Identification Methods And Characteristic Analysis Of Alfvén Waves In Solar Wind

Solar wind is a kind of supersonic plasma flow derived from the solar atmosphere.Because of its high conductivity,the interplanetary magnetic field and solar wind plasma freeze together and travel outward to interplanetary space.Alfvén waves are found to exist widely in solar wind through satellite observation,which plays an important role in the study of coronal heating and solar wind acceleration.On the basis of the characteristics of high correlation between the velocity disturbance of the Alfvén wave and the magnetic field disturbance,the Walén test is usually used to determine whether the wave signal to be analyzed has an Alfvén wave and the purity of the Alfvén wave in the observation research.In previous studies,it was found that pure Alfvén waves(Walén test results were close to 1)were commonly seen in 0.3 ~ 0.68 AU,it was found out that improving the identification method of Alfvén waves in solar wind can help to identify pure Alfvén waves at 1 AU.To better identify Alfvén waves in solar wind,this paper uses the combination of ensemble empirical mode decomposition(EEMD)and time-based intrinsic correlation(TDIC)techniques to process and analyze single satellite data at 1 AU based on the traditional identification methods of Alfvén waves in solar wind.Traditional methods can be divided into two categories to identify Alfvén waves in the solar wind: one is the Walén test based on de Hoffmann-Teller frame(HT frame),which is a very effective diagnostic method on the premise of accurately determining the HT frame.However,when the solar wind structure is unstable,it is difficult to establish a suitable time-varying HT frame based on the traditional methods.The other is the Walén test based on a modified time series of velocity.This method gets rid of the restriction of HT frame,but it faces the problem of high frequency error in practice.This paper attempts to use EEMD and TDIC methods to deal with the two problems.An analysis of a fluctuation case at 1AU(18:16-18:52,14 October 2002)showed that:(1)The EEMD method can help us to obtain a time-varying HT frame,which can improve the traditional identification methods based HT frame.(2)With the combination of EEMD and TDIC methods,the filtering scheme can be selected flexibly.The experimental results show that compared with the traditional identification method based on the modified time series of velocity,the Alfvénic parameters are significantly improved after using the time series of velocity reconstructed by EEMD and TDIC analysis for Walén test.Based on the research results in this paper,it can be considered that the Walén test using EEMD and TDIC analysis is an effective method to identify Alfvén waves in the solar wind,especially when there are unstable transient structures in the solar wind,the analysis results of new method about Alfvénic parameters are better than the results of traditional methods.The new method uses unsteady data analysis technology for the first time,which is of great significance for studying the Alfvén waves and Alfvénic characteristics in the solar wind.