Introduction

The zodiacal light (ZL) is well known to be sunlight scattered by interplanetary dust particles. Therefore, by observing the ZL one is able to probe the overall large scale properties of the interplanetary dust complex, including its 3-dimensional density structure.

Major contributions to the night sky brightness derive from four sources: BS - bright stars that are individually resolved by a given telescope, IS - integrated starlight and the diffuse Galactic light, ZL - zodiacal light, and AG - airglow emitted by the Earth atmosphere. Each of these is affected by both extinction and scattering by atmospheric constituents. The telescope's field of view intercepts not only the attenuated, directly transmitted brightnesses of the four sources but also that part of IS, ZL and AG that is scattered into the field of view from other parts of the sky (ADL - atmospheric diffuse light). Lack of accurate information about the BS distribution, difficulties in correcting for the diffusely-scattered light of extended astronomical sources by the Earth's atmosphere, and the changing and uncertain nature of the AG have made it very difficult to obtain an accurate, high spatial resolution map of the ZL brightness from ground observations.

A machine-readable star catalogue has been prepared that includes all stars resolvable by the telescope, thereby permitting accurate determination of their brightnesses in the telescope's reference system and removal of their brightness contributions, BS. Photo-polarimetric measurements of the IS by the Pioneer 10 and 11 space probes from beyond

"This work was supported by Korea Research Foundation Grant KRF-2000-DP-0441.

the asteroid belt have provided high resolution maps of diffuse starlight over most of the sky [1,2]. A semi-empirical reduction methodology has recently been developed [3] to make time-dependent corrections for the ADL. Combining these elements with atmospheric extinction measurements into a single reduction methodology, we have been able to isolate the ZL over extended areas of the sky.

2. OBSERVATIONAL DATA AND REDUCTION

From archives of night sky observations by Weinberg and Mann [4] at Mt. Haleakala, Hawaii, we selected observations from one night, 21/22 August 1968, to demonstrate this new reduction methodology and to derive a high spatial resolution map of ZL brightness. The telescope was used in an almucantar scan mode to repeatedly observe at 5080A and 5300A over a full 360° of azimuth at 8 zenith distances, at 5° interval from 45° to 80°. In this way, 11 sets of almucantar scans (88 scans in total) were obtained for each of the wavelengths. In our scheme of data reduction, parameters such as the zenith extinction optical depth r0, the telescope's effective field of view (FOV), and the calibration factor C are all simultaneously determined from the same set of data from which the ZL brightnesses are derived. This ensures internal consistency in the reduction procedure and also minimizes errors in the subtraction process.

By using 93 bright stars which are identified with distinct peaks in the scan profiles, values were determined for r0 and C-FOV simultaneously. To take account of the changing atmospheric properties, we divided the 11 sets of data into 5 subgroups and determined t0 for each group. The average values of the time-dependent r0s are 0.183 and 0.173 for 5080A and 5300A, respectively, and the relative amplitude of the time variation amounts to about 10%. Details of the procedure can be found in Kwon et al. [5,6].

3. REMOVAL OF THE BS, IS, AND ADL COMPONENTS

Positional and photometric information have been assembled on 8372 stars resolved by the instrument that are brighter than 6.5 mag in the visual, and this information has been stored in a single database called STARSUB. The STARSUB Catalogue is searched for those bright stars which come into the FOV along each scan path, and the brightness profiles of these stars as seen in the telescope reference system are synthesized. We also synthesized the IS profiles using the aforementioned Pioneer 10 and 11 data set. The sum of the two profiles is compared graphically with the total observed profile. Synthesized profiles of IS+BS were constructed for each of the 176 scans by adjusting trial values for the zenith extinction optical depth r0 and effective extinction optical depth re(j [3] until the comparison was satisfactory. Subtraction of the synthetic profile from the observed profile removes the directly-transmitted contributions of BS and IS from the observed brightness. But, in addition to ZL, the residual still contains the directly-transmitted AG and diffusely-scattered components of IS, ZL, and AG.

ADL is one of the most difficult components to remove in night sky observations, with numerous attempts but limited success [7,8,9 and others]. In this study we utilized an effective optical depth re(f and calculated the ADL brightness as a function of zenith distance using the quasi-diffusion method (QDM), which solves the problem of radiative transfer in an anisotropically scattering spherical atmosphere [3],

Figure 1. Isophotal contours of observed ZL brightness plotted in an Aitoff projection in the ecliptic coordinate system. Contour levels are in Sio brightness units 70, 80, 90, 100, 120, 150, 200, 250, 300, 400, 500, 700, and 1000. The ZL distribution clearly shows asymmetries between the northern and southern hemispheres (ecliptic latitudes) and between the morning and evening (east and west of the Sun) sides.

Figure 1. Isophotal contours of observed ZL brightness plotted in an Aitoff projection in the ecliptic coordinate system. Contour levels are in Sio brightness units 70, 80, 90, 100, 120, 150, 200, 250, 300, 400, 500, 700, and 1000. The ZL distribution clearly shows asymmetries between the northern and southern hemispheres (ecliptic latitudes) and between the morning and evening (east and west of the Sun) sides.

Figure 2. ZL brightness calculated using the 3-dimensional cosine model with the symmetry plane being placed at inclination angle ¿=2° and ascending node $7=80°. Brightness contours are on the same level as in Figure 1. The model calculations agree with major observational details, especially with the southward shifts of the Gegenschein peak and morning ZL cone.

Figure 2. ZL brightness calculated using the 3-dimensional cosine model with the symmetry plane being placed at inclination angle ¿=2° and ascending node $7=80°. Brightness contours are on the same level as in Figure 1. The model calculations agree with major observational details, especially with the southward shifts of the Gegenschein peak and morning ZL cone.

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