Results from two-dimensional particle-in-cell simulations of collisionless magnetic reconnection with zero guide field discussed in this paper reveal that around the time when the reconnection rate peaks, electron velocity distributions become highly structured in magnetic islands and open exhausts. Rings, arcs, and counterstreaming beams are generic and lasting components of the exhaust electron distributions. The temporal dependence of electron distributions provides a perspective to explain an outstanding discrepancy concerning the degree of electron anisotropy in reconnection exhausts and enables inference of the reconnection phase based on observed anisotropic electron distributions. Some of the structures predicted by our simulations are confirmed by measurements from the Cluster spacecraft during its encounter with reconnection exhausts in the magnetotail.

We present the turbulence spectra of magnetic and electron density fluctuations in situ measured by Voyager 1 in the local interstellar medium from 2012 to 2019. The magnetic spectrum shows a Kolmogorov power law with a one-dimensional power-law index −5/3 at mk ≤ 10−8.8, where k is a wavenumber and m is the unit meter. A bulge of enhanced magnetic power is found at mk = 10−8.8–10−8.2. Meanwhile, the electron density spectrum also shows a Kolmogorov power law with a one-dimensional power-law index −5/3 in the inertial range. A bulge of enhanced power is found around the kinetic scales of mk ≈ 10−5–10−1. Based on the observational data, the relationships between the outer scale of the turbulent system and the powers of electron density and magnetic fluctuations are obtained. We then calculate the spectra locally for six individual time periods, within which the electron density and magnetic fluctuations are simultaneously observed. It is found that the power of perpendicular magnetic fluctuations is usually higher than that of parallel magnetic fluctuations, which indicates the dominance of Alfvén waves in turbulence spectrum. Part of the observed turbulence spectra reveal that the normalized parallel magnetic power exhibits a much higher intensity than the normalized electron density power in the local interstellar medium of low to moderate plasma beta (β = 0.1–0.8). This dominance in the parallel magnetic power cannot be explained by the linear magnetohydrodynamic modes alone and may be associated with the arc/spherically polarized Alfvén mode.

No abstract

Coherency (C) and ellipticity (ε) of electromagnetic ion cyclotron (EMIC) waves are studied using Cassini data in the Earth's dayside low‐latitude magnetosphere from L = 7 to 10. The results are compared with linear kinetic theory, 1‐D and 2‐D simulations. The EMIC waves are observed to occur in packets with multiple wave cycles. The wave cycles within a wave packet are observed to have the same general propagation angle θkB0 and polarization. In observations and 2‐D simulations, EMIC waves have a mixture of circular and elliptical polarization for θkB0<30°. This scattered ellipticity values are due to the superposition of multiple wave modes. For wave propagation angles 30° <θkB0<60°, the waves are highly elliptical (0.2<|ε|<0.7), where ε is the ratio of minor to major axis of the polarization ellipse. For θkB0>60°, the waves are nearly linearly polarized (|ε|≤0.1). This general trend is in good agreement with linear kinetic theory and 1‐D simulations. Observations indicate right‐hand (RH) wave packets to be interspersed with the left‐hand (LH) wave packets. We show for the first time from linear theory that ion temperature anisotropies can generate RH waves at large propagation angles and for plasma beta βi>0.05. The observed mixture of RH and LH waves in the magnetosphere could be due to this direct generation of RH waves. Observations and simulations show that EMIC waves are coherent with 0.5 < C < 1.0 for θkB0≤50°. Here C is measured as the maximum value of cross‐correlation coefficient between the transverse magnetic field components of the wave.