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