The influence of the brightness of the asteroidal dust bands on the Gegenschein Tadashi Mukai, a, * Masanori Fujino, a Masateru Ishiguro, b Ryosuke Nakamura, c Munetaka Ueno, d Fumihiko Usui, d and Suk Minn Kwon e a Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan b Planetary Science Division, ISAS, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan c Earth Observation Research Center, National Space Development Agency of Japan, Tsukuba, Ibaraki 305-8505, Japan d Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan e Department of Science Education, Kangwon National University, Chunchon 200-701, Korea Received 3 November 2001; revised 20 October 2002 Abstract The position and shape of the Gegenschein’s maximum brightness provide information on the structure of the interplanetary dust cloud. We show that the asteroidal dust bands, extended near the anti-solar point, play an important role in determining both the position of the maximum brightness and the shape of the Gegenschein. After removing the asteroidal dust bands from an observation of the Gegenschein on November 2, 1997, it was found that the maximum brightness point shifted -0.4° in ecliptic latitude, i.e., to the south of the ecliptic plane, at an ecliptic longitude of 180°, in contrast to a latitude value of +0.1° when the dust bands were included. Furthermore, the part of the Gegenschein to the south of the ecliptic plane was brighter than the northern part at the time of observation. Referring to the cloud model of T. Kelsall et al. (1998, Astrophy. J. 508, 44 –73), it can be estimated that the ascending node of the symmetry plane of the dust cloud is 57° -3° +7° when its inclination is 2.03°  0.50°. © 2003 Elsevier Science (USA). All rights reserved. Keywords: Asteroids; Dust bands; Zodiacal light; Gegenschein 1. Introduction The Gegenschein appears as a part of the zodiacal light at large solar elongation angles, near the anti-solar point. The Gegen- schein’s brightness is enhanced by back-scattered light from in- terplanetary dust grains that lie along the line of sight. Therefore, the intensity and spatial distribution of its brightness provide in- formation on the nature of the interplanetary dust cloud in the region beyond the orbit of the Earth, i.e., on the spatial distribution of the dust grains and their scattering properties. It is generally believed from previous observations of the Gegenschein (i) that the point of maximum brightness of the Gegenschein is displaced from the anti-solar point to the north of the ecliptic plane in spring and to the south in autumn (Tanabe, 1965), and (ii) that the shape of the Gegenschein is an oval, elongated in an east–west direction (Roosen, 1971). In addition, (iii) the maximum brightness of the Gegenschein is about 180 S 10J and its excess brightness is about 40 S 10J , where 1 S 10J = 1.18  10 -8 Wm -2 ster -1 m -1 at a wavelength of 0.55 m (Leinert et al., 1998). The definition of the maximum brightness point and the shape of the Gegen- schein were poor, due to a low resolution of 4°, and conse- quently the data of the longitude of the Gegenschein through- out the year were scattered in photometric observations carried out before the 1990s (see, e.g., Leinert, 1975; Roosen, 1971, 1975). The general features described above as (i)–(iii), how- ever, have been confirmed by recent CCD observations of the Gegenschein (Ishiguro et al., 1998, 1999b; James et al., 1997), where the angular resolution was increased to, respectively, 4.'8, 3.'0 (at Norikura Observatory) and 7.'7 (at Kiso Obser- vatory), and 2.'5 pixel -1 . CCD images of the Gegenschein * Corresponding author. Fax: +81-78-803-6483. E-mail address: mukai@kobe-u.ac.jp (T. Mukai). R Available online at www.sciencedirect.com Icarus 162 (2003) 337–343 www.elsevier.com/locate/icarus 0019-1035/03/$ – see front matter © 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0019-1035(03)00003-4