分类： 地球科学 >> 空间物理学 提交时间： 2016-12-26
Liu, Ying D.
Hu, Huidong
Wang, Chi
Luhmann, Janet G.
Richardson, John D.()
Yang, Zhongwei
Wang, Rui1()
摘要： As a follow-up study on Sun-to-Earth propagation of fast coronal mass ejections (CMEs), we examine the Sun-to-Earth characteristics of slow CMEs combining heliospheric imaging and in situ observations. Three events of particular interest, the 2010 June 16, 2011 March 25, and 2012 September 25 CMEs, are selected for this study. We compare slow CMEs with fast and intermediate-speed events, and obtain key results complementing the attempt of Liu et al. to create a general picture of CME Sun-to-Earth propagation: (1) the Sun-to-Earth propagation of a typical slow CME can be approximately described by two phases, a gradual acceleration out to about 20-30 solar radii, followed by a nearly invariant speed around the average solar wind level; (2) comparison between different types of CMEs indicates that faster CMEs tend to accelerate and decelerate more rapidly and have shorter cessation distances for the acceleration and deceleration; (3) both intermediate-speed and slow CMEs would have speeds comparable to the average solar wind level before reaching 1 au; (4) slow CMEs have a high potential to interact with other solar wind structures in the Sun-Earth space due to their slow motion, providing critical ingredients to enhance space weather; and (5) the slow CMEs studied here lack strong magnetic fields at the Earth but tend to preserve a flux-rope structure with an. axis generally perpendicular to the radial direction from the Sun. We also suggest a "best" strategy for the application of a triangulation concept in determining CME Sun-to-Earth kinematics, which helps to clarify confusions about CME geometry assumptions in the triangulation and to improve CME analysis and observations.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-26
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.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： In 2012 March the Sun exhibited extraordinary activities. In particular, the active region NOAA AR 11429 emitted a series of large coronal mass ejections (CMEs) which were imaged by the Solar Terrestrial Relations Observatory as it rotated with the Sun from the east to west. These sustained eruptions are expected to generate a global shell of disturbed material sweeping through the heliosphere. A cluster of shocks and interplanetary CMEs were observed near the Earth, and are propagated outward from 1 AU using an MHD model. The transient streams interact with each other, which erases memory of the source and results in a large merged interaction region (MIR) with a preceding shock. The MHD model predicts that the shock and MIR would reach 120 AU around 2013 April 22, which agrees well with the period of radio emissions and the time of a transient disturbance in galactic cosmic rays detected by Voyager 1. These results are important for understanding the "fate" of CMEs in the outer heliosphere and provide confidence that the heliopause is located around 120 AU from the Sun.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Liu, Ying D.
Hu, Huidong
Wang, Rui
Yang, Zhongwei
Zhu, Bei
Liu, Yi A.
Luhmann, Janet G.
Richardson, John D.
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.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The superthermal ions at a quasi-parallel collisionless shock are considered to be generated during the reformation of the shock. Recently, hybrid simulations of a quasi-parallel shock have shown that during the reformation of a quasi-parallel shock the large-amplitude upstream low-frequency waves can trap the reflected ions at the shock front when they try to move upstream, and then these reflected ions can be accelerated several times to become superthermal ions. In this paper, with the Cluster observations of a quasi-parallel shock event, the relevance between the large-amplitude upstream low-frequency waves and the superthermal ions (about several keV) have been studied. The observations clearly show that the differential energy flux of superthermal ions in the upstream region is modulated by the upstream low-frequency waves, and the maxima of the differential energy flux are usually located between the peaks of these waves (including the shock front and the peak of the upstream wave just in front of the shock front). These superthermal ions are considered to originate from the reflected ions at the shock front, and the modulation is caused due to the trapping of the reflected ions between the upstream waves or the upstream waves and the shock front when these reflected ions try to travel upstream. It verifies the results from hybrid simulations, where the upstream waves play an important role in the generation of superthermal ions in a quasi-parallel shock.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Predicting the arrival times of coronal mass ejections (CMEs) and their related waves at Earth is an important aspect of space weather forecasting. The Shock Propagation Model (SPM) and its updated version (SPM2), which use the initial parameters of solar flare-Type II burst events as input, have been developed to predict the shock arrival time. This paper continues to investigate the influence of solar disturbances and their associated CMEs on the corresponding interplanetary (IP) shock's arrival at Earth. It has been found that IP shocks associated with wider CMEs have a greater probability of reaching the Earth, and the CME speed obtained from coronagraph observations can be supplementary to the initial shock speed computed from Type II radio bursts when predicting the shock's arrival time. Therefore, the third version of the model, i.e., SPM3, has been developed based on these findings. The new version combines the characteristics of solar flare-Type II events with the initial parameters of the accompanying CMEs to provide the prediction of the associated IP shock's arrival at Earth. The prediction test for 498 events of Solar Cycle 23 reveals that the prediction success rate of SPM3 is 70%-71%, which is apparently higher than that of the previous SPM2 model (61%-63%). The transit time prediction error of SPM3 for the Earth-encountered shocks is within 9 hr (mean-absolute). Comparisons between SPM3 and other similar models also demonstrate that SPM3 has the highest success rate and best prediction performance.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
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.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： We numerically study the detailed evolutionary features of the wave-like disturbance and its propagation in the eruption. This work is a follow-up to Wang et al., using significantly upgraded new simulations. We focus on the contribution of the velocity vortices and the fast shock reflection and refraction in the solar corona to the formation of the EUV waves. Following the loss of equilibrium in the coronal magnetic structure, the flux rope exhibits rapid motions and invokes the fast-mode shock at the front of the rope, which then produces a type II radio burst. The expansion of the fast shock, which is associated with outward motion, takes place in various directions, and the downward expansion shows the reflection and the refraction as a result of the non-uniform background plasma. The reflected component of the fast shock propagates upward and the refracted component propagates downward. As the refracted component reaches the boundary surface, a weak echo is excited. The Moreton wave is invoked as the fast shock touches the bottom boundary, so the Moreton wave lags the type II burst. A secondary echo occurs in the area where reflection of the fast shock encounters the slow-mode shock, and the nearby magnetic field lines are further distorted because of the interaction between the secondary echo and the velocity vortices. Our results indicate that the EUV wave may arise from various processes that are revealed in the new simulations.
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