised April 26, 1984) The temperature dependent vaporization rate for meteoric particles is ap- plied to estimate the production profile of the upper atmospheric sodium. It is shown that the earlier result estimated using a constant temperature that con- trols the vaporization process for meteoric particles leads to an underestimation of the thickness of the production profile. A typical production profile of sodium calculated by taking account of a mass distribution of the meteoric particles shows the peak height of about 93km and a thickness (10-1) of about 26km. This thickness is significantly wider than that of a usually observed sodium layer, so that effective removal process of sodium is required for shaping the sodium layer. 1. Introduction The maintenance mechanism of the upper atmospheric sodium layer discovered about half a century ago has not yet been established.The sodium layer is usual- ly observed in the height region of about 90km with a peak number density ranging from 103cm-3to 104cm-30f atomic sodium and a layer thickness(10-1) Of about 20km(GIBBON and SANDFORD,1971;MEGIE and BLAMONT,1977; SIMoNICH et al., 1979). The topside scale height of the layer is usually 1/2～1/3 of the atmospheric scale height in the same region, and a sharp cutoff of the bottomside is observed. The production source and the loss sink of atomic sodium are required to exist inside or near the sodium layer to maintain such a sharp profile. If this production source and loss sink were absent, the sodium layer would disperse within an order of several days due to diffusion. Atmospheric dust was suggested to be one of the important sources of atomic sodium (HUNTEN and WALLACE, 1967; DONAHUE and MEIER, 1967; Flocco and VISCONTI, 1973). It was formerly suggested that atomic sodium would be sublimated from the dust surface being warmed by the sunlight in the upper atmosphere. FIOCCO et al. (1974) estimated the dust temperature to be about 400K, and concluded that the dust distributing in an adequate height region might create the sodium layer. However, they adopted vaporization parameters for solid sodium which are not valid for the estimation of the dust sublimation. Apart from the dust-source hypothesis, some piece of evidence for upward transport 305 306 N. MARUYAMA of sea salt particles during the wintertime have been presented by HUNTENand GODSON(1967) and MEGIE et al. (1978). From the early stage of the investigation, the meteor ablation process has been proposed frequently as the source of atomic sodium (CHAPMAN,1939; JUNGE et al., 1962; GADSDEN,1968). This hypothesis is based on the fact that the meteor height ranges from about 80km to 100km, and that the sodium layer is enhanced during meteor showers (HAKEet al., 1972; ARUGAet a!., 1974; MEGIE and BLA- MONT,1977). In addition to the above, Na + , Si + and Fe + found in the upper atmosphere are considered of meteoric origin, and this makes the meteor hypothesis more plausible (ZBINDENet al., 1975). GADSDEN(1968) showed that the ablation profile of the meteoric particles having an initial radius ranging from 10-3cm to 3×10-2cm and entering into the atmosphere with an initial velocity of 25km Sec-1 were consistent with the required production profile of atomic sodium. In his estimation, however, he neglected the temperature dependence of vaporiza- tion for meteoric particles, so that the ablation profile was narrowed due to suppressing the commencement of vaporization in result. In the present paper, the meteor ablation as the production source of atomic sodium is calculated using the temperature dependent vaporization rate for meteoric particles. And the production profile of atomic sodium to evaluate more realistic features is estimated by taking account of a mass distribution of meteoric par- ticles. The material constants required in the calculation for meteor ablation are estimated for the composition of chondrite as it is a representative of meteoric particles. An effect of atmospheric dust in the sodium layer is also discussed from the viewpoint of atomic sodium adsorption to the dust surface. 2. Vaporization Rate Vaporization rate for various materials is known as temperature dependent (DUSHMAN, 1962). Accordingly, meteor ablation is considered to be controlled by such temperature dependence. However, the parameters characterizing the temperature dependence of meteoric particles are not consistent among authors due to the difference of meteoric materials assumed. In this paper, chondrite is assumed as a representative meteoric material that produces the atmospheric sodium, and the material constants including the above parameters to calculate the meteor ablation are tried to estimate as follows. In thermal equilibrium between the gas and other phases, the following Clausius-Clapevron's equation is established. (1) where Q is the heat of vaporization of the meteoric material in cal mol-1, R the gas constant of 1.978ca1 mol-1 K-1, P the gas pressure in dyne cm-2, and T the temperature in Kelvin. When the heat of vaporization Q is assumed as Production of the Upper Atmospheric Sodium 307 a material constant, then the following equation is obtained. (2) where C is an integral constant. The rate, W, of vaporization on the surface is obtained by employing the above pressure P as follows; (3) where W is in g cm-2. sec-1, M the mean atomic weight of the gas, and R the gas constant expressd in 8.31×107 erg mol-1 K-1. The heat of vaporization Q tends to increase in the material with higher boiling temperature Tb (Fig. 1). When Q is assumed proportional to Tb with a proportional constant A, then both Q and C in Eq. (2) are estimated as follows; Q=ATb, (4) Fig. 1. The heats of vaporization for various materials tend to be proportional to their boiling temperatures with the proportional constant of 22.5cal mol-1 K-1 (solid line) Referred data; BUTsuRI-JOSUHYO (1978). 308 N. MARUYAMA C=lnPb+A/R, (5) where Pb is the gas pressure at the boiling temperature Tb and is equal to 1 atm of 1.02 × 106 dyne cm-2. If chondrite is assumed to be the typical material for meteoric particles, it is expected to be composed of much Si, Fe and other metallic elements. The boiling temperatures of these elements are near 3000K, so that this value is adopted to estimate C=25.2 and Q=67.5kcal mol-1, using A=22.3cal mol-1 K-1(Fig. 1). And the other material constant to calculate the meteor ablation are estimated on the basis of the chemical composition of chondrite (Table 1)as the mass density ρm=3.54 g cm-3 and the mean atomic weight M=24.3, respectively. The latent heat and the specific heat are estimated as Lm=Q/M=1.16×1011 erg g-1 and Cm=3R/M=1.03×107 erg g-1 K-1, respectively. Equation (3) can be expressed by using the above constants as follows: (6) where C1=1.90×107g cm-2 sec-1 K-1/2, C2=3.41×104K. These parameters are comparable to those adopted by KORNBLUM (1969) for "Opik stone" with C1=123×107g cm-2 sec-1 K-1/2, C2=3.11×104K, and are different from those adopted by LEBEDINEC and SUSKOVA (1968) for "Stone meteoroid" with Table 1. Chemical composition of chondrite. Composition ratio has been referred from RIKA-NENPYO (1971), and mass densities from BUTSURI-JOSUHYO(1978) and RIKAGAKU-JITEN(1975). Production of the Upper Atmospheric Sodium 309 C1=6.92×1010g cm-2 sec-1 K-1/2, C2=5.78×104K. The latter is more difficult to vaporize than the former or "chondrite" estimated here, and is similar to "Tektite" in Kornblum's Table where some of experimentally determined values are listed. On the other hand, the estimated specific heat Cm and the mass densi- ty pm are almost the same as those used by other authors (GADSDEN, 1968; LEBEDINEC and SUSKOVA, 1968; KORNBLUM, 1969). The estimated latent heat Lm is about twice a large as that for other authors, however, it plays a role in the energy equation for meteoric particle as Lm W which is not so different from "Opik stone" . The material constants estimated here seem natural for the com- position of chondrite, so that these values are adopted to calculate the sodium production profile from meteor ablation. Apart from the vaporization of meteoric particles itself, FIOCCO et al. (1974) proposed the atmospheric dust being warmed by the sunlight to be the produc- tion source of the sodium layer. They adopted the temperature dependent sublima- tion rate applicable to solid sodium. However, the dust cannot be considered to be composed of pure sodium, and atomic sodium on the dust surface seems in chemical bond with neighboring atoms different from sodium. Generally, the heats of chemical bond are larger than that of the vaporization for solid sodium. For example, the heats of chemical bond analogized with the dissociation energies for Na-O2, Na-O, and Na-Cl (MURAD and SWIDER, 1979) are 55.8, 60.1, 97.2kcal mol-1, respectively; therefore, they are more than twice as large as that of the vaporization being 25.8kcal mol-1 (BUTSURI-JOSUHYO,1978). If such large heats are applied to evaluate the role of the dust, no possibility as to the production source of the sodium is obtained, i.e., the sodium sublimation from the dust surface is remarkably exceeded in number by the sodium adsorption to the surface in the sodium layer.