Low-energy Ions and Electrons Observed near the Termination Shock by Voyager 1 in 2004 (93 AU)

From about 2002.6 to at least 2004.7, the Low Energy Charged Particle (LECP) instrument on Voyager 1 (85-93 AU, N34^o) has observed remarkable variations in low-energy ion (40 keV to 30 MeV) and electron (26 keV to 1.5 MeV) intensities, ion energy spectra, and ion angular distributions. Comparable phenomena are not observed at Voyager 2 (66-74 AU, S25^o). This report will focus on Voyager 1 LECP data acquired during the period 2004.1 to ∼2004.9. During 2004.1-2004.7 Voyager 1 observed a second major event (E2) associated with the termination shock (TS), the first major event (E1) having been observed during 2002.6-2003.1 [Krimigis et al., Nature, 426, 2003]. Voyager 1 also observed TS-associated energetic particle activity during the intervening period ∼2003.1-2004.1; but, intensities at ion energies <1 MeV were relatively low and sporadic. In February 2004 ion intensities at Voyager 1 increased to levels that continue to exceed those reached during E1. Ion intensities include superposed small-scale ( ∼few hours) and quasi-recurrent ( ∼13 days) variations, with the larger of these variations exhibiting little or no velocity dispersion over energies from 40 keV (speed ∼2 AU/day) to 30 MeV (speed ∼36 AU/day). Significant intensity increases of electrons with energies from at least 26 keV to 1.5 MeV (speed ∼160 AU/day) occurred during 2004.1-2004.2 and 2004.55 to at least 2004.7. As with the ions, these electron increases include smaller-scale, bursty ( ∼few hours to ∼day) components, with the largest peaks time-coincident (non-dispersive) with those of the ions. Ion anisotropies during E2 are mainly unidirectional outward (away from the sun) along the near-azimuthal direction, very similar to those observed during E1. By contrast, electron intensities observed during the two large increases in 2004 (and also during E1) have nearly isotropic angular distributions. During E1, the energy spectrum of ions 40-4000 keV was well represented by a single power-law in energy with spectral index s ∼1.5. Thus far during E2 the spectral slope has shown more energy dependence, being notably harder at lower energy, with the index s ∼1 at 40 keV increasing with energy to s ∼1.5 at 4 MeV.