Note that the mass scale is one to two orders of magnitude greater than some previously used; for example, Jacchia (1948) derived a scale of 0.15 g for a ** f, zero magnitude meteorite. The older scales were based on theoretical estimates of the conversion efficiency of kinetic energy into light. The mass scale used in Table 5 -- 1 was derived on the assumption that the motion of the glowing trail is related to the momentum transfer to the trail by the meteorite, permitting the calculation of the mass if the velocity is known (Cook and Whipple, 1958).

A concentration distribution has been derived from radar observations sensitive to the fifteenth magnitude (Manning and Eshleman, 1959). Extrapolation of this relationship through the thirtieth magnitude covers the range of micrometeorites. The approximate equation is ** f, where n is the number of ** f with electron line density greater than or equal to ** f, and q is proportional to the mass of the meteorite. Therefore, n is inversely proportional to the radius cubed and in fair agreement with the inverse 7 2 power derived from 1958 Alpha and 1959 Eta data. At the fifteenth magnitude, ** f, and at the twenty-fifth magnitude, ** f. These extrapolated fluxes are about an order of magnitude less than the values from the satellite data and the figures in Whipple's table. The extrapolation may be in error for several reasons. The observational data determining the concentration distribution have a range of error which is magnified in the extension into the micrometeorite region. The solar electromagnetic -- and corpuscular-radiation pressure and the associated Poynting-Robertson effect increase in effectiveness as the particle size decreases and modify the distribution and limit sizes to larger than a few microns. Also, it has been suggested that the source of all or part of the dust may not be the same as that for visual or radar meteorites (Best, 1960), and the same distribution would not be expected.