太阳风密度对磁暴主相形态调制作用的研究

The shpae of a geomagnetic storm's main phase refers to the speed of SYM-H variations. When an interplanetary magnetic field has a southward component, the same solar wind electric field would result in a differed development speed of the main phase under a different solar wind density.A high solar wind density would lead to an accelerated development of the main phase, while a low one would come up with a slow development if the solar wind electric field is the same. The development of the storm's main phase would become very slow in speed, or even come to a standstill, when the solar wind density reaches a given low threshold,even when the southward component is strong and sustained for a long time. This kind of phenomenon is called the modulation of the shape for a storm's main phase caused by solar wind density. We will split each target geomagnetic storm's main phase into several stages, based on the changing speed of SYM-H variations.By analyzing the shape of the main phase of each storms stronger than -50 nT occurred since 1997, we will devide the speed of SYM-H variations into several levels.We will analyze the the properties of solar wind parameters corresponding to the each level of the speed of SYM-H variation to investigate the physical mechanism of solar wind density modulating the storm's main phase and try to sort out the solar wind density threshold that defines the development of a storm's main phase, establish a formula showing the relationship between solar wind parameters (including the solar wind density) and SYM-H variations, and define the solar wind conditions (including the solar wind density) causing a moderate storm,intense storm and great geomagnetic storm.

磁暴主相形态是指磁暴主相期间SYM-H变化的快慢。当行星际磁场出现南向分量时，同样的太阳风电场，不同的太阳风密度导致磁暴主相发展速度不同，高的太阳风密度导致磁暴主相发展速度快，低的太阳风密度导致磁暴主相发展速度慢，当太阳风密度低到一定程度时，即使有强的、持续时间长的行星际磁场南向分量或太阳风电场，磁暴主相的发展速度很缓慢甚至导致磁暴主相发展停滞，这种现象我们称之为太阳风密度对磁暴主相的调制。依据SYM-H变化的快慢，我们把每一个磁暴主相分成若干个阶段。通过分析第23太阳活动周以来中等以上所有磁暴主相的形态，我们将把SYM-H变化快慢分成若干等级。针对每一个等级，我们分析其对应的太阳风条件，研究太阳风密度对磁暴主相的调制作用并寻找太阳风密度对磁暴主相发展有影响的阈值，建立含有太阳风密度的太阳风参数与SYM-H变化之间的关系式并提出新的产生中等磁暴、大磁暴和特大磁暴的含有太阳风密度的太阳风条件

太阳风与磁层相互作用产生磁暴的过程，是空间天气现象中最为重要的空间天气现象之一，是空间天气研究的核心问题之一。长期以来，人们在研究太阳风与磁暴的关系时，通常最为关注的两个太阳风参数是行星际磁场的南向分量和太阳风电场的极值与磁暴强度的相关性。通过对太阳风参数与磁暴关系的逐个分析，发现太阳风密度是影响磁暴强度的关键参数。为此，我们分析了一些典型磁暴的主相期间，太阳风参数与磁暴主相发展过程关系，同时也统计分析了磁暴主相期间太阳风参数的时间积分与磁暴强度的关系。统计结果显示，行星际磁场南向分量的时间积分与磁暴强度的相关性差，磁暴主相期间太阳风电场的时间积分与磁暴强度的相关性中等，而磁暴主相期间环电流演化过程的注入项的时间积分与磁暴强度的相关性非常好，由于注入项中有太阳风的密度参数，从而在统计上证明了太阳风密度是影响磁暴强度的关键参数。该研究为后续进一步开展太阳风参数与磁暴关系，以及依据太阳风参数建立磁暴短临预报模式打下了基础。