Journal of Geophysical Research: Space Physics最新文献

Although Strong Thermal Emission Velocity Enhancement (STEVE) and subauroral ion drifts (SAID) are often considered in the context of geomagnetically disturbed times, we found that STEVE and SAID can occur even during quiet times. Quiet-time STEVE has the same properties as substorm-time STEVE, including its purple/mauve color and occurrence near the equatorward boundary of the pre-midnight auroral oval. Quiet-time STEVE and SAID emerged during a non-substorm auroral intensification at or near the poleward boundary of the auroral oval followed by a streamer. Quiet-time STEVE only lasted a few minutes but can reappear multiple times, and its latitude was much higher than substorm-time STEVE due to the contracted auroral oval. The THEMIS satellites in the plasma sheet detected dipolarization fronts and fast flows associated with the auroral intensification, indicating that the transient energy release in the magnetotail was the source of quiet-time STEVE and SAID. Particle injection was weaker and electron temperature was lower than the events without quiet-time STEVE. The plasmapause extended beyond the geosynchronous orbit, and the ring current and tail current were weak. The interplanetary magnetic field (IMF) B_z was close to zero, while the IMF B_x was dominant. We suggest that the small energy release in the quiet magnetosphere can significantly impact the flow and field-aligned current system.

The Martian ionosphere-thermosphere (I-T) coupling is variable due to complex variations of the driving factors such as atmospheric tides and crustal magnetic fields. In this study, variability of the I-T coupling in longitude structures was investigated using a series of data segments of the MGS ionospheric measurements. Measurements in each data segment can cover different longitudes, and the solar forcing and local solar time just change a little. Ionospheric and thermospheric longitude variations are statistically correlated. Ionospheric peak electron density (N_mM₂) decreases while ionospheric main peak height (h_mM₂) increases with increasing neutral scale height (H_n) along longitudes. These correlated longitude variations are consistent with the photochemical coupling that H_n longitude disturbances induce ionospheric longitude structure through photochemical processes. Statistically, N_mM₂ is a better indicator than h_mM₂ for the H_n disturbances in the lower thermosphere. H_n longitude variation intensity is a crucial factor affecting the photochemical I-T coupling in longitude structures; it is closely related to N_mM₂ longitude variation intensity and tends to decline with increasing altitudes. The I-T coupling in longitude structures tends to decline near the terminator, which is in line with the declining longitude variation of H_n with increasing altitudes since h_mM₂ significantly increases near the terminator. Moreover, it tends to enhance at high solar activity level due to increased photoionization rate. The I-T coupling in longitude structures also shows seasonal dependence, as seasonal variation of h_mM₂ can affect the H_n longitude variation intensity nearby the ionospheric main peak.

To explore the asymmetry in ion and electron heating at Earth's magnetotail at mid-tail distances ( −30 $��R_E�$), we analyze near-simultaneous observations of reconnection outflows from two opposite sides of reconnection sites at those distances using Magnetospheric Multiscale (MMS) and Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) data. We report a pronounced temperature asymmetry between the earthward and tailward reconnection outflows. The asymmetry is more significant for electrons than for ions: Earthward moving ions are only three times hotter than tailward ones, but earthward moving electrons are 5–20 times hotter than tailward ones. The closed field-line topology on the earthward side of the reconnection region, as opposed to the open topology on the tailward side, is likely a critical contributor to this asymmetry. These findings cast light on the underlying mechanisms of particle heating and energization in magnetotail reconnection, highlighting the significant role of Earth's dipolar magnetic field. This study offers insights for refining magnetic reconnection models, emphasizing the importance of incorporating realistic magnetic field topologies to accurately simulate the heating and energization processes observed in space plasma environments.

The rapidly expanding fleet of low-altitude CubeSats equipped with energetic particle detectors brings new opportunities for monitoring the dynamics of the radiation belt and near-Earth plasma sheet. Despite their small sizes, CubeSats can carry state-of-the-art instruments that provide electron flux measurements with finer energy resolution and broader energy coverage, compared to conventional missions such as POES satellites. The recently launched CIRBE CubeSat measures 250–6,000 keV electrons with extremely high energy resolution, however, CIRBE typically only measures locally-trapped electrons and cannot directly measure the precipitating electrons. This work aims to develop a technique for identifying indications of nightside precipitation using the locally-trapped electron measurements by the CIRBE CubeSat. This study focuses on two main types of drivers for nightside precipitation: electron scattering by the curvature of magnetic field lines in the magnetotail current sheet and electron scattering by resonance with electromagnetic ion cyclotron (EMIC) waves. Using energy and pitch-angle resolved electron fluxes from the low-altitude ELFIN CubeSat, we reveal the features that distinguish between these two precipitation mechanisms based solely on locally-trapped flux measurements. Then we present measurements from four CIRBE orbits and demonstrate the applicability of the proposed technique to the investigation of nightside precipitation using CIRBE observations, enabling separation between precipitation induced by curvature scattering and EMIC waves in nearby regions. Our study underscores the feasibility of employing high-energy-resolution CIRBE measurements for detecting nightside precipitation of relativistic electrons. Additionally, we briefly discuss outstanding scientific questions about these precipitation patterns that could be addressed with CIRBE measurements.

Magnetic dips are the localized depression of magnetic field in the inner magnetosphere and are suggested to play an important role in the formation of the butterfly distribution of relativistic electrons in the radiation belts. In this study, we conduct test-particle simulations to trace the electrons' trajectory within a magnetic dip and evaluate the response of PAD based on the long-term averaged flux of electrons between $��L�$ = 3–6 from Van Allen Probes. Our results show that the electron dynamics are significantly changed by magnetic dips, especially at the dip center. In a magnetic dip, the electrons' energy, $��L�$-shell, and pitch angle decrease, and the variation in $��L�$-shell is more significant than the pitch angle and energy. Based on the observational electron fluxes, the electron butterfly-like distributions are well reproduced by the simulation. Moreover, the parameterizations reveal that the butterfly distribution of electrons is closely related to the electron's energy, location, and depth of the magnetic dip. A negative radial gradient of electron flux also plays a potentially crucial role in the formation of the electron butterfly distribution. Our study provides deep insights into the evolution of the butterfly distribution of relativistic electrons within magnetic dips.

Understanding the temporal and spatial variations in the ideal inert tracer helium can provide insight into the dynamic evolution of the thermosphere. The magnitude of the thermospheric winter helium bulge was inversely correlated with the level of solar activity. However, this feature has been found to be not reproduced by the Thermosphere-Ionosphere Electrodynamic General Circulation Model (TIEGCM), and the associated physical mechanisms remain unknown. Using the Thermosphere-Ionosphere-Mesosphere Electrodynamic General Circulation Model (TIME-GCM), we found that mesospheric gravity wave drag (GWD) is a factor contributing to this inverse correlation. Specifically, the summer-to-winter circulation in thermosphere becomes the main cause of the helium bulge, as mesospheric GWD can play a role in strengthening this circulation. The GWD contributions to temperature change below the lower thermosphere do not depend prominently on solar activity. However, because the temperature impacts on the pressure gradient force are height-integrated according to the background temperature of the neutral gas, the higher background temperature in the thermosphere at the solar maximum corresponds to a relatively weaker response in pressure gradient force in the thermosphere. Therefore, the response of the thermospheric circulation that might be expected to accompany increasing solar activity is suppressed due to the influence of mesospheric GWD, which results in a decrease in the magnitude of the winter helium bulge with increasing solar activity. Thus, our results demonstrated that lower atmosphere forcing can play a significant role in the response of thermospheric helium to solar activity.

This study investigates the relative significance of gravity wave and gravity dynamo effects in large-scale wave structure (LSWS) development using the coupled Sami3 is Also a Model of the Ionosphere (SAMI3) and Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (SD-WACCM-X). Simulations show significant vertical E × B drift perturbations associated with gravity waves in the F region after ∼1700 LT, leading to LSWS near midnight. Notably, LSWS can occur independently of gravity-driven dynamo current, emphasizing the significance of the gravity wave wind dynamo mechanism. However, LSWS exhibits more pronounced vertical E × B drift perturbations, indicating the involvement of background wind fields. Both gravity wave and background wind dynamo effects cause LSWS to grow vertically by ∼20 km and extend to ±10° in latitude. Gravity-driven Pedersen current, therefore, plays a role in amplifying the upwelling growth and equatorial plasma bubble development. Furthermore, simulations demonstrate the emergence of predawn ionospheric irregularities in the bottomside F layer, even without gravity-driven currents, attributed to concentric gravity waves over the magnetic equator. A comparison between FORMOSAT-7/COSMIC2 and SAMI3 ion density is also conducted. These findings emphasize the significant influence of gravity waves and background wind fields on the formation of LSWS and irregularities.

Large-scale patterns of the steady-state magnetosheath plasma flow and their dependence on the interplanetary magnetic field (IMF) have been reconstructed for the first time on the basis of large multi-year multi-mission pool of spacecraft observations, concurrent interplanetary data, and an empirical high-resolution model. The flow model architecture builds upon a recently developed magnetosheath magnetic field representation by flexible expansions of its toroidal and poloidal components in a coordinate system, naturally conformed with the magnetopause and bow shock shapes. The model includes two physics-based flow symmetry modes: the first one treats the magnetosphere as an axisymmetric unmagnetized obstacle, whereas the second mode takes into account the geodipole tilt, an important factor in the reconnection effects. The spacecraft data pool includes 1-min average data by Themis (2007–2024), Cluster (2001–2022), and MMS-1 (2015–2024) missions, as well as OMNI interplanetary data. The model drivers include the solar wind particle flux, IMF components, and the geodipole tilt angle. The model calculations faithfully reproduce the average plasma flow geometry and substantial effects have been found of the IMF orientation and magnitude, a principal factor that defines electromagnetic forces inside the magnetosheath. A strong dependence of the magnetosheath flow patterns on the Earth's dipole tilt indicates an important contribution of reconnection effects at the magnetopause to the solar wind particle transport around the dayside magnetosphere.

This study examines the role of winds in wintertime sporadic E layer intensification (WEsLI) in 2009 from a global viewpoint. Previous studies showed that sporadic E layer (EsL) intensity had increased for 20–30 days in some winters, although intense EsLs do not form generally in winter. A recent study found that vertical ion convergence (VIC) driven by intensified migrating semidiurnal (SW2) tides caused WEsLI at middle latitudes in 2009. However, no studies have investigated the global distributions and generation mechanisms of WEsLI in 2009. Herein, we employed FORMOSAT-3/COSMIC radio occultations to investigate the global distributions of WEsLI in 2009. Distributions of VIC driven by winds obtained from the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy were compared with global WEsLI distributions to elucidate the role of winds in WEsLI. We found that WEsLI in 2009 occurred at geomagnetic low/middle latitudes except between $��60�°�$W and $��80�°�$E. WEsLI was observed below 120 km altitudes, especially at 12–17 local times. WEsLI was attributable to VIC driven by SW2 tides, migrating diurnal tides, and eastward propagating diurnal tides with wavenumber 3. Tidal amplifications were possibly related to mesospheric/stratospheric atmospheric variations such as sudden stratospheric warming, zonal mean zonal winds, and quasi-biennial oscillations. WEsLI in 2009 is further evidence of the coupling between EsLs and mesospheric/stratospheric atmospheric variations through tidal modifications.