Acceleration of heavy ions by perpendicular collisionless shocks: Impact of the shock front nonstationarity

摘要

Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi‐perpendicular shock can be nonstationary. One responsible mechanism proposed for this nonstationarity is the self‐reformation of the front itself being due to the accumulation of reflected ions. Important consequences of this nonstationarity are that not only the amplitude but also the spatial scales of fields components at the shock front (ramp and foot) are strongly varying within each cycle of the self‐reformation. On the other hand, several studies have been made on the acceleration and heating of heavy ions but most have been restricted to a stationary shock profile only. Herein, one‐dimensional test particle simulations based on shock profiles fields produced in PIC simulation are performed in order to investigate the impact of the shock front nonstationarity on heavy ion acceleration (He, O, Fe). Reflection and acceleration mechanisms of heavy ions (with different initial thermal velocities and different charge‐mass ratios) interacting with a nonstationary shock front (self‐reformation) are analyzed in detail. Present preliminary results show that: (1) the heavy ions suffer both shock drift acceleration (SDA) and shock surfing acceleration (SSA) mechanisms; (2) the fraction of reflected heavy ions increases with initial thermal velocity, charge‐mass ratio and decreasing shock front width at both stationary shocks (situation equivalent to fixed shock cases) and nonstationary shocks (situation equivalent to continuously time‐evolving shock cases); (3) the shock front nonstationarity (time‐evolving shock case) facilitates the reflection of heavy ions; (4) 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. The impact of the shock front nonstationarity on the heavy ions spectra within the shock front region and the downstream region are detailed separately. Present results are compared with previous experimental analysis and theoretical models of solar energetic particles (SEP) events. The variations of Fe/O spectra in high energy part have been retrieved, and the nonstationary effects of shock front strongly amplify these variations.