University:
Boston University
Course:
CAS PY 105 | Physics 1
Academic year:

2023
Views:

42

Pages:

26
Author:

cesureo1ig

Magnetic Reconnection: explosions in space and astrophysical plasma Magnetic Energy Dissipation in the Universe • The conversion of magnetic energy to heat and high speed flows underlies many important phenomena in nature – – – – solar and stellar flares Neutron star “quakes” magnetospheric substorms disruptions in laboratory fusion experiments • More generally understanding how magnetic energy is dissipated is essential to model the generation and dissipation of magnetic field energy in astrophysical systems – accretion disks – stellar dynamos – supernova shocks • Known systems are characterized by a slow buildup of magnetic energy and fast release  magnetic explosion – mechanism for fast release? – Why does the energy release occur as an explosion? Magnetic Free Energy • A reversed magnetic field is a source of free energy B xxxxxxxxxxxxxxxxxxxxxxxxx x J •Can imagine B simply self-annihilating •What happens in a plasma? Images of the Sun • Tsuneda ‘96 • Yohkoh Soft x-rays Normal B at photosphere TRACE •Soft X-ray emission peaks in solar active regions which are where B is large Magnetar flares • Isolated neutron stars with: – B ~ 1015 Gauss – Strongest B-fields in universe. • Giant Flare (SGR 1806-20) – Dec. 27, 2004, in our galaxy! – Peak Luminosity: 1047 ergs/sec. – Largest supernova: 4 x 1043 ergs/sec. – Cause: Global crust failure and magnetic reconnection. – Could be a source of short duration gamma ray bursts. Rhessi data: Hurley et al., 2005 Resistive Diffusion B xxxxxxxxxxxxxxxxxxxxxxxxx x J B y ¶B hc ¶ B = 2 ¶t 4p ¶y 2 • Diffusion of magnetic flux tr = 4pa / hc = 2 2 resistive time 2 Characteristic Diffusion Times Resistive Time Observed Energy Release Time 1 - 10 sec 50 sec Laboratory Tokamaks Solar Flares ~ 104 years Magnetospheric Substorms ~ ¥ ~ 20 min 30 min Resistive dissipation does not explain the observations Frozen-in Condition • In an ideal plasma (=0), the fluid moves so that the magnetic flux through any fluid element is preserved. Therefore in the absence of dissipation the plasma and magnetic field move together Energy Release from Squashed Bubble magnetic tension • Magnetic field lines want to become round Energy Release (cont.) w L • Evaluate initial and final magnetic energies – use conservation law for ideal motion • magnetic flux conserved • area for nearly incompressible motion Wf ~ (w/L) Wi << Wi •Most of the magnetic energy is released R Flow Generation • Released magnetic energy is converted into plasma flow 2 1 2 B rv = 2 8p 2 B 1/ 2 v » vA º ( ) 4pr tA = L / v A •Alfven time A is much shorter than observed energy release time •The energy release time is bracketed by A and R . Magnetic Reconnection • Strong observational support for this general picture • Reconnection is driven by the release of magnetic tension of newly reconnected magnetic field lines Magnetic Reconnection Simulation • Hall MHD simulation • d Intense currents Kivelson et al., 1995 Fast Flows at the Magnetopause Scurry et al. ‘94 Viking images of polar aurora • Elphinstone et al. ‘91 • The aurora are produced by energetic electrons from magnetic reconnection Reconnection in Solar Flares • X-class flare:  ~ 100 sec. • Alfven time: • A ~ L/cA ~ 10 sec. => Alfvenic Energy Release • Half of B-energy => energetic electrons! F. Shu, 1992 Impulsive flare timescales • Hard x-ray and radio fluxes – 2002 July 23 X-class flare – Onset of 10’s of seconds – Duration of 100’s of seconds. RHESSI and NoRH Data (White et al., 2003) Role of Resistivity • The frozen-in condition implies that in an ideal plasma (=0) no topological change in the magnetic field is possible – tubes of magnetic flux are preserved – magnetic reconnection requires resistivity some other dissipation mechanism • A measure of resistivity is the Lundquist number tr S= tA Magnetic Nozzle in the Magnetohydrodynamic (MHD) model • Formation of macroscopic Sweet-Parker layer V ~ ( /L) CA ~ (A/r)1/2 CA << CA •Slow reconnection •sensitive to resistivity •macroscopic nozzle Resistive MHD Solution • Slow reconnection due to nozzle produced by SweetParker current layer – Biskamp, 1986 Transition from MHD to Hall reconnection with plasmoids • Sweet-Parker layers break up to form plasmoids (Biskamp ’86, Laureiero et al ‘05) – For S > 104 – Faster reconnection because of shorter Sweet-Parker layer • Can plasmoids produce fast MHD reconnection in the corona? Daughton et al ‘09 Bhattacharjee et al. (2009) Cassak et al. (2009) Uzdensky et al. (2010) Shepherd and Cassak (2010) Huang et al (2011) 22 Resistive MHD reconnection • Flux diffusion across the Sweet-Parker layer ¶y ¶y hc ¶ y +Vin = 2 ¶t ¶x 4p ¶x 2 2 • Balancing flux convection and diffusion » » Slow reconnection even with plasmoids! Failure of the MHD Model • Reconnection rates too slow to explain observations – solar flares – sawtooth crash in tokamak plasmas – magnetospheric substorms • Some form of anomalous resistivity is often invoked to explain discrepancies – strong electron-ion streaming near x-line drives turbulence and associated enhanced electron-ion drag • Non-MHD physics at the small spatial scales where the frozenin condition is broken produces fast reconnection consistent with observations – Coupling to dispersive waves is critical Magnetic Reconnection beyond the MHD model • What happens when the “slingshot” occurs at very small spatial scales? – The MHD model is no longer valid  no Alfven wave – What drives the slingshot? • A class of “dispersive” waves dominate • Whistler and kinetic Alfven waves Role of Dispersive Waves • Coupling to dispersive waves at small scale is the key to understanding magnetic reconnection – rate of reconnection independent of the dissipation – no macroscopic nozzle

Magnetic Reconnection - Explosions In Space and Astrophysical Plasma

Related Documents

Recommended Documents

Description

Related Documents

Free up your schedule!

Take 5 seconds to unlock