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# Solar Wind: Neutral Sheet Model, Stream Interactions

This module allows you to experiment with a phenomenological model of stream interactions based on a parameterized treatment of the physics (described by Hakamada, K., and S.-I. Akasofu, Simulation of Three-Dimensional Solar Wind Disturbances and Resulting Geomagnetic Storms, Space Sci. Rev., 31, 3-70, 1982), and to “simulate” plasma and field data that might be observed by spacecraft at various heliocentric distances.

## Stream Interactions Model

Spacecraft measurements show that the solar wind velocity depends on the distance from the heliospheric current sheet. The velocity can be 2 to 3 times as high near the magnetic “poles” of the Sun (the location of polar coronal holes) as it is near the current sheet. This “heliomagnetic latitude” dependence produces a high speed/low speed “stream” structure that is especially prominent at low heliographic latitudes when the dipole tilt is significant or a strong quadrupole contribution to the Sun’s field is present.

When there is a non-zero dipole tilt or finite quadrupole moment the solar rotation brings different source velocities to the base of a given radial direction in the inertial frame. Since the plasma transport is radial, high speed plasma that is launched behind lower speed plasma will plow into it and create a “stream interaction” region. Due to the fact that the magnetized plasmas do not interpenetrate, the interaction speeds up the slower plasma and slows down the faster plasma. A pressure ridge is thus formed that takes a spiral form for the same reasons as the interplanetary magnetic field (IMF).

The distribution of IMF magnitude also shows the interaction regions since it too is compressed. Collisionless shocks may eventually form as the pressure ridges steepen with increasing heliospheric distance, but this usually happens beyond 1 AU.

The graphs on the left show, from top to bottom:

• The neutral sheet on the Sun’s surface where the radial component of the magnetic field is zero
• Magnetic field lines in the rotational equator carried outward like spiral threads in the flow attached on one end to the rotating source region
• The latitudinal profile of the velocity measured along meridians of the rotational equator, but referenced to the magnetic equator (neutral sheet)

The graphs on the right show the predicted 1 AU data versus time in steady state as the Sun rotates every 27 days, from top to bottom:

• Velocity
• Density
• Dynamic pressure