Observation Of Field-Aligned Currents In The Plasma Sheet Boundary Layer

Field-aligned currents are the media of energy and momentum transfer between the solar wind,magnetosphere and ionosphere,and play a important role in the solar wind—magnetosphere—ionosphere coupling process.Study about the characteristics of field-aligned currents can help us to understand the relationship of different processes occurring in the magnetosphere.This thesis is to investigate the field-aligned currents in the plasma sheet boundary layer which connects the nightside ionosphere and the magnetotail,by using the high accuracy measurements of plasma and magnetic field observed by the four Magnetosphere Multiscale spacecraft,and analyze the properties of the kinetic-scale(close to local ion inertial length)field-aligned currents in the plasma sheet boundary layer,such as their spatial occurrence and current carriers.Also,we find and analyze the observational features of the sub-proton scale field-aligned currents by using the high temporal plasma data.1.Observation of the sub-proton scale field-aligned currents.Field-aligned currents in the plasma sheet boundary layer involve dynamical small-scale fine structures(below the local ion inertial length).Using high temporal measurements of plasma,we find that the large-scale field-aligned currents investigated before is the integrated effect of those small-scale currents.Bipolar or single directed small-scale field-aligned currents are still existing where no large-scale field-aligned currents is found,which indicates that the coupling process between the tail and the inner magnetosphere or ionosphere could be more dynamical,and not just a simple energy in and energy out process.However,the source and the exact role of those small-scale field-aligned currents need further investigation of observation and/or simulation.2.Observation of the kinetic-scale field-aligned currents.The close separation of the Magnetospheric Multiscale spacecraft enable us to study the kinetic-scale field-aligned currents,whose scale are close to the local ion inertial length.Such field-aligned currents show a trident distribution pattern in the plasma sheet boundary layer in tail,i.e.,the occurrrence rates are large in the midnight and the two near-flank region;also,their occurrence rate is higher in the northern hemisphere(summer)than that in the southern hemisphere,especially for the earthward field-aligned currents.Generally,the beam-type field-aligned currents appear equally in the region where 0.1<?<1.0,while the occurrence of the flow-type field-aligned currents linearly decreases from?=1.0 to ?=0.1;the occurrence rate of field-aligned currents is higher close to the central plasma sheet.The magnitude of field-aligned currents remain constant generally with respect to ?,but would increase when close to the Earth.Under high geomagnetic activity,the occurrence rate and magnitude of field-aligned currents both increase.Those results suggest that the field-aligned currents correlate to the tail magnetic reconnection,and also reflect the feedback effect of the ionosphere environments to the field-aligned currents and the tail.3.By improving the method to calculate the field-aligned currents,we can better determine the species and energy range of the current carriers from the particle energy flux measurements.The main carriers for field-aligned currents is the electrons.It is probably that ion currents would occur within the plasma sheet boundary layer but accompany with opposite electron currents.The thermal electrons with energy between 0.5Te? and 5Te?(Te? is the electron parallel temperature)is the main contributor to field-aligned currents,but,as the geomagnetic activity enhances,the contribution from the cold electron with energy below 0.5Te? would increase.The cold electrons come from the ionosphere,and this reflect the ionosphere's response to magnetotail processes during active period.4.Field-aligned currents exist in the center of the structure named flux rope.A lot of flux ropes would appear when the Venusian ionosphere is unmagnetized,and the study shows that the occurrence of them will increase when the altitude decreases,and,however,their spatial scale would decrease as the altitude decreases.In the terminator region where the solar zenith angle is large,the axial direction of those structures are generally parallel to the Venusian surface.The research on the flux ropes in Venus could increase our knowledge of the current system on other terrestrial planets and other planetary space environment,and at the same time,help us to understand the role and coupling of different current systems on Earth.This thesis reveals that the sources region of field-aligned currents include the tail magnetic reconnection site and the low-latitude boundary layer and the modulation and feedback effect of ionosphere in the magnetosphere-ionosphere coupling,through a investigation of the spatial occurrence and carriers of field-aligned currents in the plasma sheet boundary layer.Studying the currents on other terrestrial planet could help us understanding the formation of current system in terrestrial space environment and the role of them in energy transfer process.