Localized Structure

The inhomogeneous structure of the zodiacal cloud was initially revealed by applying the Fourier filtering method (see e.g. [3]) to data acquired by infrared satellites. A question to be addressed is how such structure can be found using ground-based instruments operating at visible wavelengths?

3.3.1. Dust bands

As shown in Figure 6, the surface brightness profile of the Fourier-filtered morning zodiacal light demonstrates the existence of dust bands at visible wavelengths. The brightness of the dust bands detected at a solar elongation angle of 78° and an ecliptic latitude of 0° (the Themis/Koronis dust bands) is about 55"io© which corresponds to 2% of the surface brightness of the ZL. The ability to detect such a faint feature was attained when fluctuation of the sky brightness across the CCD was reduced to less than O.995'io0.

Figure 6. Three-dimensional profile of the Fourier-filtered profile, where the smooth component of the ZL has been removed. The isophotomap in the upper plane denotes the excess brightness in units of Sio© [5].

Several undefined dust bands were reported by [3], based on analysis of infrared satellite data. To find the corresponding features at visible wavelengths, 'snapshots' of the ZL taken in different seasons covering a wide range of solar elongation angles will become a powerful tool. The limitations of the IRAS data were noted in [21], e.g. limited coverage in elongation angle e of 60°<e<120°. DIRBE shifted the coverage to 64°<e<124°. In principle, however, CCD photometry can take images of the ZL in the range 30°<e<180° from the ground. Specifically, an extension of the dust bands in the region of the Gegenschein was discovered in [5J. We will continue to elucidate the e-dependence of the brightness of the dust bands in order to study their origin and dynamical evolution over time.

3.3.2. Dust Trails

The results of a survey of cometary dust trails based on IRAS data [22] have found eight trails associated with known short-period comets. It is hard to recognize any slight enhancement of the IDPs along a cometary orbit at visible wavelengths, because less enhanced dust particles in the foreground/background contribute to the scattered light and mask the dust trails. This effect arises because the brightness of scattered light depends on the Sun-target-observer angle (the phase angle a), as well as on the number density of the IDPs. The negative result of a search for a dust trail along the orbit of asteroid 3200 Phaethon [23] may be caused by the observed condition of less efficiency in the scattering function observed in a=120°~140°.

A meteor shower is believed to be when the Earth is passing through a dust tube associated with a dust trail. If we can see the sky near the location of the radiant of the meteor shower at this time, it should be possible to see enhancement of dust grains inside the tube. On 17th November 1998, CCD photometry of the Leonid meteoroid stream region was performed [24]. Thanks to a CCD instrument similar to that used by [5], they found an excess of brightness about (2~3)% above the background ZL in the direction of the dust trail of comet 55P/Tempel-Tuttle, the parent of the Leonid meteoroid stream. This result was the first detection of dust trails at visible wavelengths, and from inside a dust trail.

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