Cometary ion acceleration processes

In situ spacecraft measurements at comets Giacobini-Zinner and Halley in 1985 and 1986 have revealed that the turbulent plasma environment of the extended coma of a comet could lead to the acceleration of cometary ions to energies well above their initial kinetic energies at production. A tentative scenario exists in which second-order Fermi (stochastic) acceleration is important at large distance from the comets in energizing the cometary ions while at close range ({most positive}5 x 10⁴ km) to the bow shock first-order Fermi (diffusive shock) acceleration will play a role in hardening the ion energy spectra. Many of the kinetic aspects of the particle-wave interaction remain to be tested and developed by quantitative numerical simulations.