科研之友
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摘要
Context. Measurements of the sunspots area, of the magnetic field in the interplanetary medium, and of the heliospheric current sheet (HCS) position, reveal a possible north-south (N-S) asymmetry in the magnetic field of the Sun. This asymmetry could cause the bending of the HCS of the order of 5-10 deg in the southward direction, and it appears to be a recurrent characteristic of the Sun during the minima of solar activity. Aims. We study the N-S asymmetry as inferred from measurements of the deflection of polar coronal hole jets when they propagate throughout the corona. Methods. Since the corona is an environment where the magnetic pressure is greater than the kinetic pressure we can assume that the magnetic field controls the dynamics of plasma. On average, jets follow magnetic field lines during their propagation, highlighting their local direction. We measured the position angles at 1 R-circle dot and at 2 R-circle dot of 79 jets, based on the Solar TErrestrial RElations Observatory (STEREO) ultraviolet and white-light coronagraph observations during the solar minimiun period March 2007 April 2008. The average jet deflection is studied both in the plane perpendicular to the line of sight and, for a reduced number of jets, in 3D space. The observed jet deflection is studied in terms of an axisymmetric magnetic field model comprising dipole (g(1)), quadrupole (g(2)), and esapole (g(3)) moments. Results. We found that the propagation of the jets is not radial, which is in agreement with the deflection due to magnetic field lines. Moreover, the amount of the deflection is different between jets over the north and those from the south pole. A comparison of jet deflections and field line tracing shows that a ratio g(2)/g(1) similar or equal to -0.5 for the quadrupole and a ratio g(3)/g(1) similar or equal to 1.6-2.0 for the esapole can describe the field. The presence of a non-negligible quadrupole moment confirms the N-S asymmetry of the solar magnetic field for the considered period. Conclusions. We find that the magnetic deflection of jets is larger in the north than in the south of the order of 25-40%, with an asymmetry that is consistent with a southward deflection of the heliospheric current sheet of the order of 10 deg, consistent with that inferred from other independent datasets and instruments.
- 出版日期2015-11
