Ion acceleration at the contact surface of Comet P/Halley

Suprathermal ions form a major component of the plasma within the inner coma of Comet P/Halley. A close inspection of these ions shows them to contain at least two separate populations: one population covers the energy range below about 400 eV, and a second population exists in a narrow energy band centered between 3 and 4 keV. Similar distributions are found in two regions upstream of the contact surface. It is postulated that the suprathermal ions observed upstream of the contact surface were created at the contact surface. Sudden movements of the contact surface, in the cometary frame of reference, in response to changes in the magnetic field pressure, or cometary production rate, give rise to an induction electric field able to accelerate the local thermal cometary ions up into the tens of electron volts range. Higher energies can be attained with the reacceleration of these ions in subsequent movements of the contact surface. Besides creating the ion enhancements observed upstream of the contact surface, this acceleration mechanism wil release large amounts of suprathermal ions into the inner coma.