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| Paper: |
The North-South Asymmetry of the Heliospheric Current Sheet: Results of an MHD Simulation |
| Volume: |
474, Numerical Modeling of Space Plasma Flows (ASTRONUM2012) |
| Page: |
179 |
| Authors: |
Usmanov, A. V.; Goldstein, M. L. |
| Abstract: |
A displacement of the heliospheric current sheet (HCS) south of the
helioequator by ∼10° was proposed by Simpson et al. (1996)
as a possible explanation of the north-south asymmetry in the galactic
cosmic rays observed by Ulysses during its first fast transit in 1994–1995.
The idea was not supported by magnetic field measurements on Ulysses and,
on this ground, was dismissed by Sipson et al. (1996). In addition, Erdös & Balogh (1998) argued that any north-south symmetry was
unlikely as there should be flux balance between the magnetic sectors of
opposite polarity. Nonetheless, many in the scientific community have
accepted the original suggestion of Simpson et al. (1996) that a
displacement of the HCS was responsible for the cosmic ray asymmetry. In
this paper, using a magnetohydrodynamic model of the solar corona and solar
wind that includes both dipole and quadrupole magnetic source terms, we
show that a north-south asymmetry of the magnetic field on the Sun does not
give rise to a displacement of the HCS. The lack of displacement of the HCS
results from a latitudinal redistribution of magnetic flux near the Sun
where the plasma βll1. The latitudinal redistribution is a direct
consequence of the magnetic field gradient between pole and equator. Near
the Sun, the latitudinal gradient in magnetic field generates meridional
flows directed equatorward that tend to relax the gradient in the magnetic
field (to make it more latitude-independent) as heliocentric distance
increases. If there is an asymmetry between north and south magnetic field
strength then the meridional flows are also asymmetric (i.e., stronger in
the hemisphere of stronger magnetic field). Because the magnetic fluxes
(positive and negative) in the hemispheres must be equal, the
redistribution shifts the HCS in the direction of the hemisphere with a
weaker field and brings the field strength on both sides of the HCS into
balance by ∼16 R☉. At larger distances, where the magnetic field is
relatively weak (βgg1), the HCS can be displaced if there is a
difference in total pressure between the hemispheres. |
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