Magnetotail Configuration And Dynamics During Magnetospheric Substorms

Magnetospheric substorms are one of the most common physical phenomena in the Earth's magnetosphere,which affect a wide range of regions and spatial scales,including the coupling process of solar wind and magnetosphere as well as the coupling process of magnetosphere and ionosphere.High energy particles produced by the magnetospheric substorm precipitated into the ionosphere and upper atmosphere,which has a great impact on radio wave propagation and communication on the ground,and even shut down the ground power grid.The study of the physical phenomena near the magnetotail during substorms has great scientific significance and potential application value for understanding the dynamic process of substorms and avoiding some space weather disasters.The difference between the near-Earth neutral line model and the near-Earth current sheet interrupt model is where the current sheet's thinnest occurs in the magnetotail,and an intuitive response to the sheet's thinning is the change in pressure.No matter where the current sheet is interrupted,a field-aligned current will be generated near the magnetic tail.The field-aligned current,which can transport matter and energy between the magnetosphere and ionosphere,is an important component of the substorm current wedge,which in turn is a major feature of the substorm.The substorm current wedge will generate additional B_Zcomponents,making the magnetic field become more dipole-like from the tail-like structure,resulting in magnetic field dipolarization.Based on the observation data of several satellites in the near magnetotail and geosynchronous orbit,this paper systematically studies the important phenomena,such as the field-aligned current,magnetotail pressure and magnetic field dipolarization,which are closely related to the substorm current wedge during the substorm.The main work and achievements of this paper are summarized as follows:The first is the formation of vortex-associated downward field-aligned currents during substorms.In the past observational studies,the variation of field-aligned current during substorms is basically caused by pressure gradient.A vortex-associated field-aligned current phenomenon has been found in this paper.The substorm occurred at 04:17 UT on December20,2007.During the substorm,the THEMIS satellite ground station observed not only the aurora changes,but also the D-component and H-component changes.The magnetic field data of the ground station show that there is downward field-aligned current flowing through the station during the substorm.Fortunately,THEMIS's two satellites,THB and THC,are located right near the ground stations along the magnetic field line.Analysis of magnetic and plasma data from both THB and THC satellites show that both satellites observed an increase in magnetic field B_Z,or dipolarization,during the substorm.According to the positions of the two satellites and the time of dipolarization,it can be judged that the dipolarization is moving towards the magnetotail.Based on the velocity vector hodograph of the THC satellite,we found that the THC observed a clockwise vortex.Further analysis showed that the size of the vortex is 2 R_E and it moved towards the magnetotail at a velocity of 23.9 km/s.From the reversal of the B_Y component of the magnetic field,it can be judged that there is a field-aligned current flowing from the magnetosphere to the ionosphere at this place.By means of Faraday's Law and the time variation of vortex's curl,we calculated and analyzed the current density of the field-aligned current and the cause of its generation.It is found that the field-aligned current is not entirely generated by vortices,and the pressure gradient also contributes.When the substorm occurs,the thinning of the plasma sheet can also be seen to evolve from magnetotail to Earth.Second,the statistical analysis of the total pressure of the magnetotail during the growth phase of the substorm.The pressure distribution in magnetospheric plasma is still one of the challenges in magnetospheric physics.During the growth phase of the substorm,the convection of the magnetosphere increases,and the magnetic flux of the magnetotail increases,and the total pressure of the magnetotail(P_tot),as the index of energy storage and release of the magnetotail,also changes.The pressure reflects the thinning and expansion of the plasma sheet.In the analysis of the substorm event occurred in December 20,2007,the plasma sheet thinning process evolved from the outside to the inside,indicated that the total pressure in different regions of the magnetotail would be different during the substorm.The magnetic pressure,thermal pressure and total pressure(the sum of the magnetic pressure and thermal pressure)of the magnetotail are calculated based on the observed data of Cluster-?and THEMIS satellites.The time when the B_Zcomponent of the interplanetary magnetic field starts to turn south is the beginning time of the substorm growth phase,and the time when the AL index suddenly drops is the end time of the substorm growth phase.The events of the total pressure change of the magnetotail during the growth phase of 168 substorms are determined.Through statistical analysis,it is found that the variation of total pressure at the near-Earth magnetotail is greater than that in the middle magnetotail.The variation of the total pressure at the near-Earth is more affected by the solar wind dynamic pressure than that of the middle magnetotail.In the near-Earth,the magnetic pressure increases with the increase of the solar dynamic pressure,and the change of thermal pressure is less affected by the solar dynamic pressure.In the middle magnetotail,the variation of thermal pressure increases with the increase of solar wind dynamic pressure.The?-value of solar wind plasma has little effect on the variation of total pressure in either the near-Earth or middle magnetotail.Thirdly,the dipolarization types of geosynchronous orbit magnetic fields during substorms are analyzed.Dipolarization is a major feature of substorms.When dipolarization occurs,the B_Z component continues to increase,and then remains basically unchanged.In the past studies,the dipolarization of the total magnetic field intensity increased during the substorm.Zhang et al.[2007]statistically analyzed the dipolarization process of the decrease of the total magnetic field intensity in the near-Earth magnetotail.In this paper,dipolarization near geosynchronous orbit during substorms is analyzed based on magnetic field observations from GOES 10,GOES 11,and GOES 12 satellites.Through case analysis,it is found that there are two different dipolarization at the beginning of the substorm near the geosynchronous orbit.One is that B_Z increases and B_t also increases;the other is that B_Zincreases and B_t decreases.A total of 79 dipolarization events were identified near the geosynchronous orbits.The statistical results show that the dipolarization of the total magnetic field decreases occurs in the higher latitude region,while the dipolarization of the total magnetic field increases mainly occurs in the lower latitude region.The solar wind dynamic pressure and solar wind plasma?have no effect on the two kinds of dipolarization.Under different IMF conditions,the dipolarization of substorms near the geosynchronous orbit are different.The increase of total magnetic field intensity mainly occurrs when IMF B_Xdecreases.The dipolarization with decrease of B_t mainly occurs when IMF B_X decreases.The dipolarization with increase of B_t mainly occurs when IMF B_Y increases.The dipolarization with decrease of B_t mainly occurs when IMF B_Y was reduced.The dipolarization with increase of B_t mainly occurs when IMF B_Z was southward.The dipolarization with decrease of B_tmainly occurs when IMF B_Z northward turning.Current substorm models are based on the assumption of a southward interplanetary magnetic field.By analyzing the variation of the total pressure of the magnetotail during the growth phase of the substorm and the dipolarization process near the synchronous orbit,the existing substorm model can be supplemented or a new substorm model can be built to better understand the solar wind-magnetosphere coupling.