Journal of Geophysical Research: Space Physics最新文献

The wind dynamo in the ionosphere leads to differential motion of ions and electrons, which in turn sets up electric fields and currents. Observations show that daytime lower thermospheric horizontal winds have large vertical gradients. Numerical modeling conducted approximately 50 years ago demonstrated that the zonal wind shears in the ∼130–180 km altitude range can generate off-equatorial relative minima (dips) in the daytime height-integrated eastward current density, appearing as westward sidebands north and south of the equatorial electrojet (EEJ). This study observationally confirms this connection for the first time by combining Ionospheric CONnection explorer zonal wind profiles and Swarm latitudinal zonal currents. We demonstrate observationally that the magnitude of the EEJ sideband current is proportional to the strength of westward turning winds with altitude in the Pedersen conductivity dominated region. Additional numerical experiments explain the importance of wind shear in different altitude regions in generating the sideband current. This study contributes to the better understanding of the neutral wind effect on the local current generation.

Plasmaspheric hiss plays an important role in the loss of radiation belt electrons via cyclotron resonant interactions. The cold plasma approximation is widely used in the evaluation of hiss-driven electron losses, which however can break down during disturbed periods of geomagnetic storms and substorms. The kappa particle velocity distribution, characterized by a pronounced high-energy tail, is well-established to model the profile of superthermal plasma under disturbed geomagnetic conditions. In the present study, by calculating the electron bounce-averaged pitch angle diffusion coefficients with kappa plasma dispersion relations, we investigate the sensitivity of hiss-induced cyclotron-resonant electron scattering loss to the spectral index κ under a variety of superthermal plasma conditions. Our results demonstrate that, with increasing κ, the diffusion coefficients of ∼20–100 keV radiation belt electrons significantly decrease at lower pitch angles and increase at higher pitch angles. In contrast, for electrons at higher energies, the diffusion coefficients tend to increase at lower pitch angles and decrease at relatively higher pitch angles. We also find that decrease of L-shell and increase of α* and temperature anisotropy tend to weaken the hiss-driven pitch angle scattering efficiency of electrons at energies from tens to hundreds of keV with a dip at ∼30–50 keV, while the scattering of higher energy electrons can be enhanced. This study confirms the important role of superthermal plasmas in the hiss-driven electron loss processes and should be carefully incorporated in future modeling of radiation belt electron dynamics.

A necessary condition for the generation of Geomagnetically Induced Currents (GICs) that can pose hazards for technological infrastructure is the occurrence of large, rapid changes in the magnetic field at the surface of the Earth. We investigate the causes of such $��d�B�/�d�t�$ events or “spikes” observed by SuperMAG at auroral latitudes, by comparing with the time-series of different types of geomagnetic activity for the duration of 2010. Spikes are found to occur predominantly in the pre-midnight and dawn sectors. We find that pre-midnight spikes are associated with substorm onsets. Dawn sector spikes are not directly associated with substorms, but with auroral activity occurring within the westward electrojet region. Azimuthally-spaced auroral features drift sunwards, producing Ps6 (10–20 min period) magnetic perturbations on the ground. The magnitude of $��d�B�/�d�t�$ is determined by the flow speed in the convection return flow region, which in turn is related to the strength of solar wind-magnetospheric coupling. Pre-midnight and dawn sector spikes can occur at the same time, as strong coupling favors both substorms and westward electrojet activity; however, the mechanisms that create them seem somewhat independent. The dawn auroral features share some characteristics with omega bands, but can also appear as north-south aligned auroral streamers. We suggest that these two phenomena share a single underlying cause. The associated fluctuations in the westward electrojet produce quasi-periodic negative excursions in the AL index, which can be mis-identified as recurrent substorm intensifications.

The 23–24 April 2023 double-peak (SYM-H intensities of −179 and −233 nT) intense geomagnetic storm was caused by interplanetary magnetic field southward component B_s associated with an interplanetary fast-forward shock-preceded sheath (B_s of 25 nT), followed by a magnetic cloud (MC) (B_s of 33 nT), respectively. These interplanetary structures were led by a coronal mass ejection erupted from the Sun in association with an M1.7 X-ray flare. At the center of the MC, the plasma density exhibited an order of magnitude decrease, leading to a sub-Alfvénic solar wind interval for ∼2.1 hr. Ionospheric Joule heating accounted for a significant part (∼81%) of the magnetospheric energy dissipation during the storm main phase. Equal amount of Joule heating in the dayside and nightside ionosphere is consistent with the observed intense and global-scale DP2 (disturbance polar) currents during the storm main phase. The sub-Alfvénic solar wind is associated with disappearance of substorms, a sharp decrease in Joule heating dissipation, and reduction in electromagnetic ion cyclotron wave amplitude. The shock/sheath compression of the magnetosphere led to relativistic electron flux losses in the outer radiation belt between L* = 3.5 and 5.5. Relativistic electron flux enhancements were detected in the lower L* ≤ 3.5 region during the storm main and recovery phases. Equatorial ionospheric plasma anomaly structures are found to be modulated by the prompt penetration electric fields. Around the anomaly crests, plasma density at ∼470 km altitude and altitude-integrated ionospheric total electron content are found to increase by ∼60% and ∼80%, with ∼33% and ∼67% increases in their latitudinal extents compared to their quiet-time values, respectively.

The occurrence of small-scale and intense ionospheric currents that can contribute to geomagnetically induced currents have recently been discovered. A difficulty in their characterization is that their signatures are often only observed at single widely spaced (typically 200–400 km) ground geomagnetic stations. These small-scale structures motivate the examination of the maximum station separation required to fully characterize these small-scale signatures. We analyze distributions of correlation coefficients between closely spaced mid-latitude and auroral zone ground magnetometer stations spanning day to month long intervals to assess the separation distance at which geomagnetic signatures appear in only one station. Distributions were analyzed using periods that included low and high geomagnetic activity. We used data from pairs of magnetometer stations across North America within 200 km of each other, all of which were separated primarily latitudinally. Results show that while measurements remain largely similar up to separations of 200 km, large and frequent differences appear starting at around 130 km separation. Larger differences and lower correlations are observed during high geomagnetic activity, while low geomagnetic activity leads to frequent high correlation even past 200 km separation. Small but identifiable differences can appear in magnetometer data from stations as close as 35 km during high geomagnetic activity. We recommend future magnetometer array deployment in the auroral and sub-auroral zone to have separations of 100–150 km. This enables the monitoring of large scale effects of geomagnetic storms, better temporal and spatial resolution of substorms, and observations of small-scale current signatures.

The moderate geomagnetic storm on 1 December 2023 is interesting that the aurora is unexpectedly observed in Beijing. During this storm, the behaviors of the equatorial electrojet (EEJ) might deserve to be explored. When the polarization of the daytime electric field changes westward, the direction of EEJ turns westward as well, termed the counter electrojet (CEJ). Using the total electron content from Global Navigation Satellite System, observed EEJ from Tatuoca station and low-orbit Swarm satellites, and the corresponding simulations from the Thermosphere-Ionosphere Electrodynamic General Circulation Model, the polarization oscillations of EEJ responses (ΔEEJ) during the geomagnetic storm on 1 December 2023 are explored. A significant polarity oscillation is established in the Swarm-observed ΔEEJ at 9.6 and 12.8 local time (LT). A similar oscillation is also found at 15 LT. The strong polarity oscillations of ΔEEJ are dominated by the electric field, with ignorable effects from the ionospheric Cowling conductivity. The correlation coefficient between ΔEEJ and electric field changes (ΔE) is larger than 0.87, confirming the roles of the electric field. In the pre-noon sector, the quasi-oscillations of ΔEEJ are controlled by both the neutral winds and the prompt penetration electric field (PPEF). At noon, ΔEEJ is primarily driven by the PPEF, with minor effects from neutral winds. At afternoon, ΔEEJ is attributed to the PPEF, and the effects of neutral winds are ignorable.

Magnetic field measurements in the Earth's magnetosheath are compared with four analytical models, namely Kobel and Flückiger (1994), (https://doi.org/10.1029/94ja01778), Romashets and Vandas (2019), (https://doi.org/10.1029/2018ja026006), Vandas and Romashets (2019), (https://doi.org/10.1016/j.pss.2019.07.007), and Romashets et al. (2010), (https://doi.org/10.1016/j.jastp.2010.10.010). The bow shock and the magnetopause are modeled as paraboloids in all models except that of Vandas and Romashets (2019), (https://doi.org/10.1016/j.pss.2019.07.007), in which they are assumed to be spheroids. 34 magnetosheath crossings by spacecraft are analyzed in detail and it is concluded that the model by Kobel and Flückiger (1994), (https://doi.org/10.1029/94ja01778) performs the best.

The paper contains a detailed analysis of the formation of an auroral spiral based on hitherto not published observations by the all-sky camera in Kilpisjärvi, Northern Finland. We conclude that spirals appearing during a substorm form by a modification of the interface between tail and magnetosphere, the location of the generator current of the westward electrojet. Driven by the arriving flow bursts, this current is subject to ruptures by the appearance of a sequence of hook-like structures. These structures can move eastward with speeds up to 3 km/s. The propagation is attributed to a constructive magnetic fracture process driven from behind by the power of the arriving flow bursts. Poleward bending and extension of a hook-like structure, followed by a turning to the west and then equatorward, is the first step in spiral formation. It becomes the primary spiral arm, if a poleward arm grows out of weaker auroral structures, poleward and eastward of it. We suggest that the upward field-aligned currents related to the bright spiral arms are largely balanced by adjacent downward currents. The electric fields associated with the connecting Pedersen currents are consistent with the counter-clockwise motion. An important additional ingredient in the observed configuration is an eastward directed flow field, which is the generator of an additional upward current and possibly crucial for the spiral formation. Electric field data from literature throw confusing light on the propagation of a spiral, whether like a vessel in the ocean or by incorporating the magnetic flux ahead of it.

Energetic proton precipitation from the magnetosphere plays an important role in the magnetosphere-ionosphere-thermosphere coupling and energy transfer. Proton precipitation causes hydrogen emissions, such as Hβ (486.1 nm), and also triggers the excitation of other emission lines such as the blue-line (427.8 nm) and the green-line (557.7 nm). In light of the growing availability of ground-based proton auroral measurements in recent years, we revisit the proton auroral modeling in this study, with more focus on the application for interpreting ground observations. An accurate simulation of these optical emissions requires a comprehensive understanding of particle transport and collisions in the upper atmosphere, where the simultaneous consideration of precipitating protons, newly generated energetic hydrogen atoms, and secondary electrons is critical. For this purpose, we couple a 3D Monte-Carlo proton transport model and an electron transport model. The integrated model framework can compute the emission rates of most major auroral emission lines/bands resulting from proton precipitation, along with self-consistent calculation of the ionospheric electron density variations. The model results show improved agreement with ground optical observations in terms of the Hβ yield and the green-to-Hβ ratio compared to previous model studies. Our new model is a valuable tool for quantifying excitation and ionization due to proton aurora. It has the potential to leverage ground observations to infer precipitating conditions at high altitudes and even for studying magnetospheric activity.