  Context. We are creating the AKARI mid-infrared all-sky diffuse maps. Through
a foreground removal of the zodiacal emission, we serendipitously detected a
bright residual component whose angular size is about 50 x 20 deg. at a
wavelength of 9 micron. Aims. We investigate the origin and the physical
properties of the residual component. Methods. We measured the surface
brightness of the residual component in the AKARI mid-infrared all-sky maps.
Results. The residual component was significantly detected only in 2007
January, even though the same region was observed in 2006 July and 2007 July,
which shows that it is not due to the Galactic emission. We suggest that this
may be a small cloud passing near the Earth. By comparing the observed
intensity ratio of I_9um/I_18um with the expected intensity ratio assuming
thermal equilibrium of dust grains at 1 AU for various dust compositions and
sizes, we find that dust grains in the moving cloud are likely to be much
smaller than typical grains that produce the bulk of the zodiacal light.
Conclusions. Considering the observed date and position, it is likely that it
originates in the solar coronal mass ejection (CME) which took place on 2007
January 25.
