Particle accelerationin magnetic reconnection

Seiji ZENITANI
National Astronomical Observatory of Japan

Outline• 1. Acceleration mechanisms in nonrelativistic
magnetic reconnection– Single X-line systems– Multi X-line systems• 2. Particle acceleration in relativistic magnetic
reconnection– Relativistic X-line acceleration– 3D problem• 3. Summary

Magnetic reconnection (MRX)Sun• Fundamental process in plasma systems• Conversion from magnetic energy to plasmaenergy -- bulk flow energy, plasma heating,or nonthermal particle acceleration

Earth’s MagnetosphereJetJetObservations in the Earth M’sphere• Nonthermal electrons (and ions) duringreconnection events• Numerical modeling as well as ultrahighresolution observation (MMS missionsince 2015/09) will help us to understandparticle acceleration in MRX

Oieroset+ 2002 PRLImada+ 2011 JGRStructure of the MRX site

Be-XlineB
B
e-• Particles get unmagnetized• Larmor radius > scale lengthStructure of the MRX siteEB
e-B
B
EE (Reconnectionelectric field)e-EX-line acceleration• Speiser motion (Speiser 1965)• Fast bounce motion between the field reversal (±B0)• Slow gyration about the normal field (Bn)
• Energy gain ~eEL

B0+B0electronLBnEEe--B0
Bird view

Speiser 1965 JGR

Side view

Electron acc. in particle-in-cell (PIC) simulations
Front-view

Front-view

Top-view

Top-view

E

grad B driftof electrons
Hoshino 2001+ JGREnergyMulti-step electron acc. in PIC simulation• Polarization electric field traps
and accelerates electrons(Hoshino 2005, SZ+ 2016a)• Upper limit: ~Ep/B• Combination of three acc.
mechanisms

Hoshino 2005 JGRMagnetic islands in multiple MRX system• Magnetic islands try to be round by the tension force• Electrons spin up at the contracting edges• Efficient in the nonrelativistic regime• Maximum energy: ~curvature radius Rc-E⊥ e-Rc
Drake+ 2006 NatureIsland merger acceleration• Variant of X-line acceleration• Accumulated magnetic field (B)and the islands’ speed (V) leadto strong E ~ VBn• Powerful, but impulsive

Energy evolution

BnEE
Oka+ 2010 ApJ

VTanaka+ 2010, 2011 PoPMRX in multiple current-sheets systems• Bath of magnetic bubbles• Multiple reflections bycontracting islands and/orreconnection jets

• First-order Fermi acc. claimed

Drake+ 2010 ApJ

Hoshino 2012 PRL

Interim Summary• Acceleration mechanisms in magnetic reconnection– X-line acceleration– Surfing acceleration (only for electrons)– Island contraction– Island merger– Statistical acceleration in multiple current sheet system• Combination of these mechanisms lead to generationof nonthermal particles• Most of theories remain phenomenological– Acc. efficiency– Physical/numerical parameter spaces

Magnetic reconnection
in high-energy settings• Relativistic magnetic
reconnection is expected
Gamma-ray flares

Crab pulsar @X-ray

Striped wind

Tavani 2011 Science

Coroniti 1990 ApJ

“Relativistic” reconnection?
• Magnetization parameters• Poynting flux : plasma energy flux (Zenitani+ 2009)• Available magnetic energy per particle

B02
m ⌘4⇡ 2 nmc2

B02
" ⌘4⇡ 2 w

• Alfvén speed• cA approaches c for higher σ• σ >> 1 for relativistic reconnection

Vjet ⇡ cA = cr

"1+"

m ⇡"

Relativistic MRX = particle accelerator
Energy spectra

Zenitani & Hoshino 2001, 2007 ApJ• Spectral index: -1 (acceleration site), -2.x (entire box)
• Online version: http://th.nao.ac.jp/MEMBER/zenitani/files/reconnection.mov

Relativistic X-line acceleration
EE2 - B2 >0

Cerutti+ 2012, 2013 ApJ

Em0 =Bn
SZ & Hoshino 2007 ApJ

m01(vc)2Top view

Resonant acc. at the jet front
BnTimeBn
E

Bn
EZenitani & Hoshino 2007 ApJ

Outflow direction• Reconnection sweeps the magnetic field at the jet front• Lucky energetic particles surf with the magnetic front

3D problem: the kink instability
B• Kink mode corrugates thecurrent sheet in 3D• This mode grows faster than
the reconnection (tearing)
mode

Strong heating

Turbulent
Plasma mixing

Zenitani & Hoshino 2005 PRL

3D onset problem• MRX does occur in a twisted geometry• MRX may dominate after the kink mode saturates

B

Zenitani & Hoshino 2005 PRL

KinkReconnection

Sironi & Spitkovsky 2014 ApJ

Current status of 3D onset problem• Possibilities• Magnetic twist: guide-field (Zenitani & Hoshino 2005, Cerutti+ 2014)• Parameter window for MRX (Liu+ 2011)• Sufficiently large system (Sironi & Spitkovsky 2014)• Different models (Kagan+ 2013, Guo+ 2014)• Some concerns

MRXGrowthrate• Sufficient # of particles?

Kink• Particle loading(Zenitani 2015)• Vlasov theory needed
nonrela.

thin RCS

T/mc2
relativistic

thick RCS
Zenitani & Hoshino 2007 ApJ

Harder spectra in higher-σ regimes
p=1.5

p=2
p=3

Nonrela.

Kagan+ 2015 SSR

Ultra-relativistic

Guo+ 2014 PRL• Once it occurs, relativistic MRX is an efficient accelerator• Harder spectral index for higher-σ (Guo+ 2014, SIroni & Spitkovsky 2014)• Similar indexes in 2D and 3D• Which acc. mechanisms are working? (Nalewajko+ 2015)

Relativistic MRX in an ion-electron plasma

Melzani+ 2014 A&A• Ions do accelerate. Heating or nonthermal acceleration?

• More acceleration in multiple current-sheet systems?
• More research neededSummary• Acceleration mechanisms in magnetic reconnection– X-line acceleration– Surfing acceleration (only for electrons)– Island contraction– Island merger– Stochastic acceleration in multiple current sheet system• Relativistic magnetic reconnection– Relativistic X-line acceleration– 3D trigger problem needs to be better understood– Harder spectra for higher magnetization– Ions do accelerate– Which acc. mechanisms are working?

Magnetic reconnection is a favorable particle accelerator.
Stay tuned for upcoming researches!