Quasi-periodic acceleration of electrons in the flare on 2012 July 19

by Jing Huang et al.*

2017-02-28Solar Radio Science Highlights

We

study the quasi-periodic pulsations (QPPs) of nonthermal emission in an

M7.7 class flare on 2012 July 19 with spatially resolved observations

at microwave and HXR bands and with spectral observations at decimetric,

metric waves. Microwave emission at 17 GHz of two footpoints, HXR

emission at 20–50 keV of the north footpoint and loop top, and type III

bursts at 0.7–3 GHz show prominent in-phase oscillations at 270 s.

Through the estimation of plasma density around the loop top from EUV

observations, we find that the local plasma frequency would be 1.5 GHz

or even higher. Thus, type III bursts at 700 MHz originate above the

loop top. Quasi-periodic acceleration or injection of energetic

electrons is proposed to dominate these in-phase QPPs of nonthermal

emission from footpoints, loop top, and above. In the overlying region,

drifting pulsations (DPS) at 200–600 MHz oscillate at a distinct period

(200 s).

Its global structure drifts toward lower frequency, which is closely

related to upward plasmoids observed simultaneously from EUV emission.

Two individual systems of quasi-periodic acceleration of electrons are

proposed to coincide in the bi-direction outflows from the reconnection

region.

Observations

The flaring loop structure could be determined by a microwave

observation, which evolves in three phases. QPPs appear at the second

phase with intensive emission from loop legs and weak emission around LT

(as seen in Figure 1). We analyze the fluxes of selected regions in the

flaring loop and find that the emissions of FPs and region S1 have

oscillations at 270 s. The loop legs have smoother light curves but stronger emission.

Figure 1.Microwave images at 17 and 34 GHz during the flare process.

The HXR emissions observed by RHESSI

have two FPs and an LT. The emission of LT at 10–20 keV is increased

continuously, which is thermal emission, reflecting the heating of local

plasma by injected energetic electrons. The one at 20–30 keV,

increasing with oscillatory peaks at 270s,

is a mixture of thermal and nonthermal emission. The nonthermal

emission at 30–50 keV of LT and NFP shows obvious oscillation at a

period of 270 s.

Figure 2.Left

panel: HXR sources superposed on Microwave sources at 17 GHz. Right

panel: The flux of each HXR source at four energy bands.

From spectral observations (Figure 3), quasi-periodic type III bursts with periods of 270s

cover a wide range at 0.7–3 GHz. The individual type III burst drifts

toward high frequency, which indicates a downward movement of energetic

electron beams. The local plasma frequency near LT is estimated at about

1.5 GHz or even higher frequency. This indicates that the emission of

type III bursts at lower frequency originated from above LT and

energetic electrons, oscillating at 270" role="presentation">s,

transport downward from the acceleration site to LT, loop legs, and

FPs. The well-related fluxes between type III bursts and HXR LT and NFP

suggest a common origin between them. DPS at 200–600 MHz, occurring

simultaneously with an upward plasmoid at high altitude, have an

oscillating period of 200s.

The similarity of the velocity of DPS source and plasmoids indicates

that they are closely related to each other. It means that the energetic

electrons in plasmoids oscillate at a distinct period.

Figure 3.Decimetric and metric spectra observed by SBRS and YNRS

Liu et al. (2013) found

bi-direction outflows originating between an erupting flux rope and

underlying flare loops. They suggested that the primary loci of particle

acceleration and plasma heating are in the reconnection outflow

regions, rather than the reconnection site itself. Our results are

consistent with the above result and obtain further evidence of

different oscillatory periods of these two acceleration processes. The

oscillatory energetic electrons produced by quasi-periodic acceleration

at 270s

in downward outflows is responsible for nonthermal emission in the

underlying flare loop, and the quasi-periodic acceleration of electrons

at 200s in upward plasmoids generates DPS in the overlying region.

Conclusions

Two individual processes of acceleration

are proposed to take place at different locations out of the

reconnection site. The electrons quasi-periodically accelerated at 270 s

in downward outflows generate in-phase oscillations of HXR LT and type

III bursts. Furthermore, the ones precipitating directly to FPs, partial

of injected ones, emit quasi-periodic microwave and HXR emission there.

In upward plasmoids, electrons are quasi-periodically accelerated at

200 s,

which subsequently produces an oscillatory DPS. The outflows from the

reconnection site disturb the local plasma and generate MHD

oscillations, which modulates the acceleration process. The different

conditions in local plasma would result in distinct periods.

Based on the recent paper:

Huang,

J., Kontar, E., Nakariakov, V., & Gao, G. (2016). QUASI-PERIODIC

ACCELERATION OF ELECTRONS IN THE FLARE ON 2012 JULY 19The Astrophysical Journal, 831(2) DOI:10.3847/0004-637X/831/2/119

*Complete list of authors:Jing Huang^1,2, Eduard P. Kontar³, Valery M. Nakariakov⁴, and Guannan Gao⁵

¹Key Laboratory of Solar Activities, National

Astronomical Observatories, Chinese Academy of Sciences, Beijing,

100012, China; huangj@bao.ac.cn

²Sate Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, 100190, China

³School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

⁴Physics Department, University of Warwick, Coventry CV4 7AL, UK

⁵Yunnan Observatory, Chinese Academy of Sciences, Kunming, Yunnan 650011, China

References:

Liu, W., Chen, Q., & Petrosian, V. 2013, ApJ, 767, 168