Energetic Particles in the Solar Atmosphere

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Energetic Particles in the Solar Atmosphere

• Introduction
• Observations and Limits
• Acceleration Models
• Particles and Fields
• Conclusions

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Energetic particles in the solar atmosphere
Atmosphere lt 2R? above photosphere Energeti
c having a non-thermal distribution
(e.g. a beam or conic, T gt Tcor)
Several major sources of accelerated
particles Flares - relativistic electrons, ion
beams, neutrons, pions inferred from
hard X-rays, gamma-rays, radio, in-situ
particles CMEs - relativistic electrons,
ion acceleration inferred from
fast-drifting structures in Type II radio
emission, SEPs Solar wind -
non-thermal heavy ion distributions
spectral line widths and in situ ? ion T?
100-200 MK Active regions and quiet
sun - fast electrons Type I radio
noise storms, micro-flares
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Flare Electrons
Diagnostics Hard X-rays radio emission
(gyrosynchrotron, plasma emission) in situ
measurements optical and UV lines and continuum
RHESSI/TRACE flare on solar limb
looptop HXR source from Masuda et al (1994)
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Flare Electrons - Hard X rays
Hard X-rays produced by electron-proton
bremsstrahlung of electrons ? 30keV

• Impulsive flares 1036-1037 electrons/s
  accelerated above 20keV
  
• total energy in gt
  20keV electrons 1031 ergs
• Gradual flares electron poor (in situ), but
  still show weak HXR emission
• acceleration out of thermal distribution to
  100keV in 1s
• need efficient and continuous replenishment of
  particles
• - typical flare volume will be depleted
  within 1s
• Highly time variable emission, possibly also
  high spatial fragmentation
• Coronal acceleration site (Masuda 94,
  Aschwanden 96)

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Flare Electrons - Radio
1) decimetric radiation - plasma emission
generated by relativistic electron beams.
Correlated decimetric and HXR time structures
(e.g. Aschwanden et al 1993) point to same or
closely related electron populations
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Flare Electrons - Radio
2) cm/mm radiation - gyrosynchrotron emission
from electrons 10keV-few MeV
Nobeyama (17GHz) and Yohkoh HXT Strong cm radio
emission occurs above location of weak HXR
emission and vice versa ? electron mirroring
injection at pitch angles lt 30o
Weak magnetic mirror
Strong magnetic mirror
(cartoon after Lee Gary 2000)
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Flare Ions
Diagnostics - flare gamma-ray spectroscopy, in
situ particle measurements Impulsive flares
(in situ) 3He/4He 1 compared to coronal value
5x10-4 (in situ,
?-rays) Ratios Ne/O, Mg/O, Si/O, Fe/O
increased by factor 2-8 over
coronal values Gradual flares (in situ) ?
coronal abundance ratios (?-rays) ?
abundance ratios consistent with impulsive events

• Ion Acceleration from thermal distribution to
  100 MeV in 1s
• Proton acceleration rate 1035/s for several
  10s of seconds
• Protons with E lt 1 MeV unknown
• Coronal acceleration site?

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EUV and UV particle diagnostics
UV/EUV flare footpoint/ribbon emission is well
correlated with high energy HXRs
TRACE 195 /HXT M2
TRACE 171 / RHESSI 30-50keV
From Fletcher Hudson 01
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EUV and UV particle diagnostics
UV/EUV flare footpoint/ribbon emission is well
correlated with high energy HXRs
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Impulsive Phase UV/EUV Flare Kernels
Pixels are 325km x 325 km

• (E)UV kernels can be used to estimate beam
  precipitation areas (like Ha)
• precipitation areas as small as 1016cm2
• small bundles of fieldlines active for 1
  min

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Energetic particles in quiet sun

• Lin et al. (2001) find evidence for non-thermal
  particles in very small events
• (GOES A/B) seen in BATSE data
• RHESSI shows many similar events - can even be
  imaged!
• Brown et al (1997) propose a non-thermally driven
  evaporative contribution
• to microflare intensity increases (i.e. not
  just due to Joule heating)

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DC field acceleration
Reconnecting current sheet (e.g. Speiser,
Syrovatskii, Martens, Litvinenko, Somov)
Reconnecting B-field component generates
accelerating E-field in z-direction 1-10
V/cm Particles remain in sheet until drifts or
gyro-orbits take them out Addition of a Bz
magnetises electrons (and protons), forcing them
to follow the field in z-direction
y
z
x
Ez
Post-flare arcade
Relates maximum particle energy, and ratio of
species, to global field configuration
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Stochastic Resonant Acceleration (e.g. Melrose,
Benz, Miller, Ramaty)
e.g. Turbulent cascade of MHD waves (Miller and
collaborators.)
107cm
Fast mode ? Electron accn Shear Alfven
? ion accn
Magnetic Reconnection/ Reorganisation
Fast mode waves Shear Alfven waves

• Ion gyrofrequency
• First resonances with Fe - low q/m ratio not
  abundant enough to damp waves
• Later resonances with Ne, Mg, Si sufficiently
  numerous to damp waves
• Cascade ceases before reaching C, N, O, He
• (3He/4He ratio requires a separate mechanism)

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Other acceleration mechanisms
Approaching shocks - Tsuneta (1997) - Fermi
acceleration
Current sheet
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Other acceleration mechanisms
Approaching shocks - Tsuneta (1997) - Fermi
acceleration
Fast mode shock from outflow
Current sheet
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Other acceleration mechanisms
Approaching shocks - Tsuneta (1997) - Fermi
acceleration
Fast mode shock from outflow
Current sheet
Slow mode shocks
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Other acceleration mechanisms
Collapsing trap - Somov Kosugi (1997) -
betatron acceleration between
moving magnetic mirrors in a collapsing
magnetic trap
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Other acceleration mechanisms
Collapsing trap - Somov Kosugi (1997) -
betatron acceleration between
moving magnetic mirrors in a collapsing
magnetic trap
dipolarised field line
Magnetic mirror regions
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Particles and Field

• Ultimately, energy for the accelerated
  particles comes from the re-organisation
• and dissipation of magnetic fields.
• Can we relate the evolution of particle
  distributions to the evolution of the
• flare magnetic field?

Q 1 - understand location and evolution of flare
sources in magnetic field configuration. Below,
Metcalf et al. show location of HXR and WL
sources with respect to photospheric
intersection of coronal magnetic separatrix
surfaces
(also, Demoulin, Mandrini, Somov, Aulanier..)
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Particles and field
Q 2 - examine behaviour of source brightness and
spectrum as flare evolves
Rapidly reconfiguring magnetic fields should in
principle provide a high energy input rate for
acceleration of particles
RHESSI 30-50kev source motions show
photospheric mappings of evolving coronal field
2-D configuration ? - correlation between
source separation velocity and intensity?
(from Fletcher Hudson 2002)
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Separations
Flux and spectral index

• No very clear relationship between footpoint
  separation rate and intensity
• Possible tendency for intervals of high
  separation rate to correlate with flux increases.

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Conclusions

• The Sun is a prodigious particle accelerator
• Generally, detailed models of (kinetic) particle
  acceleration mechanisms are not
• integrated with (MHD) evolution of magnetic
  field
• Pre-RHESSI, information on accelerated electrons
  has been quite rich, but ions
• and protons are relatively unexplored
• RHESSI in combination with TRACE and ground-based
  optical/radio
• observations offers opportunities to study
  particles and fields simultaneously