Particle Acceleration In Non-stationary Perpendicular Collisionless Shocks

Collisionless shocks are of fundamental interests in space physics, plasma physics and astrophysics. They are commonly believed to be important sources for high-energy particles, such as Solar Energetic Particles (SEPs), Anomalous Cosmic Rays (ACRs) and Galactic Cosmic Rays (GCRs). Previous space observations, experiments, hybrid and Particle-in-cell (PIC) simulations evidenced that supercritical, quasi-perpendicular shocks are nonstationary and suffer a self-reformation (i.e. the foot amplitude increases, and then it becomes a new ramp to replace the old ramp) for low case. We investigate the evolution of a nonstationary, supercritical, perpendicular shock, the impact of shock front reformation and ripple on the ion acceleration mechanisms. The main results are shown as follows:1. The evolution of the electric field at a nonstationary perpendicular shockBy using one-dimensional full particle-in-cell simulation, the evolution of the electric field at a nonstationary, supercritical perpendicular shock is investigated. The contributions of the ion Hall, Lorentz and electron pressure terms of the generalized ohm law to the electric field are analized. During the evolution of the perpendicular shock, a new ramp is formed in front of the old ramp, and its amplitude becomes larger and larger. At last, the new ramp exceeds the old one, and such a nonstationary process can be formed periodically. When the new ramp begins to be formed in front of the old ramp, the Hall term becomes more and more important (its amplitude becomes comparable to the Lorentz term). The electrostatic field along the shock normal is dominated by the Hall term when the new ramp exceeds the old one.2. Shock front nonstationarity and proton acceleration in supercritical perpendicular shocksBy separating the incoming ions into reflected (R) and directly transmitted (DT) parts, the mechanisms of proton acceleration in a nonstationary perpendicular shock are investigated in detail. Test particle simulations are performed where the time-evolving shock profiles are issued from self-consistent 1-D PIC simulations. Both shell and Maxwellian incoming proton distributions are used. In both cases, most energetic particles correspond to reflected protons, 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, the whole shock front needs to be included in order to study ion acceleration in self-reforming shocks. In addition, 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 protons cyclically varies in time with a period equal to the self reformation cycle, which is in agreement with previous self-consisten full particle simulations. The reflected protons not only come from the distribution wings of the incoming ions but also from the core part, in contrast with previous hybrid simulation results based on stationary perpendicular shocks. Furthermore, the proton velocity distributions at non-stationary perpendicular shocks are also investigated. The downstream protons have a ring-core distribution. The ring and core parts correspond to the R and DT subpopulations, respectively.3. Acceleration of heavy ions by perpendicular collisionless shocks: impact of the shock front nonstationarityTest particle simulations based on shock profiles fields issued from 1-D PIC simulation are performed in order to investigate the impact of the shock front non-stationarity on heavy ion acceleration (3He2+,4He2+/O8+,O7+ etc.). Reflection and acceleration mechanisms of heavy ions (with different initial thermal velocities and different charge-mass ratios) interacting with a non-stationary shock front are analyzed in detail. Present preliminary results show that: (i) the heavy ions suffer both shock drift acceleration (SDA) and shock surfing acceleration (SSA) mechanisms; (ii) the fraction of reflected heavy ions increases with initial thermal velocity, chargemass ratio and decreasing shock front width at both stationary shocks (situation equivalent to fixed shock cases) and non-stationary shocks (situation equivalent to continuously time-evolving shock cases); (iii) the shock front non-stationarity (time-evolving shock case) facilitates the reflection of heavy ions; (iv) a striking feature is the formation of an injected monoenergetic heavy ions population which persists in the shock front spectrum for different initial thermal velocities and ions species. Present results are compared with previous experimental analysis (obtained by ACE ULEIS and WIND LEMT) on Fe/O energy spectrum. The shock front self-reformation can drastically change the high energy part of the Fe/O spectrum.4. Impact of the self-reformation and rippling of a supercritical perpendicular collisionless shock on the dynamics and the energy spectra of pickup ionsBy using test particle calculations based on shock profiles issued from 1-D and 2-D PIC simulations, the impact of the shock front self-refomation and rippling on the acceleration processes and the resulting energy spectra of PI's (H+) at a strictly perpendicular shock are investigated. PI's are represented by different shell distributions (variation of the shell velocity radius). The contribution of shock drift acceleration (SDA), shock surfing acceleration (SSA) and directly transmitted (DT) PI's components to the total energy spectra is analyzed. Present results show that: (i) both SDA and SSA mechanisms can be invoked as pre-acceleration mechanisms for PI's but their relative energization efficiency strongly differs; (ii) SDA and SSA always work together but SDA- and not SSA- reveals to dominate the formation of high energy PI's in self-reformation or small ripple cases. In addition, shock front ripple also can make SSA PI's suffer multi-bounces in rippling scale; (iii) large shock front ripple along ambient magnetic field line helps SSA PI's in the high energy part of their downstream energy spectrum; (iv) for high shell velocity around the shock velocity, the middle energy range of the total energy spectrum follows a power law.