Smooth Component

Figure 3. The mosaic of the zodiacal light observed in December 15 (evening; right images) and 16 (morning; left images), 1998 at Mauna Kea (4200 m, Hawaii).

Figure 3 shows a mosaic of four pieces of our CCD images of the ZL, obtained at Mauna Kea (4200 m, Hawaii) on December 15 (evening) and 16 (morning; local time), 1998 using a cooled CCD camera. The reference report on the diffuse night sky brightness [12] was a useful guide to excluding other contributions to the sky brightness from our observed data. A recent analysis [13] of the smooth component of the evening ZL shows that the inclination of the plane of symmetry of the ZL is close to i=2.2°±0.9° and the longitude of its ascending node is $7=53°±7°. There is a large discrepancy in fi between this value and the 96°±15° deduced from photometry of the ZL [14] (DLR), while i is close to DLR's value of 1.5o±0.4°. Since DLR's result was based on data averaged over a fairly long time period, whereas a 'snapshot' of the zodiacal cloud taken in the evening direction in December was used in [13], the difference in i7 may suggest that the symmetry plane varies with the season.

To obtain a reliable 3-D model for the zodiacal cloud, we have to compare the observed 2-D brightness distribution with that calculated along a line of sight based on a cloud model. A sophisticated method for model calculation was presented in [15], and Yoshishita et al. [13] similarly addressed model fitting. The absolute intensity of the airglow in the zenith direction iAa(zenith) is a key parameter in estimating the airglow brightness at a certain zenith distance based on the van Rhijn function (see [12]). The optimized model[13] yields I Aa(zenith) = 41510®, while IAa(zenith) — 1005io® was estimated in [5]. This reduction of Iag{zenith) in [13], compared with [5], arose from the effective removal of airglow lines in new filter shown in Figure 2, whereas the old filter used in [5] has an effective wavelength of 440±60 nm. One of the scientific motivations at Kobe University for building a new detection system is to establish the absolute values of the ZL and the airglow, as reported in [11].

An advantage of CCD photometry of the ZL is that we can take a 'snapshot' of the zodiacal cloud at any season. From comparison of the previous observations noted above, we suspect that the zodiacal cloud cannot be represented by a simple structure with a single plane of symmetry. We will continue to take 'snapshots' of the zodiacal cloud in different seasons to examine its spatial structure in detail.

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