Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh51c02h&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH51C-02
Physics
2100 Interplanetary Physics, 2111 Ejecta, Driver Gases, And Magnetic Clouds, 2134 Interplanetary Magnetic Fields, 3210 Modeling, 7819 Experimental And Mathematical Techniques
Scientific paper
We have examined 2(1)/(2)D static magnetic flux rope configurations (∂ /∂ t ≈ 0, ∂ /∂ z ≈ 0, but Bz!= 0, z being the flux-rope axis) to determine their axis orientations from single-spacecraft data taken as the flux rope convects past the spacecraft. In a proper frame of reference, (x,y,z), the spacecraft trajectory projected onto the transverse (x-y) plane is a straight line along x across the flux rope structure. The distance from the center of the flux rope to the spacecraft trajectory is defined as the impact parameter. We have developed a multi-step, composite magnetic variance analysis method (the HS method) to accurately determine the axis of flux rope structures having right-left symmetry such as the Lundquist model of an axially symmetric force-free configuration. We will present the results of our analysis for right-left symmetric flux rope models of circular, elliptical and non-elliptical cross-sections (in the x-y plane) for comparison with results from Lepping's method [Lepping et al., 1990], in which the intermediate variance direction, obtained from magnetic variance analysis on unit normalized magnetic vectors (i.e., B), is used as the approximation for the true axis of the Lundquist model. Under certain conditions, our method yields the exact axis orientation, while the result from Lepping's method shows increasing angular error with increasing impact parameter. More general cases of non-symmetric, static flux-rope structures can also be analyzed, since they (as well as the symmetric cases) are governed by the Grad-Shafranov equation, ∇ t2A=-μ 0dPt/dA, where the magnetic potential A is used to describe transverse magnetic field, Bx=∂ A/∂ y and By=-∂ A/∂ x, and Pt(A)=p(A)+Bz2(A)/2μ0. For these asymmetric cases, the determination of the optimal axis orientation is accomplished by searching for the minimum in the fitting residue of Pt(A) subject to certain constraints. This method does not give a unique answer for structures that are right-left symmetric. Examples are presented to illustrate the procedures, to highlight pitfalls, and to show estimates.
Hu Qiang
Sonnerup B. U. Ö.
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