In situ measurements of the solar wind have been available for almost 60 years, and in that time plasma-physics simulation capabilities have commenced, and ground-based solar observations have expa...

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JA026005

Nine Outstanding Questions of Solar Wind Physics.

Abstract
This paper briefly reviews proton acceleration at interplanetary shocks. This is key to describing the acceleration of heavy ions at interplanetary shocks because wave excitation—and hence particle scattering—at oblique shocks is controlled by the protons and not the heavy ions. Heavy ions behave as test particles, and their acceleration characteristics are controlled by the properties of proton-excited turbulence. As a result, the resonance condition for heavy ions introduces distinctly different signatures in abundance, spectra, and intensity profiles, depending on ion mass and charge. Self-consistent models of heavy-ion acceleration and the resulting fractionation are discussed. This includes discussion of the injection problem and the acceleration characteristics of quasi-parallel and quasi-perpendicular shocks.


Particle Acceleration by Interplanetary Shocks

The acceleration of thermal solar wind protons at spherical interplanetary shocks driven by coronal mass ejections is investigated. The solar wind velocity distribution is represented using $\kappa$-functions, which are transformed in response to simulated shock transitions in the fixed-frame flow speed, plasma number density, and temperature. These heated solar wind distributions are specified as source spectra at the shock from which particles with sufficient energy can be injected into the diffusive shock acceleration process. It is shown that for shock-accelerated spectra to display the classically expected power-law indices associated with the compression ratio, diffusion length scales must exceed the width of the compression region. The maximum attainable energies of shock-accelerated spectra are found to be limited by the transit times of interplanetary shocks, while spectra may be accelerated to higher energies in the presence of higher levels of magnetic turbulence or at faster-moving shocks. Indeed, simulations suggest fast-moving shocks are more likely to produce very high-energy particles, while strong shocks, associated with harder shock-accelerated spectra, are linked to higher intensities of energetic particles. The prior heating of the solar wind distribution is found to complement shock acceleration in reproducing the intensities of typical energetic storm particle events, especially where injection energies are high. Moreover, simulations of $\sim$0.2 to 1 MeV proton intensities are presented that naturally reproduce the observed flat energy spectra prior to shock passages. Energetic particles accelerated from the solar wind, aided by its prior heating, are shown to contribute substantially to intensities during energetic storm particle events.

/pdf/acceleration-of-solar-wind-particles-by-traveling-1nqhmpwwzh.pdf

Acceleration of solar wind particles by traveling interplanetary shocks

Measurements with the ACE, STEREO, and GOES spacecraft during the first 5.8 years of solar cycle 24 show that the number of large Solar Energetic Particle (SEP) events is reduced by ~32% compared to this point of cycle 23, while the fluences of >10 MeV/nucleon ions from H to Ni are reduced by factors ranging from 4 to ~10. A comparison of H, O, and Fe energy spectra from the ten largest events of the two cycles shows that the spectral breaks that are typically observed in SEP energy spectra are occurring ~3 times lower in energy/nucleon than in cycle 23. We investigate the origin of these cycle-to-cycle fluence, spectral and composition differences by evaluating possible factors that include: 1) the properties of the associated CMEs; 2) the interplanetary magnetic field strength; and 3) the density of suprathermal seed particles. These properties are evaluated in the context of existing SEP acceleration models. We conclude that both the reduced magnetic field strength and the reduced seed particle densities are contributing to the reduction in SEP output during cycle 24. In particular, we point out that in the standard model for SEP shock acceleration the maximum energy achieved is a strong function of the rate at which protons are injected into the shock acceleration process.

Investigating the Causes of Solar-Cycle Variations in Solar Energetic Particle Fluences and Composition

/pdf/acceleration-of-solar-wind-particles-by-traveling-2wyus9f5hx.pdf

Acceleration of Solar Wind Particles by Traveling Interplanetary Shocks

In the solar system, our Sun is Nature’s most efficient particle accelerator. In large solar flares and fast coronal mass ejections (CMEs), protons and heavy ions can be accelerated to over ~GeV/nucleon. Large flares and fast CMEs often occur together. However there are clues that different acceleration mechanisms exist in these two processes. In solar flares, particles are accelerated at magnetic reconnection sites and stochastic acceleration likely dominates. In comparison, at CME-driven shocks, diffusive shock acceleration dominates. Besides solar flares and CMEs, which are transient events, acceleration of particles has also been observed in other places in the solar system, including the solar wind termination shock, planetary bow shocks, and shocks bounding the Corotation Interaction Regions (CIRs). Understanding how particles are accelerated in these places has been a central topic of space physics. However, because observations of energetic particles are often made at spacecraft near the Earth, propagation of energetic particles in the solar wind smears out many distinct features of the acceleration process. The propagation of a charged particle in the solar wind closely relates to the turbulent electric field and magnetic field of the solar wind through particle-wave interaction. A correct interpretation of the observations therefore requires a thorough understanding of the solar wind turbulence. Conversely, one can deduce properties of the solar wind turbulence from energetic particle observations. In this article I briefly review some of the current state of knowledge of particle acceleration and transport in the inner heliosphere and discuss a few topics which may bear the key features to further understand the problem of particle acceleration and transport.

Particle acceleration and transport in the inner heliosphere

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Particle acceleration and transport in the inner heliosphere