Statistical Study On The Magnetic Cloud Boundary Layer And Its Geomagnetic Effects

Magnetic Cloud Boundary Layer(MCBL)is one type of small-to-meso scale solar wind disturbance structures in interplanetary space.It is formed due to the interaction of magnetic cloud and the ambient solar wind with distinctly different magentic field and plasma features.It is found in previous studies that magnetic field reconnection,excitation of various types of plasma waves,magnetic field turbulence and plasma heating can occur within the MCBLs.On the other hand,when the magnetic cloud boundary layer crosses the Earth's magnetosphere,it will strongly compress the magnetosphere and change the state of the terrestrial environment dramatically.Therefore,investigation on the physical process in the MCBL and its geomagnetic effects is a topic worthy of in-depth study for space weather prediction.Previous research mainly focused on the analysis of typical event analysis.In this dissertation,the MCBL events during around two solar cycles were identified.Based on the list of typical MCBLs,the magnetic field and plasma characteristics were statistically analyzed.Furthermore,their geomagnetic effects were discussed detailly.The main results are as follows:According to the criteria of the MCBL proposed by Wei et al.,typical MCBL events with magnetic field decrease at 1 AU from 1995 to 2015 based on the WIND observations were identified.The list of typical MCBLs was used to further research on statistical characteristics of MCBL.The statistical analysis of the magnetic field and plasma characteristics of typical MCBL events confirms that the magnetic field and plasma characteristics in MCBL are significantly different from that in the ambient solar wind and the magnetic cloud body.The B_z component of the magnetic field in MCBL has much stronger fluctuation compared with the ambient solar wind and the magnetic cloud body with its polarity changing more frequently.Almost for all the typical MCBL events,the solar wind dynamic pressure inside is stronger than its ambient ambient solar wind and magnetic cloud body.The average value of the dynamic pressure of the 77 typical MCBL events is 6.61 n Pa(for the magnetic cloud sheath region,the average is 4.78 n Pa,and for the magnetic cloud body it is 2.46 n Pa).Hence the magnetic cloud boundary layer is a strong dynamic pressure enhancement region.It is found that the?-?angular distribution of the magnetic field in 32 MCBL events(41.6%)shows an asymmetrical"U"or inverted"U"distribution,and the directional angle span is about 180°,indicating that there may be a reverse magnetic field in MCBL.Most of the studies on ICME and MC only distinguished the sheath region from the MC body.The research in this study indicates that in addition to the sheath region and MC body,MCBL is a special structure with unique magnetic field and plasma feartures,which does not belong to adjacent sheath or magnetic cloud body.It is a special structure caused by the interaction between MC and the front sheath(or the ambient solar wind).The inherent sustaining strong southward magnetic field and dynamic pressure enhancement make the MCBLs much geoeffective.During the passage of the MCBL through the Earth's magnetosphere,although the time scale is relatively shorter,it has the ability to drive the initial and main phases of geomagnetic storms.Among the discussed 77 typical MCBLs,13 events(16.9%)drove the initial phase of the magnetic storm,and 9 events(11.7%)drove the main phase of the magnetic storm.Some MCBLs with sustaining strong southward magnetic field and additional polarity reverse may trigger geomagnetic substorms.Geomagnetic substorms could be accompanied by geomagnetic storms,or occur as isolated substorms.The statistical analysis shows that there are 25(32.5%)and 18(23.4%)MCBL events that trigger the two types of geomagnetic substorms,respectively.MCBL is usually a dynamic pressure enhancement region,which is able to strongly compress the magnetosphere.By using the Shue magnetopause model,the minimum distance r_0min of the subsolar point of the magnetopause during the MCBLs cross the magnetosphere is calculated.There were 11 events(14.3%)that pushed the magnetopause into the geosynchronous orbit(with r_0min?6.6 R_E);and 42 events(54.5%)with r_0min?8.0 R_E.It could lead to catastrophic space weather events when the magnetopause is compressed and pushed into geosynchronous orbit.Observations from the GOES spacecraft have also confirmed that a few MCBLs can cause such rare space weather events.It indicates that in addition to ICME,CIR,which is regarded to be very geoeffective,MCBL is also a kind of important source of magnetospheric space weather events.