Voyager LECP Pages
Krimigis, S. M., R. B. Decker, T. P. Armstrong, C. J. Mosley, D. C. Hamilton, and G. Gloeckler, Low-Energy Oxygen Ions Observed at Voyagers 1 and 2, Spring AGU Meeting, 2002, EOS Trans. AGU, 83(19), Spring Meet. Suppl, Abstract SH31B-05, 2002. Draft:
We report on data from the LECP (Low Energy Charged Particle) instruments on Voyagers 1 and 2. Our focus is on observations in the outer heliosphere of low-energy oxygen ions with energies in the ranges (1) ~ 4-16 keV/nuc and (2) ~130-250 keV/nuc. The lower energy component (1) is detected as a result of being boosted by solar wind convection into the sunward looking sector of thin solid-state detector. The higher energy component (2) is identified by careful corrections of rate channel data. We concentrate on the solar inactive periods ~1984-1986 and ~1995-1998. It is during these periods that component (1) is most easily detected because during solar minima the solar wind speed tends to be higher and interplanetary proton intensities, which reduce instrumental sensitivity for detecting the convected oxygen ions, tend to be lower. We have interpreted component (1) as pickup oxygen ions having speeds up to few times the solar wind speed, i.e., in the low-energy portion of the high-energy tail of the pickup ion distribution. During 1996-1997, the three-point intensity spectrum at Voyager 1, which covers 6.3-16.3 keV/nuc oxygen, is consistent with a model wherein pickup oxygen ions are stochastically accelerated to form the high-energy tail, which deceases exponentially with energy. However, component (2), which is at an energy well below the low-energy turnover of anomalous cosmic ray oxygen, has an intensity that lies far above the model-predicted tail. In fact, a simple linear fit to components (1) and (2) is consistent with an oxygen ion intensity that decreases with energy as a power law with a power law index ~2. This result is remarkably similar in form to the high-energy tails observed over a broad energy range at ACE. We will report on our progress in enlarging our data sets and in interpreting the results and their implications for acceleration processes in the solar wind.
Roelof, E. C., R. B. Decker, R. E. Gold, G. M. Simnett, L. J. Lanzerotti, C. G. Maclennan, and T. P. Armstrong, Reappearance of Recurrent Low Energy Particle Events in the Northern Heliosphere: Ulysses, Voyager 1/2, and IMP8, 1997 Spring AGU Meeting, 1997 Spring Meeting Supplement to EOS, S259, 1997. Abstract SH51B-5. Draft:
|Quasi-recurrent ~26-day increases of 40-65 keV electrons have been measured in the high-latitude northern hemisphere of the heliosphere during 11 solar rotations, from October 1995 through July 1996, by the HISCALE energetic particle detectors on Ulysses. They do not appear on all rotations, but when they do, they are associated with increases in 0.5-1.0 MeV protons (preceding them by several days) and sometimes with decreases in galactic cosmic rays. The northern recurrences form two series shifted half a solar rotation with respect to each other, unlike the very regular and more intense series of 21 recurrences observed by the same instrument throughout the mid-to-high latitude southern hemispheres from mid-1993 to the beginning of 1995 [Roelof et al., Astron. Astrophys. 316, 481, 1996]. Correlated energetic particle measurements from IMP8 at Earth and Voyager 1/2 at 42-62 AU establish that recurrent events during this period were indeed stronger in the southern heliosphere than in the north. The variability of the northern recurrences is attibuted, using a generalization of the model of Fisk [JGR 101, 15547, 1996], to temporal changes during 1966 in the near-sun polar magnetic field configuration. These changes would affect the connection of Ulysses via magnetic field lines to the corotating interaction regions (CIRs) at lower latitudes >10 AU beyond the spacecraft, where the low energy particles are accelerated and the galactic cosmic rays are modulated. The observed evolution of the northern polar coronal structure, as revealed in FeXIV (5303Å) synoptic maps and confirmed by SOHO/EIT FeXII (195Å) extreme ultra-violet images, is indeed that which is required, according to the model, to explain the evolution of the low-energy particle recurrences as observed by Ulysses/HISCALE in the northen heliosphere.|
Armstrong, T. P., M. Boufaida, G. Giacalone, L. J. Lanzerotti, C. G. Maclennan, E. C. Roelof, G. M. Simnett, and K. A. Sayle, Evidence for Shock Acceleration to 2-4 MeV/Nucleon of Interstellar Helium in the 1-5 AU In-Ecliptic Region From Ulysses, Voyagers 1 and 2, and IMP8 Gradient Studies, Fall AGU Meeting, December 1995, 1995 Fall Meeting Supplement to EOS, F455, 1995. Abstract SH21A-19. Draft:
The spatial gradients of daily-averaged fluxes of protons and alpha particles have been derived in several energy-nucleon passbands from simultaneous observations at 1 AU (with the IMP 8 CPME instrument) and in the ecliptic 1-5 AU with the Ulysses HISCALE instrument and Voyager 1 and 2 (with the LECP instrument). A positive radial gradient of the 0.3 to 0.5 MeV proton fluxes suggests that interplanetary acceleration, probably associated with shocks, becomes effective in the 2-3 AU region. Protons in the 2-4 MeV energy interval, by contrast, exhibit a negative radial gradient and suggest less efficient shock acceleration in this region. However, the 2-4 MeV/nucleon helium nuclei fluxes show a positive radial gradient for all the Ulysses and Voyager observations, implying that the He/proton abundance ratio increases strongly with heliocentric distance. Values of He/H<5 at 0.35-1 MeV/nuc have been observed directly in individual CIR events at Ulysses (Simnett et al., GRL, in press). Interstellar He atoms are ionized to He+ ions which are picked up by the solar wind and can be accelerated by interplanetary shocks, both flare and CIR-generated, in the 1-5 AU region via shock drift acceleration. Interstellar H is much less abundant than He within 3 AU and may not be significantly injected into this process at this distance. Results of the evaluation of the gradients by several approaches will be shown along with theoretical arguments concerning the injection process. These observations extend to higher energies the argument of Gloeckler et al. (JGR 99, 17637, 1995) that the acceleration of interstellar helium that is proposed as a source of anomalous cosmic rays begins in the 1-5 AU region.
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