Ion pickup and acceration in magnetic reconnection exhausts J. F. Drake University of Maryland M. Swisdak University of Maryland T. Phan UC Berkeley E. Quatert UC Berkeley R. Lin UC Berkeley S. Lepri U Michican T. Zurbuchen U Michican P. Cassak University of Delaware M.A. Shay University of Delaware

Ion acceleration during magnetic reconnection In solar flares energetic ions up to GeVs have been measured –In impulsive flares have enhancements of ions over coronal abundances linked to Q/M In the solar wind see energy proportional to mass Heated ions in solar wind reconnection events (Gosling et al, 2005; Phan et al 2006) Ubiquitous v -5 energetic ion distribution in the quiet solar wind (Fisk and Gloeckler 2006)

Heavy ion data from Ulysses The thermal spread of the various ion species is nearly mass independent (von Steiger and Zurbuchen 2006) –Energy proportional to mass

Wind observations of solar wind exhaust 300R E event (Phan et al., 2006)  T p ~ 7eV)  T  ~ 30eV)

Proton spectra of the form f  v -5 are often observed What about the anisotropy of energetic ions? Similarity to spectra from the Fermi mechanism is striking Universal ion spectrum in the quiet solar wind Gloeckler et al, 2006

Impulsive flare energetic ion abundance enhancement During impulsive flares see heavy ion abundances enhanced over coronal values Enhancement linked to Q/M Mason, 2007

Ion pickup in reconnection exhausts Ions moving from upstream cross a narrow boundary layer into the Alfvenic reconnection exhaust The ion can then act like a classic “pick-up” particle, where it gains an effective thermal velocity equal to the Alfvenic outflow c A The result is roughly energy proportional to mass ( Fujimoto and Nakamura, 1994)

Ion acceleration during anti-parallel reconnection PIC simulation with m i /m e =25 Focus on ion heating well downstream of the x-line Sharp increase of T i in region of large Hall fields –The Hall fields drive the outflow exhaust

Test particle trajectories Ions cross a narrow boundary layer into the region of large Hall electric field E y that drives the outflow Initial motion along E y rather than in E  B direction (e.g., Wygant et al 2005) –acts likes a pickup ion in the solar wind –Gains a non-zero magnetic moment 

Fields and frame transformation in the outflow The boundary layer that bounds the outflow exhaust is not a switch-off slow shock –Never seen in PIC simulations Move to the local field-line frame to eliminate the electric fields (v x = -1.35c A ) moving frame

Particle motion in 1-D fields Similar to Buechner and Zeleny ‘86 –Additional sharp boundary from the Hall field B z Particles see a sharp kink in B as enter exhaust –Generates finite  Within the layer Speiser-like orbit

Ion temperature in reconnection outflows: anti-parallel case Speiser-like Fermi reflection  conservation violation at the outflow boundary Total temperature change –Similar to Krauss-Varban and Welsch (2007) Note energy proportional to mass as is typically seen in energetic particles in the solar wind

Wind solar wind exhaust data Data from 22 solar wind reconnection exhaust encounters –All with small guide fields Proton temperature increase in exhaust is given by m p  v 2 (eV) 3  T p (eV)

Heavy ions from SWICS Lepri and Zurbuchen, 2008 Heliospheric current sheet reconnection event (Gosling et al 2007)

Mass dependence of temperature increase *Fe has low statistics at this time resolution

Ion acceleration with a guide field Structure of electric field changes dramatically from anti-parallel case –Large E y drives the outflow (cE y /B z =c Ax ) Large guide field can magnetize the protons –  conserved for protons Can’t eliminate E everywhere with a frame transformation B z0 = 5.0

Test particles in Hall MHD reconnection fields Protons and alpha particles remain adiabatic (  is conserved) –Only mass 6 and above behave like pickup particles  because of large guide field EyEy B z0 = 5.0

Ion energy gain –Irreversible portion The ions that act like pickup particles -- those with high M/Q -- gain much more energy What is the threshold for acting like a pickup particle? For coronal parameters (n ~ 3  10 9 /cm 3, T ~ 3  10 6 o K) proton threshold is 60G

Ion temperature in reconnection outflows: guide field case Shift to moving frame in exhaust to eliminate electric fields there –Calculate dynamics in adiabatic and non-adiabatic limits Non-adiabatic ions  same as no guide field Adiabatic ions –Very small for a strong guide field

Production of energetic ions during flares Pickup in solar wind exhausts can seed ions to Alfvenic velocities. Ions must interact with many reconnection exhausts magnetic islands to reach GeV energies

A multi-island acceleration model Narrow current layers spawn multiple magnetic islands in guide field reconnection –The generic case Secondary islands seen in observations – FTEs at the magnetopause –Magnetotail –Downflow blobs in the corona In 3-D magnetic islands will be volume filling Explore particle acceleration in a multi- island environment

Multi-island acceleration Consider a reconnection region with multiple islands in 3-D with a stochastic magnetic field Electrons are accelerated through Fermi reflection as each island undergoes contraction(Kliem, 1994, Drake, et al., 2006) Thermal ions are too slow too slow to undergo many bounces –Ions accelerated through pickup seed the ions so they can be accelerated through Fermi reflection. Evidence that pickup is important? u in C Ax

Abundance enhancements in impulsive flares Ion pickup criterion can be rephrased as a threshold on magnetic island width w c. –Higher M/Q ions have lower island width thresholds Rate of production of pickup ions –Take powerlaw distribution of island widths: P(w) ~ w -  –Match the observations if  ~ 6.26  reasonable

Fermi acceleration for super-Alfvenic particles Steady state kinetic equation for electrons or ions in 2-D reconnection geometry Energy gain linked to release of magnetic energy Powerlaw distributions of energetic particles with spectral indices that depend on the incoming plasma  –For the solar corona v 2 f ~ E -1.5 f ~ v -5 Universal spectrum for low  Consequence of firehose marginal stability due to anisotropy of accelerated particles

Implications for the solar wind observations Do these ideas about ion acceleration translate to the solar wind observations? –Is dissipation in solar wind turbulence dominated by local reconnection processes? –Bent field lines accelerate ions in reconnection. Do similar processes occur in Alfvenic turbulence? If so then marginal firehose condition should also play a role in Alfvenic turbulence –Source of the v -5 velocity distributions?

Conclusions Ions entering magnetic reconnection exhausts can act like pickup particles –Produces Alfvenic thermal spread on with energy proportional to mass - - confirmed with Wind and ACE observations –During reconnection with a guide field small Q/M ions much more easily behave like pickup particles and are more easily accelerated than large Q/M ions High energy particle production during magnetic reconnection typically involves the interaction with many magnetic islands –Not a single x-line –Using the pickup threshold for heavy ions can explain impulsive flare heavy ion abundance enhancements with a simple model Fermi acceleration within contracting magnetic islands can energize super-Alfvenic ions seeded by the pickup process –Produces f(v) ~ v -5 distributions  consequence of firehose marginal stability