The Research On The Magnetic Structure Of Current Sheets In Magnetotail

The geometric structures and the distribution properties of the magnetic field in the magnetotail current sheet (CS) have been studied systematically based on the multi-point observations of Cluster and the associated analysis methods. The main results could be summarized as follows:1. It is found that the flattened current sheet (FCS) has sufficiently strong within neutral sheet (NS). The geometric structures of magnetic field lines (MFLs) are spiral-like, which are left-handed (right-handed) spiral structures for ( ).The normal directions are generally northward. The half-thickness of the NS is much less than the minimum curvature radius of the MFLs in the CS. The current density in the NS is field-aligned and mainly duskward. The thermal motion of the electron is adiabatic but non- adiabatic for proton. The main current carriers in the NS are electrons. A statistical survey shows that there is one positive correlation between in the FCS and IMF , and the FCS may appear at all phases of substorms. Around the midnight meridian ,the strength of enhances at the center of FCS relative to that in the CS boundaries and lobes, forming symmetric distribution approximately about the center of CS, but it becomes asymmetric type at the non-midnight meridian due to the influence of the tail flaring effect.2. The CS in magnetotail may often flaps up and down, inducing the kink-like flapping waves propagating towards both flanks, which results in the local tilted CS generally.1) The investigation for the tilted CS shows that the geometric structures of the MFLs are the same with that in the normal CS but slipped from each other to some extent. The half-thickness of the NS, , has relation with the minimum curvature radius of the MFLs, ,and the tilted angle of CS, h ? Rcmincos?.The current density embeds in NS and reaches the maximum in the center of CS where the and the field-aligned component dominate, and could be more intense in the more tilted CS. The profile of the current density can become bifurcated or asymmetric type in some cases. 2) For the study of the CS flapping motion, the kink-like waves can be induced and launched from the midnight towards both flanks. A method based on the magnetic gradient proposed so that the propagation direction of the flapping waves can be determined intuitively. Sometimes, the CS can flap locally without flapping waves involved. When spacecrafts cross the CS successively as the flapping waves passing by, the sign of and would change alternatively in each crossing. The phase velocity of the flapping waves is tens km/s, their wavelength could be several Earth radius. The propagation of the flapping waves is synchronized within the lengthwise scale ? 8 RE ?x??RE in the tail at least. The flapping motion can occur in all substorm phase with endurance time exceeding to several hours. The discussions of the mechanism to trigger the flapping motion suggest the large-scale CS flapping motion could be the intrinsic oscillation of magnetotail, probably induced by the up-down motion of plasma, and the scenario of the triggering process and propagation have been explained qualitatively.3. On the basis of the Cluster's observations on the magnetic geometry of MFLs in the tilted CS, the analytic properties of the flapping current sheets could be studied systemically. The approximate model for the magnetic field in the flapping current sheets has been obtained by analogy with Harris current sheet, so that the associated physical quantities could be directly calculated and analyzed. The applications of the magnetic rotation analysis to the tilted current sheets have quantitatively revealed the spatial distribution of the curvature of magnetic field lines and the half-thickness of the neutral sheet, and the physical relationship between them as well. The obtained results are consistent well with the actual observations of Cluster.4. The distribution properties of the magnetic field in the tail CS have been explored statistically with the 4-sec FGM data of the Cluster mission in the period June-November of the years 2001- 2005. The average strength of the magnetic field and its component in the current sheet are weaker in the midnight but stronger near both flanks, which implies that averagely the thinner CS appear in the midnight and thicker ones near both flanks. The tail CS flaps frequently in both flanks, especially in the dawn flank, but relatively calms in the midnight. The negative component and the FCS have the higher probability to occur in the magnetic local time 21:00-01:00, indicating that the magnetic activities, e.g., the magnetic reconnection and current disruption would occur more frequently there. Statistically, the probability distribution of the component and the tilted angle of the MFLs in the CS are of normal distribution approximately, and the occurrence number of the FCS is about the 1/3 of that of the normal CS. The magnetic field and the component in the CS are mainly varied from 1nT to 10nT. The in the tail central CS is averagely twice of the IMF at 1AU, their positive correlation coefficient is higher in the FCS.