The Zodiacal Light and the Gegenschein as Phenomena of the Earth's Atmosphere

Abstract

A quantitative atmospheric theory of the zodiacal light and the gegenschein is developed. Neutral particles sprayed out in all directions from the earth's atmosphere are ionized at 50,000 to 70,000 km levels by the ultra-violet light of the sun. Because of the wabble of the earth's magnetic field with the rotation of the earth ions near the equatorial plane stay for some time at these high levels to form a ring around the earth, ions at high latitudes fall quickly back to the earth to give aurorae. The gravitational magnetic drift of the ions forces the ion ring into a long oval stretching out away from the sun to ${10}^{6}$ km. The pressure of the sunlight warps the oval into the plane of the ecliptic and makes the ions stream out like a comet's tail. The ions are fluorescent; they absorb the sun's ultra-violet light and emit a part of the absorbed energy as visible light. The oval ring is the zodiacal light; the comet tail ion stream is the gegenschein. The zodiacal cones in December are somewhat to the south of the cones in June; the evening cone is south and north of the morning cone in March and September, respectively. These theoretical inferences are in qualitative accord with observation. The theory suggests that the spectrum of the zodiacal light and the gegenschein be different from that of sunlight.Quantitative estimates lead to a low ion density in the zodiacal ring, less than ${10}^{3}$ ions \ifmmode\cdot\else\textperiodcentered\fi{} ${\mathrm{cm}}^{\ensuremath{-}3}$ and to a rate of escape of the terrestrial atmosphere of ${10}^{6}$ particles \ifmmode\cdot\else\textperiodcentered\fi{} ${\mathrm{cm}}^{\ensuremath{-}2}$ \ifmmode\cdot\else\textperiodcentered\fi{} ${\mathrm{sec}}^{\ensuremath{-}1}$ or ${10}^{\ensuremath{-}6}$ of the entire atmosphere in ${10}^{6}$ years. These are perhaps under-estimates. The theory indicates that helium escapes more rapidly than the other gases because of its lightness and higher ionization potential. This may account for the small amount of helium in the atmosphere, which is regarded as being surprisingly low in the face of the estimated large rate of supply from the earth.Variations in the zodiacal light, from the observations of Jones in 1853-1855 and of more recent observers, are shown to occur usually during magnetic storms. Similarly Barnard's observations of the variations in the gegenschein fall in with magnetic disturbances, although more data are needed to establish the connection clearly. The variations are in accord with the atmospheric theory.