Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh22c..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SH22C-01
Physics
2104 Cosmic Rays, 2114 Energetic Particles, Heliospheric (7514), 2162 Solar Cycle Variations (7536), 7859 Transport Processes
Scientific paper
In an ongoing series of papers, we report modeling results and observations from 1991 to 1999 related to the inner and outer heliospheric transport of anomalous cosmic rays (ACRs) having significantly lower energies (0.3 to 40 MeV/nucleon) than have been previously studied. The measurements (which are from the last A > 0 recovery phase) provide crucial information on the transport of H, He, and O at energies below the ACR spectral peaks. A consistent picture that has emerged from our systematic study of these data---primarily from the Voyager 1 & 2 probes, but also including various 1 AU spacecraft---includes the following key features: (1) The initial ACR recovery at all observable rigidities takes about 1 year, suggesting that the dominant timescale is that which arises from the global variation of the interplanetary (IP) transport properties (not the rigidity-dependent ``relaxation'' time needed for equilibrium to develop). (2) The IP transport properties of the outer heliosphere reach a quasi steady state by mid-1994, which lasts until renewed modulation becomes evident around 1999. (3) The distinct exponential growth of the low-rigidity ACR intensities and the nearly constant intensities at higher rigidities are primarily due to the motion of the Voyager spacecraft through spatial intensity gradients, not to continued temporal evolution. (4) Most of these effects can be modeled in a time-dependent manner with a spherically symmetric transport model, suggesting a reduced role for drifts at low rigidities in the outer heliosphere. (5) Latitudinal intensity gradients for ACRs having rigidities below 2 GV are negative (not positive as expected for the A > 0 phase), again consistent with weak drifts for these particles and suggesting an important contribution from the latitudinal dependence of the solar wind velocity. In this paper we will summarize these observations and interpretations, highlighting those aspects that are felt to be generally unrecognized. The possible adjustments to our present understanding of the transport of ACRs through IP space will be outlined and we will emphasize the opportunities there are for modelers and theoreticians to utilize the largely untapped resource that our large dataset represents. Until these unique lower-energy ACR measurements are explained, a complete understanding of the transport and acceleration of energetic particles in the heliosphere cannot properly be claimed.
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