Particle Acceleration in Solar Energetic Particle (SEP) Events and Solar Flares
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Transcript of Particle Acceleration in Solar Energetic Particle (SEP) Events and Solar Flares
Particle Acceleration in Solar Energetic Particle (SEP) Events and Solar Flares
R. P. Lin
Physics Department & Space Sciences Laboratory University of California, Berkeley, CA, USA
For fall 2009 semester: School of Space Research, Kyung Hee University, Yongin,
Gyeonggi 446-701, South Korea,
Thanks to the RHESSI team & the Wind 3DP team
Forbush, S., Phys. Rev. Lett., 70, 771, 1946
“…unusual increases… nearly simultaneous with a
solar flare…”
“…may have been caused by charged particles
actually being emitted by the Sun…”
The Sun is the most energetic particle accelerator in the solar system:
- Ions up to ~ 10s of GeV - Electrons up to ~100s of MeV
Acceleration occurs in transient energy releases, in two (!) processes:
- Large Solar Flares, in the lower corona- Fast Coronal Mass Ejections (CMEs), in the
inner heliosphere, ~2-40 solar radii
Bastille Day 2000 Solar Flare
23 July 2002 X4.8 Flare
(Lin et al 2003)
Thermal Plasma ~3x107 K
Accelerated Electrons~10 keV to >10s MeV
Accelerated Ions ~1 to >100s of MeV
SPECTROSCOPY
Π0 Decay
Nonthermal Bremsstrahlung
Thermal Bremsstrahlung
Solar Flare Spectrum
Positron and NuclearGamma-Ray lines
T = 2 x 107 KT = 4 x 107 K
hot loop
HXRfootpoints
soft X-rays hard X-rays
-rays
photosphere
RHESSI Energy Coverage
Large solar flares are the most powerful explosions in the solar system
Up to ~ 1032- 10 33 ergs released in ~ 10 – 103 sFlare-accelerated ~20-100 keV electrons contain
~10-50% of total energy releasedIn large flares, >~1 MeV ions contain comparable
energy
Particle acceleration is intimately related to flare energy release
The total energy released by all flares, down to microflares/nano-flares may be significant for the heating of the solar corona
Imaging spectroscopy
Krucker & Lin 2004
e-e-
HXR HXR
vinvin
vfp
e-e-
HXR HXR
vinvin
t1
t2
vfp
?
?
HXR source motions in magnetic reconnection
models
photosphere Bfp
Bc
vin = coronal inflow velocity Bc = coronal magnetic field
strengthvfp = HXR footpoint velocityBfp = magnetic field strength
in HXR footpoint ~ photospheric value
vin Bc = vfpBfp
Velocity-HXR flux correlationRough correlation between v and HXR flux
d = B v a dt
Reconnection rate d/dt= B v a
~ 2x1018 Mx/s
E = vB ~ 5 kV/m
v= velocityB= magnetic field strengtha=footpoint diameter
-ray imaging with RHESSI
• Before RHESSI, no imaging in the -ray range available• RHESSI -ray imaging at 35” and 180” resolution
(compared to 2” for HXRs, i.e. electrons)• Low photon statistics: integration over total flare duration needed• 2.2MeV line is best candidate
(Hurford et al. 2003, 2006)
photosphere
p+ p+
-ray footpoints?
low density
high density
Where are the -ray footpoints relative to the HXR footpoints?
Protons vs Electrons
>~30 MeV p (2.223 MeV
n-capture line)
> 0.2 MeV e (0.2-0.3 MeV
bremsstrahlung X-rays)
e & p separated by ~104 km, but close to flare ribbons
Energetics – 23 July 2002 Flare• Accelerated Electrons: > ~2 x 1031 ergs
~3 x 1028 ergs/s = ~3 x 1035 (~50 keV) electrons/s for ~600s
• Accelerated Ions (>2.5 MeV) : ~ 1031 ergs~ 1028 ergs/s = ~1033 (~10 MeV) protons/s for ~1000s
• Thermal Plasma: ~ 1031 ergs + losses
• dE/dt = ~5x1028 ergs/s energy release rate for ~1000s
Ion acceleration
Kahler 1994:Compare ion release time near Sun with CME front altitudeCME is already severalSolar radii away from the Sun
Tylka & Lee, 2006
Large (L)SEP events - tens/year at solar maximum
- >10 MeV protons (small e/p ratio)- Normal coronal composition
(but sometimes 3He & Fe/O enhanced)- Normal coronal charge states, Fe+10
(but sometimes enhanced )
- SEPs seen over >~100º of solar longitude
- associated with: - Fast Coronal Mass Ejections (CMEs)- Large flares (but sometimes missing) - Gradual (hours) soft X-ray bursts (also called Gradual SEP events)
• Acceleration by fast CME driven shock wave in the inner heliosphere, 2-40 solar radii
Mewaldt et al 2005
(Mewaldt et al. 2004)
Mewaldt et al, 2005
If these SEPs are accelerated by CME-driven shocks, they use
a significant fraction of the CME kinetic energy (up to
20%)
(see also Emslie et al. 2004).
Tycho Supernova Remnant
X-ray picture from Chandra
Diffusive shock acceleration: rg >> d
Compressive discontinuity of the plasma flow leads to acceleration of particles reflecting at both sides of the discontinuity: diffusive shock acceleration (1st order Fermi)
u1u2
R = u1/u2
in the shock rest frame
where u = u1-u2
1st order acceleration
shock compression
Δp~ (u/v) p
Gopalswamy et al. 2004
January 20, 2005 SEP event
(from Mewaldt et al. 2005)
Very short time to maximum intensity (30 min)
Very hard spectrum
2002 Aug 03, X1.0
2004 Sep 19, M1.9
SHH
SHS
“soft-hard-soft” (SHS) vs. “soft-hard-harder” (SHH)(non-thermal HXR spectral behavior during flares)
I E Power law spectrum:
lower gamma value = flatter, ‘harder’ spectrum
SHS behavior:(more common)
SHH behavior:(~15-20% of flares)
time
HX
R E
mis
sion
Results• 60 of the 84 events had determinable spectral behavior and SEP occurrence• 23 of 60 had only partial observations and no SHH, finally leaving 37 events
SEPs? Yes: No:
Yes: 12 0 No: 6 19
SHH?
very large source (>200 arcsec)expanding and rising
October 27, 2002CME velocity ~2000 km/ssize
motion
very large source (>200 arcsec)expanding and rising
October 27, 2002CME velocity ~2000 km/ssize
motion
300”
very large source (>200 arcsec)expanding and rising
October 27, 2002
~400 km/s
~800 km/s
CME velocity ~2000 km/ssize
motion
300”
HXR emission from electrons in magnetic structures related to coronal mass ejections.
speed of CME front ~ 2000 km/s
filament behind ~ 1000 km/s
X-ray spectrum
14 MK, low EM (1e46 cm-3) density ~108 cm-3
relatively hard/flat spectrum (~3.3)extending down to ~10 keV
Number of non-thermal electrons are 10% of number of thermal electrons.
NASA Solar Probe + ESA Solar OrbiterSolar Sentinels
Protons >30 MeV
(2.223 MeV Line Fluence, corrected for
limb darkening)
(Shih et al 2009)
GOES Soft X-ray Peak flux
Cliver et al, 1994
Discussion of models
turbulence(e.g. Liu et al. 2007)
Contracting islands(Drake et al. 2006)
time evolution given by acceleration and escape.
Drake et al. • extended acc. region• all electrons are acc.• power law distribution• 1~1 stops contraction • 1~1 stops acceleration
Gradual SEP event with 3He (Mason, Mazur, & Dwyer, 1999)
Tylka & Lee, 2006
Tylka & Lee 2006
Zurbuchen 2004
Solar Reconnection – Breakout CME Model
• 3D simulation Lynch et al (2007)
• Null-point ahead of ejected plasmoid
• Bipolar reconnection behind
Computed wave spectra (Leamon et al 1998)
Reames & Ng 1998
Proton spectrum:
RHESSI Gamma-rays
(lines) vs
SEPs at 1AU(blue points)
Comparison of SEP spectral indices from in-situ and g-ray data
(Krucker, Mewaldt, Murphy, & Share, RHESSI workshop 2005)
W56
W56E08
Electron -3He-rich SEP events
- ~1000s/year at solar maximum- dominated by:
- electrons of ~0.1 (!) to ~100 keV energy
- 3He ~10s keV/nuc to ~MeV/nucx10-x104 (!) enhancements
- heavy nuclei: Fe, Mg, Si, S enhancements
- high charge states
- associated with: - small flares/coronal microflares - Type III radio bursts - Impulsive soft X-ray bursts (so also
called Impulsive SEP events)
LSEP event with 3He (Mason, Mazur, & Dwyer, 1999)
Gloeckler et al 2003