宇宙线小波分析在大地磁暴预报中的应用

Application of wavelet analysis of cosmic ray in prediction of great geomagnetic storms

利用Morlet小波变换方法对北京宇宙线台站的地面宇宙线强度在地磁暴前后的变化特征进行分析,得到:1)在平静期,北京宇宙线数据存在准24h周期性的特征,且通过分析周期为12h的Morlet小波"模",发现值稳定,且小于0.6;2)以90天为时间窗口,对2004年7月地磁暴前后的小波频谱变化进行详细分析,发现当发生大地磁暴时,宇宙线的静日准24h周期被打破,其他周期的波动开始增强.进一步研究发现,周期12h的波动在大地磁暴数小时到1天左右会出现显著增强,这一现象在2001、2002和2004年期间的大地磁暴前得到验证.3)Morlet小波"模"数据的急速增大是发生地磁暴的先兆特征,当小波模变化达到一定的阈值就可能发生大磁暴.本文分析了周期为12h时小波的模数据,对强地磁暴事件进行统计,选定阈值0.6,并通过2003年的6次大地磁暴进行预报验证,结果表明该方法不仅能够对大地磁暴事件进行预报,而且提前量满足预报需求,为基于宇宙线实测数据预报地磁暴方法提供了重要基础.

Severe geomagnetic storms,which are mainly triggered by CME,can cause systematic violence in the geomagnetosphere,thus remain a key factor in space weather forecast.Unlike satellite at L1 point,ground base cosmic ray can receive the approaching of CME through the fluctuation feature of cosmic ray flux.CME always carries a strong magnetic field and ignites shock in front of it,which can alter the normal routine of cosmic ray particles and can be recorded on the earth.In this work,Morlet wavelet is employed to analyze the characters of cosmic ray flux evolution,and much effort is expended to recognize the special signal before CME.Suitable parameters and threshold are chosen to recognize the effective CME,permitting it to become the candidate factors for severe geomagnetic storms.Periodic revolution is an important character of cosmic ray in which the 24 hperiod is the most dominant during the quiet phase.When CME approaches the earth,the instinct regularpattern will be broken and other frequency grows.In this work,Morlet wavelet is applied to analyze the change in the character of cosmic ray intensity of the Beijing station（latitude：40.04°N,longitude：116.4°E,China）,and key point is set to be the revolution of cosmic ray intensity.Before and after severe magnetic storms,three peculiarities are derived.Firstly,the 24 h periodicity exists and is distinct during quiet time.The module of Morlet wavelet of 12 h periodicity is of stability less than 0.6.Secondly,by analyzing the spectrum variation of wavelet before and after the great geomagnetic storm on 25 th July,2004 with a 90-days window,it is found that the 24 hperiodicity was disrupted when the strong geomagnetic storm occurred,while the other different periodicity became obvious,especially fluctuation of 12 hperiodicity.After further investigation,we find that fluctuation of 12 hperiodicity became stronger a few hours to one day before the strong geomagnetic storm,which proves that the CME with the intensive magnetic field could highly alter the transportation and change the normal pattern of period distribution of cosmic ray.The verification with the all 13 events in 2002,2004 and 2005shows that all the modules of Morlet wave surpass 0.6.And finally,rapid increase in module of Morlet wavelet may become a precursor of a severe magnetic storm.Through applying module threshold to other storm events,the variation pattern of Morlet wave module is validated,and it is concluded that the threshold could be a useful factor in geomagnetic storm prediction.Based on understanding of cosmic ray flux modulation process and principle as CME reaching,we think the flux of cosmic rays can produce abnormal fluctuations,and destroy the original period of 24 h.After analyzing the 12 h wave signal intensity changes of 13 strong geomagnetic storm events in 2002,2004 and 2005,it is found the signal strength is increased significantly and quickly before the geomagnetic storm comes.After doing some statistics with all the severe geomagnetic events with Kp≥8in 2002,2004 and 2005,the Morlet wave module reached their maximum more than 5hrs before the storm commence.The method is tested with the all events in 2003 and the result turned out to be encouraging that 5out of 6（83%）great geomagnetic storms could be predicted and leading time was more than 10 hrs.For the one absence,the author thought that the CME only played a partial role and high stream from coronal hole also a main factor because large coronal hole emerged that time and solar wind revolution exhibited come coronal property.This proved that the threshold of module could be the candidate parameter for prediction for severe geomagnetic storm.Cosmic ray observation stations in Beijing received the cosmic ray flux.The period of 12 h abnormal increasing of wavelet signal mode can be an important precursor of a geomagnetic storm.It can be applied in the prediction of geomagnetic storms and used as an early warning of geomagnetic storms.This study suggests that the cosmic ray wave characters can be used to prognosis the properties of the coming CMEs be L1 satellites detect,which will improve the accuracy and leading time.More wave analysis methods and data from more cosmic ray stations are required for further investigations to this issue.