Particle Acceleration at 5 au Associated with Turbulence and Small-scale Magnetic Flux Ropes

• Published in: The Astrophysical journal 2019-02, Vol.872 (1), p.4
• Main Authors: Zhao, L.-L., Zank, G. P., Chen, Y., Hu, Q., le Roux, J. A., Du, S., Adhikari, L.
• Format: Article
• Language: English
• Subjects: acceleration of particles; Astrophysics; Charged particles; Current sheets; Energetic particles; Fluctuations; Heliospheric current sheet; Interplanetary magnetic field; Islands; Magnetic clouds; Magnetic fields; Magnetic flux; Magnetic islands; magnetic reconnection; Magnetohydrodynamic turbulence; Particle acceleration; Particle intensity; Power law; Power spectral density; Proton flux; Protons; shock waves; solar wind; Spacecraft; Spectra; Turbulence; Ulysses spacecraft
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aafcb2

An observational analysis of an atypical energetic particle event near 5 au measured by the Ulysses spacecraft is presented. A previous study has attributed the unusual enhancement of energetic proton flux to the presence of small-scale dynamic magnetic islands or flux ropes. Here, we find that the generation of these magnetic islands may be related to the interaction of a stream interaction region (SIR) and the heliospheric current sheet (HCS). Particles are accelerated and trapped within the SIR structure characterized by a forward shock-reverse wave pair. Analysis of the energetic particle intensity spectra shows that the 63 keV-4.4 MeV protons form a power-law (PL) spectrum near the shock, with a slope much steeper than predicted by the diffusive shock acceleration (DSA) theory. Double PL spectra are found in the region of unusual particle flux enhancement, and the lower energy part of the spectrum gets harder farther away from the shock. In comparison, we discuss a later forward wave-reverse wave pair with an embedded large-scale magnetic cloud (MC). In contrast to small-scale magnetic islands, the MC corresponds to a decrease in energetic proton fluxes. A power spectral density analysis suggests that the turbulence level increases in the compression regions, and the majority of the observed turbulence power resides in the two-dimensional (2D) component because the spacecraft velocity is almost perpendicular to the interplanetary magnetic field. This is consistent with the enhanced generation of magnetic flux ropes, which are instrinsic to quasi-2D MHD turbulence, in the SIR. An automatic Grad-Shafranov reconstruction technique is used to identify flux rope structures within the period of interest, and their detailed parameters are included in the paper. The observational evidence suggests that the interaction of shock/compressional waves with the HCS may be a key element in generating small-scale dynamic magnetic islands, which subsequently accelerate charged particles and complement the classical DSA mechanism.