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We survey crossings of Jupiter's dawn magnetopause during the Juno prime mission to identify and characterize Jupiter's magnetopause boundary layer. Using plasma and magnetic field observations from Jovian Auroral Distributions Experiment and Juno Magnetic Field investigation, we identify 53 boundary layer events from the 62 magnetopause crossings studied here. We find that the boundary layer generally exhibits mixed properties of magnetosheath and magnetosphere electron distributions, including lower characteristic electron energies and denser ion populations than in the magnetosphere, but higher characteristic electron energies and less dense ion populations than in the magnetosheath. Boundary layer proton speeds are on average slower than both the magnetosheath and magnetosphere. Other proton parameters in the boundary layer have intermediate values between the magnetosheath and magnetosphere. Through ion composition analysis in regions adjacent to the magnetopause, we find evidence of solar wind and magnetospheric plasma in the boundary layer that suggests plasma is transported across the magnetopause in both directions. This mass and energy transport may be the result of solar wind interactions such as magnetic reconnection and Kelvin-Helmholtz instabilities. However, many boundary layer events do not exhibit local signatures of these solar wind interactions and plasma may be transported by a non-local process or diffusively transported.

Volumetric Reconstruction of Ionospheric Electric Currents From Tri‐Static Incoherent Scatter Radar Measurements

JGR:Space physics - Thu, 08/22/2024 - 05:10

Abstract

We present a new technique for the upcoming tri-static incoherent scatter radar system EISCAT 3D (E3D) to perform a volumetric reconstruction of the 3D ionospheric electric current density vector field, focusing on the feasibility of the E3D system. The input to our volumetric reconstruction technique are estimates of the 3D current density perpendicular to the main magnetic field, j ⊥, and its covariance, to be obtained from E3D observations based on two main assumptions: (a) Ions fully magnetized above the E region, set to 200 km here. (b) Electrons fully magnetized above the base of our domain, set to 90 km. In this way, j ⊥ estimates are obtained without assumptions about the neutral wind field, allowing it to be subsequently determined. The volumetric reconstruction of the full 3D current density is implemented as vertically coupled horizontal layers represented by Spherical Elementary Current Systems with a built-in current continuity constraint. We demonstrate that our technique is able to retrieve the three dimensional nature of the currents in our idealized setup, taken from a simulation of an active auroral ionosphere using the Geospace Environment Model of Ion-Neutral Interactions (GEMINI). The vertical current is typically less constrained than the horizontal, but we outline strategies for improvement by utilizing additional data sources in the inversion. The ability to reconstruct the neutral wind field perpendicular to the magnetic field in the E region is demonstrated to mostly be within ±50 m/s in a limited region above the radar system in our setup.

Statistics and Models of the Electron Plasma Density From the Van Allen Probes

JGR:Space physics - Thu, 08/22/2024 - 05:00

Abstract

We use the full NASA Van Allen Probes mission (2012–2019) to extract the electron plasma density from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) and Electric Field and Waves (EFW) instruments and discuss the evolution of the plasmasphere. We generate new statistics including mean and standard deviations of the plasma density with respect to L-shell, magnetic local time (MLT), and various geomagnetic indices. These statistics are generated to be applied in radiation belt physics and space weather codes (with fits provided). The mean plasmasphere is circular around Earth with respect to MLT for Kp < 1. The mean 100 cm−3 level line is above L = 5 and mean 10 cm−3 level expands above the Van Allen Probes apogee for Kp < 1. The outer electron belt lies within the plasmasphere for 60% of all times. As activity increases (Kp > 2), a gradual MLT asymmetry forms with higher mean density in the afternoon sector due to plumes expanding outward. Conversely, the mean density decreases on the dawn and night sectors. The mean density is between ∼500 and ∼50 cm−3 between L ∼ 4 and L ∼ 6 during quiet and moderately active times (Kp < 3), representing ∼80% of all times. Statistics in regions of high density below L = 2 are underdefined for intense activity. The highest standard deviation of density represents a factor 2.5 to 3 times the mean above L = 5 and for active times. We find the percent difference between the EFW and EMFISIS densities is bounded by ±20% for quiet and moderate activity (Kp < 5) and goes up to ±100% for extreme activity.

The Spatiotemporal properties of afterslip following the 2016 Kaikoura earthquake in New Zealand from GPS observations: Its complementary and inherited patterns with coseismic slip

Geophysical Journal International - Thu, 08/22/2024 - 00:00

SummaryThe Mw 7.8 Kaikoura earthquake, which occurred on November 13, 2016, ruptured a complex system of strike-slip and reverse faults in northeastern South Island, New Zealand. However, the postseismic afterslip behavior and its relationship to the coseismic slip remain incompletely understood. This study investigates the spatiotemporal characteristics of afterslip following the mainshock by using four years of position data from 58 continuous GPS (cGPS) stations, considering the viscoelastic relaxation. Meanwhile, this study considers the contributions of crustal and the interface faults when exploring the combined effect of afterslip and viscoelastic relaxation. Results reveal substantial coseismic deformation northeastern of the epicenter, and postseismic displacements exhibit a continuation of the northeastward evolution. The primary coseismic slip occurred along the Kekerengu and Jordan Thrust faults, while secondary slip was accommodated by the Humps fault and the shallow subduction interface. Two primary afterslip zones are identified: one extending downdip from the secondary coseismic slip areas, and the other adjacent to shallow primary coseismic slip areas near the seismogenic Needles and Hope faults. The afterslip distribution exhibits a spatially complementary pattern to the coseismic slip areas, suggesting that velocity-strengthening zones may have hindered coseismic rupture propagation. The total seismic moment released by afterslip is estimated at ∼2.51×1020 N·m (Mw ∼7.53), approximately 30% of the coseismic moment. Meanwhile, about 80% of the postseismic seismic moment is ascribed to the slip along the southern subduction interface, suggesting the subduction fault plays an important role during postseismic slip. Temporal evolution modeling highlights that roughly 55% of the total afterslip moment was released within the initial three months. Postseismic afterslip dominated during the first month following the earthquake, with a slip rate of ∼10 mm/day. This rate subsequently decreased to ∼5 mm/day over the following two to three months, indicating that the majority of postseismic afterslip occurred shortly after the mainshock. In contrast to the earlier afterslip stages, the latter stages show continued movement along the Needles fault and the subduction interface. Cumulative peak slips have reached 2 cm since mid-2018, with fault slip rates decreasing to approximately 0.6-1.0 mm/day. This indicates ongoing afterslip at shallow faults and the subduction interface, with a steady slip rate over time. Importantly, the cumulative Coulomb stress changes induced by both coseismic slip and afterslip have increased the earthquake hazards potential near the Wellington fault, a densely populated region warranting further investigation.