Slide 1

Clusters of small eruptive flares produced by magnetic reconnection in the Sun. Vasilis Archontis 1 & Viggo H. Hansteen 2,3 1 Mathematical Institute, St Andrews University 2 Institute of theoretical astrophysics, University of Oslo 3 Lockheed Martin Solar and Astrophysics Laboratory 14 th European Solar Physics Meeting, Dublin Ireland Slide 2 Overview Numerical experiments: initial conditions. Flux emergence at/above the photosphere. Plasmoid-induced-reconnection. Fragmentation of current layers / intermittent heating. Onset of small flares (nano/micro-flares). Heating of the active corona. Slide 3 Bifrost simulations Hansteen 2004, Hansteen, Carlsson, Gudiksen 2007, Martnez Sykora, Hansteen, Carlsson 2008, Gudiksen et al 2011 Slide 4 Numerical set-up Initial ambient field of B~0.1 G with inclination of 45 o with respect to z axis. Flux sheet (B x =3300 G at bottom boundary) within [x,y]=[0-24,3-16 Mm] for 105 min. CZ (z=-2.5 Mm). PHOT./CHR. (z=0-2.5 Mm). T~ 5 10 3 O(10 5 ) K. TR (z~2.5-4 Mm). COR. (z~4 Mm). T ~ O(10 6 ) K. 24x24x17 Mm, 504x504x496 grid. Convection zone Photosphere/Chromosphere Transition region Corona X Y Z Emerging field (flux sheet). Ambient magnetic field (oblique, space filling). Convection is driven by optically thick radiative transfer from the photosphere. Radiative losses in the chrom. include scattering, optically thin in the corona. Field-aligned thermal conduction is included. Hyper-diffusion is included. Stratification & magnetic field Archontis & Hansteen, 2014 Slide 5 1 st phase: emergence to the photosphere B-flux elements pile up (surface) for ~15 min. B phot = 500-600 G (max=1-1.5 kG). B-field emerges above the surface after ~2hrs. Chromospheric temperature structure set by acoustic shocks, oscillations etc. until magnetic field emerges into outer atmosphere. Photospheric and chromospheric intensity little changed by flux emergence during 1 st phase. Larger granules appear at the beginning of 2 nd phase. Vertical slices at y~10 Mm. Horizontal slices at z~700 km above phot. Intensity: continuum, 630 nm & CaII 854.2 nm. Slide 6 The emerging field enters the corona. Emerging loops: dense and cool (adiabatic expansion). Photosphere: granule size change, bright points. Chrom/corona: local temperature increase. (Low) chromosphere intensity and contrast increase. Acoustic shock structure severely modified by magnetic field as waves are expelled from cool emerging bubbles. Magnetic loops interact (e.g. reconnect). 2 nd phase: emergence above the photosphere See Ortiz et al. 2014, ApJ 781, 126. for this phase. Slide 7 Multi-scale emergence of magnetic flux Slide 8 Evolution across the current sheet Long, thin current layer. Tearing instability plasmoids. Ejection of plasmoids reconnection X-ray temperatures. Jets (V~200-400 km/s, T~2.5 mK). Small flare loops (T~ 2 mK). Heat conduction Chrom. heating. Lifetime of flaring: