J

D2A spacecraft and Tenerife data

1.5 ± 0.4

96 ± 15

[3]

Modified fan model

2.2 ±0.9

53 ±7

This work

Figure 1. The circle mark in the center is the Sun. The numbers indicate the position of the Earth at the beginning of the respective month. The 7 is vernal equinoctial direction, and the shaded fan-shape denotes the sky area of observation in the evening of December 16th.

2. OBSERVATIONS

We observed the evening zodiacal light at Mauna Kea (4200m, Hawaii) during 5h14m — 7,135m(i7T) in December 16, 1998. The resulting images are shown in Figure 3 of [4]. We used a cooled CCD camera with a fish-eye lens(/=16mm,.F=2.8). To reduce the contamination of the airglow, we used a special filter, which has no prominent airglow emission lines in its available wavelength region of 39-524 nm [4], Our observations cover the range of helioecliptical longitude of 50° < A — A® < 120°, and ecliptic latitude of -25° < ¡3 < 25°. The field of view is 51° x 34° and the angular resolution is 2.0' pixel-1 without a binning. The exposure time was set to 10 mins, and the temperature of the CCD chip was kept at —30°C.

3. DATA REDUCTION

First, we applied the basic data reduction procedures to the raw data, such as dark current subtraction and flat fielding. It is well-known that a frame taken by a fish-eye lens shows strong 'vignetting', that is, the detected intensity in the central part of the frame is brighter than that in the edge of frame. The vignetting, as well as the pixel-to-pixel variation in sensitivity, can be corrected by flat-fielding. The reference frames for flat-fielding were taken inside the integrating sphere, which can provide a uniform illumination on the wide FOV of our fish-eye lens. Furthermore, we remove the stars by using the computer program and eliminate the inhomogeneous structure of the zodiacal light, such as the dust bands existing around the ecliptic plane, by applying the Fourier-filtering procedure. Thanks to this technique, we got rid of the structure with angular scales between 5° and 20°, i.e. dust bands components.

The residual components inside the image frame, i.e. the sky brightness (I0bs) observed from the ground-based site, consists of the light from several different sources. Namely, they are the zodiacal light(/^£,), the airglow from upper atmosphere^c), the integrated starlight of unresolved stars(//sl), and the light scattered by Earth's atmosphere(/sca). It is shown that lobs - {IzL + I AG + //sl)exp(-Te//(z)) + Isca (1)

where Tejj(z) denotes the effective optical depth for the extinction of diffuse light source at zenith distance z. We have derived the optical depth r(z)=0.16/cos z for point source by the photometry of standard stars. Then, we assume tejj(z) ~ 0.75r(z) [5]. The Ijsl is deduced from an interpolation of the Pioneer's data cited in [6], and the Isca comes from [7]. The zenith angle dependence of IAg is assumed from the van Rhijn function [8].

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