RHESSI Observations ofIon Acceleration

Albert Y. Shih

2009 Sep 2

Flare

electrons

protons, alphas, heavy ions

neutrons

bremsstrahlung

nuclear de-excitation, positron annihilation

neutron-capturephotosphere

Gamma rays

Thermalizationtime delay of ~ 100 secondsspatial separation of < 1 arcsec

n (cm-3)

1011

1012

1013

1014

1015

corona

Versus >300 keV electrons

• Direct proportionality

• Dotted lines are factors of 2 from the best-fit line

• Some spread is due to incomplete coverage (triangles)

• Almost all flares fall within 1 σ of spread

(Shih et al. 2009, ApJL)

Versus thermal emission

• GOES class (emission from hot plasma) as a measure of flare size

• Gray region has not been systematically searched

• Direct proportionality above a threshold?

• Below threshold: excess heating

(Shih et al. 2009, ApJL)

Comparison conclusions

• >~20 MeV ions and >300 keV electrons are proportionally accelerated over >3 orders of magnitude– Relative ion to electron acceleration in models is often

sensitive to certain parameters, not constrained to a ratio

– Cf. loop-length explanation by Emslie et al. (2004)

• >~20 MeV ion acceleration and flare heating may be proportional above a threshold of ion acceleration, with relative excess heating below

• Flare e/p ratios are significantly greater than ratios in gradual SEP events, comparable to impulsive SEP events (cf. Ramaty et al. 1993)

Protons versus >50 keV electrons

Direct proportionality

3

50

2.2 106.1

F

F

Date GOES class >1 hr γ>50 γ>300 HA (°) CME? Type II?

2002 Feb 26 C9.6 N 3.40 2.56 79 none N

2002 Jul 23 X4.8 N 3.11 2.72 73 fast Y

2003 Apr 26 M7.0 N 2.77 2.86 73 slow Y

2003 Jun 17 M6.8 Y 2.90 2.64 59 fast Y

2004 Nov 10 X2.5 N 3.05 2.81 47 fast Y

2005 Jan 17 X3.8 Y 3.34 2.54 34 fast Y

2005 Jan 19 X1.5 Y 2.55 2.62, 2.36 51 fast Y

2005 Jan 20 X7.1 Y 2.90 2.49 61 fast Y

2003 May 27 X1.4 N 3.51 2.73 17 fast Y

2004 Jul 15 X1.8 N — 1.73 56 fast N

2004 Jul 15 X1.6 N 3.54 2.05 46 none N

2004 Jul 16 X1.3 N 4.41 3.17 43 none N

2005 Sep 13 X1.7 N 4.27 1.82 18 fast N

Effect of albedo

Low-energy excess conclusions

• Some flares appear to have an excess, soft bremsstrahlung component <300 keV

• Albedo may explain some of the apparent excess and the softer spectral index

So, is ion acceleration occurring in all flares?

Next: can try to look for the neutron-capture line by summing together many flares

Summed-flare line fluxes

2002 Average Total time 2.2 flux (γ/cm2s) 2.2 fluence (γ/cm2)

X X2.2 2.4 hours (6.6 ± 0.4) × 10-3 57 ± 4

M M2.2 1.2 days (1.8 ± 1.2) × 10-4 18 ± 12

C C3.4 7.8 days (6.7 ± 5.0) × 10-5 45 ± 34

quiet C1.3 79 days (3.7 ± 2.4) × 10-5 252 ± 168

2003 Average Total time 2.2 flux (γ/cm2s) 2.2 fluence (γ/cm2)

X X4.2 2.8 hours (4.8 ± 0.6) × 10-3 49 ± 6

M M2.4 1.0 days (4.5 ± 1.8) × 10-4 40. ± 16

C C3.0 6.1 days (4.8 ± 7.6) × 10-5 25 ± 40.

quiet B9.0 100. days (2.6 ± 3.0) × 10-5 222 ± 266

Error estimates are preliminary

Comparison with individual flares

Lines offixed ratio

Summed-flare conclusions

• Relative to their GOES class, small flares (and quiet-time emission) appear to be roughly proportionally as productive as the average large flare, which is much less than individual gamma-ray line flares

Finally: what can we say about changes in ion acceleration or atmospheric conditions as a flare progresses?

Temporal variability analysis00:27:20–00:32:56 UT 00:32:56–00:43:20 UT

Low-FIP: Fe , Mg , Si

High-FIP: Ne , C , O

Temporal variability results2002 July 23 2003 October 28 2003 November 2

0.29±0.08 0.80±0.14

Low-FIP/high-FIP ratiosLow-FIP/high-FIP ratios

0.36±0.05 0.74±0.20

Temporal variability conclusions

• Several flares exhibit temporal variability of line flux ratios

• This variability may be due to variations in the spectral index or in ambient abundances, and in some flares the pattern of variation is consistent with low-FIP enhancement

• Low-FIP enhancement suggests that the interaction region is more corona-like

Conclusions

• Proportional acceleration between >~20 MeV ions and >300 keV electrons falls apart at lower electron energies (partially due to albedo?)

• Summed-flare analysis has not yet detected small-flare ion acceleration, but sensitivity is low

• Temporal variability shows intriguing behavior , to be analyzed with physics-based photon models

• Gordon Hurford’s talk on Saturday: Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS)

Gamma-Ray Imager/Polarimeterfor Solar flares (GRIPS)

• Recently funded NASA LCAS balloon mission

• 3D position-sensitive germanium spectrometer– High spectral resolution

– Compton-scatter track reconstruction

• Imaging approach similar to RHESSI, but takes advantage of position sensitivity in the detectors

• Increased angular resolution for gamma rays

• Measures gamma-ray polarization “for free”

• Proves technologies for a future space mission

GRIPS componentsMulti-pitch rotating modulator (MPRM)

Spectrometer/polarimeter

Gordon Hurford’stalk on Saturday

Energy range ~20 keV to >~10 MeV

Spectral resolution ~2 keV FWHM at 662 keV

Boom length 8 m

Angular resolution 12.5 to 162 arcsec

Other components include:Cryostat/cryocoolerElectronics, with ASICsAnti-coincidence BGO shieldSolar aspect system

Con-US test flight:

spring 2012