Magnetic Reconnection and Particle

1
Magnetic Reconnection and Particle Acceleration
in Solar Flares L.Fletcher University of
Glasgow, U.K.
UC Berkeley Feb-5-2002
2
Motivation
To provide an overview of current ideas about the
role of, and evidence for, magnetic reconnection
in solar flares, and its relationship to
accelerated particles
Outline

• The standard flare model
• Observational evidence for coronal reconnection
• Accelerated particle properties
• Models for particle acceleration
• 3D magnetic fields
• Conclusions

UC Berkeley Feb-5-2002
3
Solar Flare Basics

• Solar flares - fast and transient releases of
  energy in the solar atmosphere - are
• one manifestation of the solar magnetic cycle

• The flare energy comes from non-potential energy
  stored in the pre-flare coronal
• magnetic field

• The release of this energy is thought to be
  facilitated by magnetic reconnection
• taking place in the corona.

• The general pattern is
• Rapid release phase, energy in the form of
  fast ions and electrons
• followed by
• Gradual equilibriation phase, possibly further
  slow energy input

UC Berkeley Feb-5-2002
4
Compact and Two Ribbon Flares
Solar Flares are often classified into two types
compact flares and two ribbon flares. Ha flare
imaging observations show a subset of flares with
(spreading) ribbons of emission from the
chromosphere.

• Two ribbon flares tend to be
• larger and more energetic
• electron poor, coronal ion abundances
• more likely to be associated with an eruption
• Extended gradual phase indicates
• energy input into the corona and
• chromosphere for up to several hours.

Image BBSO
UC Berkeley Feb-5-2002
5
Flare Movies
2 ribbon flare BBSO Ha
Compact Flare - TRACE 1216A
movie2
UC Berkeley Feb-5-2002
6
The Standard Flare Model - ubiquitous features
Coronal Reconnection at current sheet or X-line
- in wake of rising filament or flux rope - as
result of shear applied along an arcade, - due
to reconnection of anti-parallel and initially
open magnetic fields
Acceleration of particles associated with this
coronal reconnection
flare ribbons generated in various wavelengths
at chromosphere due to collisional excitation
or conductive heating
Heated Chromospheric material evaporates upwards
Forms (arcade of) post-flare loops
UC Berkeley Feb-5-2002
7
Standard model - Carmichael-Sturrock-Hirayama-Ko
pp-Pneuman
Sturrock 1966
UC Berkeley Feb-5-2002
8
Standard model
This has evolved to keep pace with observations
Sturrock 1966
Tsuneta 1997
UC Berkeley Feb-5-2002
9
Observational Support for Reconnection
(1) Coronal Cusps
Shape lends qualitative support to coronal
reconnection temperature structure. Also,
late-phase arcade.
UC Berkeley Feb-5-2002
10
Observational Support for Reconnection
(2) Supra-Arcade Downflows in LDEs
Evidence for shrinking back of field
lines following coronal reconnection
McKenzie Hudson 1999
UC Berkeley Feb-5-2002
11
Observational Support for Reconnection
(3) A Reconnection Inflow
Yokoyama et al 2000
MA(inflow) 0.001-0.03
UC Berkeley Feb-5-2002
12
Observational Support for Reconnection
(4) chromospheric upflows and downflows
Particularly elegantly demonstrated by Czaykowska
et al (1999) using CDS post-flare observations.
Blueshifts on outer part of arcade Redshifts on
inner part
UC Berkeley Feb-5-2002
13
The Role of Reconnection in Particle Acceleration
Reconnection is generally agreed to be a
necessary ingredient in flares, as it permits the
field to reconfigure and release stored
energy Is it a necessary ingredient in flare
particle acceleration? YES - reconnection
geometries provide the current sheets in
which particles are accelerated by DC electric
fields or... NO - particles can be accelerated
by shock acceleration or by resonant
acceleration, in the absence of reconnection
UC Berkeley Feb-5-2002
14
Characteristic Signatures of a Flare
Impulsive Hard X-rays (33-53keV)
Gradual soft X-rays (1keV)
typical time profile from Yohkoh HXT and SXT
Long duration event with evidence for extended
particle acceleration.
UC Berkeley Feb-5-2002
15
Observed Properties of Accelerated Particles
Canonical Values Electrons - tacc from thermal
to 10s-100s of keV 1s
1036-1037 e/s with Egt20keV total energy
content 1031 ergs fragmentation into
smaller micro-bursts (values imply that coronal
flaring volume will be depleted of electrons
within 1s ? number problem) Ions - tacc
from thermal to 10s-100s of MeV 1s
1035 protons/s above 1MeV for several 10s of
seconds total proton energy content is
comparable to electrons total heavy ion
energy comparable to that in protons Electron
and ion acceleration simultaneous to within 1s
UC Berkeley Feb-5-2002
16
Energetic particles in Solar Flares
Accelerated electrons (gt 8keV) are present even
in very small GOES C flares
The energy in accelerated ions is comparable to
that in accelerated electrons
Ramaty et al. 1995
Lin et al., 2001
UC Berkeley Feb-5-2002
17
Mechanisms of Particle Acceleration

• There are three mechanisms most often discussed
  in the context of
• acceleration of solar flare particles
• DC acceleration
• Stochastic resonant acceleration
• Shock Acceleration
• The last of these three is generally thought of
  as a secondary
• accelerator of particles, in the presence of a
  shock associated
• with a coronal mass ejection. We shall
  concentrate on the DC
• and resonant models.

UC Berkeley Feb-5-2002
18
DC electric field models - no current sheet

• DC electric field, aligned with magnetic field,
  in a coronal loop
• Electron accelerated if the field is greater than
  the Dreicer field,
• Ed such that the force exerted by the electric
  field exceeds
• drag force from e-e Coulomb collisions
• Ed typically 10-4 V/cm
• Modified by pitch-angle scattering and e-ion
  collisions, but basically
• electrons with speeds greater than some critical
  speed will be
• freely accelerated - runaway

Problems wrong HXR spectrum (too flat)
limits on current achievable, due to
self-field of beam
UC Berkeley Feb-5-2002
19
DC electric fields with current sheet

• Super-Dreicer fields occur within the context of
  magnetic
• reconnection.
• Litvinenko (1996) proposed that reconnection
  above a flare
• loop/arcade could generate a field on the order
  of 10V/cm
• parallel to solar surface and
• normal to loops in post-flare arcade

UC Berkeley Feb-5-2002
20
DC electric fields with current sheet
Character of particle orbit and energy gain are
controlled by magnetic structure in current
sheet Magnetic field cannot change the particle
energy, but can limit the time it spends in the
current sheet. e.g. the component of the
magnetic field perpendicular to the plane of the
current sheet magnetises particle energy
gain determined by particle gyroradius
(Litvinenko 1996)
Litvinenko Somov (1993) showed that E ltlt 1keV
for electrons
UC Berkeley Feb-5-2002
21
DC electric fields with current sheet
The third component of the magnetic field,
parallel to the current sheet can be invoked to
keep particles in the sheet. Electrons will be
efficiently magnetised and accelerated
Acceleration length sheet width ?
B/B? Typical estimated and modelled solar
values give electron energies in agreement with
observations, especially in the vicinity of the
current sheet. Protons will not be efficiently
accelerated in this manner, though a recent paper
investigating a fast reconnection solution
(Heerikhuizen, Litvinenko Craig 2002) may have
solved this
UC Berkeley Feb-5-2002
22
Resonant Stochastic Acceleration
Charged particles resonate with a turbulent
spectrum, hopping stochastically from one
resonance to another as their energy increases
or decreases (e.g. Miller Vinas 1993, Miller
1998) Frequency matching condition
w-k v - lW/g 0 Protons
and ions (mostly) resonate with high-frequency
Alfvenic waves Electrons resonate with
electromagnetic electron cyclotron waves
Miller 1997 presents a turbulent cascade scenario
from a large-scale Alfvenic perturbation down to
scales necessary for acceleration.
UC Berkeley Feb-5-2002
23
14 July 2000 Flare seen by TRACE
The 14-Jul-00 flare exhibited moving EUV ribbons
(seen by TRACE), which spread apart as the flare
progressed
UC Berkeley Feb-5-2002
24
Ribbons and Field
As reconnection progresses in the corona, the
footpoints of just reconnected fieldlines are
illuminated (by particles/ heat conduction)
leading to the appearance of flare ribbons. The
footpoint ribbons sweep across the magnetic field
Predictions Footpoint ribbons move faster
through weak field Equal flux swept out on
either side of the neutral line
UC Berkeley Feb-5-2002
25
Movement of EUV ribbons
In some instances, ribbons move slowly through
low field regions. Footpoints in low B
regions Ribbons straighten out as they slow down.
Longitudinal fluxes swept out on both sides of
the neutral line are not equal
Total longitudinal flux swept out by ribbons
(1020 Mx)
UC Berkeley Feb-5-2002
26
Flare Ribbons
We can estimate the coronal electric field
(assuming 2D symmetry) Poletto Kopp 1986,
Forbes 1999 Electric field given by the
rate at which closed magnetic flux A increases
E0

HAO January 28 2002
27
3-D fields
In reality, one must take into account the actual
distribution of magnetic sources in the
photosphere and construct the 3-D field - even
in the apparently axisymmetric case of 2-ribbon
flares
Here there is also a growing literature
(Demoulin, Priest, Mandrini, Amari, Longcope,
Gorbachev, Somov)
Sweet (1959) demonstrated presence of nulls and
separators in multipolar field
In 3-d this gives separatrix surfaces and their
intersections, separators (from Priest
Schrijver 2000)
UC Berkeley Feb-5-2002
28
E.g. Mandrini et al 1995
(from a series of papers with Demoulin and
collaborators)
Linear FFF extrapolation from photospheric vector
field showing intersection of separator with
source plane
Ha-0.6A image showing 2-ribbon event
UC Berkeley Feb-5-2002
29
Yurchyshyn et al.
Using FF extrapolation fit to SXT data, the
authors find 3 separate flux systems -
interaction leads to excitation of flare ribbons
UC Berkeley Feb-5-2002
30
Current sheets in 3-D fields
Numerical simulations of Galsgaard show extremely
complex current sheets in multipolar reconnecting
fields - these must be integrated with treatments
of particle accelerate (Nordlund)
UC Berkeley Feb-5-2002
31
Conclusions
Observations exist which strongly support the
presence of magnetic reconnection in at least
some solar flares Advocates of acceleration in
reconnecting current sheets and acceleration by
stochastic resonance would both claim that their
models can explain most of what is observed
(Resonance may just have the edge, in
explaining ion abundances) Reality is that there
is a long way to go in both integrating MHD and
kinetic treatments of the process, and also
extending our picture to truly 3D magnetic
geometries.
UC Berkeley Feb-5-2002