Periodic Acceleration of Electrons in Solar Flares

1. Periodic Acceleration of Electrons in Solar Flares 1S. Kamio, 2M. Shimojo, 2A. Asai, 1H.Isobe, and 2K. Shibasaki 1Kwasan and Hida Observatories, Kyoto University 2Nobeyama Solar Radio Observatory, NAOJ

2. Introduction • Hard X-ray, microwave, and gamma-ray are produced by high-energy particles during the impulsive phase of flares. Behavior of these emissions can provide clues for understanding the particle acceleration process in the flare • Some papers have reported quasi-periodic pulsation (10-100s) in hard X-ray and microwaveKane et al.(1983), Nakajima et al.(1983), Kiplinger et al.(1983) and so on.

3. Loop oscillation • Oscillation of EUV loopsTRACE observation found coronal loop oscillations induced by flares.e.g. Nakarikov et al.(1999) • Miyagoshi et al.(2004)They performed three-dimensional MHD simulation of coronal loop oscillation. The period of oscillation is equal to the Alfven transit time along the loop.

4. Quasi-periodic pulsations • Asai et al.(2001)Pulsation in spatially resolved microwave and hard X-ray data. Oscillation period6.6s remote microwave 17GHz flux

5. Pulsation periods • Asai et al.(2001)The period of pulsation is similar to the Alfven transit time along the flare loop. • Kundu et al.(2001)Variation of the oscillation period is due to density increase. They assumed a disturbance propagating along the loop in Alfven speed. • Our goal: • To examine if the relationship is a common feature • Estimate the relationship between macroscopic magnetic environment and particle acceleration

6. Data selection Surveyed data from Jun 1992 to Dec 2000 • Selection criterion- Observed with Yohkoh/HXT and NoRH- HXT M1 maximum count > 4 (cts/sec/sc)- 4 or more peaks are clearly seen • Selected 3 flares (from 368)- 1 Dec 1997 in NOAA 8113 (M1.2)- 10 Nov 1998 in NOAA 8375 (C7.9, studied by Asai et al.)- 24 Apr 2001 in NOAA 9433 (M2.1)

7. Flare 1: 1-Dec-1997 • Pulsation period: 6.4s(Wavelet analysis) • Microwave peaks delayedabout 1s from HXT peaks. movie

8. Estimating physical quantities • EM and Temperature inferred from Yohkoh/SXT filter ratio analysis. • Potential field calculation from MDI using MAGPACK2 • Estimated configuration (cartoon)HXR sources: Foot pointsRadio source: Loop top Loop Length: 1x109cmLoop width: 4x108cm Density: 2x1010cm-3 Temperature: 13MK Field strength: 240G

9. Flare 2: 10 Nov 1998 • Flare in the west limb(Studied by Asai et al.) • Pulstation in HXR and remote microwave source microwave HXR Pulsation Period: 6.6s

10. Configuration • Flare loop: 11.5x104 km (estimated)Bright in SXR • Faint loop: 25.0x104 kmHXR in the south foot pointMicrowave in the north foot point Pulsation delay between microwave and HXR is about 1 sec Microwave 2 1 HXR Yohkoh/SXT image

11. Flare 3: 24 April 2001 • Pulsating remote source in microwave Pulsation Period: 5.2s movie

12. Possible configuration • Magnetic field eruption followed by the magnetic reconnection. • Microwave pulsation at the foot point of faint loop. • Non-thermal electron produced by the flare emitted HXR and microwave.

13. What determines the period? • Periodic pulsations may be related to the loop oscillation. • The periods of several oscillation modes are calculated using observed quantities. Ls: Loop Length (in 108cm) rs: Loop radius (in 108cm) B: Field strength n10: Density (in 1010cm-3) T: Temperature (j=1) (m=1) oscillation modes in a uniform magnetic loop

14. Pulsation periods • Periods of fast sausage mode is the closest to the observed periods. • Pulsation may be related to the fast sausage mode oscillation.

15. Flux Spectral index • HXR index becomes gradually softer, while radio index become harder.same as nominal flares • Index variation at flux peaks: opposite trend in HXR and microwaveEach peak shows the similar behavior of a single flare. HXR index hard soft Radio index hard soft

16. Interpretation • Spectral index α<0Microwave pulsation is caused by non-thermal electrons (gyrosynchrotron) • HXR sources are located near foot point of flare loopsEmission by thick target model • Each peak of pulsation indicate the same index variation as nominal flares.Modulation of non-thermal particle acceleration • The pulsations of remote microwave source are delayed from HXRThe delay is flight time of non-thermal electrons from HXR source to microwave source

17. Tsuneta and Naito 1998 Loop oscillation flare-loop Model of pulsation Our results suggest that the pulsation period is related to the fast sausage mode oscillation of coronal loops. The pulsation may be caused by the modulation of particle acceleration. • The oscillation of loops affect the length of fast shock region and hence change the efficiency of particle acceleration or injection.(proposed by Asai et al. (2001))

18. Summary • We studied 3 flares indicating pulsation in HXR and microwave. • Observed pulsation periods are close to the estimated periods of fast sausage mode oscillation of flare loop. • The evolution of spectral index in each flux peak indicated the same characteristics of nominal flares. • Pulsation may be caused by the modulation the efficiency of particle acceleration. • RHESSI will provide more detailed spectral data in HXR and γ-ray

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20. Flux Spectral index (cont.) • HXR index becomes gradually softer, while radio index gradually harder. • Index variation at flux peaks HXR index hard soft Radio index hard soft