The phase curve of cometary dust: Observations of comet 96P/Machholz 1 at large phase angle with the SOHO LASCO C3 coronagraph

We have analyzed brightness and polarization data of comet 96P/Machholz, obtained with the SOHO-LASCO C3 coronagraph at phase angles up to 167° and 157°, respectively. The polarization data are characteristic of a typical dusty comet. Within error limits the corresponding trigonometric fit describes the new data measured at larger phase angles as well as those of the previously known range. In the phase angle range from 110° to 167° the brightness increases almost linearly by about two orders of magnitude. The gradient is independent of wavelength. From the absence of a diffraction spike we conclude that the grains contributing significantly to the scattered light must have a size parameter $x = 2\pi r/\lambda \ge 20$, i.e. have a radius larger than 1 μ m. Fits of the data with Mie calculations of particles having a power law distribution of power index ≈ 2.5 provide a best fit refractive index $\emph{m}$ = 1.2 + $\emph{i}$0.004. In the framework of effective medium theory and on the assumption of a particle porosity $P= 0.5$ this leads to a complex refractive index of the porous medium $\emph{m}$ = 1.43 + $\emph{i}$0.009. A higher refractive index is possible for more porous grains with very low absorption. The large particle sizes are in qualitative agreement with findings derived from the analysis of the motion of cometary dust under solar radiation pressure (Fulle and coworkers, see [CITE]; [CITE] 1997) and with the in-situ measurements of the dust of Halley's comet.