|The Large Scale Solar Wind|
How does the solar wind affect the solar magnetic field?
The solar wind drags the solar magnetic field, which is frozen into the highly conductive plasma, like string in a stream being dragged from its source. This creates the interplanetary magnetic field, which extends to the very boundary of the heliosphere. As the Sun rotates approximately every 27 days, the solar wind emerging from coronal holes is spun out like water streams from a garden sprinkler.
Because the magnetic field is dragged parallel to the flow field, while remaining anchored to the Sun, it becomes wrapped into a spiral or Archimedean pattern, known as the Parker spiral (a), after Eugene Parker, who first described it. Close to the Sun, the field is radial; at 1 AU, or Astronomical Unit, the distance from the Sun to the Earth, it is at approximately 45 degrees from radial; beyond 15 AU, it is almost 90 degrees from radial.
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The Parker spiral. The field is radial close to the Sun, about 45 degrees from radial at 1 AU, and almost 90 degrees from radial beyond 15 AU. Figure courtesy of Steve Suess.
The Voyager spacecraft has measured the magnetic field strength(b) and orientation all the way to 60 AU, well beyond Pluto, the furthest planet, and has confirmed Parker's theory of the interplanetary magnetic field structure and strength.
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Like a large magnet, the interplanetary magnetic field is dipolar (c). During solar minimum, the Sun's field is very regular, with large unipolar regions. The magnetic field will point outward in one hemisphere and inward in the other. Near the equator the field changes sign. Thus there is a boundary where the interplanetary magnetic field goes to zero.
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The interplanetary magnetic field, showing the solar wind flow (red lines), the magnetic field lines (blue lines), and the neutral current sheet (green dotted line)
Because the field is dragged out by the solar wind, this thin boundary is sheet-like, and we call it the neutral current sheet.
Neutral Current Sheet:
Differences between the axis of solar rotation and the magnetic field dipolar axis cause ripples in the current sheet, so that it resembles the folds of a ballerina's skirt (d). At solar maximum, the increased solar magnetic activity complicates the structure of the current sheet.
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