Abstract
Abstract We augment the heliospheric network of galactic cosmic ray (GCR) monitors using 2012–2017 penetrating radiation measurements from the New Horizons (NH) Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), obtaining intensities of ≳75 MeV particles. The new, predominantly GCR observations provide critical links between the Sun and Voyager 2 and Voyager 1 (V2 and V1), in the heliosheath and local interstellar medium (LISM), respectively. We provide NH, Advanced Composition Explorer (ACE), V2, and V1 GCR observations, using them to track solar cycle variations and short-term Forbush decreases from the Sun to the LISM, and to examine the interaction that results in the surprising, previously reported V1 LISM anisotropy episodes. To investigate these episodes and the hitherto unexplained lagging of associated in situ shock features at V1, propagating disturbances seen at ACE, NH, and V2 were compared to V1. We conclude that the region where LISM magnetic field lines drape around the heliopause is likely critical for communicating solar disturbance signals upstream of the heliosheath to V1. We propose that the anisotropy-causing physical process that suppresses intensities at ∼90° pitch angles relies on GCRs escaping from a single compression in the draping region, not on GCRs trapped between two compressions. We also show that NH suprathermal and energetic particle data from PEPSSI are consistent with the interpretation that traveling shocks and corotating interaction region (CIR) remnants can be distinguished by the existence or lack of Forbush decreases, respectively, because turbulent magnetic fields at local shocks inhibit GCR transport while older CIR structures reaching the outer heliosphere do not.
Highlights
Since their discovery by Victor Hess (1912) over a century ago, cosmic rays have been recognized as a fundamental component of Earth’s external radiation environment
The development of the New Horizons (NH)/Particle Spectrometer Science Investigation (PEPSSI) cosmic ray monitor and integration of the resulting data into the largest array of in situ cosmic ray observations, extending from 1 au to the interstellar medium, has led to several conclusions: (1) galactic cosmic ray (GCR) are clearly observed at both Advanced Composition Explorer (ACE) and NH, revealing that GCR minimum occurred at 2014.8 and 2015.2 at 1 au and 31 au, respectively
(2) Seventy-two percent of solar disturbances observed by NH before and during solar cycle maximum (2012–2015) are associated with Forbush decreases, whereas only 20% are associated after solar maximum (2016–2017), supporting the interpretation that the earlier events are propagating shocks bringing turbulent magnetic fields and the later events are remnants of corotating interaction region (CIR) lacking magnetic fields that strongly affect GCRs
Summary
Since their discovery by Victor Hess (1912) over a century ago, cosmic rays have been recognized as a fundamental component of Earth’s external radiation environment. The longest-standing interpretation centers on the heliospheric termination shock (TS) (e.g., Cummings & Stone 2013), but ACRs can potentially be accelerated in the heliosheath (HS), for example via magnetic reconnection (e.g., Drake et al 2010; Zank et al 2015; Zhao et al 2019). These particles are influenced by their source populations (interstellar pickup ions) as well as their transport to and through regions local to the observer. These particles are influenced by their source populations (interstellar pickup ions) as well as their transport to and through regions local to the observer. Zank et al (2018) and Zhao et al (2019) have recently reported on their theoretical progress and simulations of pickup ions, demonstrating the importance of pickup ions in mediating the solar wind and finding agreement with NH and V2 plasma observations
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