Distinguishing the Rigidity Dependences of Acceleration and Transport in Solar Energetic Particles
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Distinguishing the Rigidity Dependences of Acceleration and Transport in Solar Energetic Particles Donald V. Reames1
Received: 19 June 2020 / Accepted: 30 July 2020 / Published online: 13 August 2020 © Springer Nature B.V. 2020
Abstract In solar energetic particle (SEP) events, the power-law dependence of element abundance enhancements on their mass-to-charge ratios [A/Q] provides a new tool that measures the combined rigidity dependences from both acceleration and transport. Distinguishing between these two processes can be challenging. However, the effects of acceleration dominate when SEP events are small or when the ions propagate scatter-free, and transport can dominate the temporal time evolution of large events with streaming-limited intensities. Magnetic reconnection in solar jets produces positive powers of A/Q from +2 to +7 and shock acceleration produces mostly negative powers from −2 to +1 in small and moderate SEP events where transport effects are minimal. This variation in the rigidity dependence of shock acceleration may reflect the non-planar structure, complexity, and temporal time variation of coronal shocks themselves. Wave amplification by streaming protons in the largest SEP events suppresses the escape of ions with low A/Q, creating observed powers of A/Q from +1 to +3 upstream of the accelerating shock, decreasing to small negative powers downstream. For shock acceleration, the powers of A/Q are correlated with the energy spectral indices of He, O, and Fe, yet unexplained departures exist. Keywords Solar energetic particles · Solar system abundances · Coronal mass ejections · Shock acceleration
1. Introduction The processes of acceleration and transport of solar energetic particles (SEPs) can depend upon particle velocity and magnetic rigidity. What is often called particle “energy” [E], quoted as MeV amu−1 , is actually a measure of velocity E = E A = Mu (γ − 1) ≈ 1/2Mu β 2 , where E is the total kinetic energy, A is the atomic mass, Mu = mu c2 = 931.494 MeV,
B D.V. Reames
[email protected]
1
Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742-2431 USA
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γ = (1 − β 2 )−1/2 , and β = v/c is the particle velocity relative to the speed of light [c]. The magnetic rigidity, or momentum per unit charge, is P = pc/Qe = Mu βγ A/Q in units of MV, and it determines an ion’s magnetic interactions and scattering by Alfvén waves, for example. When we measure the abundances of different elements at a constant velocity, this rigidity dependence causes relative enhancements in SEP ion abundances that often vary as a power-law function of their mass-to-charge ratio A/Q, relative to their average or source abundances in the corona. This power-law dependence was first noticed by Breneman and Stone (1985) for the elements with atomic numbers 6 ≤ Z ≤ 30. These observations have been extended for many SEP events in recent years, and especially by including H and He (e.g. Reames, 2019b, 2020), but the relative contributions of acceler
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