The density of cometary protons upstream of comet Halley's bow shock

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Cometary protons picked up by the solar wind were detected by the high energy range spectrometer of the Giotto ion mass spectrometer starting at a cometocentric distance of ∼12×106 km. On the average, the density of cometary protons varied approximately as the inverse square of the cometocentric distance, reaching a value of 0.11 cm−3 just outside the bow shock. The data can be successfully fit to models that include substantial amounts of both slow (∼1 km/s) and fast (≥8 km/s) H atoms beyond the bow shock. Large local variations in the density of picked‐up protons can be explained on the basis of variations in the direction of the interplanetary magnetic field in upstream regions where pitch angle scattering was weak.