Abstract
Measurements of the magnetic field in the interplanetary medium, of the sunspots area, and of the heliospheric current sheet position, reveal a possible North-South asymmetry in the magnetic field of the Sun. We study the North-South asymmetry as inferred from measurements of the deflection of polar coronal hole jets when they propagate throughout the corona. Since the corona is an environment where the magnetic pressure is greater than the kinetic pressure, we can assume that magnetic field controls the dynamics of plasma. On average, jets during their propagation follow the magnetic field lines, highlighting its local direction. The average jet deflection is studied both in the plane perpendicular to the line of sight, and, for a reduced number of jets, in three dimensional space. The observed jet deflection is studied in terms of an axisymmetric magnetic field model comprising dipole We measured the position angles at 1 rs and at 2 rs of the 79 jets from the catalogue of Nistico et al 2009., based on the STEREO ultraviolet and white-light coronagraph observations during the solar minimum period March 2007-April 2008. We found that the propagation is not radial, 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. Comparison of jet deflections and field line tracing shows that a ratio g2/g1 ~ -0.5 for the quadrupole and a ratio g3/g1 ~ 1.6-2.0 for the esapole can describe the field. The presence of a non-negligible quadrupole moment. 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 which 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.
Highlights
The solar corona is an environment highly structured by the strength and the topology of magnetic fields
Indirect estimates of the coronal magnetic field are obtained from extrapolations techniques, such as the potential field source surface (PFSS) and the non-linear force-free field (NLFFF) models (Wiegelmann & Sakurai 2012), by radiophysical methods and coronal seismology (e.g. Nakariakov & Ofman 2001)
Since the corona is an environment with a plasma-β parameter less than 1, we assumed that jets on average propagate along the magnetic field lines
Summary
The solar corona is an environment highly structured by the strength and the topology of magnetic fields. Evidence came from direct measurements of the photospheric magnetic field by magnetograms, which were extrapolated to the solar wind source surface by Hoeksema (1995), who showed that the magnetic field strength in the Sun’s south polar cap was 60% larger than in the north polar cap during some solar minima. Erdos & Balogh (2010) studied and compared magnetic field data of Ulysses during the first latitude scan (coincident with the minimum of solar cycle in 1994–1995) and the third scan (minimum of solar cycle in 2007–2008) They found that the radial component of the magnetic field (normalised to 1 AU) of the south hemisphere is greater by a factor 1.12–1.21 than that of the north.
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