Clusters of small eruptive
flares produced by
magnetic reconnection in
the Sun.
14th European Solar Physics Meeting, Dublin Ireland
Vasilis Archontis1 & Viggo H. Hansteen2,3
1Mathematical
Institute, St Andrews University
2Institute of theoretical astrophysics, University of
Oslo
3Lockheed Martin Solar and Astrophysics
Laboratory
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.
Bifrost simulations
Hansteen 2004, Hansteen, Carlsson, Gudiksen 2007,
Martínez Sykora, Hansteen, Carlsson 2008, Gudiksen et al 2011
Numerical set-up
Stratification & magnetic field
Z
Y
X
Emerging field (flux
sheet).
Ambient magnetic field
(oblique, space filling).
Archontis & Hansteen, 2014
• CZ (z=-2.5 Mm).
• PHOT./CHR. (z=0-2.5 Mm). T~ 5 103 –
O(105) K.
• TR (z~2.5-4 Mm).
• COR. (z~4 Mm). T ~ O(106) K.
• 24x24x17 Mm, 504x504x496 grid.
• 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.
• Initial ambient field of B~0.1 G with
inclination of 45o with respect to z axis.
• Flux sheet (Bx=3300 G at bottom
boundary) within [x,y]=[0-24,3-16 Mm] for
105 min.
1st phase: emergence to the
photosphere
• Vertical slices at y~10 Mm.
• Horizontal slices at z~700 km above phot.
• Intensity: continuum, 630 nm & CaII 854.2
nm.
• B-flux elements pile up (surface) for ~15
min.
• Bphot = 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 1st
phase.
• Larger granules appear at the beginning of
2nd phase.
2nd phase: emergence above
the photosphere
• 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).
See Ortiz et al. 2014, ApJ 781, 126. for this
phase.
Multi-scale emergence of magnetic flux
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: <=100 s.
• Energy release: 1025 – 1026 erg.
CSHKP (2D
view)
Standard reconnection flare
model(s)
Carmichael 1964, Sturrock 1966, Hirayama 1974, Kopp-Pneuman
1976
Shibata, et.al. ApJ 451, L83, 1995.
Plasmoid ejection associated
with LDE flare (Yohkoh/SXT)
Evolution along the current sheet
• Not one but several plasmoids.
• Patchy reconnection – spatially
intermittent heating.
• Cluster of small flares.
• Fragmentation of current.
• Many sites of acceleration.
• One flare stimulates(?) the other.
“Sympathetic” flaring?
• Larger energy release: the
composite effect of the adjacent
small flares.
• Energy release: 1027 – 1028 erg.
3D view: plasmoids, heating, jets,
flares.
•
•
•
•
Isosurfaces: (red) T≥5.5 105 K, (blue) ρ ≈ 10-12 g cm-3 .
zslice=3.14 Mm, t=8800 s.
Intense heating where strong currents.
“Sympathetic” eruptions / flaring.
• Eruptions evolve into helical jets (EUV/X-ray).
• Cusp-shaped flare loops.
Temperature, vertical velocity (VZ) and
energy.
• Plasma heating (1-6 mK) by small
flares.
• Reconnection-driven acceleration.
• Heating-Energy: good correlation.
• Short-lived bursts of energy.
•
Superposition of small flares (10251026 erg).
•
“Individual” energy emissions
~(1027) erg.
• Lifetime of small flares: 30 s – 3
min.
• Flares at various heights.
• ~ 20 flares in 38 min.
• For coronal flares: average energy
flux
~ 2.1x106 - 107 erg s-1 cm-2 .
•
•
•
•
•
•
•
Discussion/Summary
Ejection of plasmoids leads to ‘patchy’ reconnection and, thus,
spatially intermittent heating.
Plasmoids share field lines, thus the eruption of one plasmoid initiates
the "sympathetic" eruption of others.
Eruption of plasmoids evolves into helical jets. Velocities comparable
to local Alfvén speed.
Average lifetime of individual small flares is of order 30 s - 3 minutes.
Plasma heated to 1-6 MK.
Some larger flares have energies of O(1027) erg, but many events are
superpositions of several small flares; 1025-1026 erg.
For the flares in the corona: average energy flux O(106) ergs s-1 cm-2.
Non-negligible contribution of heating in the active corona from small
flares.