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The Particle Acceleration and Transport in the Heliosphere (PATH) numerical code was developed to understand solar energetic particle (SEP) events in the near-Earth environment. We discuss simulation results for the 13 December 2006 SEP event. The PATH code includes modeling a background solar wind through which a CME-driven oblique shock propagates. The code incorporates a mixed population of both flare and shock-accelerated solar wind suprathermal particles. The shock parameters derived from ACE measurements at 1 AU and observational flare characteristics are used as input into the numerical model. We assume that the diffusive shock acceleration mechanism is responsible for particle energization. We model the subsequent transport of particles originated at the flare site and particles escaping from the shock and propagating in the equatorial plane through the interplanetary medium. We derive spectra for protons, oxygen, and iron ions, together with their time-intensity profiles at 1 AU. Our modeling results show reasonable agreement with in situ measurements by ACE, STEREO, GOES, and SAMPEX for this event. We numerically estimate the Fe/O abundance ratio and discuss the physics underlying a mixed SEP event. We point out that the flare population is as important as shock geometry changes during shock propagation for modeling time-intensity profiles and spectra at 1 AU. The combined effects of seed population and shock geometry will be examined in the framework of an extended PATH code in future modeling efforts.

Products of nuclear reactions between primary radiation belt protons and constituents of the tenuous upper atmosphere form a collocated secondary radiation belt. A calculation of the time-dependent secondary intensity provides a model specification of this environmental component for low- and medium-altitude satellite orbits. It is based on an earlier model of the radiation belt protons, the novel feature being a determination of the secondary source function from nuclear reaction cross sections. All long-lived secondary products are included, isotopes of H and He being dominant while the heavier Li to O isotopes are present at relatively low levels. Secondary protons are shown to be a minor correction to the primary radiation belt.

Measurements of the energy spectra of SEP events over a broad energy range (~0.1 to 100 MeV/nuc) show that all large SEP events have spectral beaks organized by the charge-to-mass ratio (Q/M) of the ions. In this paper we present preliminary results of a multi-spacecraft study of the Q/M-dependence of spectral breaks in 11 SEP events and investigate whether the deduced Q/M dependence is correlated with other characteristics of the events.

The extraordinary period from late October through early November 2003 was marked by more than 40 coronal mass ejections (CME), eight X-class flares, and five large solar energetic particle (SEP) events. Using data from instruments on the ACE, SAMPEX, and GOES-11 spacecraft, the fluences of H, He, O, and electrons have been measured in these five events over the energy interval from ∼0.1 to >100 MeV/nucleon for the ions and ∼0.04 to 8 MeV for electrons. The H, He, and O spectra are found to resemble double power laws, with a break in the spectral index between ∼5 and ∼50 MeV/nucleon which appears to depend on the charge-to-mass ratio of the species. Possible interpretations of the relative location of the H and He breaks are discussed. The electron spectra can also be characterized by double power laws, but incomplete energy coverage prevents an exact determination of where and how the spectra steepen. The proton and electron fluences in the 28 October 2003 SEP event are comparable to the largest observed during the previous solar maximum, and within a factor of 2 or 3 of the largest SEP events observed during the last 50 years. The 2-week period covered by these observations accounted for ∼20% of the high-energy solar-particle fluence over the years from 1997 to 2003. By integrating over the energy spectra, the total energy content of energetic protons, He, and electrons in the interplanetary medium can be estimated. After correcting for the location of the events, it is found that the kinetic energy in energetic particles amounts to a significant fraction of the estimated CME kinetic energy, implying that shock acceleration must be relatively efficient in these events.

We report here, on the observation of Jovian electrons during the time period 1992 to 2002, using instruments on board SAMPEX and IMP8 at 1 AU. The Jovian electron flux diminished greatly from early 1996 to the end of 1997 and recovered subsequently and was observed till the end of 2001. The decrease in the Jovian flux was seen in three distinct instruments lasting for about two Jovian synodic periods. Such a dramatic and persistent decrease has not been observed before. The observed decrease could be due to changes at the source or variations in interplanetary conditions affecting transport of these particles. The latter may be solar cycle dependent as in the heliospheric modulation of cosmic rays. Long-term measurements from IMP8 suggest that solar cycle related propagation effects may not be responsible for the observed decrease. We suggest that either a change in the Jovian source strength or a softening of the Jovian electron energy spectrum produced the observed attenuation.

From the end of 1997 to early 1998, the relatively steady-state solar-minimum conditions provided an ideal period to study the solar modulation of anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs). Using observations of the energy spectra of the most abundant elements (H, He, C, N, 0, and Ne) from an array of spacecraft and instruments, we calculate the radial gradients for ACRs and GCRs over a wide range of rigidity. The GCR radial gradient is near zero for all rigidities, out to 70 AU. The ACR radial gradient shows a strong rigidity dependence in the middle heliosphere, which is expected, but the rigidity dependence is not present in the outer heliosphere, and is not obvious in the inner heliosphere.