Detection of a new “chemical” boundary at comet Halley

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Abstract
Plasma observations near comet Halley indicate that around 1.6×105km from the nucleus a newly discovered sharp boundary (cometopause) separates the solar wind controlled external and the heavy cometary ion dominated internal regions. Such a discontinuity was previously not predicted by theoretical models. Inside the cometopause (in the cometary plasma region) the protons and heavy ions move with different speeds: the heavy ion velocity is less than a few km/s throughout this region, while the protons decelerate from several tens of km/s (observed near the cometopause) to a few km/s (near 1.5×104km).