Brightness and Fluctuation of the Mid-Infrared Sky from AKARI
  Observations towards the North Ecliptic Pole
  We present the smoothness of the mid-infrared sky from observations by the
Japanese infrared astronomical satellite AKARI. AKARI monitored the north
ecliptic pole (NEP) during its cold phase with nine wavebands covering from 2.4
to 24 micron, out of which six mid-infrared bands were used in this study. We
applied power spectrum analysis to the images in order to search for the
fluctuation of the sky brightness. Observed fluctuation is explained by
fluctuation of photon noise, shot noise of faint sources, and Galactic cirrus.
The fluctuations at a few arcminutes scales at short mid-infrared wavelengths
(7, 9, and 11 micron) are largely caused by the diffuse Galactic light of the
interstellar dust cirrus. At long mid-infrared wavelengths (15, 18, and 24
micron), photon noise is the dominant source of fluctuation over the scale from
arcseconds to a few arcminutes. The residual fluctuation amplitude at 200
arcseconds after removing these contributions is at most 1.04 +/- 0.23 nW m^-2
sr^-1 or 0.05% of the brightness at 24 micron and at least 0.47 +/- 0.14 nW
m^-2 sr^-1 or 0.02% at 18 micron. We conclude that the upper limit of the
fluctuation in the zodiacal light towards the NEP is 0.03% of the sky
brightness, taking 2{\sigma} error into account.
