Physical Roles of Interstellar-Origin Pickup Ions at the Heliospheric Termination Shock: Impact on the Shock Front Microstructures and Nonstationarity

Abstract

The nonstationary dynamics of the heliospheric termination shock in the presence of pickup ions (PUI) is analyzed by using a one-dimensional particle-in-cell simulation code. This work initially stimulated by Voyager 2 data focusses on this nonstationarity for different percentages of PUIs and for different Alfvén Mach numbers M A . Solar wind ions (SWIs) and PUIs are described, respectively, as Maxwellian and shell distributions (with a zero/finite thickness). For a moderate M A , present results show that(1) the shock front is still nonstationary even in the presence of 25% of PUIs; its instantaneous velocity varies, which is in favor for shock multicrossing; (2) the presence of PUIs tends to smooth out the time fluctuations of fieldamplitude and of microstructurewidths at the front and overshoot; (3) the shock has a multiple overshoot, which is analyzed by identifying the contributions of SWIs and the PUIs; (4) as the PUI percentage increases, the shock moves faster and the downstream compression becomes weaker, which is explained by a Rankine-Hugoniot model; (5) the reflection rate of SWIs and PUIs decreases as the PUI percentage increases; (6) the shock structure is almost insensitive to the shell thickness;and (7) for the PUIs dominated shock case (PUI=55%), the shock becomes stationary. However, for higher M A regime, the front nonstationarity persists even in the PUI=55% case. In summary, high M A regime allows to compensate the smoothing of the microstructures and the time fluctuations of the shock front brought by the presence of PUIs.

A Comparison of Particle-in-cell and Hybrid Simulations of the Heliospheric Termination Shock

“…The models agree closely for the profiles of the total (thermal + pickup) proton density (panel (d)), the proton velocity along the shock normal (panel (c)), and |B| (panel (b)). The secondary peaks downstream of the shock front, most visible in |B| and n, have been noted in previous simulations (Lembège & Yang 2016;Kumar et al 2018) and arise from the reflection and transmission of gyrating thermal ions. Perhaps the most notable differences occur in the profile and magnitude of E x , the cross-shock electric field, which is ≈two to three times larger and ≈two times sharper in the PIC simulation (note that this implies the cross-shock potentials are roughly equivalent).…”

“…) Simulations of a strictly perpendicular 1D shock that included a population of PUIs showed the development of an extended foot and crossshock electric field that resembled those found in previous hybrid simulations (Scholer et al 2003;Chapman et al 2005), but did not find efficient PUI acceleration via shock surfing. Later work demonstrated that the concentration of PUIs has a significant effect on the transfer of bulk dynamic energy to the thermal energy of solar wind protons (Matsukiyo & Scholer 2014;Lembège & Yang 2016). Companion 2D full-particle simulations recovered similar results regarding the structure and dynamics of the shock front and the dynamics of the partition of energy (Yang et al 2015).…”

“…The addition of a small thermal spread to the PUIs, drawn from a Maxwellian with T = 0.025, somewhat smears their initial distribution. This choice resembles the distribution of a freshly ionized PUI population; more sophisticated PUI distributions have been suggested (e.g., Chen et al 2013), but the rapid evolution at the shock seen in this work and others (Lembège & Yang 2016) suggests the details have minimal effects on the results. While Giacalone & Decker (2010) include a third, even higher-energy population, we do not, because the Larmor radii of such ions would equal or exceed L y .…”

In Situ Observations of Interstellar Pickup Ions from 1 au to the Outer Heliosphere

“… 2018 ; Lee et al. 1996 ; Lembège and Yang 2016 ; Matsukiyo and Scholer 2014 ; Richardson et al. 2008a ; Yang et al.…”

Energy Power Spectra Measured at an Interplanetary Shock by the New Horizon's SWAP Experiment: 1D Full Particle Simulations versus Observations

“…To answer the above questions, we use a one-dimensional (1-D) full electromagnetic particle-in-cell (PIC) code named Epoch (Arber et al, 2015), to simulate the interaction of the solar wind with a supercritical, collisionless, interplanetary shock propagating within a quasi-perpendicular angular range. The shock is produced by the injection method as in previous PIC simulations (Lee et al, 2005;Matsukiyo & Scholer, 2007;Yang et al, 2015;Lembege & Yang, 2016. To reproduce self-consistently the affect of PUIs and SW ions, five particle species are introduced within our PIC code: three solar wind populations (electrons, SW-H + and SW-He 2+ ions) and two pickup populations (PUI-H + and PUI-He + ).…”