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Abstract In this Letter, we report observations of magnetic switchback (SB) features near 1 au using data from the Wind spacecraft. These features appear to be strikingly similar to the ones observed by the Parker Solar Probe mission closer to the Sun: namely, one-sided spikes (or enhancements) in the solar-wind bulk speed V that correlate/anticorrelate with the spikes seen in the radial-field component B R . In the solar-wind streams that we analyzed, these specific SB features near 1 au are associated with large-amplitude Alfvénic oscillations that propagate outward from the Sun along a local background (prevalent) magnetic field B 0 that is nearly radial. We also show that, when B 0 is nearly perpendicular to the radial direction, the large-amplitude Alfvénic oscillations display variations in V that are two sided (i.e., V alternately increases and decreases depending on the vector Δ B = B − B 0 ). As a consequence, SBs may not always appear as one-sided spikes in V , especially at larger heliocentric distances where the local background field statistically departs from the radial direction. We suggest that SBs can be well described by large-amplitude Alfvénic fluctuations if the field rotation is computed with respect to a well-determined local background field that, in some cases, may deviate from the large-scale Parker field. 

A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvén-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ( $$P_\textrm {AWb}$$ ) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $$r$$ , which we denote by $$\chi _{H}(r)$$ , and the fraction that is transferred via work, which we denote by $$\chi _{W}(r)$$ . We find that $$\chi _{W}(r_{A})$$ ranges from 0.15 to 0.3, where $$r_{A}$$ is the Alfvén critical point. This value is small compared with one because the Alfvén speed $$v_{A}$$ exceeds the outflow velocity $$U$$ at $$r < r_{A}$$ , so the AWs race through the plasma without doing much work. At $$r>r_{A}$$ , where $$v_{A} < U$$ , the AWs are in an approximate sense ‘stuck to the plasma’, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $$r=r_{A}$$ , so the total rate at which AWs do work on the plasma at $$r>r_{A}$$ is a modest fraction of $$P_\textrm {AWb}$$ . We find that heating is more effective than work at $$r < r_{A}$$ , with $$\chi _{H}(r_{A})$$ ranging from 0.5 to 0.7. The reason that $$\chi _{H} \geq 0.5$$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed scale heights between the coronal base and $$r_{A}$$ . A given amount of heating produces more magnetic moment in regions of weaker magnetic field. Thus, paradoxically, the average proton magnetic moment increases robustly with increasing $$r$$ at $$r>r_{A}$$ , even though the total rate at which AW energy is transferred to particles at $$r>r_{A}$$ is a small fraction of $$P_\textrm {AWb}$$ . 

Abstract The Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called “switchbacks” (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals—and regions of solar wind plasma measured just before and after each SB—to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of an SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small-scale structures at the SB edges. 

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere.