Other
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..140..156o&link_type=abstract
Icarus, Volume 140, Issue Icarus, pp. 156-172.
Other
1
Scientific paper
Owing to simultaneous dust and plasma wave observations onboard the Vega mission to Comet Halley, previous studies have found that the two double probe antennas, short (of APV-N experiment) and long (APV-V), (i) responded to plasma clouds induced by impacts of relatively large particles, (ii) the target area was comparable to the whole spacecraft projection, and (iii) the mass thresholds depended on the ambient plasma conditions. Subsequently, the response mechanisms have been identified, and it was shown that if impacts became continuous, the sensitivity of the antennas to individual plasma clouds was reduced or even cancelled. In the present paper, about 30 short-time events of continuous impact (CIEs), recognized in the Vega-2 records from the two experiments mostly near the closest approach to (at ~10^4 km from) the nucleus, are investigated. The high-resolution APV-N waveforms reveal that the respective dust formations were structured. A few types of structure, all belonging to one family, have been distinguished. The basic structure, as seen along the Vega-2 pass, is a sequence of particle clouds. CIEs have time scales shorter than or comparable to the time resolution of the dust experiments (spatial scale less than 200 km) and do not correlate with the SP-1 observations (m<=10^-10 g) nor with the published SP-2 fluxes (m<=5.8x10^-8 g). But, these dust data, combined with an integral criterion for continuous impact, provide a constraint which implies that the particles responsible were bigger than 10^-9-10^-8 g. The data from the DUCMA V-detector confirm positively this inference for about 1/3 (~10) of CIEs and indicate that particles (much) bigger than 10^-7 g were decisive in generating several other events. Using an argument from the dusty gas dynamics, it is shown that the small-scale dust structures were not jets but have originated from the disintegration of particle aggregates. An estimate of the total mass contained within a dust structure leads to values of 1-10 kg. Besides CIEs near closest approach, a pair of exceptionally prolonged events has been recorded by APV-V at relatively large distances (~4x10^4 km). The dust data show that the mass distribution across the respective dust formations was highly variable.
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