K. Jockers

Observations of scattered light from cometary dust and their interpretation

This review begins with a discussion of the techniques needed for observations of scattered light from cometary dust. After an introduction into the basic concepts of the scattering process, observations of the phase curves of brightness, colour and polarization are covered. Images of colour and polarization are presented and the observed relation of colour and polarization in jets and shells is discussed. The interpretation of the measurements is based on the power law size distributions of dust grains observed from space. The power index must lie between 2 and 4 to provide the mass budget and visibility of the dust coma in accordance with the basic facts of cometary physics. Application of mechanical (radiation pressure) theory to cometary images allows us to derive related power law distributions for comets not explored by spacecraft. Grain scattering models are presented and compared with observations. A prediction is made of the spatial distribution of Stokes parameters U and V in the presence of aligned particles. Up to now such patterns have not been observed. Future work should include the exploration of comets at small and possibly very small phase angles and a detailed comparison of polarization and colour images of comets with thermal images and with models based on mechanical theory.

Bifurcation of force-free solar magnetic fields: A numerical approach

Numerical calculations of two-dimensional force-free fields as models of solar active regions are presented. For a given ‘toroidal’ component of the photospheric magnetic field two branches of solutions are numerically obtained which merge at the critical point of maximum allowed toroidal magnetic field. Depending on boundary conditions magnetic islands may or may not form. The results are discussed with respect to their relevance to the flare process.

Color and polarization as indicators of comet dust properties and evolution in the near-nucleus coma

We present CCD colorimetric and polarimetric observations of comets Hale-Bopp, Hyakutake, and Tabur and an analysis of their change in color and polarization with the distance from the nucleus that may indicate dust evolution in the coma. We concentrate on trends along the solar-antisolar direction in the innermost coma, where all three observed comets demonstrate a correlation between color and polarization. Comparisons to laboratory studies of scattering by complex dust structures reveal that the observed correlation is most likely a result of evaporation/destruction of a dark (organic) material. We study the evolution and properties of the dust in the central coma of these comets using a tentative model of cometary dust as an ensemble of core-mantle particles with sublimating mantles. Calculations for a variety of optical properties, core-mantle mass ratios, and dust size distributions showed that this model could describe the observed change in color and polarization in cometary comae when realistic ranges of input parameters are used. The best fit particles have silicate cores of radius 0.05–0.2 μm with a porous organic-carbon mantle of mantle/core mass ratio 0.5–1. Within the frames of the model the peculiarity of the color and polarization for comet Hale-Bopp (both increase with the distance from the nucleus unlike the decrease observed in comets Hyakutake and Tabur) can be understood in terms of a compact organic mantle on the Hale-Bopp dust grains, whereas the Hyakutake and Tabur dust have more porous mantles. We show how polarimetric color restricts the range of satisfactory refractive indices of both the core and the mantle materials.

Images of polarization and colour in the inner coma of comet Hale-Bopp

On March 31 and April 1, 1997, simultaneous photometry and polarimetry of comet Hale-Bopps dust was conducted with the two-channel focal reducer of the Max-Planck-Institute for Aeronomy attached to the 2 m telescope of Pik Terskol Observatory (Northern Caucasus). Interference filters at642 nm and 443 nm selected red and blue narrow-band continuum windows. The observations have been averaged over the one hour of timethe comet could be observed. The polarization maps cover an area of about1 arcmin2 around the nucleus. The values of polarization degree measured close to the nucleus agree very well with observations obtained with aperture polarimetry. They are lower than in the surrounding coma by about 1%. In our field of view the polarization increases along the sun-comet line from the solar to the antisolar side by about 3%. The dust shells are visible in the polarization images. The polarization in the shells is higher by 1 to 2%and this increase is higher in the red than in the blue range. Therefore the ratio of red to blue polarization (≈ 1.2) increases in the shells by ≈ 0.03. In principle, the polarization excess in the shells, the ratio of red/blue polarization and the higher integrated polarization as compared to other comets can be explained by an excess of particles of radius of about 0.1 μm. Such particles, however, are subject to strong radiation pressure and will be pushed back into the tail before they reach the observed location of the shells. Real Rayleigh particles cannot explain the observed increase in the ratio of red/blue polarization. One therefore cannot exclude the possibility that the excess polarization in the shells is caused by fluffy aggregates via effects which are presently not well understood. The colour map shows features not well related to intensity and polarization, perhaps another dust shell of a different particle size.

Pickup water group ions at comet Grigg-Skjellerup

The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the ‘bow shock’ at comet Grigg-Skjellerup. The results show that the distributions of ions are ring-like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r−2 dependence.

CCD imaging and aperture polarimetry of comet 2P/Encke: are there two polarimetric classes of comets?

We present results of imaging and aperture polarimetry of the dust of comet 2P/Encke at phase angles 91–105°, obtained during the 2003 apparition. We investigate how strongly molecular emissions transmitted by the filters used in the observations can affect the resulting polarization of cometary dust. This problem is of particular importance for so-called gas-rich comets like comet 2P/Encke which has particularly strong molecular emission as compared to its dust continuum. Aperture polarimetry in the wide-band UBVR filters was performed at the 2.6-m Shain telescope and 1.25-m telescope of the Crimean Astrophysical Observatory on November 17–24. From these measurements a dust polarization of ≈

Composition of cometary dust from polarization spectra

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Anomalous wavelength dependence of polarization of Comet 21P/Giacobini-Zinner

% is derived, which puts the comet in the class of comets with low polarization. The imaging observations of comet 2P/Encke were carried out at the 2-m telescope of the Bulgarian National Astronomical Observatory on November 20–22, 2003. Narrow-band filters centered on the 0–7–0 transition of the A 2A_1 –

In-situ observations of a bi-modal ion distribution in the outer coma of comet P/Halley

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A novel quadruple beam imaging polarimeter and its application to Comet Tanaka-Machholz 1992 X*

2B_1 electronic band system of NH 2 (662 nm) and on an adjacent red continuum at 642 nm were employed. The polarization of NH 2 averaged over the 0–7–0 vibronic transition amounts to ≈