Resonant ion heating by high-frequency Alfvén waves has long been believed to be the primary dissipation mechanism for solar coronal heating, and these high-frequency Alfvén waves are considered to be generated via cascade from low-frequency Alfvén waves. In this study, we report an unusual harmonic Alfvén event from in situ observations by the Van Allen Probes in the magnetosphere, having an environment similar to that in the solar corona. The harmonic Alfvén waves, which propagate almost along the wave vector of the fundamental waves, are considered to be generated due to the interaction between quasi-parallel Alfvén waves and plasma density fluctuations with almost identical frequency. These high-frequency harmonic Alfvén waves can then cyclotron resonantly heat the heavy ions. Our observations provide an important insight into solar corona heating by Alfvén waves.

Whistler‐mode waves are electromagnetic waves pervasively observed in the Earth's and other planetary magnetospheres. They are considered to be mainly responsible for producing the hazardous radiation and diffuse aurora, which heavily relies on their properties. Density irregularities, frequently observed in the Earth's magnetospheres, are found to change largely the properties of whistler‐mode waves. Here we report, using Van Allen Probes measurements, whistler‐mode waves strongly modulated by two different density enhancements. With particle‐in‐cell simulations, we propose wave trapping caused by field‐aligned density irregularities (ducts) may account for this phenomenon. Simulation results show that whistler‐mode waves can be trapped inside the enhanced density ducts. These trapped waves remain quasi‐parallel and usually get much larger amplitudes than those unducted whistler waves during propagation away from the magnetic equator, and tend to focus at a spatially narrow channel, consistent with observations. Our results imply density irregularities may be significant to modulate radiation‐belt electrons.

To improve the accuracy and generalization ability of hyperspectral image classification, a feature extraction method integrating principal component analysis(PCA) and local binary pattern (LBP) is developed for hyperspectral images in this paper. The PCA is employed to reduce the dimension of the spectral features of hyperspectral images. The LBP with low computational complexity is used to extract the local spatial texture features of hyperspectral images to construct multi-feature vectors. Then the gray wolf optimization(GWO) algorithm with global search capability is employed to optimize the parameters of kernel extreme learning machine(KELM) to construct an optimized KELM model, which is used to effectively realize a hyperspectral image classification (PLG-KELM) method. Finally, the Indian pines dataset, Houston dataset and Pavia University dataset and an application of WHU-Hi-LongKou dataset are selected to verify the effectiveness of the PLG-KELM. The comparison experiment results show that the PLG-KELM can obtain higher classification accuracy, and takes on better generalization ability for small samples. It provides a new idea for processing hyperspectral images.

With 64‐month magnetic data from Van Allen Probes, we have studied not only the global distribution, wave normal angle (θ), and ellipticity (ε) of electromagnetic ion cyclotron (EMIC) waves, but also the dependence of their occurrence rates and magnetic amplitudes on the AE* index (the mean value of AE index over previous 1 hr). Our results show that H+ band waves are preferentially detected at 5 ≤ L ≤ 6.5, in the noon sector. They typically have small θ (<30°) and weakly left‐hand polarization but become more oblique and linearly polarized at larger magnetic latitudes or L‐shells. With the increase of AE* index, their occurrence rate significantly increases in the noon sector, and their source region extends to dusk sector. He+ band waves usually occur in the predawn and morning sectors at 3 ≤ L ≤ 4.5. They generally have moderate θ (30 °  − 40°) and left‐hand polarization and also become more oblique and linearly polarized at larger latitudes or L‐shells. There is a clear enhancement of occurrence rate and amplitude during active geomagnetic periods, especially in the dusk and evening sectors. O+ band waves mainly occur at 3 ≤ L ≤ 4 in the predawn sector. They have either very small θ (<20°) or very large θ (>50°), and typically linear or weakly right‐hand polarization. During active periods, they mostly occur at the midnight sector and L < 3.5. As a valuable supplement to previous statistical studies, our result provides not only a more compresentive EMIC wave model for evaluating their effects on the radiation belt, but also detailed observational constraints on generation mechanisms of EMIC waves.