Similarities between in situ measurements of local dust light scattering and dust flux impact data within the coma of 1P/Halley

In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density are only available for one comet, 1P/Halley. These data, returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DIDSY) aboard the European Space Agency spacecraft Giotto, are re-assessed in light of the recent improvements in the analysis and calibration of the OPE data after the encounter of the spacecraft with the comet 26P/Grigg-Skjellerup. We find that the local brightness and dust flux are remarkably consistent (even though the DIDSY data samples a limited range of particle mass) for distances from the nucleus in the 10(3) to 10(5) km range, and broadly speaking both data sets exhibit a -2 radial gradient (albeit with localised features). An interesting deviation from the -2 radial gradient is seen for the local brightness as Giotto approaches within 2000 km of the nucleus (which corresponds to Giotto crossing from the anti-sunward side of the terminator plane to the sunward side). These two in situ data sets, and their similarities and correlation, now offer an excellent diagnostic test of the effectiveness and validity of cometary coma models.