Electron Acceleration at the Solar Flare Reconnection Outflow Shocks

1
Electron Acceleration at the Solar Flare
Reconnection Outflow Shocks
Gottfried Mann, Henry Aurass, and Alexander
Warmuth Astrophysikalisches Institut Potsdam, An
der Sternwarte 16, D-14482 Potsdam,
Germany e-mail GMann_at_aip.de
Huge solar event on October 28, 2003 produced
highly relativistic electrons seen in the hard
X- and ?-ray radiation by INTEGRAL
2
Electron Acceleration in Solar Flares
basic question particle acceleration in
the solar corona energetic electrons ?
non-thermal radio and X-ray radiation

• electron acceleration mechanisms
• ? direct electric field acceleration (DC
  acceleration)
• (Holman, 1985 Benz, 1987 Litvinenko,
  2000
• Zaitsev et al., 2000)
• ? stochastic acceleration via
• wave-particle interaction
• (Melrose, 1994 Miller et al., 1997)
• ? shock waves
• (Holman Pesses,1983 Schlickeiser, 1984
  
• Mann Claßen, 1995 Mann et al., 2001)
• ? outflow from the reconnection site
• (termination shock)
• (Forbes, 1986 Tsuneta
  Naito, 1998
• Aurass, Vrsnak Mann, 2002)

HXR looptop
HXR footpoints
3
Outflow Shock Signatures During the Impulsive
Phase
Solar Event of October 28, 2003

• X17.2 flare
• RHESSI INTEGRAL data (Gros et al. 2004)
• termination shock radio signatures start
• at the time of impulsive HXR rise
• signatures end when impulsive
• HXR burst drops off

The event was able to produce electrons up to 10
MeV.

4
Relativistic Shock Drift Acceleration I

• fast magnetosonic shock ? magnetic field
  compression
• ? moving magnetic mirror
• ? reflection and acceleration
• reflection in the de Hoffmann-Teller frame (see
  e.g. Ball Melrose, 2003 (non-rel. appr.))
• Lorentz-transformations laboratory frame ?
  shock rest frame ? HT frame ? back
• motional electric field has been removed
• ? conservation of kinetic energy
• conservation of magnetic moment
  
• electrostatic cross-shock potential
  (Goodrich Scudder, 1984 Kunic et al., 2002)

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Relativistic Shock Drift Acceleration II

? transformation of the particle velocities
? reflection conditions
6
Discussion I
basic coronal parameters at 150 MHz (? 160 Mm
for 2 x Newkirk (1961)) (Dulk McLean,
1978) (flare plasma)
shock parameter
total electron flux through the shock

7
Discussion II
electron distribution function in the corona
Kappa-distribution
kinetic definition of the temperature

8
Discussion III
relativistic electron production by shock drift
acceleration
phase space densities
The density of 8.54 MeV electrons is enhanced by
a factor of 1.5 ?106 with respect to the
undisturbed level in the phase space.

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Summary

• ? The termination shock is able to efficiently
  generate energetic electrons
• up to 10 MeV.
• ? Electrons accelerated at the termination
  shock could be the source of
• nonthermal hard X- and ?-ray radiation in
  chromospheric footpoints
• as well as in coronal loop top sources.
• The same mechanism also allows to produce
  energetic protons (lt 16 GeV).

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Radio Observations of Coronal Shock Waves

• type II bursts ? signatures of shocks
  in the solar radio radiation
• (Wild McCready, 1950 Uchida, 1960 Klein et
  al., 2003)
• two components
• backbone (slowly drifting ? ? 0.1
  MHz/s)
• ? shock wave
• (Nelson Melrose, 1985
• Benz Thejappa,
  1988)
• ? herringbones (rapidly drifting ? ? 10
  MHz/s)
• ? shock accelerated
• electron beams
• (Cairns Robinson, 1987
• Zlobec et al., 1993
  type II burst during the
  event
• Mann Klassen, 2002)
  on June 30, 1995

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Plasma Emission Interpretation of Solar Radio
Spectra
radio wave emission ? plasma emission
? drift rate
heliospheric density model (Mann et al., AA,
1999)
frequency in MHz
height from center of the Sun in Mio. km
height frequency velocity
drift rate
dynamic radio spectrogram ? height-time diagram
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Characteristics of Termination Shock Signatures

• no or very slow drift
• at comparatively
• high frequencies
• (320-420 MHz)
• split band structure
• herringbones

• characteristics closely resemble ordinary
  type II bursts ? shock

• but no drift, stationary in radioheliogams
  ? standing shock

• located above flaring region
  ? termination
  shock

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Discussion III
comparison with usual type II bursts ?
Coronal shock waves (type II bursts) are usually
not able to produce a large number of
energetic electrons than during at a flare (Klein
et al., 2003). ? Usual type II bursts appear
below 100 MHz (fundamental radiation). ?
Type IIs related with the termination shock
appear around 300 MHz ? Comparing electron
fluxes in the upstream region quiet
corona at 70 MHz and 1.4 MK flaring
plasma at 300 MHz and 10 MK (maximum
temp. 38 MK)