HST Observation of Mg ⁺in Outburst from Comet D/Shoemaker-Levy 9

Abstract

Ultraviolet spectra of fragment G1 of Comet D/Shoemaker-Levy 9 taken 4 days before impact (on 14 July 1994) by the Faint Object Spectrograph of the Hubble Space Telescope show a brief outburst of magnesium ions. The Mg⁺emission decays back to its quiescent (undetectable) level with a time constant of ∼1 min, consistent with the ions being swept out of the field of view by the motional electric field induced by Jupiter's rotating magnetic field. Although this event occurred when the G1 fragment was inside the jovian magnetosphere, the role of energetic trapped electrons in producing the observed event is not clear. As such, the mechanism that produced the transient population of Mg⁺remains unidentified. A continuum enhancement that was observed ∼18 min after the Mg⁺outburst was almost certainly caused by the passage of a faint (V∼ 19) late-type star through the spectrograph aperture.