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Abstract The Parker Solar Probe (PSP) and Wind spacecraft observed the same plasma flow during PSP encounter 15. The solar wind evolves from a sub-Alfvénic flow at 0.08 au to become modestly super-Alfvénic at 1 au. We study the radial evolution of the turbulence properties and deduce the spectral anisotropy based on the nearly incompressible (NI) MHD theory. We find that the spectral index of thez⁺spectrum remains unchanged (∼−1.53), while thez⁻spectrum steepens, the index of which changes from −1.35 to −1.47. The fluctuating kinetic energy is on average greater than the fluctuating magnetic field energy in the sub-Alfvénic flow while smaller in the modestly super-Alfvénic flow. The NI MHD theory well interprets the observed Elsässer spectra. The contribution of 2D fluctuations is nonnegligible for the observedz⁻frequency spectra for both intervals. Particularly, the magnitudes of 2D and NI/slab fluctuations are comparable in the frequency domain for the modestly super-Alfvénic flow, resulting in a slightly concave shape ofz⁻spectrum at 1 au. We show that, in the wavenumber domain, the power ratio of the observed forward NI/slab and 2D fluctuations is  ∼15 at 0.08 au, while it decreases to  ∼3 at 1 au, suggesting the growing significance of the 2D fluctuations as the turbulence evolves in low Mach number solar wind. 

Abstract A multispecies energetic particle intensity enhancement event at 1 au is analyzed. We identify this event as a corotating interaction region (CIR) structure that includes a stream interface (SI), a forward-reverse shock pair, and an embedded heliospheric current sheet (HCS). The distinct feature of this CIR event is that (1) the high-energy (>1 MeV) ions show significant flux enhancement at the reverse wave (RW)/shock of the CIR structure, following their passage through the SI and HCS. The flux amplification appears to depend on the energy per nucleon. (2) Electrons in the energy range of 40.5–520 keV are accelerated immediately after passing through the SI and HCS regions, and the flux quickly reaches a peak for low-energy electrons. At the RW, only high-energy electrons (∼520 keV) show significant local flux enhancement. The CIR structure is followed by a fast-forward perpendicular shock driven by a coronal mass ejection (CME), and we observed a significant flux enhancement of low-energy protons and high-energy electrons. Specifically, the 210–330 keV proton and 180–520 keV electron fluxes are enhanced by approximately 2 orders of magnitude. This suggests that the later ICME-driven shock may accelerate particles out of the suprathermal pool. In this paper, we further present that for CIR-accelerated particles, the increase in turbulence power at SI and RWs may be an important factor for the observed flux enhancement in different species. The presence of ion-scale waves near the RW, as indicated by the spectral bump near the proton gyrofrequency, suggests that the resonant wave–particle interaction may act as an efficient energy transferrer between energetic protons and ion-scale waves. 

Abstract We study the solar cycle dependence of various turbulence cascade rates based on the methodology developed by Adhikari et al. that utilizes Kolmogorov phenomenology. This approach is extended to derive the heating rates for an Iroshnikov–Kriachnan (IK) phenomenology. The observed turbulence cascade rates corresponding to the total turbulence energy, fluctuating magnetic energy density, fluctuating kinetic energy, and the normalized cross helicity are derived from WIND spacecraft plasma and magnetometer data from 1995 through 2020. We find that (i) the turbulence cascade rate derived from a Kolmogorov phenomenology and an IK phenomenology changes with solar cycle, such that the cascade rate is largest during solar maximum and smallest during solar minimum; (ii) the turbulence energy Kolmogorov cascade rate increases fromθ_UB(angle between mean magnetic field and velocity) = 0° to 90° and peaks nearθ_UB= 90°, and then decreases asθ_UBtends to 180°; (iii) the 2D turbulence heating rate is larger than the slab heating rate; (iv) the 2D and slab fluctuating magnetic energy density cascade rates are larger than the corresponding cascade rates of the fluctuating kinetic energy; and (v) the total turbulence energy cascade rate is positively correlated with the solar wind speed and temperature and the normalized cross-helicity cascade rate. Finally, we find that the total turbulent energy Kolmogorov cascade rate is larger than the IK cascade rate.