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200 -200 -100 0 100 200 Day of the year in 1998

-200 -100 0 100 Day of the year in 1998

200 -200 -100 0 100 200 Day of the year in 1998

Figure 2. Annual brightness variations towards the North and South Ecliptic Poles at 25/xm [15]. Black dots are ISOPHOT observations transferred to the DIRBE photometric system (see text), plus signs are values extracted from the DIRBE weekly maps. Both the ISOPHOT and DIRBE values are colour corrected.

tion was used: at 7.3pm the DIRBE 4.8 and 12pm points were connected with a Planck function; at 150/zm a spline interpolation in the logarithmic space among the 60, 90, and 240/zm DIRBE bands was performed. The DIRBE 140/zm band was not used in this interpolation, because it introduced higher noise (see Fig. 1).

The resulting ISOPHOT vs. DIRBE relationships for the selected 5 ISOPHOT filters are shown in Fig. 1. The 60 and 150/iin values are averages over the 3x3 and 2x2 pixels of the C100 and C200 camera, respectively. The figure shows that over the whole brightness range of the zodiacal light the calibrations of the two instruments are related by simple linear transformations. Note that for apertures and FCS powers different from those given in Fig. 1 the coefficients of the linear fits may be different. At 7.3, 12, and 25/zm ISOPHOT and DIRBE have almost identical zero points. At 60 and 150^m a zero point difference seems to be present. In most filters the slope of the linear relationships differ significantly from unity. The random scatter is a few percent of the absolute brightness, and this is the level of precision which can be expected when ISOPHOT data are transformed into the DIRBE photometric system or vice versa. When the DIRBE 140^m data point was included in the interpolation (Fig. 1 lower middle panel) the scatter was significantly higher than in the case when the 140^m point was ignored (Fig. 1 lower right panel), suggesting that the calibration of this DIRBE band is of somewhat lower accuracy.

These results demonstrate that the ISOPHOT and DIRBE databases of extended sky brightness can be transformed into each other's photometric system with an accuracy of a few percent. This result offers the possibility to supplement the DIRBE data with ISOPHOT observations from the Sept-Dec period when DIRBE was not cooled, and to check the DIRBE 140/zm photometry by using the less noisy ISOPHOT data.

2.2. Example: 25/xm photometry of the North and South Ecliptic Poles

As a test of the photometric transformations described above, we re-analysed a set of ISOPHOT 25/xm observations of the North and South Ecliptic Poles, analysed already by Holmes & Dermott [15]. In Fig. 2 we plotted the DIRBE observations for the 9 months when DIRBE was operational, and overplotted the ISOPHOT measurements transformed into the DIRBE photometric system using the relationship in Fig. 1. The two data sets are in good agreement, with a remaining scatter around a smooth fit to the DIRBE data of 0.31 MJy/sr (2.0%) and 0.18MJy/sr (1.1%) for the North and South Ecliptic Poles, respectively.

2.3. Concept of a zodiacal light model for ISOPHOT

The derivation of a zodiacal light model for all ISOPHOT filters is a calibration task, since observations of faint extended emission (diffuse nebulae, cirrus, extragalactic background) has to be corrected for the interplanetary contribution. Based on the ISOPHOT data alone we cannot create an independent global model for the zodiacal light; instead we use the following strategy: we (1) adopt the DIRBE zodiacal light model [4] consisting of a fan shape cloud, the asteroidal bands, and the Earth's resonant dust ring; then (2) apply the relationships of Fig. 1 (or actually a set of similar relationships for different apertures and FCS powers) to transform the DIRBE model predictions to the ISOPHOT photometric system. Values of the DIRBE zodiacal light model for a given position and date can be extracted from the DIRBE Sky and Zodi Atlas (DSZA) (available at http://cobe.gsfc.nasa.gov/cobel/DIRBE/DSZA/). Derivation of a complete set of transformation relationships between ISOPHOT and DIRBE is in progress.

The adapted DIRBE model can be considered as the first version of the ISOPHOT zodiacal light model. The predictions of this model can be checked by spectral decomposition (zodiacal light + galactic cirrus + extragalactic background) of spectral energy distributions observed by ISOPHOT towards dark fields (Sect. 5).

3. SUBSTRUCTURES IN THE ZODIACAL DUST: ASTEROIDAL BANDS AND COMETARY TRAILS

The main contribution to the zodiacal light comes from dust grains in the 20-200 /jm size range [16]. The lifetime of individual particles of this size at 1 AU is about 104yrs, and to maintain the zodiacal cloud in a steady state an average dust input of a 9 x 106 g s-1 is required [17]. Possible dust sources are collisions within the asteroidal belt, active comets, satellites and planetary rings, as well as a stream of interstellar dust particles. The relative contributions of these sources are not well determined, and may depend on the position of the observer within the IDC. ISO observed the asteroidal bands and cometary trails, the most important places where fresh dust is released. The new measurements contribute to our knowledge on the dust ejection mechanism, and can help to quantify the fraction of dust particles of asteroidal and cometary origin.

3.1. Asteroidal Bands

The asteroidal bands were discovered as two local maxima in the IRAS scans across the Ecliptic Plane at (5 ~ 0° and as shoulders at (5 » ±10° [18]. They were further investigated using the COBE/DIRBE database [19]. The amplitude of the bands is 1-3%

Ecliptic latitude [deg]

15 -10 -5 0 Ecliptic latitude [deg]

Ecliptic latitude [deg]

15 -10 -5 0 Ecliptic latitude [deg]

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