# Presentation: 488933

## Flare Acceleration 

- _for GeV Particles_
- Markus Aschwanden
- Lockheed Martin Solar and Astrophysics Laboratory
- Ground Level Enhancement (GLE)
- Comparative Data Analysis Workshop (CDAW), LMSAL, Jan 6-9, 2009

## Energy spectrum of the largest flares

- 1 GeV

- 1 MeV

- 1 keV

## Hard X-ray and Gamma-ray Emission Mechanisms

-                                                       Observed	   Primary
-                                                       photons            particle energy
- ____________________________________________________
- Bremsstrahlung continuum           20 keV-1 MeV  20 keV-1 MeV
-                                                       >10 MeV          10 MeV-1 GeV
- Nuclear de-exitation lines             0.4...6.1 MeV   1-100 MeV/nucl
- Neutron capture line                     2.2 MeV            1-100 MeV/nucl
- Positron annihilation radiation      0.511 MeV        1-100 MeV/nucl
- Pion decay radiation                     10 MeV-3 GeV 0.2-5 GeV
- Neutrons in space                        10-500 MeV     10 MeV-1 GeV
- Neutrons induc.atmos. cascades 0.1-10 GeV       0.1-10 GeV
- Neutron decay protons in space  20-200 MeV      20-400 MeV

## Electron-dominated bremsstrahlung spectrum observed 

- Electron-dominated bremsstrahlung spectrum observed
- up to 50 MeV in 1991-Jun-30 flare, gamma rays detected
- up to >1 GeV in 1991-Jun-11 flare (Kanbach et al. 1992)

## Gamma rays detected with EGRET/CGRO during 1991-Jun-11

- flare up to >1 GeV              (Kanbach et al. 1993; Mandzhavidze & Ramaty 1992)

## Slide 6

## The 2005-Jan-20 flare shows up to >200 MeV gamma rays.

- Grechnev et al. 2008

- The impulsive phase of GLE, pion-decay gamma rays
- closely correspond in time.

## Temporal coincidence of hard X-ray and gamma-rays.

## Impulsive gamma-rays and GLE coincide within ~1 minute.

- Grechnev et al. (2008)

## What are the maximum energies to which particles can be accelerated in solar flares ?

- * No known high-energy cutoff of electron
-   bremsstrahlung spectrum
- * Highest energies of observed bremsstrahlung >100 MeV
- * Highest gamma-rays reported >1 GeV with EGRET/CGRO
-    in 1991-Jun-11 flare: spectrum could be fitted with a
-    composite of a proton generated pion neutral spectrum
-    and a primary electron bremsstrahlung component

## DC Electric field acceleration

- 	- sub-Dreicer field: maximum velocity to which
-         electrons can be accelerated is limited by the
-         value of the Dreicer field, ED~2 *10-10 n/T
-         Energies of 10-100 MeV can be obtained for
-         high densities n~1012 cm-3 and low temp. T=2 MK,
-         ED=0.03 V/cm, if electrons are continuously
-         accelerated over a current channel with a length
-         of L=10-100 Mm (Holman 1996). Such large-scale
-         DC fields conflict with TOF measurements
-         (Aschwanden et al. 1996).

## Reconstruction of height

- of electron acceleration region
- in Masuda flare:  L/s ~ 1.5-2.0
- (Aschwanden et al. 1996)

## Measurements of

- ratio of
- electron time-of-flight
- distance L to
- flare loop half length s
- L/s = 1.43 +/- 0.30
- (Aschwanden et al. 1996)
- L/s = 1.6 +/- 0.6
- (Aschwanden et al. 1998, 1999)

## 2) Acceleration in super-Dreicer electric fields

-     Maximum energies for accelerated particles is
- 	Emax=e E*L(t)=e E c t
-     yielding super-Dreicer fields of E>100 V/m,
-     electron acceleration up to a few 10 MeV in
-     electron-rich flares, or protons with energies up
-     to several GeV in large graual flares
-     (Litvinenko 2003). Super-Dreicer fields in
-     current sheets do not conflict with TOF
-     measurements (Aschwanden et al. 1996).

## Particle Acceleration in current sheets  

- Fast (subsecond) time structures of hard X-ray and radio
- pulses in solar flares suggest small-scale, fragmented,
- bursty magnetic reconnection mode.

## Magnetic island formation by tearing mode instability

- (Furth, Killeen, & Rosenbouth  et al. 1963)
- Magnetic X-point and O-points form 
- coalescence instability (Pritchett & Wu 1979)
- Magnetic island formation + coalescence instability 
- regime of impulsive bursty reconnection
- (Leboef et al. 1982; Tajima et al. 1987; Kliem 1998, 1995)

## Karpen et al. 1995, 1998

- Schumacher & Kliem 1997
- Kliem et al. 1995, 1998
- Kliem, Karlicky, & Benz 2000

## Convective electric field : Econv = - u/c  B

- Convective electric field : Econv = - u/c  B
-      (convective flow speed  u ~ (0.01-0.1) vA
- Particle orbit near magnetic O-point in magnetic island shows
-      largest acceleration kick due to B-drift next X-point

- (Kliem 1994)

## Particle acceleration near X-point (chaotic orbits)

- (Hannah et al. 2002)

## 3) Stochastic Acceleration

-     can generate 10 MeV electrons and 1 GeV protons
-     if a sufficiently high wave turbulence level is assumed
-     (Miller et al. 1997). There is no diagnostic of wave
-     turbulence level available yet!

## -Electron acceleration by whistler waves

- Miller et al. (1996)

- Ion acceleration by Alfven waves
- Enhanced ion abundances in flares reproduced by stochastic acc.
-          (C, O, Ne, Mg, Si, Fe, but some problems with He3/He4)

##  Particle Kinematics and Propagation 

- Each particle transport
- process has its characteristic
- energy-dependent timing
- that can be used for diagnostic
- acceleration  dE/dt > 0
- injection [pitch angle, (t)]
- time-of-flight  t(E) ~ t/v(E)
- trapping: collisional deflection
-      time  t(E) ~  E 3/2 / ne
- -energy loss:
-      tloss << tTOF

- Radio: electron beams along open and/or closed field lines produce
-        plasma emission (radio type III, J, U, N, RS bursts)
- Predictions: bi-directional beams (type III+RS pairs)
-        correlated pulses in radio (type III) and hard X-rays

- Upward versus downward acceleration in flares

## Electron beam trajectories diagnosis for radio dynamic spectra:

- Electron beam trajectories diagnosis for radio dynamic spectra:
-       - open magnetic field lines (type III)
-       - closed magnetic field lines (type J, U)
-       - downward propagating electron beams (type RS)

## Acceleration region bracketed by upward/downward electron beams:

- Acceleration region bracketed by upward/downward electron beams:
- --> triple correlations between radio type III, RS and HXR pulses

- CGRO

- Zurich
- radio

- Aschwanden et al. (1993)

## Escape into interplanetary space

- The escape of flare-accelerated
- particles into interplanetary is
- faciliated on one hand by
- temporarily opened fields
- during the magnetic reconnetion
- process.
- For instance, during the
- “magnetic break-out model”
- (Antiochos et al. 1999),
- a temporary opening of
- a secondary arcade occurs
- during the reconnection process.

## On the other hand,

- there are also
- pre-existing open
- field lines that allow
- particles to escape.
- Schrijver & DeRosa
- (2003) found from
- potential field
- extrapolations that
- a fraction of the IMF
- connects directly to
- plages of active regions
- (<10% in solar min.
- 30-50% at cycle max.)

- Schrijver & deRosa (2003)

## Type II-associated acceleration source was localized 

- Type II-associated acceleration source was localized
- in lower corona (Nancay), far below CME bowshock at 5 Rsun

- Klein et al. (1999)

## Almost every interplanetary

- type III burst, whose spiral
- field line is connected to the
- visible disk or limb of the Sun,
- is found to be associated with
- a group of solar metric type
- III bursts.
- This is the finding of a study
- With Ulysses/URAP and
- Artemis/Nancay
- (Pocquerusse et al. 1996).

## A detailed temporal correlation was found between the

- Temmer et al. (2008)

- CME acceleration dh2/dt2(t) rate and the hard X-ray flux fHXR(t)

## CONCLUSIONS : 

-         is observed up to 100 MeV for electron bremsstrahlung
-         and up to 1 GeV for protons and pions.
- 2)    The maximum energy of accelerated particles observed
-         in solar flare gamma rays implies severe constraints for
-         sub-Dreicer electric field acceleration mechanisms, but
-         is achievable with super-Dreicer fields or stochastic acceleration.
- 3)   The altitude of acceleration regions in flare sites is confined
-       to hacc=5000-35,000 km (<0.05 solar radii) according to time-of-flight
-       measurements and direct hard X-ray imaging. Acceleration
-       sites at larger distances to the Sun (as inferred for GLE events)
-       are likely to be associated with CME shocks.
- 4)  The vertical symmetry of acceleration sites warrants
-       simultaneous acceleration in upward and downward
-       direction (e.g., electric DC fields, wave turbulence
-       regions, fast shocks in Petschek-type) and allows charged
-       particles to escape into interplanetary space and produce GLEs.

- _CONCLUSIONS : _

*http://www.lmsal.com/~aschwand/ppt/2009_GLE_CDAW.ppt*

## Backup slides

## 1) Magnetic Topology in Solar Flares

- The basic configurations of X-type magentic reconnection topologies
- in solar flares entail combinations between open and closed field lines :
- bipolar, tripolar, and quadrupolar cases, in 2D as well as in 3D.

## Bipolar reconnection (examples from Yohkoh)

## Bipolar topologies

- Tripolar topologies

**B**ipolar topologies

- Kopp-Pneuman model (CSHKP)
- Tsuneta

## The standard Kopp-Pneuman (CSHKP) flare model

- with a bipolar 2D magnetic topology and a
- single flare loop seems to explain many
- simple flares, but there are also many observations
- that require 3D models, multiple flare loops,
- as well as tripolar, quadrupolar, or 3D nullpoint
- magnetic topologies.

## Bastille-Day flare, 2000-Jul-14

## Bastille-Day flare

- 2000 Jul 14:
- The footpoints
- In two-ribbon
- flares separate
- Consequence
-     of magnetic
-     reconnecion
-     to progress in
-     altitude

- Aschwanden & Alexander (2001)

## Discovery of symmetrc dual coronal hard X-ray sources

- Discovery of symmetrc dual coronal hard X-ray sources
- confirm the X-point geometry with upward/downward
- acceleration of hard X-ray producing electrons.

- TRACE 1600 A on 2002 Apr 15, 23:07 UT, overlaid with RHESSI
- 10-15 keV contours. The energies of the symmetric coronal sources
- decrease progressively with distance from X-point, as expected from
- the temperature drop of conductive cooling (Sui & Holman 2003).

- 6-8 keV
- 10-12 keV
- 6-20 keV

## Coronal hard X-ray source is found to initally drop in altitude

- before it rises in the later flare phase. This discovery is still unexplained:
- 	- relaxation of newly-reconnected field line ?
- 	- implosion after CME launch ? --> check EUV dimming !

- Sui & Holman (2003)

## The hard X-ray flux F(t) is correlated with the acceleration d2h/dt2(t) of

- the CME (Temmer et al. 2008). The upward-directed CME acceleration
- could push the X-point initially down, while it sucks it in upward direction
- after the pressure drops when the CME expands into interplanetary space.

## Does a simple HXR double footpoint source mean a single flare loop? 

- Krucker & Lin (2002)

- Does a simple HXR double footpoint source mean a single flare loop?
- Time profiles indicate multiple loops with different timing

## Propagation of

- reconnection sites
- 2002 July 23 flare:
- Motions of footpoints
- are observed to
- increase in size and
- to move systematically
- along the ribbons.
- see simulations
- of zipper effect by
- Mark Linton

- Krucker, Hurford & Lin (2003)

## The footpoints of conjugate hard X-ray footpoints are observed

- to systematically propagate along the flare ribbons (in 2002 Nov 9 flare),
- rather than apart as predicted in the Kopp-Pneuman model
- reconnection propagates along neutral line

- Grigis & Benz (2005)

## There is an increasing number

- Veronig & Brown (2004)

- of flares with absence of
- nonthermal footpoint emission,
- contrary to the expectation of
- the standard thick-target model.
- Veronig & Brown (2004)
- Interpret those events in terms
- of high densities in the coronal
- flare loop part which produce
- thermal conduction fronts,
- rather than precipitation-driven
- chromospheric evaporation.

## Where are the hard X-ray double ribbons ?

- During the 2005 May 13 flare (color=RHESSI, white contour=
- TRACE 1600 A) extended hard X-ray ribbons are seen,
- interpreted in terms of a particular sigmoid-to-arcade evolution.

- (Chang) Liu et al. (2007)

## Sigmoid-to-Arcade Evolution

- (Moore Tether-cutting model)

- Liu et al. (2007)

## Tripolar topologies

- Emerging flux model
- Heyvaerts-Priest-Rust 1977
- Soft X-ray jets (Shibata)

## 3D nullpoint spine reconnection

- Krucker et al. (2004)

## Quadrupolar topologies

- Magnetic flux transfer (Melrose)
- 3D flare geometry:
- Hanaoka, Nishio, Aschwanden

## 2)  Localization of Acceleration Region 

- Volume of field-line
- shrinking (relaxation)
- after magn. reconnection
- defines geometry of
- acceleration region :
- cusps
- double cusps
- jets
- curved hyperboloids
- spines

## Shrinkage ratio :

- length of pre-reconnection
- to post-reconnection
- field line
- = electron time-of-flight
- distance to soft X-ray
- flare loop half length
- bipolar: L/s > 1.0
- tripolar: L/s > 1.0
- quadrupolar: L/s = 1.4-1.6
- 3D fan:  L/s = 2.0

## Anatomy of hard X-ray sources

- in a solar flare:

- How do we localize the particle acceleration sources from this ?

## Acceleration regions are expected in locations where newly

- reconnected field lines relax into a force-free configuration.

- Force on accelerated
- particles :

- Energy gain from shortened
- (relaxed) force-free field line:

## Measurement of electron time-of-flight distance :

- velocity dispersion from hard X-ray energy-time delay t=L/v
- pitch angle correction (vparallel/v = cos )
- magnetic field line twist correction (Lprojected/LTOF)

## Image reconstruction with MEM-SATO algorithm (Jun Sato)

## Slide 58

## Altitudes averaged from northern and southern footpoints :

- (error bars correspond to difference between N and S)

## The height distribution of HXR emission  dI/dz(z)  is shown

- for 5 different HXR photon energies  e=5, 10, 20, 30, 40 keV

## How can flare models with particle acceleration in the

- Fletcher & Hudson (2008)

- How can flare models with particle acceleration in the
- chromosphere be reconciled with electron time-of-flight
- measurements ?

## Acceleration sources of electrons versus ions:

- The standard flare scenarios predict identical sources
- but the observations reveal different locations for
- hard X-ray electrons and 2.2 MeV producing neutrons !

- Hurford et al. (2006)

## The gamma-ray source of the 2.2 MeV neutron-capture line

- was found to be displaced by 20”+/- 6” from the 25 keV hard
- X-ray footpoints during the 2002 Jul 23 flare. A similar
- result was found for the Oct/Nov 2003 flares.
- Energized electrons and ions show displaced energy loss sites
-      (1) different acceleration sites for electrons and ions ?
-           - different path lengths for stochastic accleration (Emslie 2004)
-               - charge separation in super-Dreicer electric field (Zharkova & Gordovskyy 2004)
-      (2) different propagation paths for electrons and ions ?

## Electric field at X-point in

- impulsive bursty reconnection mode
- (Kliem et al. 2000)

- Hard X-ray pulses resulting from
- accelerated electrons dt ~ 0.1-0.3 s
- (Aschwanden et al. 1996)

## Observations show a scaling law between Hard X-ray pulse 

- Observations show a scaling law between Hard X-ray pulse
- durations and flare loop size : Tpulse ~ 0.5 s [rloop/10 Mm]
- scale invariance of magnetic reconnection region
- (Aschwanden et al. 1998)

- Lower limit of pulse durations: collisional deflection time

## a) Electric DC field acceleration :

- _a) Electric DC field acceleration_ :
- Sub-Dreicer field needs too large current sheets
-  (Holman 1985; Tsuneta 1985)
- Super-Dreicer field applicable in magnetic islands
-  (Litvinenko 1996)
- Generalization to dynamics of filamentary current sheets
- (Tajima et al. 1987; Kliem 994)

## Stochastic acceleration

- _Stochastic acceleration_
- Wave turbulence spectrum (Kolmogorov, Kraichnan)
- Particle randomly gains energy by wave-particle interactions
-       (Doppler gyroresonance condition)

## c) Shock acceleration 

- _c) Shock acceleration _

- First-order Fermi acceleration
-  (electric field  E=-(vshock/c)xB
-   in deHoffman-Teller frame)
- -Diffusive (second-order Fermi)
-  (multiple shock crossings)

## Particle orbit undergoes diffusive shock acceleration

- in a quasi-perpendicular shock (60 deg)
- by multiple crossings of the shock front
- with  magnetic mirroring upstream the shock front (x<0)
- Decker & Vlahos (1986)

## Applications of shock acceleration to solar flares :

- Somov & Kosugi (1997)

- Tsuneta & Naito (1998)

- _Applications of shock acceleration to solar flares :_
- First-order Fermi in mirror trap in flare loop cusp
- Fast shock in reconnection outflow above flare loop
- -Type II as shock front signature in interplanetary space

## Classification of acceleration mechanisms :

## Electron velocity

- _Electron velocity_
- _Dispersion:_

- _Pitch angle:_

- _Magnetic twist:_

- _Electron energy:_

- _Photon energy:_

- (Bremsstrahlung cross-section)

## Electron time-of-flight

- _Electron time-of-flight_
- (velocity dispersion)
- t1-t2 = L/v1 – L/v2
- v(E) = c [ 1 – 1/2] 1/2
- EHXR ~ 0.5 Ekin
- pitch angle correction
- magnetic field twist corr.

## Electron vs. ion acceleration :

-  gamma ray pulses delayed with respect to hard X-rays by few sec
-  time-of-flight difference between ions and electrons dt=L/ve-L/vion
-   (vion ~ ve/42)

## Electron trapping:

- _Electron trapping:_
- Weak-diffusion limt : collisional deflection time
-         ttrap (E) ~ E 3/2 / ne              ne ~ 1010-1012 cm -3

## Evidence for trapping? - tcoll(E) ~ E3/2 

- Krucker et al. (2008)

- RHESSI showed coronal 250-500 keV gamma-ray
- Continuum emission ar the looptop later in the flare
- With decay times progressively increasing with energy.
- Evidence for trapping? - tcoll(E) ~ E3/2

## RHESSI shows 6-12 thermal emission (colored map)

- Krucker & Lin (2008)

- as well as 20-30 keV nonthermal emission in occulted flares.
- Evidence for thin-target and thick-target hard X-ray emission
- in coronal regions? Trapping involved ?

## Spatial reconstruction of electron beam propagation:

- Aschwanden, Bastian, Benz, & Brosius (1992)

- _Spatial reconstruction of electron beam propagation:_
- radio dynamic spectra: type U burst turnover frequency 1.445 GHz
-    --> electron density ne=(2*1010 cm-3) at loop top
- radio image at 1.445 GHz (with VLA)
-    --> spatial location of loop top
- magnetic field extrapolation (KPNO)
-    --> footpoints and origin of electron beam acceleration

## Gyrosynchrotron emission of trapped electrons :

- Lee & Gary (2000)

- Kundu, White, Shibasaki et al. (2001)

-   pitch angle distribution of accelerated/injected electrons
- Time profile components and e-folding deconvolution :
-   separation of direct-precipitating and trap-precipiting electrons