1981年4月27日射电大爆发渐变相湍动加速封闭性环模型

CLOSE LOOP MODEL OF STOCHASTIC ELECTRON ACCELERATION IN SOLAR GRADUAL BURSTS OF APRIL 27, 1981

本文提议1981年4月27日的射电爆发渐变相模型为湍动随机加速的封闭性环模型.它能较好地解释1981年4月27日0800UT射电爆发渐变相的频谱特征,即上升时间较长、没有低于56keV的X射线爆发,没有检测到质子事件等特征.

In this paper a model of stochastic particle acceleration by whistlers in turbulent plasma in a closed magnetic loop is used to explain the observational features of the April 27, 1981 bursts during the gradual phase. The following conclusions have been drawn: 1. Intense X-and γ-ray bursts at 0831.5 UT were recorded by SMM HXRBS and SMM GRS, but no X-ray bursts below 56 keV were observed. In our model. we may attribute this to the constraint of resonance condition of the whistlerparticle interaction. 2. The power-law spectral indices at the high frequencies of 9400 and 35000 MHz as a function of time were consistent with the time profile of the gradual burst at 35 GHz. The spectral index at 0831. 5UT (time of peak) was softest. However, during the impulsive phase (0800—0823UT), they became negatively correlated with the spectral indices hardest at peaks, and softest at valleys. According to the theory of turbulent acceleration, we think that the electron acceleration rate by whistlers ε_p is inversely proportional to the speed cubically in the low energy range, but to energy of particle linearly in the high energy range. Hence low energy particle acceleration is more efficient than that of high energy. In addition, as the particle velocity is close to c, the coefficient for energy diffusion is approximately a constant. Then ε_p^2=4c^2D~;t could be obtained. Therefore ε_p∝1/ε_p∝1/ t^(1/2), that is, acceleration rate of high energy particle decreases with time. In other words, relative increase of high energy electron number decreases with time. On the contrary, relative increase of low energy electron number does not decrease. This is why the spectral indices soften with time before peak. 3. Estimate of time of acceleretion. From the acceleration rate, we have the expression for time of acceleration t=1.7(E_e^(5/12)—E_(eo)^(5/2))/(π~2e^2c^2m_e^(1/2)W_(ω He/v)~Wω_(He)~4/ω_(Pe)~4).Given W_(ω He/v)~W=4×10^(-4)erg/cm^2, we estimate the time-scale of electron acceleration from 1 keV to 100 keV to be about 1 s. It seems that the particle acceleration by whistlers is very efficient. 4. The flux densities reached their peaks almost simultaneously at 0831. 5UT(<0.05min)at the frequencies of 3000, 3100, 6100, 9400 and 35000 MHz. There were no time delays with each other. This may be caused by the simultaneous stochastic acceleration of electrons to various energy levels in a short time scale (<0.05min). 5. All the rise times of X(40—80keV), γ(56—199keV) and microwave (3.2cm) bursts at 10 peaks in the impulsive phase were less than 1 rain. However, the rise times of gradual rand microwave (no 40—80 keV X-ray bursts) were longer than 1 min. This shows that the turbulent particle acceleration by whistlers in the gradual phase is not so efficient as the acceleration in the impulsive phase.