太阳光球磁亮点的基本特征研究及其对日冕加热的贡献

Studies of Magnetic Bright Points in the Photosphere and Their Contribution to the Coronal Heating

在太阳光球表面出现的磁亮点是目前观测手段能够分辨的最小磁结构,也被认为是日冕中的磁绳在光球足点运动的可靠示踪者。磁亮点的尺度约为100～300 km,寿命从几分钟到几十分钟。磁亮点被观测到不仅具有漩涡运动现象,还有很强的振荡现象。磁亮点是在磁通量管的对流坍缩过程中形成的,这已被观测和数值模拟所验证;磁亮点的运动导致其所在的磁通量管产生振荡,或者与其他磁通量管发生扭绞。理论上认为,这些振荡会以波的形式向色球和日冕传送能量,而磁通量管之间的扭绞会在色球和日冕中发生磁重联并释放能量,从而加热色球和日冕。为了解开日冕加热和色球加热等未解之谜,对磁亮点的研究显示出它特殊的重要性。对磁亮点的基本特征、形成原理、观测证据、光球磁亮点和太阳大气其他亮点之间的关系,以及磁亮点对日冕加热贡献等方面进行了介绍和讨论。

Magnetic bright points in the photosphere are the smallest structures that the present observational technique could resolve. They are regarded as a reliable tracer of footpoints of the coronal magnetic field in the photosphere. The energy conversion and transportation caused by the motion of these footpoints is considered as one of the most important energy source of heating the chromosphere and the corona by waves or magnetic reconnection through twist magnetic tubes. Currently, we have known some important facts about the elementary structures and the basic features of magnetic bright points. For example, magnetic bright points have sizes about 100~300 km and their lifetimes range from several to tens of minutes. Furthermore, their velocities are around 1~2 km.s-1 on average in the horizontal direction. Especially, some magnetic bright points whirl along a logarithm path in granulation lanes, which can trace large scales swirling down flows. The temperature is an important parameter of the magnetic bright point as well. Studies have indicated that the whole volume of the bright point does not necessarily fill with magnetic field. Instead the magnetic field is confined in a fraction of the bright point volume. The temperature in the region filled with field could be 103 K higher than that without field, and displayed a weak dependence on the strength of the field. On the other hand, the temperature in the field-free region does not show such a dependence. Most of the numerical simulations and observations show that the formation mechanism of the bright points is related to the process of the convective collapse. Namely, magnetic bright points evolve as follows： strong plasma down flows in the flux tube, magnetic field strengthens, and then magnetic bright points appear. Some simulations and observations revealed that strong downfiows bounce back when they reach the bottom of the flux tubes and turn into strong upflows. The upflow may develop a shock front that could be energetic enough to bring the chromospheric matter into the corona producing spicules. At the same time the flow move upward, the flux tube undergoes a kind of instability associated with the convective collapse. The upflowing gas leads to the magnetic field weakening in the flux tube, the gas density and the temperature increasing, and the flux tube eventually splits. In theory, Alfv^n waves are found to be excited by the oscillation of the flux tubes in the photosphere if the footpoints of the flux tubes move at a velocity of 1~2 km.s-1. The Alfv^n wave is invoked in the photosphere and travels upward into the chromosphere and the corona, then dissipates its energy heating the chromosphere and the corona. However, as we have known, the Alfv^n wave has not been detected in the photosphere yet. Whether or not the coronal magnetic field footpoints in the photosphere could move at speed of 1~2 km.s-1 is an important criterion for producing the AlfvSn wave by the flux tube motions. Developments of large solar telescopes and advanced data processing techniques lead to the discovery of the magnetic bright points in both the photosphere and the chromosphere. Some important properties and dynamic features of the bright point have been revealed, which include their sizes, brightness, and kinematic behaviors. But the internal structure, the detailed relationship with magnetic field, interactions with the nearby granulations, and so on still remain unknown. These open questions constitute essential scientific goals of thegiant solar telescope in decades. To our knowledge, three such telescopes have been planned. They are the European Solar Telescope with aperture of 4 m, the Advanced Technique Solar Telescope with aperture of 4 m, and the Chinese Giant Solar Telescope. Successfully constructing and running these huge telescopes will bring us a brand new view of the Sun with its very fine structures.