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Abstract We examine the statistics of Alfvénic deflections in sub- and super-Alfvénic solar wind with particular focus on the magnetic deflection angleθ. Our findings are in general agreement with earlier studies suggesting thatθ > 90° rarely occurs in sub-Alfvénic regimes. We find the upper limit ofθexhibits an identifiable trend with the Alfvén Mach numberM_a, suggesting that gradual steepening of Alfvénic deflections with increasingM_ais a plausible mechanism controlling deflection angles in the young solar wind. Further analysis reveals that large velocity fluctuations (δv/v > 1) tend to be important in the largest sub-Alfvénic deflections with increasing contributions fromδv_∥approachingM_a = 1, while few deflections in the super-Alfvénic regime exhibit such large velocity perturbations. We also determine the local ratio of radial Poynting fluxS_Rto kinetic energy fluxK_Rand find that large sub-Alfvénic deflection angles tend to be dominated byS_R, while super-Alfvénic deflections are eventually dominated by theK_Rassociated with the radial solar wind flow. Our results show that near the Alfvén surface (whereM_a = 1), there is a critical region of parameter space whereδv ∼ v_aandK_R/S_R ∼ 1. We refer to this region (where $\mid {\log }_{10}\left(M_a\right)\mid <0.2$) as the conversion layer. The conversion layer may play a significant role in the evolution of magnetic defections by providing the medium for converting magnetic energy to particle energy and likely driving the formation of magnetic switchbacks in super-Alfvénic solar wind. 

ABSTRACT Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfvénic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfvénic environment where kinetic energy and turbulence become more prominent. Switchbacks (i.e. large directional deviations from the mean field) are shown to occur almost exclusively in the super-Alfvénic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfvén Mach number ($$M_\mathrm{ A}$$) and switchback parameter (Z). We find that the fluctuation of the magnetic field magnitude normalized to the mean magnetic field and of the radial velocity normalized to local Alfvén speed increase with both Alfvén Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfvén Mach number range of $$\sim$$4–6. Results are interpreted in terms of the Chandrasekhar criterion for Kelvin–Helmholtz activity. Overall, these findings are in agreement with earlier studies suggesting switchback generation through non-linear shear flow dynamics. 

Abstract Close to Earth, the solar wind is usually super-Alfvénic, i.e., the speed of the solar wind is much larger than the Alfvén speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfvénic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfvénic flow, R. Bandyopadhyay et al. performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we reexamine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency, and switchback strength using PSP data from encounters 8–19. The later orbits probe lower altitudes and experience sub-Alfvénic conditions more frequently, providing a greater statistical coverage to contrast sub- and super-Alfvénic solar wind. These later orbits also extend the observations from near solar minimum at launch to near solar maximum conditions. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfvén speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfvénic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfvénic samples are more anisotropic than the super-Alfvénic samples, in general. Further, we show that the sub- and super-Alfvénic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing >90° switchbacks in the sub-Alfvénic solar wind. 

Context. Periodicities have frequently been reported across many wavelengths in the solar corona. Correlated periods of ~5 min, comparable to solar p -modes, are suggestive of coupling between the photosphere and the corona. Aims. Our study investigates whether there are correlations in the periodic behavior of Type III radio bursts which are indicative of nonthermal electron acceleration processes, and coronal extreme ultraviolet (EUV) emission used to assess heating and cooling in an active region when there are no large flares. Methods. We used coordinated observations of Type III radio bursts from the FIELDS instrument on Parker Solar Probe (PSP), of EUV emissions by the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) and white light observations by SDO Helioseismic and Magnetic Image (HMI), and of solar flare X-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April 12, 2019. Several methods for assessing periodicities are utilized and compared to validate periods obtained. Results. Periodicities of ~5 min in the EUV in several areas of an active region are well correlated with the repetition rate of the Type III radio bursts observed on both PSP and Wind. Detrended 211 and 171 Å light curves show periodic profiles in multiple locations, with 171 Å peaks sometimes lagging those seen in 211 Å. This is suggestive of impulsive events that result in heating and then cooling in the lower corona. NuSTAR X-rays provide evidence for at least one microflare during the interval of Type III bursts, but there is not a one-to-one correspondence between the X-rays and the Type III bursts. Our study provides evidence for periodic acceleration of nonthermal electrons (required to generate Type III radio bursts) when there were no observable flares either in the X-ray data or the EUV. The acceleration process, therefore, must be associated with small impulsive events, perhaps nanoflares. 

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.