| Substorm had been one of the key subjects in space physics.Substorm is the basic process of magnetospheric dynamics,which is the process of energy input,coupling,and dissipation in geospatial space.The occurrence and development of substorm are involved in the whole process of the Sun-Earth system,including the coupling of solar wind and Earth magnetic field,the coupling of the magnetosphere,the ionosphere,and the upper atmosphere.Meanwhile,substorm onset,as an unsolved problem,has always been the focus of people’s research.Generally,substorm has identifiable and repeatable structures consisting of three phases including growth,expansion,and recovery phase.At present,the mechanism of substorm given by different models is not the same and focuses on the substorm onset model.There are two main schools on the cause of substorm.The first school(outside-in)emphasizes the role of magnetic reconnection in the midtail region(Xgsm≈-15RE to-30RE,RE is the earth radius).Substorm onset follows when a streamer approaches the thin arc.The second one(inside-out)focuses on the role of plasma instabilities in the near-Earth magnetotail region(Xgsm≈-6RE to-15RE),causing current disruption,and magnetic reconnection in the outer region following.In this case,substorm onset takes place without a streamer.The two opposing models both have observation evidence.Later some researchers proposed a more general model called "entropy stwich model" which combines the "inside-out" and "outside-in" models.In this model,substorm onset is regarded as a switch on the sequence of transport from decreasing entropy to increasing entropy.In the stable magnetosphere,entropy tends to increase tail ward.In the substorm growth phase,the bubble has a phenomenon of entropy reverse,the early arriving high entropy bubble will block the later arriving low entropy bubble tailward,forming an unstable domain.The development of the interchange instability leads to the collapse of the stretched plasma sheet,and the substorm expansion phase follows.In this paper,we present observation evidence of entropy switch model of substorm onset and make a statistical study using THEMIS data.Confined the events with THEMIS satellites location in the down tail region between-8～-12RE,and 3 hours before and after midnight,we examine two events,one with and the other without a streamer before substorm onset,and perform a statistical study with 205 events.The detailed study of two cases proves the rationality of our model.And an occurrence rate of magnetic reversal is found to be 69%and an occurrence rate of entropy reversal is found to be 77%of the total 205 events. |
