The Halloween shock of 2003 as it reached Voyager 2 at ∼73 AU - Two separate acceleration zones and two different spectra for energetic protons

Motivated by the recent observation that two separate periods of enhanced intensities of solar wind ions at ∼2 times the normal solar wind energy were observed at times of the shock arrival and at the magnetic field maximum at Voyager 2 at 73 AU, arising from the 2003 Halloween event at the Earth, we have re-examined the higher energy proton data from 0.06 to 20 MeV from the LECP and CRS instruments on V2 for this event. We find that there are two separate regions of particle acceleration in this outward propagating merged interaction region. The one near the shock has a much harder proton spectrum extending up to ∼20 MeV, but with a relative paucity of particles below ∼1.0 MeV. The other, near the time of maximum magnetic field fluctuations, is dominated by protons at energies ∼1 MeV or less with a sharp cutoff above 2 MeV. The two regions are separated spatially and the half width of the respective radial intensity distributions at each energy can be used to estimate a local diffusion coefficient. The composite spectrum from these two regions is a power law with a spectral index ∼-1.4 below 1 MeV steepening to −3.2 above ∼2 MeV. This observation has important implications astrophysically, beyond what is seen locally, because most astrophysical observations of accelerated spectra cannot resolve the two components and therefore miss the clues that help identify the particle acceleration mechanisms.