The South Atlantic Anomaly (SAA) refers to a region where the strength of the magnetic field is notably weaker compared to a dipole field. While previous studies have primarily focused on its effects ...on the inner radiation belt, this study investigates its impact on the aurora system. By analyzing 2 years' worth of data obtained by the Fengyun‐3E/ACMag instrument, we discover that magnetic fluctuations within the auroral oval are significantly weaker in the longitude sector corresponding to the SAA, as compared to those outside this area. This characteristic remains permanent and independent of seasons and geomagnetic activities. Additional investigation using Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager (DMSP/SSUSI) observations reveals a similar phenomenon in the auroral intensity. Therefore, our results demonstrate that the SAA substantially weakens the aurora system, shedding new light on the effects of magnetic anomalies on planetary auroras and magnetosphere‐ionosphere‐thermosphere coupling.
Plain Language Summary
The South Atlantic Anomaly (SAA) is a unique location on Earth where the magnetic field is weaker than normal. This region has drawn a lot of attention because its weakened magnetic field brings the inner Van Allen radiation belt unusually close to the Earth's surface, which poses a threat to satellites passing through it. Here, we uncovered another interesting aspect of the SAA: its impact on the aurora system. To investigate this, we first examined 2 years' worth of data from the ACMag instruments on the Fengyun‐3E satellite, which orbits the Earth at an altitude of 836 km in a dawn‐dusk, Sun‐synchronous orbit. Our findings reveal that the magnetic fluctuations within the southern auroral oval are significantly weaker in the region that aligns with the SAA. This weakening effect is consistently present, regardless of the season or the level of geomagnetic activity. To reinforce our results, we also analyzed auroral intensity from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on the Defense Meteorological Satellite Program (DMSP) satellite, and it corroborated the same weakening trend in this data set. In conclusion, our observations demonstrate that the SAA has a substantial impact on weakening the aurora system. This discovery deepens our understanding of how magnetic anomalies can influence planetary auroras.
Key Points
The effects of the South Atlantic Anomaly (SAA) on the terrestrial aurora system are examined using multiple instruments
Observations reveal a substantial weakening of auroral magnetic fluctuations and auroral intensity in the SAA longitude sector
The results indicate considering magnetic anomalies like the SAA is essential for comprehensively understanding planetary aurora systems
By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, we have experimentally ...produced high-brightness high-energy electron beams from a cascaded laser wakefield accelerator with peak energies in the range of 200-600 MeV, 0.4%-1.2% rms energy spread, 10-80 pC charge, and ∼0.2 mrad rms divergence. The maximum six-dimensional brightness B_{6D,n} is estimated as ∼6.5×10^{15} A/m^{2}/0.1%, which is very close to the typical brightness of e beams from state-of-the-art linac drivers. These high-brightness high-energy e beams may lead to the realization of compact monoenergetic gamma-ray and intense coherent x-ray radiation sources.
Face-centered-cubic (fcc) type high entropy alloys (HEAs) exhibit outstanding ductility even at the liquid nitrogen temperature, but they are relatively weak in strength which is far from the ...requirements for practical structural applications. One of the general concepts employed previously in alloy design is the suppression of ‘brittle’ intermetallic compound formation which usually leads to a serious embrittlement. Surprisingly, we reveal in this study that the precipitation of hard σ and μ intermetallic compounds tremendously strengthened the CoCrFeNiMo0.3 HEA but without causing a serious embrittlement. It exhibits a tensile strength as high as 1.2 GPa and a good ductility of ∼19%. A careful study of the deformation behavior reveals that the fcc matrix exhibits an extremely high work hardening exponent of 0.75, which suppresses the propagation of microcracks originated at these brittle particles. Our work presents a very successful demonstration of using complex hard intermetallic particles to manipulate the properties of fcc-type HEA systems. Furthermore, lattice distortion has been carefully measured in powder-metallurgy materials by line broadening from X-ray diffraction (XRD). It is interesting to discover that lattice planes are highly distorted in HEAs and this distortion also contributes to solid solution hardening.
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Highlights • Regional and network functional changes could be seen in the early stage of SCI. • Functional changes were associated with clinical symptom severity in SCI patients. • Functional ...reorganization may reflect a compensatory role in the recovery of SCI.
Three-dimensional (3D) topological Dirac semimetals (TDSs) represent an unusual state of quantum matter that can be viewed as "3D graphene." In contrast to 2D Dirac fermions in graphene or on the ...surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. By investigating the electronic structure of Na3Bi with angle-resolved photoemission spectroscopy, we detected 3D Dirac fermions with linear dispersions along all momentum directions. Furthermore, we demonstrated the robustness of 3D Dirac fermions in Na3Bi against in situ surface doping. Our results establish Na3Bi as a model system for 3D TDSs, which can serve as an ideal platform for the systematic study of quantum phase transitions between rich topological quantum states.
Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not ...only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. 3) and Na3Bi (refs 4, 5), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena.
Abstract
Understanding the sources of lunar water is crucial for studying the history of lunar evolution, as well as the interaction of solar wind with the Moon and other airless bodies. Recent ...orbital spectral observations revealed that the solar wind is a significant exogenous driver of lunar surficial hydration. However, the solar wind is shielded over a period of 3–5 days per month as the Moon passes through the Earth’s magnetosphere, during which a significant loss of hydration is expected. Here we report the temporal and spatial distribution of polar surficial OH/H
2
O abundance, using Chandrayaan-1 Moon Mineralogy Mapper (
M
3
) data, which covers the regions inside/outside the Earth’s magnetosphere. The data shows that polar surficial OH/H
2
O abundance increases with latitude, and that the probability of polar surficial OH/H
2
O abundance remains at the same level when in the solar wind and in the magnetosphere by controlling latitude, composition, and lunar local time. This indicates that the OH/H
2
O abundance in the polar regions may be saturated, or supplemented from other possible sources, such as Earth wind (particles from the magnetosphere, distinct from the solar wind), which may compensate for thermal diffusion losses while the Moon lies within the Earth’s magnetosphere. This work provides some clues for studies of planet–moon systems, whereby the planetary wind serves as a bridge connecting the planet with its moons.
Abstract
We present the second release of value-added catalogues of the LAMOST Spectroscopic Survey of the Galactic Anticentre (LSS-GAC DR2). The catalogues present values of radial velocity Vr, ...atmospheric parameters – effective temperature Teff, surface gravity log g, metallicity Fe/H, α-element to iron (metal) abundance ratio α/Fe (α/M), elemental abundances C/H and N/H and absolute magnitudes MV and $M_{K_{\rm s}}$ deduced from 1.8 million spectra of 1.4 million unique stars targeted by the LSS-GAC since 2011 September until 2014 June. The catalogues also give values of interstellar reddening, distance and orbital parameters determined with a variety of techniques, as well as proper motions and multiband photometry from the far-UV to the mid-IR collected from the literature and various surveys. Accuracies of radial velocities reach 5 km s−1 for the late-type stars, and those of distance estimates range between 10 and 30 per cent, depending on the spectral signal-to-noise ratios. Precisions of Fe/H, C/H and N/H estimates reach 0.1 dex, and those of α/Fe and α/M reach 0.05 dex. The large number of stars, the contiguous sky coverage, the simple yet non-trivial target selection function and the robust estimates of stellar radial velocities and atmospheric parameters, distances and elemental abundances make the catalogues a valuable data set to study the structure and evolution of the Galaxy, especially the solar-neighbourhood and the outer disc.
A simple, efficient scheme was developed to obtain near-gigaelectronvolt electron beams with energy spreads of few per-mille level in a single-stage laser wakefield accelerator. Longitudinal plasma ...density was tailored to control relativistic laser-beam evolution, resulting in injection, dechirping, and a quasi-phase-stable acceleration. With this scheme, electron beams with peak energies of 780–840 MeV, rms energy spreads of 2.4‰–4.1‰, charges of 8.5–23.6 pC, and rms divergences of 0.1–0.4 mrad were experimentally obtained. Quasi-three-dimensional particle-in-cell simulations agreed well with the experimental results. The dechirping strength was estimated to reach up to 11 TeV/mm/m, which is higher than previously obtained results. Such high-quality electron beams will boost the development of compact intense coherent radiation sources and x-ray free-electron lasers.
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