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Zodiacal light, Gegenschein and Airglow
Even more so than the aurora, these phenomena are visible only on deep, moonless nights at great distances from city lights. The zodiacal light is the brightest among the three and is outlined against the sky as a luminous wedge-shaped triangle in the position where the sun is below the horizon. This light is brightest along the Ecliptic, which is also the direction in which the triangle points. This means that the phenomenon is best seen, when the ecliptic has a steep angle to the horizon, i.e. in spring after sunset in the west, and in autumn before sunrise in the east. The most favourable circumstances for observation are those with the position of the sun about 20-30º below the horizon; this means about three hours before sunrise or after sunset. Aeroplanes sometimes offer a magnificent view of the zodiacal light, whose brightness may be comparable to that of the Milky Way.
The zodiacal light has extraterrestrial origins and is also visible from the moon. It comes into being by the scattering of sunlight on countless minute dust particles in the solar system which are concentrated in a huge disc-shaped area around the sun. The disc lies in the plane of the orbits of the planets so that we see its scattered light extending along the ecliptic.
The zodiacal light is, as may be expected from scattered light, Polarized Tangential ly with respect to the sun. Thus it differs from the unpolarized light of the Milky Way, which is produced directly by millions of faint stars. The degree of polarization of the zodiacal light amounts to a maximum of about 15-20 %. These values can be found at about 60º from the sun, but the polarization is hardly less closer to the sun or farther away from it. At sea level, the zodiacal light is usually so faint that its polarization is hardly discernible. But from a high mountain this light can sometimes be surprisingly bright and then its polarization is distinctly visible, as I know from experience.
On very dark winter nights one can see that the zodiacal light extends like a thin light-bridge all along the ecliptic, while continually decreasing in intensity of light and in polarization. Near the point exactly opposite to the sun, the brightness slightly increases again: this is the gegenschein (literally opposition light). This gleam is analogous to the heiligenschein: an increase of the brightness near the observer's shadow. The gegenschein is usually a very faint and difficult observable phenomenon. But again, from a high mountain, it can sometimes be relatively bright and is easily found as an area of slightly enhanced luminosity of several degrees in diameter, outlined against the dark sky. Its light is unpolarized, just like that of the heiligenschein.
Finally, there is also light in the nocturnal sky which does not come from the stars but from the highest layers of the atmosphere. It is called the airglow, and in favourable circumstances it can be rather bright. It is unpolarized.
source: Polarized light in Nature