Anomalous Cosmic Rays and Heliospheric Energetic Particles.

We present a review of Anomalous Cosmic Rays (ACRs), including the history of their discovery and recent insights into their acceleration and transport in the heliosphere. We focus on a few selected topics including a discussion of mechanisms of their acceleration, escape from the heliosphere, their effects on the dynamics of the heliosheath, transport in the inner heliosphere, and their solar cycle dependence. A discussion concerning their name is also presented towards the end of the review. We note that much is known about ACRs and perhaps the term Anomalous Cosmic Ray is not particularly descriptive to a non specialist. We suggest that the more-general term: "Heliospheric Energetic Particles", which is more descriptive, for which ACRs and other energetic particle species of heliospheric origin are subsets, might be more appropriate.

Probing the energetic particle environment near the Sun.

NASA's Parker Solar Probe mission1 recently plunged through the inner heliosphere of the Sun to its perihelia, about 24 million kilometres from the Sun. Previous studies farther from the Sun (performed mostly at a distance of 1 astronomical unit) indicate that solar energetic particles are accelerated from a few kiloelectronvolts up to near-relativistic energies via at least two processes: 'impulsive' events, which are usually associated with magnetic reconnection in solar flares and are typically enriched in electrons, helium-3 and heavier ions2, and 'gradual' events3,4, which are typically associated with large coronal-mass-ejection-driven shocks and compressions moving through the corona and inner solar wind and are the dominant source of protons with energies between 1 and 10 megaelectronvolts. However, some events show aspects of both processes and the electron-proton ratio is not bimodally distributed, as would be expected if there were only two possible processes5. These processes have been very difficult to resolve from prior observations, owing to the various transport effects that affect the energetic particle population en route to more distant spacecraft6. Here we report observations of the near-Sun energetic particle radiation environment over the first two orbits of the probe. We find a variety of energetic particle events accelerated both locally and remotely including by corotating interaction regions, impulsive events driven by acceleration near the Sun, and an event related to a coronal mass ejection. We provide direct observations of the energetic particle radiation environment in the region just above the corona of the Sun and directly explore the physics of particle acceleration and transport.

Observation of the 6°Fe nucleosynthesis-clock isotope in galactic cosmic rays.

Iron-60 ((60)Fe) is a radioactive isotope in cosmic rays that serves as a clock to infer an upper limit on the time between nucleosynthesis and acceleration. We have used the ACE-CRIS instrument to collect 3.55 × 10(5) iron nuclei, with energies ~195 to ~500 mega-electron volts per nucleon, of which we identify 15 (60)Fe nuclei. The (60)Fe/(56)Fe source ratio is (7.5 ± 2.9) × 10(-5) The detection of supernova-produced (60)Fe in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the (60)Fe half-life of 2.6 million years and that the (60)Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍1 kiloparsec. A natural place for (60)Fe origin is in nearby clusters of massive stars.