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# Solar Wind: Neutral Sheet Model, 3-D Structure

This module allows you to experiment with different influences on current sheet topology, including magnetic moment tilt angle (with respect to the solar rotation axis), relative contribution of quadrupole and dipole moments, and solar wind velocity.

## Heliospheric Current Sheet

If the solar magnetic field is originally dipolar before the solar wind stretches it into space, and the dipole axis is aligned with the solar rotation axis, one would expect the interplanetary magnetic field (IMF) to show different “polarities” in the northern and southern heliosphere. These oppositely directed “toward” (outward) and “away” (inward) fields should be separated by a planar current sheet in the equatorial plane.

Observations at 1 AU in the early 1970s showed that the IMF usually exhibits alternating polarities during the course of each 27-day apparent solar rotation. Sometimes there are two “sectors” per rotation, but there are often four. Several researchers (including M. Schulz and H. Alfvén) recognized that a dipole axis tilted at an angle with respect to the solar rotation axis would produce the two-sector pattern, while the four-sector pattern would be created by the addition of a solar quadrupole moment. The appearance of the heliospheric current sheet in such circumstances invoked the “ballerina skirt” analogy.

The graph on the left shows the solar surface mapped onto a Cartesian grid of latitude (vertically) and longitude (horizontally) measured versus the rotational equator and pole. Shading indicates the polarity of the magnetic field crossing the surface. The graph on the right shows a perspective view of the current sheet separating radially outward fields from radially inward fields.

This module allows you to control the tilt of the magnetic dipole axis, the amount of quadrupole field present, the viewing perspective, the velocity of the solar wind, and the polarity of the field. Note that when the tilt angle is zero, the rotational and magnetic dipole axes are aligned.

The added quadrupole moment produces four alternately directed “orange slice” regions of fields around the Sun. This moment is equivalent to the close positioning of two oppositely-directed dipoles, each in the equatorial plane separated along their lengths. The addition of this quadrupole moment to the dipole field warps the current sheet so that the region of net outward field and of net inward field penetrates above and below the rotational equator every 180° around the equator.