Ion spectra in the heliosheath and the interstellar medium from Voyager LECP/LEMPA

Abstract:

We analyze publicly available data from Voyager Low Energy Charged Particle (LECP)/Low Energy Magnetospheric Particle Analyzer (LEMPA) using a model of the instrument and data from the Cosmic Ray System (CRS) on Voyager and data from various sources at 1 AU. The model, which we developed, utilizes the Geant4 framework, is fully three-dimensional, and is based on actual drawings of LECP/LEMPA. It incorporates all relevant physical effects appropriate to the problem in a Monte Carlo style set of simulations. These simulations produce a response in the model instrument, which we convolve with the CRS and 1 AU data to produce a simulated background for the instruments as a function of time. We then subtract these backgrounds after validating with LECP/LEMPA data taken during times that are known to be background dominated. The resulting ion spectra, though preliminary, show interesting spatio-temporal and directional variations in the heliosheath, with signal-to-noise ratios of a few. Interstellar spectra show a much lower signal to noise ratio and likely represent the background generated in LECP/LEMPA due to local interstellar cosmic rays. However, we can at least place upper limits on the very low energy (< 1 MeV) local interstellar cosmic ray spectrum using this technique.