Preface
Auroral streamers are important meso-scale processes that transport plasma and magnetic energy and drive dynamic magnetosphere-ionosphere (MI) coupling and space weather. Although streamers are typically studied using imagers sensitive to energetic ((Formula presented.) 1 keV) electron precipitation, such as all-sky imagers, some are associated with low-energy ((Formula presented.) 1 keV) precipitation better captured by red-line auroral emissions. This paper reports such streamer-like red-line auroras observed poleward of a black aurora and an auroral torch, associated with a magnetospheric electron injection and braking ion flows. Using conjugate space-ground observations, quasilinear theory, and auroral forward modeling, we establish the first direct linkage between streamer-like red-line auroras and plasma sheet electron pitch-angle scattering by time-domain structures. These results underscore the importance of wave-driven diffuse auroral processes in generating low-energy auroral streamers, distinct from the conventional quasi-electrostatic coupling paradigm.
Abstract: Many planets in the solar system and across the Galaxy have hydrogen-rich atmospheres overlying more heavy element-rich interiors with which they interact for billions of years. Atmosphere–interior interactions are thus crucial to understanding the formation and evolution of these bodies. However, this understanding is still lacking in part because the relevant pressure–temperature conditions are extreme. We conduct molecular dynamics simulations based on density functional theory to investigate how hydrogen and water interact over a wide range of pressure and temperature, encompassing the interiors of Neptune-sized and smaller planets. We determine the critical curve at which a single homogeneous phase exsolves into two separate hydrogen-rich and water-rich phases, finding good agreement with existing experimental data. We find that the temperature along the critical curve increases with increasing pressure and shows the influence of a change in fluid structure from molecular to atomic near 30 GPa and 3000 K, which may impact magnetic field generation. The internal temperatures of many exoplanets, including TOI-270 d and K2-18 b, may lie entirely above the critical curve: the envelope is expected to consist of a single homogeneous hydrogen–water fluid, which is much less susceptible to atmospheric loss as compared with a pure hydrogen envelope. As planets cool, they cross the critical curve, leading to rainout of water-rich fluid and an increase in internal luminosity. Compositions of the resulting outer, hydrogen-rich and inner, water-rich envelopes depend on age and instellation and are governed by thermodynamics. Rainout of water may be occurring in Uranus and Neptune at present.
The federally endangered sister species, Eucyclogobius newberryi (northern tidewater goby, NTG) and E. kristinae (southern tidewater goby) comprise the California endemic genus Eucyclogobius, which historically occurred in all coastal California counties. Isolated lagoons that only intermittently connect to the sea are their primary habitat. Reproduction occurs during lagoon closure, minimizing marine dispersal and generating the most genetically subdivided vertebrate genus on the California coast. We present a new genome assembly for E. newberryi using HiFi long reads and Hi-C chromatin-proximity sequencing. The 980 Mb E. newberryi reference genome has an N50 of 34 Mb with 22 well-described scaffolds comprising 88% of the genome and a complete BUSCO (Benchmarking Universal Single-Copy Orthologs) score of 96.7%. This genome will facilitate studies addressing selection, drift, and metapopulation genetics in subdivided populations, as well as the persistence of the critically endangered E. kristinae, where reintroduction will be an essential element of conservation actions for recovery. It also provides tools critical to the recovery of the genetically distinct management units in the NTG, as well as broader ecological and evolutionary studies of gobies, the most speciose family of fishes in the world.
An intense surge in the equatorial electron temperature (Te) at sunrise, known as the morning Te overshoot, has been one of the defining ionospheric features since its discovery early in the Space Age. Despite decades of study, the behavior of the morning overshoot during geomagnetic storms remains poorly understood. We report a two-stage response of the morning Te overshoot to geomagnetic activity, uncovered by a neural network model. Electron temperatures show an initial enhancement during the storms main phase, followed by a drastic depletion exceeding 1000 K and disappearance of the overshoot in the recovery phase. This two-phase change aligns with the early influence of westward prompt penetration electric field, overtaken by the development of the eastward disturbance dynamo later in the storm. These electric field changes affect vertical plasma drifts that redistribute electron densities, modifying ionospheric cooling rates. Our findings provide new insights into the dynamics of one of the most widely studied ionospheric features and showcase the potential of new-generation digital twin models of near-Earth space environment to reveal previously unrecognized physical patterns.
The collisionless plasmas in space and astrophysical environments are intrinsically multiscale in nature, behaving as conducting fluids at macroscales and kinetically at microscales comparable to ion and/or electron gyroradii. A fundamental question in understanding the plasma dynamics is how energy is transported and dissipated across scales. Here, we present spacecraft measurements in the terrestrial foreshock, a region upstream of the bow shock where the solar wind population coexists with the reflected ions. In this region, the fluid-scale, ultralow-frequency waves resonate with the reflected ions to modify the velocity distributions, which in turn cause the growth of the ion-scale, magnetosonic-whistler waves. The latter waves then resonate with the electrons, and the accelerated electrons contribute to the excitation of electron-scale, high-frequency whistler waves. These observations demonstrate that the chain of wave-particle resonances is an efficient mechanism for cross-scale energy transfer, which could redistribute the kinetic energy and accelerate the particles upstream of the shocks.
Shock waves, the interface of supersonic and subsonic plasma flows, are the primary region for charged particle acceleration in multiple space plasma systems, including Earths bow shock, which is readily accessible for in-situ measurements. Spacecraft frequently observe relativistic electron populations within this region, characterized by energy levels surpassing those of solar wind electrons by a factor of 10,000 or more. However, mechanisms of such strong acceleration remain elusive. Here we use observations of electrons with energies up to 200 kiloelectron volts and a data-constrained model to reproduce the observed power-law electron spectrum and demonstrate that the acceleration by more than 4 orders of magnitude is a compound process including a complex, multi-step interaction between more commonly known mechanisms and resonant scattering by several distinct plasma wave modes. The proposed model of electron acceleration addresses a decades-long issue of the generation of energetic (and relativistic) electrons at planetary plasma shocks. This work may further guide numerical simulations of even more effective electron acceleration in astrophysical shocks.
The weakly ionized plasma in the Earths ionosphere is controlled by a complex interplay between solar and magnetospheric inputs from above, atmospheric processes from below, and plasma electrodynamics from within. This interaction results in ionosphere structuring and variability that pose major challenges for accurate ionosphere prediction for global navigation satellite system (GNSS) related applications and space weather research. The ionospheric structuring and variability are often probed using the total electron content (TEC) and its relative perturbations (dTEC). Among dTEC variations observed at high latitudes, a unique modulation pattern has been linked to magnetospheric ultra-low-frequency (ULF) waves, yet its underlying mechanisms remain unclear. Here using magnetically conjugate observations from the THEMIS spacecraft and a ground-based GPS receiver at Fairbanks, Alaska, we provide direct evidence that these dTEC modulations are driven by magnetospheric electron precipitation induced by ULF-modulated whistler-mode waves. We observed peak-to-peak dTEC amplitudes reaching ∼ 0.5 TECU (1 TECU is equal to 10 6 electrons/ m 2 ) with modulations spanning scales of ∼ 5-100 km. The cross-correlation between our modeled and observed dTEC reached ∼ 0.8 during the conjugacy period but decreased outside of it. The spectra of whistler-mode waves and dTEC also matched closely at ULF frequencies during the conjugacy period but diverged outside of it. Our findings elucidate the high-latitude dTEC generation from magnetospheric wave-induced precipitation, addressing a significant gap in current physics-based dTEC modeling. Theses results thus improve ionospheric dTEC prediction and enhance our understanding of magnetosphere-ionosphere coupling via ULF waves.
