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. (2014a, 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.

[1] Previous particle-in-cell simulations have evidenced that quasiperpendicular shocks are nonstationary and suffer a self-reformation on gyro scale of the incoming ions due to the accumulation of reflected ions. In this paper, by separating the incoming ions into reflected and directly transmitted parts, we investigate the detailed mechanisms of ion acceleration in a nonstationary perpendicular shock. Test particle simulations are performed where the shock profiles are issued from self-consistent one-dimensional full particle-in-cell simulations. Both shell and Maxwellian incoming ion distributions are used. In both cases, most energetic particles correspond to reflected ions, and the associated acceleration mechanisms include both shock drift acceleration (SDA) and shock surfing acceleration (SSA). Two types of results are obtained. First, if we fix the shock profiles at different times within a self-reformation cycle, the mechanisms of particle acceleration are different at different profiles. SDA process appears as the dominant acceleration mechanism when the width of the ramp is broad (and overshoot amplitude is low) whereas both SDA and SSA contribute as the width of the ramp is narrow (and overshoot amplitude is high). For the different shock profiles concerned herein, SDA process is more efficient (higher resulting ion energy gain) than the SSA process. Second, in order to investigate ion acceleration in self-reforming shocks, not only the ramp but also the variations of the whole shock front need to be included. In the continuously time-evolving shock, SDA remains a dominant acceleration mechanism whereas SSA mechanism becomes more and more important with the increase of the initial particle energy. The percentage of reflected ions cyclically varies in time with a period equal to the self reformation cycle, which is in agreement with previous full particle simulations. The reflected ions not only come from the distribution wings of the incoming ions but also from the core part, in contrast with previous results based on stationary shocks.

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. (2013) 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 a speed 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 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.

On 2012 September 30 -October 1 the Earth underwent a two-step geomagnetic storm. We examine the Sun-to-Earth characteristics of the coronal mass ejections (CMEs) responsible for the geomagnetic storm with combined heliospheric imaging and in situ observations. The first CME, which occurred on 2012 September 25, is a slow event and shows an acceleration followed by a nearly invariant speed in the whole Sun-Earth space. The second event, launched from the Sun on 2012 September 27, exhibits a quick acceleration, then a rapid deceleration and finally a nearly constant speed, a typical Sun-to-Earth propagation profile for fast CMEs (Liu et al. 2013). These two CMEs interacted near 1 AU as predicted by the heliospheric imaging observations and formed a complex ejecta observed at Wind, with a shock inside that enhanced the pre-existing southward magnetic field. Reconstruction of the complex ejecta with the in situ data indicates an overall left-handed flux rope-like configuration, with an embedded concave-outward shock front, a maximum magnetic field strength deviating from the flux rope axis and convex-outward field lines ahead of the shock. While the reconstruction results are consistent with the picture of CME-CME interactions, a magnetic cloud-like structure without clear signs of CME interactions (Lugaz & Farrugia 2014) is anticipated when the merging process is finished.

We report on a clear solar-cycle variation of the Sun’s shadow in the 10 TeV cosmic-ray flux observed by the Tibet air shower array during a full solar cycle from 1996 to 2009. In order to clarify the physical implications of the observed solar cycle variation, we develop numerical simulations of the Sun’s shadow, using the potential field source surface model and the current sheet source surface (CSSS) model for the coronal magnetic field. We find that the intensity deficit in the simulated Sun’s shadow is very sensitive to the coronal magnetic field structure, and the observed variation of the Sun’s shadow is better reproduced by the CSSS model. This is the first successful attempt to evaluate the coronal magnetic field models by using the Sun’s shadow observed in the TeV cosmic-ray flux.

[1] Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi-perpendicular shock can be rippled. Recent two-dimensional simulations have focused on two different types of shock front rippling: (1) one characterized by a small spatial scale along the front is supported by lower hybrid wave activity, (2) the other characterized by a large spatial scale along the front is supported by the emission of large amplitude nonlinear whistler waves. These two rippled shock fronts are self-consistently observed when the static magnetic field is perpendicular to (so called "B 0 -OUT" case) or within (so called "B 0 -IN" case) the simulation plane, respectively. On the other hand, several studies have been made on the reflection and energization of incoming ions with a shock but most have been restricted to a one dimensional shock profile only (no rippling effects). Herein, two-dimensional test particle simulations based on strictly perpendicular shock profiles chosen at a fixed time in twodimensional Particle-in-cell (PIC) simulations, are performed in order to investigate the impact of the shock front ripples on incident ion (H + ) dynamics. The acceleration mechanisms and energy spectra of the test-ions (described by shell distributions with different initial kinetic energy) interacting with a rippled shock front are analyzed in detail. Both "B 0 -OUT" and "B 0 -IN" cases are considered separately; in each case, y-averaged (front rippling excluded) and non-averaged (front rippling included) profiles will be analyzed. Present results show that: (1) the incident ions suffer both shock drift acceleration (SDA) and shock surfing acceleration (SSA) mechanisms. Moreover, a striking feature is that SSA ions not only are identified at the ramp but also within the foot which confirms previous 1-D simulation results; (2) the percentage of SSA ions increases with initial kinetic energy, a feature which persists well with a rippled shock front; (3) furthermore, the ripples increase the porosity of the shock front, and more directly transmitted (DT) ions are produced; these strongly affect the relative percentage of the different identified classes of ions (SSA, SDA and DT ions), their average kinetic energy and their relative contribution to the resulting downstream energy spectra; (4) one key impact of the ripples is a strong diffusion of ions (in particular through the frontiers of their injection angle domains and in phase space which are blurred out) which leads to a mixing of the different ion classes. This diffusion increases with the size of the spatial scale of the front ripples; (5) through this diffusion, an ion belonging to a given category (SSA, SDA, or DT) in y-averaged case changes class in non-averaged case without one-to-one correspondence.

We examine two successive flare eruptions (X5.4 and X1.3) on 2012 March 7 in the NOAA active region 11429 and investigate the magnetic field reconfiguration associated with the two eruptions. Using an advanced non-linear force-free field (NLFFF) extrapolation method based on the SDO/HMI vector magnetograms, we obtain a stepwise decrease in the magnetic free energy during the eruptions, which is roughly 20%−30% of the energy of the pre-flare phase. We also calculate the magnetic helicity, and suggest that the changes of the sign of the helicity injection rate might be associated with the eruptions. Through the investigation of the magnetic field evolution, we find that the appearance of the "implosion" phenomenon has a strong relationship with the occurrence of the first X-class flare. Meanwhile, the magnetic field changes of the successive eruptions with implosion and without implosion were well observed.

We study the role of the coronal magnetic field configuration of an active region in determining the propagation direction of a coronal mass ejection (CME). The CME occurred in the active region 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 • in latitude within 2.5 R ⊙ and 43 • in longitude within 6.5 R ⊙ with respect to the CME source region. This significant non-radial motion is consistent with the finding of Möstl et al. (2015). We use nonlinear force-free field (NLFFF) and potential field source surface (PFSS) 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 active region coronal magnetic field has a strong influence on the CME propagation direction. This is consistent with the "channelling" by the active region coronal magnetic field itself, rather than deflection by nearby structures. These results indicate that the active region coronal magnetic field configuration has to be taken into account in order to determine CME propagation direction correctly.