摘要：Detecting near-surface soil freeze-thaw cycles in high-altitude cold regions is important for understanding the Earth's surface system, but such studies are rare. In this study we detected the spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River during the period 2002-2011 based on data from the Advanced Microwave Scanning Radiometer for the Earth Observing System
摘要：A turbocharger compressor working in commercial vehicles, especially in some passenger cars, often works together with some pipes with complicated geometry as an air intake system, due to limit of available space in internal combustion engine compartments. These pipes may generate various distortions of physical parame- ters of the air at the inlet of the compressor and therefore the compressor aerodynamic performance deteri- orates. Sometimes, the turbocharging engine fails to work at some operation points. This paper investigates the effects of various swirl distortions induced by different bending-torsional intake ducts on the aerodynamic performance of a turbocharger compressor by both 3D numerical simulations and experimental measurements. It was found that at the outlet of the pipes the different inlet ducts can generate different swirl distortions, twin vortices and bulk-like vortices with different rotating directions. Among them, the bulk-like vortices not only affect seriously the pressure distribution in the impeller domain, but also significantly deteriorate the com- pressor performance, especially at high flow rate region. And the rotating direction of the bulk-like vortices is also closely associated with the efficiency penalty. Besides the efficiency, the transient flow rate through a sin- gle impeller channel, or the asymmetric mass flow crossing the whole impeller, can be influenced by two dis- turbances. One is from the upstream bending-torsional ducts; other one is from the downstream volute.
摘要：We study the solar energetic particle (SEP) event associated with the 2012 July 23 extreme solar storm, for which Solar Terrestrial Relations Observatory (STEREO) and the spacecraft at L1 provide multi-point remote sensing and in situ observations. The extreme solar storm, with a superfast shock and extremely enhanced ejecta magnetic fields observed near 1 au at STEREO A, was caused by the combination of successive coronal mass ejections (CMEs). Meanwhile, energetic particles were observed by STEREO and near-Earth spacecraft such as the Advanced Composition Explorer and SOlar and Heliospheric Observatory, suggesting a wide longitudinal spread of the particles at 1 au. Combining the SEP observations with in situ plasma and magnetic field measurements, we investigate the longitudinal distribution of the SEP event in connection with the associated shock and CMEs. Our results underscore the complex magnetic configuration of the inner heliosphere formed by solar eruptions. Examination of particle intensities, proton anisotropy distributions, element abundance ratios, magnetic connectivity, and spectra also gives important clues for particle acceleration, transport, and distribution.
摘要：The largest geomagnetic storms of solar cycle 24 so far occurred on 2015 March 17 and June 22 with D-st minima of -223 and -195 nT, respectively. Both of the geomagnetic storms show a multi-step development. We examine the plasma and magnetic field characteristics of the driving coronal mass ejections (CMEs) in connection with the development of the geomagnetic storms. A particular effort is to reconstruct the in situ structure using a Grad-Shafranov technique and compare the reconstruction results with solar observations, which gives a larger spatial perspective of the source conditions than one-dimensional in situ measurements. Key results are obtained concerning how the plasma and magnetic field characteristics of CMEs control the geomagnetic storm intensity and variability: (1) a sheath-ejecta-ejecta mechanism and a sheath-sheath-ejecta scenario are proposed for the multi-step development of the 2015 March 17 and June 22 geomagnetic storms, respectively;(2) two contrasting cases of how the CME flux-rope characteristics generate intense geomagnetic storms are found, which indicates that a southward flux-rope orientation is not a necessity for a strong geomagnetic storm;and (3) the unexpected 2015 March 17 intense geomagnetic storm resulted from the interaction between two successive CMEs plus the compression by a high-speed stream from behind, which is essentially the "perfect storm" scenario proposed by Liu et al. (i.e., a combination of circumstances results in an event of unusual magnitude), so the "perfect storm" scenario may not be as rare as the phrase implies.
摘要：We study the role of the coronal magnetic field configuration of an active region (AR) in determining the propagation direction of a coronal mass ejection (CME). The CME occurred in the AR 11944 (S09W01) near the disk center on 2014 January 7 and was associated with an X1.2 flare. A new CME reconstruction procedure based on a polarimetric technique is adopted, which shows that the CME changed its propagation direction by around 28 degrees in latitude within 2.5 R-circle dot and 43 degrees in longitude within 6.5 R-circle dot with respect to the CME source region. This significant non-radial motion is consistent with the finding of Mostl et al. We use nonlinear force-free field and potential field source surface extrapolation methods to determine the configurations of the coronal magnetic field. We also calculate the magnetic energy density distributions at different heights based on the extrapolations. Our results show that the AR coronal magnetic field has a strong influence on the CME propagation direction. This is consistent with the "channeling" by the AR coronal magnetic field itself, rather than deflection by nearby structures. These results indicate that the AR coronal magnetic field configuration has to be taken into account in order to determine CME propagation direction correctly.
摘要：Voyager 2 (V2) observed multiple crossings of the heliospheric termination shock (TS) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. Here, for the first time, we present two-dimensional particle-incell (PIC) simulations of the TS self-consistently including pickup ions (PUIs), and compare the simulation results with V2 observations. We find that (1) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion kinetic energy is transferred to the thermal energy of PUIs. The PIC simulation indicates that, for the upstream parameters chosen for V2 conditions, the density of PUIs is about 25% and the PUIs gain the largest fraction (approximately 86.6%) of downstream thermal pressure. (2) The simulated heliosheath ion distribution function is a superposition of a cold core formed by transmitted solar wind ions (SWIs), with the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. The V2 Faraday cups observed the cool core of the distribution, and so they only saw the tip of the iceberg. (3) The nonstationarity of the shock front is mainly caused by ripples along the shock front which form even if the percentage of PUIs is high. These simulation results agree reasonably well with the V2 experimental data. The relevance of the shock front ripples to the multiple TS crossings observed by V2 is also discussed in this paper.