Journal of Geophysical Research: Space Physics最新文献

Equatorial plasma bubbles (EPBs) pose significant challenges to trans-ionospheric radio communication and navigation systems. Because of their disruptive effects, evaluating and predicting them is key for the development of space systems. The influence of geomagnetic activity on EPBs remains unclear, partly due to limited low-latitude neutral wind measurements. Neutral winds drive electric fields and plasma transport, both of which are essential for EPB development. This study addresses this gap using a superposed epoch analysis of key parameters: geomagnetic indices, magnetic zonal and meridional neutral winds, and the $��\sigma �$ index (an indicator of EPB occurrence derived from in situ plasma density measurements). Using data from the Ionospheric Connection Explorer satellite between 2020 and 2022, and focusing on 18:00–23:00 solar local time, our analysis reveals regional differences in EPB behavior. In South America, EPB development is suppressed during the recovery phase of geomagnetic storms, whereas this effect is not detected in other longitudinal sectors. We also observe a global westward shift in zonal winds near the peak of geomagnetic disturbances. These findings improve our understanding of EPB dynamics and highlight the regional variability in their response to geomagnetic forcing.

Electromagnetic Ion Cyclotron (EMIC) waves and magnetosonic (MS) waves are frequently observed in the Earth's magnetosphere, playing critical roles in the loss of ring current ions and relativistic electrons. Recent observations suggest a potential energy transfer from MS waves to unusual high-frequency EMIC waves mediated by low-energy protons (10–100 eV). Specifically, low-energy protons are hypothesized to be heated perpendicularly by MS waves, subsequently generating EMIC waves near the local proton cyclotron frequency. However, this hypothesis has not been demonstrated self-consistently. Here, we employ a 2D particle-in-cell (PIC) simulation to investigate this process. Our results show that cold protons can be effectively energized, achieving a large temperature anisotropy $���\left(�T_{\perp }�/�T_{\Vert }�\approx �16�\right)��$ during the excitation of MS waves. This finding supports the feasibility of energy transfer from MS waves to EMIC waves. Furthermore, our study highlights the need for further investigations into the key parameters governing the occurrence and efficiency of this energy transfer process.

A high-altitude nuclear explosion (HANE) can transport plasma along geomagnetic field lines into distant regions, directly impacting the formation of artificial radiation belts and the regions of energy deposition. However, the early stage dynamics of ambient ion motion transition to field alignment remain poorly understood. In this paper, a two-dimensional hybrid model in the field-parallel plane is employed to analyze the dynamics of debris and ambient ions, with particular focus on the transition of ambient ions to field-aligned motion. The simulations reveal that while debris ions expand nearly linearly, ambient ions are accelerated across the magnetic-compression region perpendicularly within a single gyro-period and then transition toward the geomagnetic field-aligned motion. These transitions result from changes in the magnetic field direction at the boundary of the magnetic-compression region. This process requires efficient energy transfer from debris to the ambient plasma. Through a series of simulations, we delineate the regime dependence of this transition using a dimensionless parameter $��\Gamma �=�{\rho }_d�/�R_M�$ and quantify the fraction of ambient ions streaming along geomagnetic field lines. In the simulations, under the condition in two-dimensional simulations, more than 20% of ambient ions achieve field-aligned velocities exceeding the initial debris expansion speed. This study clarifies the physical origin and parameter dependence of ambient-ion field-aligned transport, providing new insights into plasma distribution following HANE events.

A high-altitude nuclear explosion (HANE) can transport plasma along geomagnetic field lines into distant regions, directly impacting the formation of artificial radiation belts and the regions of energy deposition. However, the early stage dynamics of ambient ion motion transition to field alignment remain poorly understood. In this paper, a two-dimensional hybrid model in the field-parallel plane is employed to analyze the dynamics of debris and ambient ions, with particular focus on the transition of ambient ions to field-aligned motion. The simulations reveal that while debris ions expand nearly linearly, ambient ions are accelerated across the magnetic-compression region perpendicularly within a single gyro-period and then transition toward the geomagnetic field-aligned motion. These transitions result from changes in the magnetic field direction at the boundary of the magnetic-compression region. This process requires efficient energy transfer from debris to the ambient plasma. Through a series of simulations, we delineate the regime dependence of this transition using a dimensionless parameter $��\Gamma �=�{\rho }_d�/�R_M�$ and quantify the fraction of ambient ions streaming along geomagnetic field lines. In the simulations, under the condition in two-dimensional simulations, more than 20% of ambient ions achieve field-aligned velocities exceeding the initial debris expansion speed. This study clarifies the physical origin and parameter dependence of ambient-ion field-aligned transport, providing new insights into plasma distribution following HANE events.

This study investigates the ionospheric response to the intense geomagnetic storm of 20–21 December 2015 over the equatorial and low-latitude South American sector. Ionosonde data from São Luís (2.6°S, 44.2°W; dip angle: 3.9°S) and four additional Brazilian stations were analyzed to examine variations in the peak height of the F2 layer (hmF2) and vertical plasma drift velocities. A notable feature of this event was the occurrence of post-midnight equatorial plasma bubbles (EPBs) during the recovery phase, an uncommon storm-time response. In contrast, a suppression of post-sunset EPBs was observed on 20 December, despite enhanced vertical drifts associated with the pre-reversal enhancement. The post-midnight EPBs are attributed to disturbance dynamo electric fields (DDEFs), which uplifted the F layer to higher altitudes and created favorable conditions for the Rayleigh-Taylor instability (RTI) growth rate $��\left({\gamma }_{�R�T�}\right)�$. On 20 December evening, the vertical drifts exceeded quiet-time reference at equatorial stations, reflecting undershielding Prompt Penetration Electric Field activity. However, the absence of post-sunset EPBs under seemingly favorable RTI conditions suggests the influence of additional suppressive mechanisms. The suppression of EPBs may be attributed to disturbance meridional winds driven by DDEFs, which generate interhemispheric asymmetries in the Equatorial Ionization Anomaly. These asymmetries can increase the integrated flux-tube E-region conductivity along magnetic field lines. Additionally, enhanced chemical recombination—associated with storm-time variations in the $��O�/�N_2�$ ratio—may further reduce $��{\gamma }_{�R�T�}�$ under geomagnetically disturbed conditions.

The Geospace Dynamics Constellation (GDC) mission aims to investigate the dynamic coupling between the magnetosphere, ionosphere, and thermosphere by resolving key spatiotemporal processes at scales ranging from local to global. A key aspect is GDC's ability to reconstruct hemispheric-scale high-latitude electrodynamics with comprehensive measurements at multiple local times. This study evaluates the accuracy of GDC reconstructions of electric potential (PHI) and Joule Heating (JH) derived from the AMPERE-derived electrodynamic properties of the high-latitude ionosphere (ADELPHI) and Weimer 2005 models by comparing them to the original model outputs used as ground truth. Accuracy is assessed across four selected geomagnetic storm events with GDC reconstructions spanning from Day 91 after GDC launch to the end of the mission using a 20-day cadence. As the mission progresses, the constellation evolves into three well-separated orbital pairs with increasing local time separation, significantly improving the PHI and JH reconstruction accuracy. To assess accuracy, we compute several performance metrics. Results from both models show that for the specific application of reconstructing large-scale high-latitude electrodynamics, performance metrics improve over mission time as the constellation evolves, reflecting better sampling and constraint for the reconstructions. Notably, however, even during the early mission stage, reconstructions can sometimes be as accurate as in the later stage, depending on the fortuitous sampling of appropriate local time sectors. This quantitative assessment underscores the critical role of orbital geometry and sampling diversity in fulfilling GDC's science objectives and advancing the understanding of space weather dynamics. It also provides a tool for optimizing the GDC orbital characteristics.

Using the Low lAtitude long Range Ionospheric raDar (LARID) at Dongfang (19.2°N, 108.8°E), Hainan Island, China, we report a unique case of complex ionospheric irregularities observed on 9 June 2024. The most interesting aspect is the first-time long-range (∼2,000 km) detection of daytime ionospheric irregularities and post-sunset band-like irregularity structures by HF radar at low latitudes. By incorporating VHF radar and global navigation satellite system (GNSS) rate of total electron content index (ROTI) observations, the radio wave propagation modes of the ionospheric irregularity events observed by LARID and evolution of these ionospheric irregularities were analyzed. The daytime ionospheric irregularity echoes observed by LARID over the Indian Ocean exhibited westward drift, evident in both echo patterns and Doppler velocities. These ionospheric echoes were likely backscattered from E region field aligned irregularities by the “downleg” of the 1-hop HF ray path. The post-sunset irregularity echoes observed by LARID over Indian Ocean manifested as a band-like structure. This structure was closely attached to the bottom of ground/sea scatter echoes and showed the same range variation as the ground/sea scatters. GNSS ROTI observation revealed that irregularities primarily appeared south of the magnetic equator and drifted eastward across the eastern longitudes in later hours. Analysis indicates that the displacement of small-scale irregularities within the equatorial plasma bubble event along the magnetic field likely contributed to the band-like irregularity structure observed by LARID and the asymmetric irregularity distribution observed by GNSS.

Energetic electron precipitation, often in association with enhanced geomagnetic activity, leads to increased D-region ionization in the auroral region causing auroral absorption. A statistical auroral absorption model is presented for high-latitudes (poleward of 50° magnetic latitude) based on data collected from 2010 to 2019 from 13 wide-beam riometers at 30 MHz spanning 53.9°–86.9° magnetic latitude. Hourly maximum absorption values were sorted into bins of 5° magnetic latitude and 1 hr of magnetic local time and parameterized based on the hourly maximum global AE index for 0–100 nT, 100–200 nT, 200–300 nT, 300–400 nT, 400–500 nT, 500–600 nT, 600–800 nT, 800–1,000 nT, and ≥1,000 nT. The model uses spherical cap harmonic analysis with a maximum degree index and order of 12 and 10, respectively. Absorption is characterized by a dawnside enhancement that peaks in the pre-noon sector and spreads toward the midnight sector with increasing geomagnetic activity. The maximum equatorward and poleward extents of absorption >0.5 dB expand at a rate of 1.4°/100 nT and 0.4°/100 nT, respectively. According to this model, >0.5 dB absorption is expected in >10% of the high-latitude region for AE > 500 nT (occurs 23.6% of times), and 40% of the high-latitude region for AE ≥ 1,000 nT (occurs 3.2% of times). The statistical auroral absorption model may be used as a background model as a constraint for modeling instantaneous auroral absorption.

Eighty-five geomagnetic storms were simulated with the space weather modeling framework Geospace model using two different ionospheric conductance models. One set used the legacy conductances within the Ridley ionosphere model, while the other set used the newly developed conductance model for extreme events conductance model. As input and other model setup parameters were identical, we use this unique simulation suite to assess the impacts of ionospheric conductance on the ionospheric and magnetotail states during magnetic storms. The results show that higher conductance somewhat improves predictions of ground magnetic disturbances, as measured by the AL index. The cross-polar cap potential was lower for higher ionospheric conductance. In the magnetotail, higher ionospheric conductivity led to slightly lower ring current intensity and weakened Earthward flows, while the tail current magnitude was relatively insensitive to the conductance model. These results are an important step in understanding how the ionosphere impacts the magnetospheric state and the magnetosphere–ionosphere coupling.

Self-organization of planetary turbulence in persistent and geometrically well-defined flow features, has been attracting, recently, great scientific interest, thanks also to the spectacular images of Saturn's hexagon provided by the Voyager and Cassini missions. These flow patterns can be replicated in laboratory experiments with shallow rotating fluids, provided some characteristic non-dimensional parameters are suitably set up. In particular, we consider here prograde and retrograde (with respect to the rotation of the tank) hexagonal-shaped jets. Both Eulerian and Lagrangian data, directly reconstructed from the experiments, were analyzed and discussed. Our results are consistent with the conjecture that barotropic instability plays a key role in the genesis and maintenance of the hexagonal shape. We also show that the cross-analysis of experimental results and kinematic simulations on a simplified six-node meandering jet model allows, in principle, to formulate a scenario about the Lagrangian dispersion properties of the hexagon on Saturn in relation to its spatial and temporal characteristic scales.