Recent Developments in Particle Acceleration at Shocks: Theory and Observations

Perri, S.; Bykov, A. M.; Fahr, H. J.; Fichtner, H.; Giacalone, J.

doi:10.1007/s11214-022-00892-5

Cited by 19 publications

(10 citation statements)

References 203 publications

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“…Part of the multiply reflected PUI population could be further accelerated by the diffusive shock acceleration process provided that the particle speed is above a threshold or injection energy. The injection problem in the theories of shock acceleration is still under debate (Giacalone & Jokipii 1999;Zank et al 2006Zank et al , 2001Giacalone 2012;Giacalone et al 2022;Perri et al 2022). Parker et al (2014) argued that the injection energy must be at least a few times the upstream thermal energy so that the particle can make an initial crossing of the shock boundary.…”

Section: Diffusive Shock Accelerated Particlesmentioning

confidence: 99%

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Wang

Zank

Shrestha

et al. 2023

ApJ

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Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.

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Section: Diffusive Shock Accelerated Particlesmentioning

confidence: 99%

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Wang

Zank

Shrestha

et al. 2023

ApJ

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“…magnetic obstacle of the CME that generated the SEP event (Lario et al 2018;Perri et al 2022). Fundamental shock parameters have been computed and are shown in Table 1, although single-spacecraft shock parameter estimation is a process that is known to be subject to many sources of uncertainty (Paschmann & Daly 2000;Koval & Szabo 2008).…”

Section: Shock Crossingsmentioning

confidence: 99%

Interpretation of Flat Energy Spectra Upstream of Fast Interplanetary Shocks

Perri

Prete

Zimbardo

et al. 2023

ApJ

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Interplanetary shocks are large-scale heliospheric structures often caused by eruptive phenomena at the Sun, and represent one of the main sources of energetic particles. Several interplanetary (IP) shock crossings by spacecraft at 1 au have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. Surprisingly, in some shock events ion fluxes with energies between 100 keV and about 2 MeV acquire similar values (which we refer to as “overlapped” fluxes), corresponding to flat energy spectra in that range. In contrast, closer to the shock the fluxes are observed to depend on energy. In this work, we analyze three IP-shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity-filter mechanism for particles in a given energy range. In particular, ions with velocity parallel to the local magnetic field larger than the speed of the upstream plasma, in the reference frame of the shock, can easily propagate back upstream, while lower-energy ions tend to be confined to the shock front, thus reducing their fluxes far upstream and giving rise to flat energy spectra. The velocity-filter mechanism has been corroborated from observations of particle flux anisotropy by the Solid-State Telescope of Wind/3DP.

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“…It is likely that other acceleration mechanisms such as the diffusive shock acceleration are active deeper in the heliosheath. For recent reviews on ACRs and particle acceleration processes at collisionless shocks, see Giacalone et al (2022) and Perri et al (2022) in this journal.…”

Section: Turbulence At the Heliospheric Termination Shockmentioning

confidence: 99%

Turbulence in the Outer Heliosphere

et al. 2022

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The solar wind (SW) and local interstellar medium (LISM) are turbulent media. Their interaction is governed by complex physical processes and creates heliospheric regions with significantly different properties in terms of particle populations, bulk flow and turbulence. Our knowledge of the solar wind turbulence nature and dynamics mostly relies on near-Earth and near-Sun observations, and has been increasingly improving in recent years due to the availability of a wealth of space missions, including multi-spacecraft missions. In contrast, the properties of turbulence in the outer heliosphere are still not completely understood. In situ observations by Voyager and New Horizons, and remote neutral atom measurements by IBEX strongly suggest that turbulence is one of the critical processes acting at the heliospheric interface. It is intimately connected to charge exchange processes responsible for the production of suprathermal ions and energetic neutral atoms. This paper reviews the observational evidence of turbulence in the distant SW and in the LISM, advances in modeling efforts, and open challenges.

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Recent Developments in Particle Acceleration at Shocks: Theory and Observations

Cited by 19 publications

References 203 publications

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Interpretation of Flat Energy Spectra Upstream of Fast Interplanetary Shocks

Turbulence in the Outer Heliosphere

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