Journal of Geophysical Research: Space Physics最新文献

Using three-dimensional particle-in-cell (PIC) simulations, we study the spread of magnetic reconnection X-line. We show that structural asymmetries along the X-line direction develop during its spread. On the plane of the current sheet (i.e., corresponding to the equatorial plane of the magnetotail), sharp cusp-shaped signatures develop along the ion-drifting direction, capturing the spread of the X-line. The spreading is catalyzed by the lower ion pressure from the active diffusion region, and the X-line spreads at the ion-drifting speed of the non-reconnecting current sheet. Along the electron-drifting direction, the X-line barely spreads even though the electron-drifting speed is high within the electron diffusion region, and reconnected flux is transported toward this direction by the Hall effect. This preferential spread in the ion-drifting direction provides an additional explanation for the higher occurrence rate of reconnection events on the dusk side in Earth's magnetotail. In contrast to the laminar X-line, in a companion run, we demonstrate that the fluctuation and turbulence caused by drift-kink instability only suppress the X-line spreading. Even though the fluctuation breaks the frozen-in condition, it does not lead to the continuous onset of reconnection adjacent to the active region.

Whistler mode chorus waves, common in Earth's and planetary magnetospheres, play a crucial role in energetic electron dynamics. These waves exhibit distinctive narrowband features, intense amplitudes, and frequency chirping. While numerous studies have explored chorus wave generation, an important yet often overlooked parameter is the resonant electron pitch angle. Motivated by this gap, we conduct particle-in-cell (PIC) simulations to identify the key pitch angle governing wave-particle power transfer during interactions with chorus waves. Our findings reveal that the characteristic pitch angle, which dominates power transfer, aligns with the pitch angle that minimizes the nonlinear parameter during the central portion of a chirping element. This insight supports the use of a representative pitch angle in nonlinear theories of chorus waves to estimate wave properties.

The finite-difference time-domain (FDTD) method was previously applied to high-frequency electromagnetic wave propagation through 250 km of the F region of the ionosphere. That modeling approach was limited to electromagnetic wave propagation above the critical frequency of the ionospheric plasma, and it did not include the lower ionosphere layers or the top of the F-region. This paper extends the previous modeling methodology to frequencies below the critical frequency of the plasma and to altitudes encompassing the ionosphere. The following changes to the previous work were required to generate this model: (a) the D, E and top of the F regions of the ionosphere were added; and (b) the perfectly matched layer absorbing boundary on the top side of the grid was replaced with a collisional plasma to prevent reflections. We apply this model to the study of extremely low frequency (ELF) and very low frequency (VLF) electric power line harmonic radiation (PLHR) through the ionosphere. The model is compared against analytical predictions and applied to PLHR propagation in polar, mid-latitude and equatorial regions. Also, to further demonstrate the advantages of the grid-based FDTD method, PLHR propagation through a polar cap patch with inhomogeneities is studied. The presented modeling methodology may be applied to additional scenarios in a straightforward manner and can serve as a useful tool for better tracking and studying electromagnetic wave propagation through the ionosphere at any latitude and in the presence of irregularities of any size and shape.

“Polar” substorms are identified as substorm-like disturbances that are exclusively observed at high geomagnetic latitudes (>70° MLAT) and are absent at lower latitudes. Although “polar” substorms typically occur during periods of quiet geomagnetic activity, it is still unclear whether they can develop under extremely quiet conditions when geoeffective space weather parameters are exceptionally low. Utilizing data from the IMAGE network across the Svalbard archipelago within the longitudinal sector of (∼108–114 Mlong), we examined 92 “extremely quiet geomagnetic” intervals from 2010 to 2020, which were associated with intervals of extremely slow solar wind (ESSWs, V < 300 km/s). We discovered that “polar” substorms can occur during ESSWs, but only with the presence of a negative Bz component. A total of 32 such events were identified from 17 ESSW intervals (∼19% of all ESSW intervals). We found that “polar” substorms during ESSWs display the primary characteristics of ordinary substorms, including the accompaniment of Pi1B geomagnetic pulsations, positive subauroral or mid-latitude magnetic bays, a poleward shift of the westward electrojet, and auroral activity during their expansion phase. Additionally, it was found that the majority of “polar” substorm events during ESSWs (∼82%) were isolated substorms, developing solely in the pre-midnight sector without disturbances in other longitudinal sectors. Several “polar” substorm events have been examined in detail.

Surface charging phenomena on the lunar surface are significantly influenced by topographical features such as craters, boulders, and cavities. This study employs Particle-in-Cell (PIC) simulations to explore how the size $��d�$ of surface cavities affects charging under typical solar wind conditions. Our results show that cavities smaller than the lunar sheath thickness develop strong positive potentials at the depths due to ion currents. However, as cavity size increases beyond the sheath thickness, the influence of ion currents is reduced, resulting in a more moderate potential change inside the cavities. This transition is driven by a shift from surface-charge-dominated $��\left(��\sim �d^2��\right)�$ to space-charge-dominated $��\left(��\sim �d^3��\right)�$ electrostatic structures, as larger cavities allow for greater electron inflow and contribution of the space charge. These findings suggest that both macroscopic and microscopic surface irregularities need to be evaluated according to their spatial scale when considering a global charging environment, which would be significant for understanding dust transport and potential breakdown processes.

This study primarily examines the variability of zonal winds in the geomagnetic “quiet-time” (AE index < 190 nT; Kp index < 2) thermosphere over Alaska (60–75°N) during the winter months from November 2018 to February 2019, and demonstrates a correlation of the meteor radar observed zonal winds at heights 82–98 km with Scanning Doppler Imager observed zonal wind variability at heights 120 and 250 km during nighttime. Notably, over Alaska following a mid-winter sudden stratosphere warming (SSW) event, the zonal wind magnitudes on tidal timescale (difference between maximum eastward and minimum westward wind values over a 6-hr interval during nighttime) in the thermosphere experience about two-fold increase at observed thermosphere altitudes (98, 120, and 250 km). Additional validation of observational findings comes from SD-WACCM-X model simulations across solar-minimum (2017–2021) winters, including both non-SSW and SSW events (occurring in different winters). The model indicates that the SSW-induced response in zonal wind tidal magnitudes may be indistinguishable from the seasonal trend in late winter, while it is more pronounced when the seasonal variations is minimal during mid-winter months. By tidal diagnostics of zonal winds from meteor radar observations and SD-WACCM-X simulations at polar latitudes, a connection is established between the thermosphere zonal wind variations and the semidiurnal originating in the lower atmosphere following the SSW onset within the altitude range of 90–300 km (i.e., ionosphere-thermosphere region). Additionally, the study highlights the migrating solar semidiurnal tides as a major contributor in the variability in the polar thermosphere region with minor contribution from lunar semidiurnal tides.

Jupiter's polar aurora exhibits low brightness temperatures in Juno Microwave Radiometer (MWR) observations when the Juno spacecraft passes over the high-latitude region of the Northern Hemisphere. These cold features are observed predominantly at 0.6 GHz and show both long-term $��\sim �$ hours and short-term changes over time, that is, spans less than the 30-s spacecraft spin period. The MWR “cold spot” observations are associated with polar ultraviolet emission features that are thought to originate from high energy electron precipitation into the Jovian high latitude atmosphere. The energetic electron precipitation produces strong absorptive characteristics at microwave frequencies due to the transient formation of high-density electron regions in the lower stratosphere. In this paper, we describe progress on the analysis of Juno MWR observations of the northern aurora and simulate the effects of heating and electron impact ionization processes due to high energy particle precipitation events in Jupiter's auroral ionosphere. Electron precipitation intensities at energies up to 10 MeV inferred from the Jupiter Energetic-Particle Detector Instrument (JEDI) and Ultraviolet Spectrograph (UVS) instruments are used as a Northern Hemisphere case study to understand the energy deposition and ionization processes in the lower stratosphere, and subsequently used to estimate the microwave and ultraviolet opacity of the auroral region. The northward progression of Juno's perijove during the mission reduces the overflight altitude and allows important insights into effects produced at different length scales with respect to the auroral oval.

Geomagnetic storms transfer massive amounts of energy from the sun to geospace. Some of that energy is dissipated in the ionosphere as energetic particles precipitate and transfer their energy to the atmosphere, creating the aurora. We used the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mosaic of all-sky-imagers across Canada and Alaska to measure the amount of energy flux deposited into the ionosphere via auroral precipitation during the 2013 March 17 storm. We determined the time-dependent percent of the total energy flux that is contributed by meso-scale (<500 km wide) auroral features, discovering they contribute up to 80% during the sudden storm commencement (SSC) and >∼40% throughout the main phase, indicating meso-scale dynamics are important aspects of a geomagnetic storm. We found that average conductance was higher north of 65° until SYM-H reached −40 nT. We also found that the median conductance was higher in the post-midnight sector during the SSC, though localized conductance peaks (sometimes >75 mho) were much higher in the pre-midnight sector throughout. We related the post-midnight/pre-dawn conductance to other recent findings regarding meso-scale dynamics in the literature. We found sharp conductance peaks and gradients in both time and space related to meso-scale aurora. Data processing included a new moonlight removal algorithm and cross-instrument calibration with a meridian scanning photometer and a standard photometer. We compared ASI results to Poker Flat Incoherent Scatter Radar (PFISR) observations, finding energy flux, mean energy, and Hall conductance were highly correlated, moderately correlated, and highly correlated, respectively.

This study presents a time-domain methodology for jointly modeling magnetic field variations resulting from ionospheric and magnetospheric currents, with the ultimate goal of improving imaging of Earth's electrical conductivity at mantle depths. Three ionospheric current systems–the equatorial electrojet (EEJ), the polar electrojet (PEJ), and the mid-latitude current system (MLCS; called Sq in quiet times)–produce quasi-periodic diurnal variations (DV). The longer-period (LP) signals are primarily due to irregular fluctuations in magnetospheric currents. Traditionally, DV and LP signals are treated separately. For example, the analysis of LP signals is often based on nighttime data to reduce the effects of ionospheric sources. However, because of EM induction in the Earth, signals of ionospheric origin persist even at night. As for DV, its analysis is usually performed in the frequency domain. However, the morphology of all ionospheric sources varies from day-to-day, depending on the solar activity and the Earth's orbital position, which suggests the analysis of DV in the time domain. Moreover, the analysis of EEJ and MLCS signals is usually based on non-polar data to reduce the effects of PEJ. In this study, we present a methodology to simultaneously model magnetic fields from all sources directly in the time domain using day and night as well as non-polar and polar data. The approach uses two types (data-based and physics-based) of source parameterizations and accounts for 3-D electromagnetic induction effects. Using observatory data, we obtain continuous spatio-temporal models of multi-source external and induced magnetic fields for 1998–2021.

Asymmetry in the northern and southern hemispheres of Sq variations at the middle and low latitudes (within ±50°) is examined by analyzing observations from five pairs of geomagnetically conjugate stations of SuperMAG during quiet days. The results indicate that the asymmetries of the horizontal components of Sq variations (ASYM-X/Y) significantly vary with season and depend on latitude. First, ASYM-X of the three pairs of conjugate stations on the polar-side (44°) of and near (33° and 25°) the Sq focus are stronger from May through October than in the other seasons. ASYM-X of the two pairs of conjugate stations on the equatorial-side of focus (14° and 8°) is weak even in the June solstice. The intensity of ASYM-X on the polar-side is as strong as that of near focus from May through September and becomes as weak as that of the equatorial-side in other seasons. Second, the pattern of ASYM-Y on the polar-side is different from that at other latitudes, which basically consists of two parts. At other latitudes, ASYM-Y from April to October has a three-part pattern. The intensities of ASYM-Y in 06–10 LT and 14–18 LT decrease with increasing latitude from April to September. Finally, the north-south hemisphere asymmetry of the Sq variations is mainly caused by the asymmetry of the neutral winds. The seasonal and latitudinal variations of the asymmetry of the migrating tides, especially the zonal component, are essentially consistent with those of the Sq variations.