The turbulent state of cometary ion pickup generated fluctuations near comet P/Halley

Abstract

One of the principal effects of an outgassing comet is to introduce additional mass into the solar wind flow. This mass‐loading process results in the generation of Alfvén waves via ion‐cyclotron resonance. In the case of the Halley encounter this source of “new” fluctuations was introduced into a low speed solar wind 0.89 AU from the sun. Previous in situ observations in low speed flows at this distance have described the fluctuations as turbulently evolved or “aged.” The spectral characteristics and time dependent anisotropies of these new cometary fluctuations have been effectively explained by a conservation‐of‐energy description of the cometary ion pickup process. An alternative description has been proposed whereby turbulent cascading generates the observed spectrum, implying that the fluctuations evolve significantly in the time between their generation and their observation. Analysis of the rugged invariants and, in particular, the Alfvén ratio suggests that throughout the interaction region upwind from the comet the cometary ion pickup generated fluctuations became less evolved as the bow shock was approached. Hence turbulent cascading of the fluctuation energy is unlikely to be an effective description of the observations or to modify the spectral characteristics of these fluctuations significantly in the upwind interaction region.