Anomalous cosmic ray

ACRs were first discovered as "an unusual enhancement in the low-energy end of the helium spectrum at 1 au over the solar quiet periods of May–July 1972, which cannot be explained by conventional solar modulation theory", by Garcia-Munoz, Mason & Simpson. The particles were called "anomalous" because their presence and characteristics didn't fit with the existing understanding of cosmic rays at the time. The enhancement was particularly notable in that "the GCR [galactic cosmic rays] intensity did not decrease with decreasing energy, as was expected based on our understanding that low-energy GCRs were unable to reach 1 AU due to their interaction with the solar wind". ==Origin and acceleration==

In the standard picture, pickup ions created from interstellar neutrals are accelerated at the termination shock; numerical models and transport calculations reproduce key ACR properties under this assumption. Recent work also discusses pre-acceleration of pickup ions in the inner heliosphere. Solar modulation theory explains how the intensity of cosmic rays changes as they travel through the heliosphere, influenced by the solar wind and magnetic field. Solar modulation is a quasiperiodical change in cosmic rays intensity caused by 11- and 22-year cycles of solar activity. ACRs are also modulated by the Sun; the mechanism of transport and modulation is complex, as described by Rankin et al.: == Composition and effects on the heliosphere==

ACRs are dominated by species most abundant of the neutral gas in the very local interstellar medium. Measurements show enhanced contributions from hydrogen, helium, nitrogen, oxygen, neon, and argon, Coupled MHD–particle simulations that include an ACR pressure component find that ACRs can modify large-scale solar-wind structures in the outer heliosphere, smoothing shock fronts and reducing shock speeds. == Observations==

ACRs were measured using multiple spacecraft. In 1990, Cummings et al. used data collected by Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and the Interplanetary Monitoring Platform-8 (IMP-8) to derive gradients for oxygen and helium. ACR were also measured by WIND, Helios, Ulysses, SOHO, and other spacecraft. Recently, Parker Solar Probe measured ACRs at 1 au to 0.05 au from the Sun. Solar Orbiter measured ACRs from 1 to 0.3 au. == Voyager paradox==

at 121.6 au; in 2018, Voyager 2 crossed it at ~119 au. Voyagers detect cosmic rays using its Cosmic Ray Subsystem, under Edward C. Stone and Alan C. Cummings. Voyager 1 crossed the termination shock in 2004 without detecting the anticipated local ACR source peak, proposed by almost all models; it was called the Voyager paradox. Several alternative models were proposed: "magnetic reconnection near the heliopause", "second-order Fermi processes", According to McComas et al. (2019), the blunt termination shock is "a simple and natural extension of the previously accepted ACR acceleration mechanism", supported by observations: Using the blunt termination shock theory McComas & Schwadron (2006) predicted "the progressive unfolding of the ACR spectrum as each of the Voyagers moved out further beyond the TS into the surrounding heliosheath", which occurred as predicted. File:Cosmic Rays at Voyager 1.png|Cosmic ray measurements by Voyager 1 from 2011 to 2012, a time when it is thought to have exited the heliosphere File:PIA22924-Voyager2LeavesTheSolarSystem-20181105.jpg|Cosmic ray counts observed by Voyager 2 leaving the heliosphere on November 5, 2018. File:ACR GCR Voyager 2.jpg|Comparison of H ACR- vs. GCR-dominated rates for Voyager 2 (top). The bottom two panels compare the ACR and GCR rates separately with intervals of expansion and cooling (green) and compression and heating (red) at Voyager 2 == Further reading==