Ivan Y Vasko

and 4 more

We present statistical analysis of 16,903 current sheets (CS) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The dataset is classified into about 90.6\% non-bifurcated and 9.4\% bifurcated CSs. Most of the CSs are proton kinetic-scale structures with the half-thickness of non-bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–$5\lambda_{p}$ and 0.7–$15\lambda_{p}$ in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m$^{2}$ or 0.04–$0.4J_{A}$ in units of local Alfv\’{e}n current density. The CSs demonstrate approximate scale-invariance with the shear angle and current density amplitude scaling with the half-thickness, $\Delta \theta\approx 16.6^{\circ}\;(\lambda/\lambda_{p})^{0.34}$ and $J_0/J_{A}\approx 0.14\;(\lambda/\lambda_{p})^{-0.66}$. The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting thereby its scale-invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X-line, but magnetic reconnection is probably suppressed by other processes. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.

Zijin Zhang

and 16 more

We investigate the dynamics of relativistic electrons in the Earth’s outer radiation belt by analyzing the interplay of several key physical processes: electron losses due to pitch angle scattering from electromagnetic ion cyclotron (EMIC) waves and chorus waves, and electron flux increases from chorus wave-driven acceleration of ~100-300 keV seed electrons injected from the plasma sheet. We examine a weak geomagnetic storm on April 17, 2021, using observations from various spacecraft, including GOES, Van Allen Probes, ERG/ARASE, MMS, ELFIN, and POES. Despite strong EMIC- and chorus wave-driven electron precipitation in the outer radiation belt, trapped 0.1-1.5 MeV electron fluxes actually increased. We use theoretical estimates of electron quasi-linear diffusion rates by chorus and EMIC waves, based on statistics of their wave power distribution, to examine the role of those waves in the observed relativistic electron flux variations. We find that a significant supply of 100-300 keV electrons by plasma sheet injections together with chorus wave-driven acceleration can overcome the rate of chorus and EMIC wave-driven electron losses through pitch angle scattering toward the loss cone, explaining the observed net increase in electron fluxes. Our study emphasizes the importance of simultaneously taking into account resonant wave-particle interactions and modeled local energy gradients of electron phase space density following injections, to accurately forecast the dynamical evolution of trapped electron fluxes.

Anton V Artemyev

and 4 more

Magnetic field-line curvature scattering (FLCS) of energetic particles in the equatorial magnetotail results in isotropization of pitch-angle distributions, loss-cone filling, and precipitation above a minimum energy at a given latitude. At a fixed energy, the lowest latitude of isotropization is the isotropy boundary (IB) for that energy. Nominally, the IB (latitude) exhibits a characteristic energy dependence due to the monotonic variation of the equatorial magnetic field intensity Beq with radial distance. Deviations from this nominal IB dispersion can occur if the radial Beq variation (spatial or temporal) is non-mononotic and/or if other precipitation mechanisms prevail. With its sensitive and detailed measurements of electron spectra up to relativistic energies, ELFIN’s recent observations reveal a variety of electron IBe patterns near magnetic midnight which are repeatable enough to warrant classification. This study aims to categorize the various IBe patterns observed by ELFIN’s high-fidelity but short lived dataset (a few months), compare them with simultaneous nearby POES observations, which are made with a limited energy coverage and resolution but last for decades, and discuss their possible interpretation. The general agreement between ELFIN and POES IB observations indicate a relatively large-scale nature of IBe patterns. Surprisingly, there exists a large number (up to 2/3 of all events) of non-monotonic-or steep/multiple-IB patterns. This suggest an abundance of non-trivial tail current sheet structures or a mixed contribution of two mechanisms in the vicinity of IBe in these cases.

Ethan Tsai

and 6 more

Energetic electron losses by pitch-angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler-mode waves. The efficacy of such precipitation is primarily controlled by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low-altitude, polar-orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field-aligned wave power across a wide-swath of local-time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications – wave obliquity, frequency spectrum, and local plasma density – to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi-linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN-observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi-field aligned waves to resonate with near $\sim1$ MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler-mode wave-driven relativistic electron precipitation match empirical expectations, and should therefore be included in future radiation belt modeling.