Ion dynamics and shock front nonstationarity in supercritical perpendicular shocks:

impact of the pickup ions

Zhongwei YANG1 and Quanming LU2

1 SOA Key Laboratory for Polar Science, Polar Research Institute

of China, Shanghai, 200136, China.2 CAS Key Laboratory of Basic Plasma Physics, University of Science and Technology of China, Hefei, 230026, China

2012 West Lake International Symposium on Plasma Simulation program, ZJU

Introduction

Simulation model

Simulation results

Conclusions

04/2012Contents

Upstream of the termination shock (quasi-perpendicular), ions primarily consist of ions of two distinct components: the solar wind ions (SWs) and the pickup ions (PUI or PIs). [Vasyliunas and Siscoe, 1976; Fisk and Gloeckler, 2006; Wu et al.,

2009,2010]

1. Introduction

We thank NASA for this cartoon.

70-100AU

Interstellar neutral gas

Pickup ions

Anomalous cosmic rays (ACRs)

Solar wind protons

ANOMALOUS COSMIC RAYS, Klecker, Mewaldt et al., SSRv, 1998

1. Liewer et al. [1993] investigated the impact of PUI on the termination shock structure by using 1-D hybrid simulation.

For quasi-perpendicular shocks with 0-20% PUI, they found the PUI lead to the formation of an extended foot (quasi-stationary).

The amplitude of the extended foot increases with the PUI%.

1. Introduction

2. Similar results (extended foot) are retrieved by PIC simulations [Lee et al. 2005, Chapman et al., 2005].

Moreover, they found that 10% PUI do not modify the dynamics of the reforming shock.

Shock speed increases with PUI%.

High-energy part of the downstream distribution of the solar wind ionsdecreases with the relative density of the pickup ions.

1. Introduction

10%PUI

0%PUI

Distributions of the solar wind ions

3. Zank et al. [2010] developed a model for the termination shock. They have two assumptions of downstream proton distributions.

1. Introduction

Solar wind protons

Transmitted solar wind protons

Transmitted pickup ions

High-energy tail: reflected and accelerated pickup ions

k-distribution used in global MHD simulations (Heerikhuisen et al. 2008)

Maxwellian distributions with the downstream density and temperature for reference

Main questions (present goal):

1.First, what’s the impact of pickup ions on the shock front structure?

2. Second, whether the reforming shock solutions found previously persist no matter how large the relative density of pickup protons is?

3.Third, to what extent are acceleration mechanisms of both solar wind protons and pickup protons in the resulting self-reforming, nonstationary shock profiles?

1. Introduction

2. Simulation model

We use a one-dimensional PIC code including pickup ions to simulate the supercritical perpendicular shock. The shock is produced by the Injection / reflection wall method as in previous works [e.g. Quest, 1985; Burgess et al., 1989;

Nishimura et al., 2003; Chapman et al., 2005].(1) Setups:---------------------------------------------------------------------------------------------------θBn=90o,Injection bulk velocity Vin=3,B0=1 along y direction, mi/me=100, ωpe/Ωce=2, c=20---------------------------------------------------------------------------------------------------(2) Upstream plasma parameters:

e-: Maxwellian distributionSWs: Maxwellian distributionPUI: Shell distribution[e.g. Kucharek and Scholer, 1995; Lee et al., 2005; Wu et al., 2009; Scholer et al., 2011]

PUI%=0

PUI%=25

3. Simulation results3.1. Impact of the PIs% on the shock front reformation.

Run PIs%

0 0

1 5

2 10

3 15

4 20

5 25

6 30

… …

11 55

12 60

PUI%=55

Upstream

Downstream

3. Simulation results

2. Pickup ions 2. Pickup ions

[By and Ex are also shown for reference]

3. Simulation results

2. Pickup ions 2. Pickup ions

3. Simulation results

2. Pickup ions 2. Pickup ions

SDA ions

SSA ions

Core: DT ions

4. conclusions

1. Shock front reformation can be persisted well at supercritical

perpendicular shocks when PUI%<40.

2. Impact of the PUI% on the shock front micro-structure are as follows:

3. SDA is the most important acceleration mechanism. SSA also

exists. Injection angle domains of SSA shifts due to the Pickup ion

extended foot.

Brief summary

Low PUI% High PUI%

extended foot (dominated by PUI & Lorentz term) stationary stationary

foot in solar wind ion gyro-scale (Lorentz and Hall term are competing with each other)

nonstationary -

ramp in electron inertial scale(dominated by SWs & Hall term)

nonstationary stationary

Thank you!