Research On Typical Structures In The Slow Solar Wind

Slow solar wind is an important component of background solar wind,which is always the hot and also difficult issue among space weather research.The origin and evolution of slow solar wind is significant in both science and application.Based on multispacecraft observations and relevant models,we make a comprehensive research,through case study and statistical analysis,on typical structures in the slow solar wind in this dissertation.1.The Distribution and Nature of Heliospheric Plasma SheetHeliospheric plasma sheet(HPS),heliospheric current sheet(HCS)and true sector boundary(TSB)are important structures in the slow solar wind,and also tightly relate to the origin and evolution processes of slow solar wind.Although these structures have been studied for decades,many issues are still controversial,such as the distributions of HPS relative to HCS,and the nature of HPS.Based on the relative locations of HCS and TSB,some researchers classified the HCSs into five types.Furthermore,we find that two of them are ideal HCSs,and only these HCSs are suitable for studying the distribution of HPSs.As a result,we identify 37 pairs of HCSs with STEREO data,and also conclude their types.After picking out the ideal HCSs,we further investigate the distributions of HPS.The results show that HPS can either straddle or border(lead/follow)the HCS,and some are even absent around the HCSs.Subsequently,we select ten HPS/HCS case pairs to study the nature of HPSs based on multispacecraft observations.We find that most of the HPSs maintain their relative locations with HCSs,and the observed time differences between the STEREO spacecraft match the expected time differences very well.Therefore,the HPSs seem to be stationary structures.2.The Coincidence of Heliospheric Current Sheet and Stream InterfaceIn general,the HCSs are embedded in slow solar wind,and the stream interfaces(SIs)separate slow solar wind from fast solar wind.Observations also suggest the HCS usually leads the SI by several hours around 1 AU.Consequently,the coincidence of HCS and SI implies the slow solar wind between them is disappeared,and this is valuable for studying the evolution of slow solar wind.Based on the high time resolution data from STEREO spacecraft,we first report a special coincidence case around 1 AU.In this case,the HCS not only coincides with the SI,but also separates from the TSB for about six hours.Combined the white light coronagraph and WSA simulation results,we confirm the existence of a pseudostreamer at the source region of this case.We therefore suggest the interchange reconnection processes between the helmet streamer and pseudostreamer may be responsible for the formation of this special case.In order to further our understandings of the characteristics of the coincidence of HCS and SI,we also analyze nine coincidence cases between2007 and 2010.The analysis suggests the coincidence cases can be classified into three categories based on the types of HCSs and the existence of heat flux dropouts,and each type cases may have different formation mechanism.However,only two of the nine cases can be observed by different spacecraft,implying the coincidence cases should be unstable structures.With the PFSS simulation results and remote sensing observations,we also find the coincidence cases have a variable connection with pseudostreamers.We suppose interchange reconnection and pseudostreamer could paly a role in forming the coincidence of HCS and SI.3.Research on Small Flux RopesSmall flux ropes(SFRs)contribute to slow solar wind,but their source regions and formation mechanism are still debatable.We first present a SFR with smooth magnetic field rotations entrained by rolling back magnetic field lines around 1 AU.This kind of SFRs are expected in interchange reconnection debris,and also valuable for investigating the origin mechanism of SFRs.Based on multispacecraft observations,we find this SFR and an adjacent HPS have similar plasma signatures,density ratio of alpha particle-to-proton and heavy ion ionization states,which implies they may have a similar origin in the corona.The composition signatures and the configuration of the rolling back magnetic field lines suggest the SFR originates from the streamer belt through interchange reconnection.The origin processes of the SFR are also presented.In addition,the observations and model results further suggest that the size and spatial morphology of SFRs may make the spacecraft miss them in interplanetary space.In order to further study the origin mechanisms of SFRs,we then focus on the distribution of iron average charge states(Q<Fe>)within SFRs.Compared with latest results of large flux ropes,we analyze 24 SFRs from 1998 to 2005.The results show that the distributions of Q<Fe> within SFRs only have four types,i.e.type B to type E,but type A is absent.Therefore,we suppose there are different mechanisms under different circumstances.For the SFRs that originated from solar corona,the statistical results suggest their flux rope structures should be only formed during eruptions.However,the SFRs formed in interplanetary medium may involve with complicate magnetic reconnection processes,which may,consequently,result in complicate distributions of Q<Fe> within SFRs.The Main Innovation Points in this Dissertation:· Combined the typical structures(heliospheric plasma sheet,heliospheric current sheet,true sector boundary,stream interface and small flux rope)in the slow so- lar wind more comprehensively,we can make a more systematic study of the origin and evolution of slow solar wind;· Based on multispacecraft observations,we can detect the slow solar wind at different positions,and therefore make a better investigation of the slow solar wind;· With case study and statistical analysis,we can thoroughly explore the characteris- tics of typical structures in the slow solar wind.