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  1. Abstract Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We showmore »that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process.« less
    Free, publicly-accessible full text available December 1, 2023
  2. Abstract The formation, development, and impact of slow shocks in the upstream regions of reconnecting current layers are explored. Slow shocks have been documented in the upstream regions of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the kglobal kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropesmore »in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine–Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma β , where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow.« less
    Free, publicly-accessible full text available February 1, 2023
  3. Abstract We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet images from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves from the Fermi/Gamma-ray Burst Monitor, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highlymore »intermittent in both space and time, presumably reflecting nonuniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated nonthermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of nonthermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of nonthermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects the inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of the reconnecting magnetic flux.« less
    Free, publicly-accessible full text available February 1, 2023
  4. Electrons in earth's magnetotail are energized significantly both in the form of heating and in the form of acceleration to non-thermal energies. While magnetic reconnection is considered to play an important role in this energization, it still remains unclear how electrons are energized and how energy is partitioned between thermal and non-thermal components. Here, we show, based on in situ observations by NASA's magnetospheric multiscale mission combined with multi-component spectral fitting methods, that the average electron energy [Formula: see text] (or equivalently temperature) is substantially higher when the locally averaged electric field magnitude [Formula: see text] is also higher. Whilemore »this result is consistent with the classification of “plasma-sheet” and “tail-lobe” reconnection during which reconnection is considered to occur on closed and open magnetic field lines, respectively, it further suggests that a stochastic Fermi acceleration in 3D, reconnection-driven turbulence is essential for the production and confinement of energetic electrons in the reconnection region. The puzzle is that the non-thermal power-law component can be quite small even when the electric field is large and the bulk population is significantly heated. The fraction of non-thermal electron energies varies from sample to sample between ∼20% and ∼60%, regardless of the electric field magnitude. Interestingly, these values of non-thermal fractions are similar to those obtained for the above-the-looptop hard x-ray coronal sources for solar flares.« less
    Free, publicly-accessible full text available May 1, 2023
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  6. Abstract A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field—switchbacks (SBs). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface, termed interchange reconnection, that leads to the ejection of flux ropes (FRs) into the solar wind. Observations also suggest that SBs undergo merging, consistent with an FR picture of these structures. The role of FR merging in controlling the structure of SBs in the solar wind ismore »explored through direct observations, analytic analysis, and numerical simulations. Analytic analysis reveals key features of the structure of FRs and their scaling with heliocentric distance R, which are consistent with observations and demonstrate the critical role of merging in controlling the structure of SBs. FR merging is shown to energetically favor reductions in the strength of the wrapping magnetic field and the elongation of SBs. A further consequence is the resulting dominance of the axial magnetic field within SBs that leads to the observed characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary. Finally, the radial scaling of the SB area in the FR model suggests that the observational probability of SB identification should be insensitive to R , which is consistent with the most recent statistical analysis of SB observations from PSP.« less
    Free, publicly-accessible full text available February 1, 2023
  7. Abstract Transport equations for electron thermal energy in the high- β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te / β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scatteringmore »when L c > L / β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.« less
    Free, publicly-accessible full text available December 1, 2022
  8. Observations in Earth’s turbulent magnetosheath downstream of a quasiparallel bow shock reveal a prevalence of electron-scale current sheets favorable for electron-only reconnection where ions are not coupled to the reconnecting magnetic fields. In small-scale turbulence, magnetic structures associated with intense current sheets are limited in all dimensions. And since the coupling of ions are constrained by a minimum length scale, the dynamics of electron reconnection is likely to be 3D. Here, both 2D and 3D kinetic particle-in-cell simulations are used to investigate electron-only reconnection, focusing on the reconnection rate and associated electron flows. A new form of 3D electron-only reconnectionmore »spontaneously develops where the magnetic X-line is localized in the out-of-plane (z) direction. The consequence is an enhancement of the reconnection rate compared with two dimensions, which results from differential mass flux out of the diffusion region along z, enabling a faster inflow velocity and thus a larger reconnection rate. This outflow along z is due to the magnetic tension force in z just as the conventional exhaust tension force, allowing particles to leave the diffusion region efficiently along z unlike the 2D configuration.« less
    Free, publicly-accessible full text available October 1, 2022
  9. Abstract The origin of switchbacks in the solar wind is discussed in two classes of theory that differ in the location of the source being either near the transition region near the Sun or in the solar wind itself. The two classes of theory differ in their predictions of the switchback rate (the number of switchbacks observed per hour) as a function of distance from the Sun. To distinguish between these theories, one-hour averages of Parker Solar Probe data were averaged over five orbits to find the following: (1) The hourly averaged switchback rate was independent of distance from themore »Sun. (2) The average switchback rate increased with solar wind speed. (3) The switchback size perpendicular to the flow increased as R , the distance from the Sun, while the radial size increased as R 2 , resulting in an increasing switchback aspect ratio with distance from the Sun. (4) The hourly averaged and maximum switchback rotation angles did not depend on the solar wind speed or distance from the Sun. These results are consistent with switchback formation in the transition region because their increase of tangential size with radius compensates for the radial falloff of their equatorial density to produce switchback rates that are independent of radial distance. This constant switchback rate is inconsistent with an in situ source. The switchback size and aspect ratio, but not their hourly average or maximum rotation angle, increased with radial distance to 100 solar radii. Additionally, quiet intervals between switchback patches occurred at the lowest solar wind speeds.« less
  10. 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 βmore »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.« less
    Free, publicly-accessible full text available December 1, 2022