Polarization, linear, comet

For obtaining and studying the phase-angle dependence of linear polarization for comets, we used our own homogeneous data sets received with narrowband filters [31,34-40]. Only comet C/1982 Ml (Austin) was observed through the wideband V filter. These data were supplemented with data for comets C/1982 Ml (Austin), 67P/Churyumov-Gerasimenko [41], C/1989 XI (Austin) [42-45], C/1983 HI (IRAS-Araki-Alcock) [46], and IP/Halley [47]. All data for the blue and red domains of the continuum spectra are shown on the left and right panels in Fig. 1(a). Most measurements of polarization were made for the whole coma (several... 

Figure 3. The diaphragm dependence of polarization for comets - Encke, (LINEAR), - A2 (LINEAR), V - S4 (LINEAR), n - West, X - Hale-Bopp.

For comets, the spectral phase dependence of the degree of linear polarization is similar to that of atmosphereless solar system bodies. Moreover, for spectral... 

Figure 4. Phase angle dependence of the whole coma polarization a) Cometary data in the red domain with (x) low maximum comets, (+) higher maximum comets and (A) C/1996 01 Hale-Bopp. Data points above 120 are from C/1999 S4 (LINEAR) at its disruption [30] b) C/1996 01 Hale-Bopp data in the green domain (lower fit) and the red domain (upper fit) [20].

In this paper, we provide both a brief historical review of the polarimetric researches of comets and analysis of the observed angular and spectral characteristics of linear and circular polarization of the scattered solar light by dust particles in order to reveal principal regularities in the polarimetric properties of cometary dust. 

Parameters P n and slope AP/Aa at the inversion angle ainv = 2 ° show a tendency to increase slightly in the visible domain for dust-rich comets. This result was obtained by comparison of trigonometric fits for the phase-angle dependence of polarization for dust-rich comets in the blue and red domain [40]. This tendency was recently confirmed by observations of comet C/2000 WMl (LINEAR) [77]. It should be noted that the negative polarization branch of comet... 

Figure 6. Variations of tiie degree of linear polmization with wavelength for dust-rich comets, Hale-Bopp, and Giacobini-Zinner (a). Spectral gradient of polarization versus phase angle for dust-rich comets (b).

The split Comet A2 (LINEAR) showed a short-term variability of the polarization parameters on a time scale of a day. Inexplicable cases of polarization-plane rotation and a decrease in degree of polarization have been found for the comet [82]. The cause of such changes is rmclear. It might be interpreted as the result of solar light scattering by aligned particles. Comet A2 (LINEAR) was observed in the active phase and displayed a series of sporadic outbursts due to disruption of its nucleus. It is impossible to eliminate the short-time changes of dust properties in the comet due to separation of small short-lived splinters that may not be directly observed. 

The spatial distribution of circular polarization along the cut through the coma and nucleus in the solar and antisolar directions was investigated for comet S4 (LINEAR) [86]. Its maximum value reached up to 1%. At most times, the degree of circular polarization at the cometary nucleus was close to zero. The left handed as well as right handed polarization was observed over the coma although left-circularly polarized light was systematically observed in the sunward part of the... 

Recently Mishchenko et al. [10] predicted that at small phase angles near opposition, asteroids produce a sharp narrow spike of the measurable linear polarization due to coherent backscattering of reciprocal rays (named Opposition Effect). This mechanism operates in planets, asteroids and comets. The polarimetric observations made recently by Rosenbush et al. [11] with 125cm telescope at the Crimean Astrophysical Observatory confirm this prediction. 

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