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Ecliptic latitude [deg]

Figure 3. Upper panels: high spatial resolution (A¡3 = 3') scans across the Ecliptic taken by ISOPHOT at A - Ae = +90° on Jan. 8, 1997 {dots). At A < 25 /zm the 180" circular aperture was selected; at 60/zm the C100 camera (3x3 pixels, 43" x 43"/pix) was used. The /0=[-5,+5] degree scans were performed two days later than the [-13,5] scans, causing an offset in the absolute brightness. The 25/um scan was repeated on Jan. 8, 1998 with lower spatial resolution (A/3 = 30', triangles). The positions of the —10° and ±1.4° asteroidal bands, as predicted by [19], are marked. Lower panel: Residuals of the high resolution scans after subtracting the smooth component.

of the zodiacal light brightness, their width at half maximum is 1.5°-3.5° [19,20]. The bands are interpreted as the result of collisions in the asteroidal belt, and are associated with the asteroidal families Themis, Koronis, and Eos [19,21,22]. The discovery of the dust bands showed unambiguously that there is an important asteroidal contribution to the IDC. A detailed modelling of the bands indicates that their dust content constitutes 5-10% of the zodiacal cloud [22], Another estimate of the fraction of asteroidal dust comes from modelling of the Earth's resonant circumsolar ring and leads to a value of about 30% [23].

With ISOPHOT we observed the asteroidal bands by performing a multi-filter high spatial resolution scan (A = 12,25,60/im, A/3 = 3') across the Ecliptic at A-Ae = 88-90°.

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