Energetic heavy ions observed near comet Giacobini‐Zinner

Abstract

We report observations from the University of Maryland/Max‐Planck‐Institut ultralow energy charge analyzer (ULECA) as it flew past the comet Giacobini‐Zinner on September 11, 1985. The instrument observed energetic ions associated with the comet during the ∼8‐hour‐long period when it was within about 3×105 km of the cometary nucleus. Water group ions released from the comet underwent pickup in the solar wind, with an initial energy of ∼20 keV in the solar wind frame and ∼0–80 keV in the spacecraft frame. ULECA, whose energy range is ∼30–180 keV, is thus well suited for studies of pickup of ions. The bulk flow speed VF of the accelerated ions in this energy range was very close to the solar wind speed during the whole period and in particular slowed significantly in the mass‐loaded region near closest approach. The spectral index of the rest frame distribution function, γ, increased slowly as the cometary nucleus was approached. In the solar wind pickup region the spectral shape is consistent with ion energization by second‐order Fermi acceleration, as described by, for example, Ip and Axford (1986). In the sheath region near the cometary coma the distribution function (in the reference frame moving with the energetic particle bulk flow) was constant; thus there was no additional acceleration taking place in the region closest to the comet. The energetic particle observations presented here have a number of characteristics different from those reported at somewhat higher energies.