The Structure of the Pulsar Magnetosphere via Particle Simulation

S. Shibata (1) , T. Wada (2) , S. Yuki (3) , and M. Umizaki (3)

(1)Department of Phys.Yamagata University(2)National Astronomical Observatory of Japan(3)School of Science and Engineering, Yamagata University

magnetic field

pair

creation

Step -1 dipole no

Step -2 dipole Yes

(low rate)

Step -3 Modification by J

Yes

(high rate)

Abstract:We perform a particle simulation to explain the pulsar phenomena

from the first principles. It is shown that the outer gaps indeed appear around the null surface. The origin can be understood simply by strong charge separation by emf of the star.

The gap persists under copious pair-creation, providing the particle source of the pulsar wind as well as the gamma-ray pulses.

If pair creation rate is high, then magnetic field is opened and pairs flow out along the opened magnetic flux, i.e., the pulsar wind is formed.

If pair creation rate is low, then the wind is moderate, and strong radiation drag causes a large scale circular flow in the magnetosphere.

The wall of the polar cap flow is verified, but locates nearer to the polar axis than expected.

The most prominent activities that pulsars show are (1) the pulsar wind, (2)high-energy pulsed emissions, and (3) the radio pulses.To explain these phenomena, the outer gap, the polar cap particle accelerators and the relativistic centrifugal wind are proposed. We aim to prove existence of these from the first principles via particle simulation.

The strong magnetic field (1012G) and fast rotation cause strong charge separation in the plasmas around the neutron star. The produced electric field perpendicular to the magnetic field brings about co-rotational motion. Beyond the light cylinder on which co-rotation speed reaches the speed of light, the plasmas will flow out centrifugally. Due to deficiency of plasmas, the gaps are formed along the electric dividing ridge, so-called “null surface”.

Our simulation includes 3 steps (right table). The method of solution is described in the companion paper entitled “Method of Particle Simulation for the Pulsar Magnetosphere by use of Special Purpose Computer, GRAPE-6” in this conference.

Gaps are formed, which is unstable against electron-position pair cascade.

Particle distribution

Electrostatic clouds

Field-aligned electric field

The final state is static clouds with the gaps. The previous result by Krause-Polstorf and Michel (1985) is reproduced.

1.Strong charge separation by the magnetic neutron star produces the outer gaps.

2.The gaps persist against pair creation cascade to provide the particle source of the pulsar wind.

3.If pair creation rate is high, then magnetic field is opened and pair plasmas flow out along the open magnetic flux, though the number of particles is not enough for realistic pulsar winds.

4.If pair creation rate is low, then the wind is moderate, and large scale circulation of the pair plasma is dominant with strong radiation drag.

5. The assumption of the side wall for the polar cap is verified.

Summary of our simulation

The gap is formed around the null surface. The field-aligned electric field is marginally greater than the threshold for the pair creation and continue to produce pairs.Most of the place other than the gaps has no field-aligned electric field, and whereby the plasma tends to rotate.

Corotationregion

Corotationregion

Super-rotation

Ligh

t cyl

inde

r

The created pairs partially flow out and partially circulate in the magnetosphere. The reason why particles can flow out across the closed filed is the drift motion by radiation-drag in azimuthal direction.

ReferencesHalpern, J.P. & Holt, S.S., 1992, Nature 357, 222Mori, K. et al., 2004, ApJ., 609, 186Wada, T. & Shibata, S., 2007, MNRAS, 376, 1460 (for Step 1-2)Krause-Polstorf, J. & Michel, F.C., 1985, AA, 144, 72

IntroductionQuickLook

Ｄｅｔａｉｌ

What would happen in the simulation?

Results of Step-1

Results of Step-2

As pair creation rate is increased, the original dipole field is modified to makes open flux. In our simulation, the magnetic neutral sheet is still thick with closed magnetic flux since the pair density is not enough. However, the pair creation rated was still higher, the neutral sheet would be thinner and most of the flux is opened. In such cases, the pair plasma would be much more likely the mhd wind.

Magnetic field

Dipole field for comparison

Bφ <0 Blue Bφ >0 Red

Modification of the magnetic field

Color codes for toroidal component

Results of Step-3(Halpern & Holt ,1992)

(Mori, K. et. al. 2004)

null surface

For detail ｌ

Simulation steps

The wall of the polar cap

We find the side-wall, which is assumed in the polar cap models. The electric potential is the same as the star on this line. However, the wall is not the boundary to the dead zone, but to the returning current region.

Outer gap

The Energetic Cosmos: From Suzaku to Astro-H, Otaru, Hokkaido, Japan 29 June -2 July 2009

Rotatingplasma