Study of the Interplanetary Dust at High Ecliptic Latitudes: Doppler-Fizeau Shifts

Measurements of the Doppler shifts in the zodiacal light spectrum were already performed by James and Smeethe (1970) and more recently by Fried (1978). The results showed that dust grains of the zodiacal cloud are on prograde orbits around the Sun, but with velocities higher than what was expected: hyperbolic type velocities were found at small heliocentric distances. Using a different method of observation, our purpose is to obtain direct spectra of the light scattered by the dust grains, in order to measure the observed, averaged along the line of sight, Doppler shifts at high ecliptic latitudes. The observations are performed with a spectrograph of conventional Czerny-Turner type and described by Bucher et al. (1981) (entrance slit: 1x80mm — holographic grating: 16x140mm, 2400gr/mm, 1st order — image intensifier: VARO, 2 stages, fiber optic entrance window O=50mm and blue phosphor output). The site of the observations is the Pic du Midi Observatory (altitude 2800m); we used a 103aF film directly pressed against the output window, in order to obtain spectra with an exposure time of 10 min; the instrumental set-up was permanently oriented to the North: azimut 180°, high above horizon 20°. A spectral region has been selected near the triplet b1, b2 and b3 of MgI around 5180 A, giving absorption lines contrasted enough to be measured in any case with a comparator of Hilger-type.