Physics
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icrc...10.4247h&link_type=abstract
Proceedings of the 27th International Cosmic Ray Conference. 07-15 August, 2001. Hamburg, Germany. Under the auspices of the Int
Physics
2
Scientific paper
Differential energy spectra and time-intensity profiles of anomalous cosmic ray (ACR) oxygen are presented for 1to 29-MeV/nucleon ions observed in the inner and outer heliosphere. New low-energy measurements from the Low-energy Ion Composition Analyzer (LICA) instrument aboard the SAMPEX spacecraft, along with published lowand high-energy data from the SAMPEX, WIND, and ACE spacecraft, provide thorough 1-AU coverage of interplanetary ACR oxygen from 1993 to 1998. Outer heliospheric measurements of 0.3 to 30-MeV/nucleon ACR oxygen are determined from data returned by the Low Energy Charged Particle (LECP) experiments on the Voyager 1 (V1) and Voyager 2 (V2) probes from 1992 to 2000. These measurements extend the simultaneous timeand energy-dependent study of ACRs to lower energies than previously analyzed throughout the heliosphere and, in particular, comprise the most complete examination of the recovery of low-energy ACR ions during the positive heliomagnetic polarity (A > 0) phase of the 22-year solar cycle. We find the data to be consistent with dominantly diffusive transport of ACRs from the outer to the inner heliosphere, a relativley constant ACR source flux during the recovery period after ~1994, and a significant spatial component to the large exponential increases in low-energy ACR oxygen at V1 and V2 from 1994 to 2000 resulting from spacecraft motion. 1. Introduction It is generally believed that the so called anomalous component of cosmic rays is composed of ambient interstellar neutrals that penetrate the heliosphere, become "pick-up" ions by solar photoionization or charge exchange with solar wind plasma, are subsequently convected outward with the solar wind until undergoing acceleration at the solar wind termination shock (TS). From the TS the ACRs are thought to be transported both into and out of the heliosphere. This interpretation, although persuasive, presently lacks direct experimental confirmation at the acceleration stage. In particular, the TS has yet to be encountered by spacecraft, preventing a direct test of this region as the anomalous cosmic ray source, and the nature of the ACR source is not yet well understood, for example, in terms of its time dependence, geometry and motion. The transport of ACRs is also generally thought to be fairly well understood in terms of convection with the solar wind, diffusion resulting from interaction with the turbulent interplanetary magnetic field (IMF), and curvature and gradient drifts in the large scale IMF. The relative importance of these processes, however is not known. Since there are still unresolved questions regarding the acceleration and transport of ACRs throughout the heliosphere, it is important to make studies from as many perspectives as possible. To wit, the low-energy portion of the ACR spectrum requires further attention. In this paper low-energy ACR oxygen is studied in the inner and outer heliosphere as a function of time, energy, and position. Comparisons are made with higher-energy measurements and together an expanded understanding of the transport and acceleration of ACRs in the heliosphere is sought. 2. Data Analysis The anomalous oxygen measurements herein were made in three heliospheric regions: 1 AU - ecliptic measurements, outer heliospheric - northern heliolatitudes, and outer heliospheric - southern heliolatitudes. The new 1-AU data are from LICA, a time-of-flight mass spectrometer aboard the SAMPEX Earth-satellite, supplemented by published data from various instruments aboard the SAMPEX, ACE and WIND near-Earth spacecraft. The outer heliospheric data are from the LECP experiments on Voyager 1 and Voyager 2. From 1992 to the end of 2000 V1 varies in heliographic radius from 47 to 80 AU, and in heliographic latitude from 32º N to 34º N. During this period the V2 coordinates range from 36 to 63 AU and 5º S to 22º S. In 1996, the time period of the Voyager spectra shown in Fig. 1, the V1 and V2 coordinates are approximately 64 AU, 33º N and 49 AU, 16º S, respectively. The radial and
Hamilton Douglas C.
Hill Matthew E.
Krimigis Stamatios M.
Mazur J. E.
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