太阳活动区磁场扭绞与耀斑产率

本文以1972年10月的太阳活动区McMath 12094为范例, 研究了活动区磁场扭绞与耀斑产率的关系.先在常α无力场模型假定下, 以观测到的活动区光球磁场为边值, 对活动区在日面中心附近4天(10月28—31日), 推算出代表活动区磁场平均扭绞程度的无力因子α, 从而外推出活动区在这4天的三维磁力线形态.然后以这些资料为基础, 进一步讨论了活动区磁场演化特征, 磁场扭绞与耀斑产率的关系, 并且近似用单极场模型估算了通过活动区前导大黑子A的电流、电流密度以及因大黑子逆时针旋转造成磁场扭绞所贮存的能量.本文主要结论为:(1)活动区McMath 12094从10月27日起保持较强扭绞, 10月30日达到极大, 10月31日后扭绞减弱.活动区磁场扭绞的主要原因是光球中的磁流体力学作用所导致的前导大黑子A的逆时针旋转。(2)代表活动区磁场平均扭绞程度的无力因子α与活动区耀斑产率同步变化, 表明活动区磁场扭绞与耀斑产率成正相关.(3)通过活动区前导大黑子A的本影电流为4.3—6.6×1012A, 因扭绞产生的自由能贮存为0.44—1.11×1032erg.活动区中的电流密度达到0.96—1.47×10A·m-2.这样高的电流密度可能是该活动区高耀斑产率的重要原因. 

THE MAGNETIC FIELD TWIST AND THE FLARE OCCURRENCE RATE IN A SOLAR ACTIVE REGION

In this paper the well-known solar active region McMath 12094 of Oct. 1972 is taken-as an example to investigate the relation between the magnetic field twist and the flare occurrence rate in a solar active region. Adopting a model of constant a force-free magnetic field for the region and taking the observed photospheric longitudinal magnetic field as the boundary condition, the values of force-free factor a, which represent the average level of magnetic twist in the region, are calculated for the four days when the region was near the center of solar disk (28-31 Oct.). The nusing these a values, the three dimensional configurations of the lines of magnetic force are extrapolated. Based on these data, the evolutionary characteristics of the magnetic field as well as the relation between the magnetic field twist and the flare occurrence rate are discussed. Finally, the electric current in the preceding sunspot Aof the region the electric current density and the stored magnetic energy caused by the on of the sunspot Aare estimated under the approximative model of an unipolar magnetic field. The main conclusions obtained are as follows.1) The magnetic field twist in McMath 12094 was rather strong since 28 Oct. and reached a maximum on₃₀ Oct.. After 31 Oct. it became weaker. The principle source of the magnetic twist was the counterclockwise rotation of the preceding sunspot A, which was caused by MHDin the photosphere.(2) The values of the force-free factor a which represent the average level of the magnetic twist varied in parallel with the flare occurrence rate, indicating that the latter is proportional to the magnetic field twist in the region.(3) The electric currents through the umbra of the large preceding sunspot Aare 4.3-6.6×1012A. The stored free energies caused by the magnetic twist are 0.44-1.11×1032erg. The densities of electric current in the region rise up to 0.96-1.47×10-2 A/m2. Probably these high densities are responsible for making the active region with high occurrence rate of flares.