Journal of Geophysical Research: Space Physics最新文献

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.

The extension of the post-sunset equatorial plasma bubbles (EPBs) to mid-latitudes during geomagnetic storms has been widely reported. However, previous research has primarily focused on the initiation and spatial distribution of these EPBs, while their decay processes remain underexplored. In this study, we provide observations of the evolution and dissipation of intense EPBs during the geomagnetic storm on 23–24 April 2023, using measurements from the Global-scale Observations of the Limb and Disk (GOLD), Swarm, and COSMIC-2 satellites. During the storm main phase, intense EPBs with zonal extents of ∼2–3° and latitudinal extents reaching up to ±35° magnetic latitude (Mlat) were observed between 15°W and 5°W. Simultaneously, very low electron density regions, spanning approximately from ±30° to ±45° Mlat and 420–500 km in altitude, were observed in the mid-latitudes. During the storm, the EPBs were observed to extend gradually poleward and subsequently encountered the low electron density regions. This encounter process, along with the storm-caused electric field dynamics, led to a gradual weakening and diffusion of the EPBs. The airglow images from GOLD showed that EPBs evolved from coherent depletions into structurally diffused forms with embedded filamentary features and eventually merged into the background. Notably, a hemispheric asymmetry in EPB dissipation was observed. In the Northern Hemisphere, EPBs tended to retain their morphology for a longer time, while in the Southern Hemisphere, more rapid dissipation occurred, likely due to differing background plasma conditions and magnetic field configurations in the two hemispheres. These findings reveal a previously uncharacterized feature of EPB dissipation during geomagnetic storms.

The extension of the post-sunset equatorial plasma bubbles (EPBs) to mid-latitudes during geomagnetic storms has been widely reported. However, previous research has primarily focused on the initiation and spatial distribution of these EPBs, while their decay processes remain underexplored. In this study, we provide observations of the evolution and dissipation of intense EPBs during the geomagnetic storm on 23–24 April 2023, using measurements from the Global-scale Observations of the Limb and Disk (GOLD), Swarm, and COSMIC-2 satellites. During the storm main phase, intense EPBs with zonal extents of ∼2–3° and latitudinal extents reaching up to ±35° magnetic latitude (Mlat) were observed between 15°W and 5°W. Simultaneously, very low electron density regions, spanning approximately from ±30° to ±45° Mlat and 420–500 km in altitude, were observed in the mid-latitudes. During the storm, the EPBs were observed to extend gradually poleward and subsequently encountered the low electron density regions. This encounter process, along with the storm-caused electric field dynamics, led to a gradual weakening and diffusion of the EPBs. The airglow images from GOLD showed that EPBs evolved from coherent depletions into structurally diffused forms with embedded filamentary features and eventually merged into the background. Notably, a hemispheric asymmetry in EPB dissipation was observed. In the Northern Hemisphere, EPBs tended to retain their morphology for a longer time, while in the Southern Hemisphere, more rapid dissipation occurred, likely due to differing background plasma conditions and magnetic field configurations in the two hemispheres. These findings reveal a previously uncharacterized feature of EPB dissipation during geomagnetic storms.

Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) have long been a subject of interest in ionospheric research. However, their spatiotemporal variability across regions, local times, seasons, and solar cycles is very complicated and remains not well established. Using Total Electron Content (TEC) data from global GNSS receiver networks processed at MIT Haystack Observatory, we perform a detailed statistical analysis of MSTIDs over the Continental US (CONUS). Differential TEC data every day from 2012 to 2023 are processed using a keogram-based image processing technique to identify MSTID wave properties, including the occurrence, propagation direction, phase speed, wavelength, and period. Focusing on eastern US midlatitudes (80°W, 40°N), we extend comparisons longitudinally and latitudinally across CONUS. Our results reveal significant variability in MSTID occurrence rates and propagation directions, notably linked to solar terminators. MSTID occurrence peaks after summer sunrise (with minor maxima near winter daytime), around summer sunset, and after summer midnight. Occurrence generally correlates positively with solar activity in summer but can become negative after winter midnight. In winter, MSTIDs propagate southeastward in the morning and rotate clockwise to west-northwestward after midnight; in summer, propagation is more variable. Comparisons across the CONUS highlight strong regional differences. Our findings reflect complex drivers behind MSTIDs, including gravity waves, electrodynamic processes, and solar terminators. Their relative influences vary with local time, season, and location. This long-term analysis provides critical insights into MSTID climatology and forms a basis for in-depth investigations of MSTID generation mechanisms.

Spatial geographic variability in Earth's core magnetic field, measured at 400 km altitude but corrected for ionospheric and magnetospheric signals, correlates with electron flux levels measured by the RBSP spacecraft. A higher Earth's |B| magnitude results in lower flux over L2-6. Over 20 eV–2 MeV, at L2-4, this negative correlation is as large as −0.21, peaking at the 158 keV electrons, with the strongest effects in the 71 keV–2 MeV electrons. Despite higher L shells being well above the 400 km field measure, statistically significant correlation with the core field was still seen in higher energy 1–2 MeV electrons over L5-6. Adding Earth's geographic |B| variability as a covariate in regression or ARMAX analyses, particularly at lower L shells, results in stronger correlations between electron flux and solar wind, substorm, and ULF wave drivers, with possible nonlinearity in the associations accounted for by taking logs of the variables. At L2, substorms (measured by the SME index), ULF waves, and solar wind velocity show increased correlations with electron flux (30%, 100%, and 175%, respectively) when Earth's |B| is added as a covariate to the ARMAX regression models. Modest increases in correlation of electron flux with these possible drivers were also seen at L3-6. This argues for the addition of Earth's |B| as a covariate in models of electron response to drivers.

A number of mechanisms have been suggested to operate within the terrestrial bow shock to redistribute energy contained in the incoming solar wind flow. The majority of mechanisms involve the generation of turbulence while some are based on particle motion alone. In this paper, we investigate the possible occurrence of the Electron Trajectory Instability, that results from short scale electric field gradients. Spike-like bipolar features in electric field measurements are a commonly observed signature within the terrestrial bow shock. They are usually associated with the passage of electrostatic solitary waves associated with phase space holes in the particle distribution. Using electric field measurements, we compare different interferometric methods to determine the propagation direction, velocity, and spatial scale of these features. Based on these results, it appears that the instability criterion for the Electron Trajectory Instability is fulfilled and the electron trajectories will diverge in the presence of these structures.