Modeling the number density distribution of interplanetary dust on the ecliptic plane within 5AU of the Sun

We have used the relationship, consistent with observational data, between the radial dependence of the dust supply and the mass dependence of the number density distribution, to consider the parent bodies of interplanetary dust. We examine the number density distribution of the interplanetary dust within 5AU of the Sun on the ecliptic plane. For the model calculations, the number density equations for the ecliptic plane are solved directly by taking into account collisional destruction between particles and the PoyntingRobertson effect, and by assuming a state of equilibrium and axial symmetry in the interplanetary dust cloud. Typical models for the radial dependence of the dust input on the ecliptic plane are considered. For three typical dust groups that are characterized by their orbits–i.e., bound particles, hyperbolic particles of collisional origin, and interstellar particles–a variety of simple models of the physical parameters are considered. These include the particles’ optical properties, the mean sweep-out velocities of the dust clouds, the power law distribution of mass in the collisional fragments, the maximum size of particles, and the inner/outer boundaries. From the model calculations, the existence of the three characteristic particle groups and their input radial dependencies are found to play important roles in determining the environmental conditions of interplanetary dust and the number density distribution of the particles. The roles played by comets and asteroids are estimated by analyzing the relationship between the radial dependence of the dust input and the resultant number density distribution at 1AU. To simulate the flux curve of interplanetary meteoroids at 1AU (e.g., Grün et al. 1985), a source that directly supplies the interplanetary dust is required. It is found that the simulated number density distribution fits that observed at 1AU well, if the mass production rate of dust sources outside 1AU increases with a radial index of −3 ∼ −4 as the solar distance decreases. Such dust sources are more likely to be comets rather than asteroids. The numerical results indicate that, at 1AU, cometary dust is the major component of particles with masses m ≥ 10−6g, and almost comparable in number to asteroidal particles with masses 10−12g ≤ m ≤ 10−7g. Furthermore, we can expect that within 1AU the contribution of cometary particles increases as the solar distance decreases, due to the direct input of cometary particles. In order for the results to be consistent with the observed r−1∼−1.3 radial dependence in the number density distribution of the zodiacal cloud inside 1AU, the mass production rate by the dust source should be almost constant or decreasing as the solar distance decreases. Using a possible model for the dust sources and for the radial dependence of dust input, the number density of hyperbolic particles of collisional origin at 1AU is estimated to be ∼ 1.8× 10−4m−2 sec−1. Hyperbolic particles and the influx of interstellar particles (m ∼ 10−13g) inside 5AU increase the number density of interplanetary dust particles in the medium-sized range (10−15g ≤ m ≤ 10−6g). Interplanetary dust beyond 3AU of the Sun will, therefore, maintain a flat radial distribution of medium mass particles if the interstellar flux is significant.