冕洞相关地磁Ap指数中短期预报方法研究

Medium and Short Term Forecasting of Ap Index Related to Coronal Holes

地磁Ap指数是描述全球地磁活动水平的重要指数,过去许多参考大气模式中都用Ap指数来表述地磁活动状态,大气模式的运行需要输入地磁Ap指数,因此,地磁Ap指数的预报一直是空间环境预报中一个非常重要的内容.针对太阳活动低年冕洞引起的地磁扰动具有明显27天重现的特性,利用修正的自回归方法,对地磁Ap指数进行了提前27天的预报;采用从SOHO/EIT观测资料发展出来的描述冕洞特性的P_(ch)因子,进行了提前三天的地磁Ap指数预报.结果显示,将统计方法与物理分析相结合,进行地磁Ap指数的中短期数值预报,可以得到较好的预报效果.

Ap index is a daily measure of geomagnetic activity. In many reference atmospheric models, geomagnetic storm effects are modeled using Ap index as the driver of global density changes. Ap index is often used in making space mission plans because of the geomagnetic activity effects, and the forecasting of the Ap index plays an important role in space environment prediction. In the present study, the modified autoregressive model is used to forecast the Ap index 27 days ahead. In the modified model, the input data were reconstructed based on the 27-days periodicity of the solar rotation. The average absolute error between the forecasted and observed geomagnetic Ap index is 4.5. For short-term Ap forecasting （3-days ahead in this paper）, a new forecasting index Pch derived from the SOHO/EIT EUV images is used. Pch index is a parameter which can reflect both the area and brightness of coronal holes. Because the high speed stream will arrive at the earth orbit and cause geomagnetic disturbances about 3 days later after a coronal hole passes over the center meridian of the solar disk, the parameters of coronal holes can be used to forecast the geomagnetic disturbances. The average absolute error between the forecasted and observed geomagnetic Ap index is 3.7. According to the results of medium term and short term forecasting, the prediction accuracy can be improved by combining the statistical method with the physical analysis above.