The Outbreak Of The Overall Flow Of The Magnetic Tail Multi-satellite Studies

Bursty bulk flows often occur in the magnetotail. Since Angelopoulos et al [1994] definedBBF, due to its importance in the transport of mass and magnetic flux in the magnetosphere andsubstorm, it became a rather hot research subject in the magnetospheric phyics. Especially, thedispute on the transport of energy and magnetic flux relevant to BBF arouse more and moreinterest of magnetospheric physicists. The Cluster mission of European Space Agency is the firstmission that multi-satellites operate in the same orbit, and provides unprecedented opportunity tostudy middle and small scale phnomena in the magnetosphere.In this thesis, using the plasma and magnetic field data of three satellites of Cluster, we studythe temporal and spatial characterristics of BBF during both quiet and active times of magnetotail,as well as the role of BBF in the transport of mass and magnetic flux in the magnetosphere. Inaddition, we also compare the results of three satellites with those of single satellite.The data of Cluster in July and October in 2001 and 2002 when satellites crossed themagnetotail are used in our study. The flux gate magnetometer (FGM) provides the data ofmagnetic field, and the Cluster ion spectrometry (CIS) provides the data of the density,temperature and velocity of plasma.We first study the BBF occuring in the quiet time of magnetosphere. The duration of a typicalbursty bulk blow event of quiet time on September 7, 2001 is analysed in detail. In addition,several other bursty bulk flow events that occurred in 2001 and 2002 are also studied. The resultsshow that the duration of multi-satellites is about twice that of single satellite. It is also foundthat the selection criterion of BBF influence our results. The BBFs with longer duration duringthe quiet time of magnetotail enhances the earthward transport of mass and energy, and changesthe onset of the substorm. Our study also provides a new hint to the Pressure BalanceInconsistency (PBI).Then, we analyze 209 active time BBFs/Ang from July to October in 2001and 2002. Amongthe 209 active time BBFs/Ang, the single satellite (C1) only detected 162 BBFs/Ang. Theaverage duration of 162 BBFs/Ang observed by C1 is 604s, which is almost identical with theduration of BBF of previous studies (10 min). The average duration of 209 BBFs/Ang of threesatellites (C1, C3 and C4) is 1105 s. If considering that three satellites only occupy a limitedregion in the IPS, the average duration of BBF/Ang may be larger than 1105s. Thus the durationof active time BBFs/Ang is possibly of the same magnitude order with the duration of substorms.In order to compare the results obtained by using different selection criterion, we use theselection criterion of Raj et al [2002] to analysis the data. Among the 243 active time BBF/Raj,the single satellite (C1) only detected 134 BBF/Raj. The average duration of 243 BBFs/Ang ofthree satellites (C1, C3 and C4) is 771 s.The average duration of 134 BBF/Raj observed by C1 is527s. The difference between single satellite and three satellites based on the two selectioncriteria indicates that the number of bursty bulk flow observed by one satellite is less than that ofthree satellites. So the obseravtions of single satellite can not tell actual number of BBF. Theresults obtained by two different criteria indicate that the results based on selection criterion ofRaj et al [2002] is more sensitive to the multi satellite observations than those of Angelopoulos etal [1994].The separate distances of satellites also have influence on our results. The separation distanceof Cluster satellites in the tail (close to the apogee of Cluster) is about 2000 km from July toOctober 2001, while it is about 4000 km from July to October 2002. Among the 209 BBFs/Ang(137 in 2001 and 72 in 2002) observed by three satellites, C1 observed 162 BBFs/Ang (134 in2001 and 28 in 2002). Among the 243 BBF/Raj (158 in 2001 and 85 in 2002) observed by threesatellites, C1 observed 134 BBF/Raj (115 in 2001 and 19 in 2002). This indicates that in the caseof small separation distance (for example, 2000 km in 2001), the number of BBFs observed bysingle satellite is almost equal to the number of BBFs observed by three satellites. However inthe case of large separation distance (for example, 4000 km in 2002), the single satellite can onlyobserve 39 %( 28/72) of BBFs observed by three satellites. The selection criteria of Schodel et al.[2001] give the similar results.Furthermore, the occurrence rate of BBF in the inner plasma sheet is studied also here. Forthe 209 BBFs/Ang, the total durations of BBFs/Ang observed by single satellite C1 is 27.2 hoursand the total time of C1 in the central plasma sheet is 286.9 hours. Thus the occurrencefrequency of BBFs in the central plasma sheet obtained by single satellite is about 9.5%. Thisresult is similar to that of Angelopoulos et al. [1999]. The total duration of 209 BBFs/Ang is64.17 hours. The total time of three satellites in the central plasma sheet is about 329.94 hours.The occurrence frequency of BBFs/Ang obtained by multi satellites is therefore about 19.4%,much larger than the occurrence frequency of BBFs obtained by single satellite C1.Sometimes the BFFs could not be identidied by the observations of all three satellites. Thisdiscrepancy may be causd by various reasons. For example, a high speed flow regarded as BBFby one satellite could not be seen as BBF if we use the data of other two satellites because thepeak value of bulk velocity observed by other satellites were smaller than threshold value of BBF.When the satellite is in the requested region, the peak value of bulk velocity doesn't satisfy.Since BBF is high localized, three satellites rarely have identical observations. The entireduration of BBFs observed by C1, C3, C4 account for 55.07, 77.48, and 55.52 percents of thatobserved by three satellites jointly.The relationship between bursty bulk flows in the magnetotail and substorms is a veryinteresting problem. From July to October of 2001 and 2002, 67 substorms were identified byground observations. The single satellite observations (C1) show that among the 67 substorms,15 substorms are not accompanied by BBFs/Ang. However if we use multi satellite observations(C1, C3 and C4), only three substorms are not accompanied by BBFs/Ang. 48 substorms areaccompanied by 196 BBFs/colligation, that is to say, 71.64 percents of substorm areaccompanied by one or more BBFs/colligation. Both of the results of these two selection showthat one substorm may be accompanied by one or more bursty bulk flow.Rapid Convection Event is in fact high-speed bulk flow defined by the horizontal transportof magnetic flux. Among 306 RCEs detected by three satellites simutaneously, only 215 RCEsare detected by C1, 266 RCEs detected by C3 and only 227 RCEs detected by C4. The statisticalresults show that the average number missing ratio of single satellite is 22.8% and the averagetime missing ratio is 40.5%. So it can be concluded that the ratio of energy and magnetic fluxtransported by RCEs obtained by three satellites may be from 50 to 80 percents, which is largerthan the previous ratio obtained by single satellites (about 30-50%). 85.07% of substorm areaccompanied by one or more RCEs, which are less than the results given by the velocity criterionthat is 95.52 percents. So the relationship between BBF (selected by velocity criterion) andsubstorm is closer than that between RCE (defined by the horizontal transport of magnetic flux)and substorm.