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1. Prevalence of magnetic reconnection in the near-Sun heliospheric current sheet

   https://doi.org/10.1051/0004-6361/202039863  

   Phan, T. D. ; Lavraud, B. ; Halekas, J. S. ; Øieroset, M. ; Drake, J. F. ; Eastwood, J. P. ; Shay, M. A. ; Pyakurel, P. S. ; Bale, S. D. ; Larson, D. ; et al ( June 2021 , Astronomy & Astrophysics)

   null (Ed.)

   During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale heliospheric current sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5–107 solar radii during encounters 1, 4, and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absence of reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which are associated with “switchbacks”. Thus, the PSP findings suggest that large-scale dynamics, either locally in the solar wind or within the coronal source of the HCS (at the tip of helmet streamers), plays a critical role in triggering reconnection onset. 

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2. Analytic Model and Magnetohydrodynamic Simulations of Three-dimensional Magnetic Switchbacks

   https://doi.org/10.3847/2041-8213/ad335a  

   Shi 时, Chen 辰. ; Velli, Marco ; Toth, Gabor ; Zhang 张, Kun 琨. ; Tenerani, Anna ; Huang 黄, Zesen 泽森 ; Sioulas, Nikos ; van der Holst, Bart ( March 2024 , The Astrophysical Journal Letters)

   Parker Solar Probe observations reveal that the near-Sun space is almost filled with magnetic switchbacks (“switchbacks” hereinafter), which may be a major contributor to the heating and acceleration of solar wind. Here, for the first time, we develop an analytic model of an axisymmetric switchback with uniform magnetic field strength. In this model, three parameters control the geometry of the switchback: height (length along the background magnetic field), width (thickness along radial direction perpendicular to the background field), and the radial distance from the center of switchback to the central axis, which is a proxy of the size of the switchback along the third dimension. We carry out 3D magnetohydrodynamic simulations to investigate the dynamic evolution of the switchback. Comparing simulations conducted with compressible and incompressible codes, we verify that compressibility, i.e., parametric decay instability, is necessary for destabilizing the switchback. Our simulations also reveal that the geometry of the switchback significantly affects how fast the switchback destabilizes. The most stable switchbacks are 2D-like (planar) structures with large aspect ratios (length to width), consistent with the observations. We show that when plasma beta (β) is smaller than one, the switchback is more stable asβincreases. However, whenβis greater than 1, the switchback becomes very unstable as the pattern of the growing compressive fluctuations changes. Our results may explain some of the observational features of switchbacks, including the large aspect ratios and nearly constant occurrence rates in the inner heliosphere.

    

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3. A Solar Source of Alfvénic Magnetic Field Switchbacks: In Situ Remnants of Magnetic Funnels on Supergranulation Scales

   https://doi.org/10.3847/1538-4357/ac2d8c  

   Bale, S. D. ; Horbury, T. S. ; Velli, M. ; Desai, M. I. ; Halekas, J. S. ; McManus, M. D. ; Panasenco, O. ; Badman, S. T. ; Bowen, T. A. ; Chandran, B. D. ; et al ( December 2021 , The Astrophysical Journal)

   Abstract One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfvénic magnetic field reversals termed switchbacks . These δ B R / B ∼  ( 1 ) fluctuations occur over a range of timescales and in patches separated by intervals of quiet, radial magnetic field. We use measurements from PSP to demonstrate that patches of switchbacks are localized within the extensions of plasma structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma β and pressure, and by depletions in the magnetic field magnitude and electron temperature. These intervals are in pressure balance, implying stationary spatial structure, and the field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in Carrington longitude with a steeper leading edge and a small (∼1°) edge of hotter plasma and enhanced magnetic field fluctuations. Some structures contain suprathermal ions to ∼85 keV that we argue are the energetic tail of the solar wind alpha population. The structures are separated in longitude by angular scales associated with supergranulation. This suggests that these switchbacks originate near the leading edge of the diverging magnetic field funnels associated with the network magnetic field—the primary wind sources. We propose an origin of the magnetic field switchbacks, hot plasma and suprathermals, alpha particles in interchange reconnection events just above the solar transition region and our measurements represent the extended regions of a turbulent outflow exhaust. 

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4. Interchange reconnection as the source of the fast solar wind within coronal holes

   https://doi.org/10.1038/s41586-023-05955-3  

   Bale, S. D. ; Drake, J. F. ; McManus, M. D. ; Desai, M. I. ; Badman, S. T. ; Larson, D. E. ; Swisdak, M. ; Horbury, T. S. ; Raouafi, N. E. ; Phan, T. ; et al ( June 2023 , Nature)

   Abstract The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating 1,2 and interchange reconnection 3–5 . The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft 6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’ 7,8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts. 

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5. Magnetic Reconnection as the Driver of the Solar Wind

   https://doi.org/10.3847/1538-4357/acaf6c  

   Raouafi, Nour E. ; Stenborg, G. ; Seaton, D. B. ; Wang, H. ; Wang, J. ; DeForest, C. E. ; Bale, S. D. ; Drake, J. F. ; Uritsky, V. M. ; Karpen, J. T. ; et al ( March 2023 , The Astrophysical Journal)

   Abstract We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere. 

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