Summary and Conclusions

Abstract

In this chapter we summarize our current understanding of SEPs, of properties of the sites of their origin and of the physical processes that accelerate or modify them. These processes can leave an indelible mark on the abundances of elements, isotopes, ionization states, anisotropies, energy spectra and time profiles of the SEPs. Transport of the ions to us along magnetic fields can impose new variations in large events or even enhance the visibility of the source parameters as the SEPs expand into the heliosphere. We lack physical models that can follow the complexity of SEP abundance variations.

What is our current understanding of solar energetic particles (SEPs)?

• 1. All acceleration of the SEPs that we see in space occurs on magnetic field lines that are open to particles of that magnetic rigidity. We also see γ rays and neutrons from nuclear reactions of similar SEPs on closed field lines in solar flares, but no charged products of those nuclear reactions are seen in space. Neither the primary nor the secondary ions can escape, instead they heat the flares.

• 2. There are two acceleration sites for the SEPs we see in space: solar jets and CME-driven shock waves. (A) “Impulsive” SEP events, accelerated at solar jets, appear to involve two physical mechanisms, magnetic reconnection and resonant wave-particle absorption. Both produce striking, and identifiable, enhancements of abundances of chemical elements and isotopes. (B) For “gradual” SEP events the dominant mechanism is acceleration by CME-driven shock waves, but the seed population may be complex, and abundances are also modified by pitch-angle scattering during transport in large events.

• 3. Impulsive SEP events are small and brief. Solar jets, where acceleration occurs, are associated with slow, narrow CMEs. Magnetic reconnection in jets, sampling ions of 2–4 MK plasma in active regions, cause abundance enhancements rising as a steep power law in A/Q by factors up to ~1000 from H to Pb. Wave-particle resonance causes large, highly variable, enhancements in ³He/⁴He by factors up to 10,000 that vary strongly with the ion energy and may sometimes cause rounded, steep, low-energy spectra of ions with gyro-frequencies near the second harmonic of the ³He gyro-frequency. The waves may be generated by the copious streaming electrons that also produce type III radio bursts. Acceleration may occur near 1.5 R_S and ions traverse enough material for electron stripping to attain equilibrium velocity-dependent Q, but not enough energy loss to disrupt the strong high-Z enhancements that are seen. “Pure” impulsive events (SEP1), lack shock acceleration. Local shocks can reaccelerate SEP1 ions plus “excess” protons from the ambient corona (SEP2).

• 4. Gradual SEP events are large, energetic, and intense. They sometimes accelerate multi-GeV protons, and they have long durations and broad spatial extent, often exceeding ~180°. They are associated with fast, wide CMEs that drive shock waves that accelerate ions from ambient coronal plasma of ~0.8–1.6 MK in ~69% of the events (SEP4). In 24% of gradual events the shock waves pass through solar active regions where they sample a seed population that includes ambient plasma laced with residual suprathermal ions from pools fed by multiple small solar jets (SEP3). The seed population and source-plasma temperature can vary across the face of a shock. The location of high-energy spectral breaks or knees depends upon both shock properties and A/Q of the ion species, causing complex abundance variations at high energies. Shock waves begin to form near 1.5 R_S, first accelerating electrons that produce type II radio bursts; acceleration of SEPs can begin above the tops of magnetic loops by 2–6 R_S, depending upon longitude around the CME.

• 5. Self-amplified Alfvén waves become increasingly important in larger gradual SEP events. Pitch-angle scattering by proton-amplified waves limits particle intensities at the streaming limit, alters initial element abundance ratios after onset, rapidly broadens angular distributions, and flattens low-energy spectra during the early intensity-plateau period. Preferential scattering of ions with lower A/Q during transport causes regions of relative A/Q-dependent abundance enhancements or depletions in space that evolve with time. This Q-dependence allows determination of the source plasma temperature. In contrast, non-relativistic electrons and particles from small impulsive SEP events travel scatter free. Larger events are increasingly dominated by self-generated waves, but SEPs become scatter-free again later in the reservoir behind the CME (see (7) below). Wave growth and scattering depend upon rigidity, spatial location, and time during a large SEP event.

• 6. Can we always distinguish impulsive and gradual events? Usually, but not always. Shocks often reaccelerate residual impulsive suprathermal ions with pre-enhanced abundances. Some SEP events, called “impulsive” because of their high Fe/O enhancement, for example, may have also undergone reacceleration by a shock wave. These (SEP2) events may be distinguished by their abundance of excess protons sampled from the ambient plasma. Impulsive seed ions reaccelerated by wide, fast shock waves from pools (SEP3) are usually distinguished by lower Fe/O and fewer abundance fluctuations.

• 7. Reservoirs are large volumes of adiabatically-trapped SEPs seen late in gradual events. Particles are magnetically trapped between the CME and the Sun with negligible leakage. Intensities of all species and energies are spatially uniform but all decrease with time as the trapping volume expands. Early workers mistook this slow decline as slow spatial diffusion. Actually, particles in reservoirs propagate nearly scatter-free since most waves have been absorbed. Reservoirs often provide the high-energy particles that slowly precipitate to produce long-duration, spatially-extensive, energetic γ-ray events when they scatter into the magnetic loss cone and interact in the denser corona below.

• 8. In large events, CMEs capture the largest share of the magnetic energy released at the Sun and SEPs can acquire as much as ~15% of a CME’s energy. In flares, SEPs capture 30–60% of the energy of magnetic reconnection, but those SEPs do not escape from closed loops, they scatter into the loss cone and dump their energy into the footpoints of the loops. Trapping creates hot, bright flares.

• 9. SEPs, at energies above a few MeV amu⁻¹, and ions of the slow solar wind, show differences in the pattern of their element abundances relative to corresponding photospheric abundances as a function of first ionization potential, FIP, specifically for the elements C, P, and S. SEPs show a source where C, P, and S is less likely to be ionized crossing the chromosphere. Such differences are determined near the base of the corona, long before acceleration, so that SEPs and the solar wind must be derived from different coronal regions. Thus, SEPs are not merely accelerated solar wind but an independent sample of the solar corona. Theory, based upon the ponderomotive force of Alfvén waves, suggests that material that will become SEPs is transported up into the corona along closed field lines as in active regions, while that forming the solar wind arrives on open field lines. It is ironic that the SEP ions accelerated on open field lines probably entered the corona on closed field loops.

• 10. The properties discussed above distinguish four patterns of element abundances shown in Table 10.1.

Table 10.1 Properties of the four SEP element abundance patterns

We have seen that some processes depend upon particle velocity and others depend upon magnetic rigidity. Early studies could not distinguish these processes using proton spectra alone. Ions highlight rigidity dependence of abundances upon A/Q at constant velocity, giving us new leverage on the underlying physics, as well as the nature of the source plasma and even an estimate of its temperature. For the SEP4 events, the abundances and energy spectral indices can be correlated. With spectra of the form E^y and abundance enhancements of the form (A/Q)^x we find y = x/2–2.; it is not yet clear why. This relationship provides new information on the “injection problem,” i.e. on the way shocks select ions from the plasma. For SEP2 and SEP3 events, the power-law abundances of ions from impulsive events are so distinctive that this signature can be followed through reacceleration by a strong shock.

Thus, some of the early mysteries of SEP origin seem to be resolved, even though many new questions have arisen. The progress has come almost entirely from the direct measurement of SEPs in space, especially from their abundances. The story is complex. It involves acceleration and reacceleration of ions that, nevertheless, carry measurable properties of their convoluted histories. We have identified the physical mechanisms that contribute to particle acceleration and developed new tools to explore them. What remains is to understand their detailed interplay. What parameters determine when and where each mechanism operates, and how can we predict their onset, their magnitude and their outcome? Other questions abound. How is it that reservoirs are so uniform and so well maintained? What causes He-poor events? We anticipate the next generation of understanding.

Theories and models of acceleration often treat element abundances as adjustable parameters – or not at all. However, we now know absolute coronal abundances sampled at the source and we need theories that follow ion injection and map those abundances through acceleration and transport into observations. Spectra and abundances are correlated, at least for many SEP4 events. How does the shock-shape matter? Element abundances probe the physics of SEPs. Only proton predictions are required for astronaut safety, but a model that could predict the complex abundance variations in a large SEP event could gain a powerful badge of quality and reliability. We have no such model today.