  In the Solar System, interplanetary dust particles (IDPs) originating mainly
from asteroid collisions and cometary activities drift to the Earth orbit due
to the Poynting-Robertson drag. We analyzed the thermal emission from IDPs that
was observed by the first Japanese infrared astronomical satellite, AKARI. The
observed surface brightness in the trailing direction of the Earth orbit is
3.7% greater than that in the leading direction in the $9{\rm \mu m}$ band and
3.0% in the $18{\rm \mu m}$ band. In order to reveal dust properties causing
the leading-trailing surface brightness asymmetry, we numerically integrated
orbits of the Sun, the Earth, and a dust particle as a restricted three-body
problem including radiation from the Sun. The initial orbits of particles are
determined according to the orbits of main-belt asteroids or Jupiter-family
comets. The orbital trapping in mean motion resonances results in a significant
leading-trailing asymmetry so that intermediate sized dust (~10-100${\rm \mu
m}$) produces a greater asymmetry than the zodiacal light has. The
leading-trailing surface brightness difference integrated over the size
distribution of the asteroidal dust is obtained to be the values of 27.7% and
25.3% in the $9{\rm \mu m}$ and $18{\rm \mu m}$ bands, respectively. In
contrast, the brightness difference for cometary dust is calculated as the
values of 3.6% and 3.1% in the $9{\rm \mu m}$ and $18{\rm \mu m}$ bands,
respectively, if the maximum dust radius is set to be $s_{\rm max} = 3000{\rm
\mu m}$. Taking into account these values and their errors, we conclude that
the contribution of asteroidal dust to the zodiacal infrared emission is less
than ~10%, while cometary dust of the order of 1 mm mainly accounts for the
zodiacal light in infrared.
