The sun's magnetic field contains both open and closed field lines. The closed ones trap energetic particles close to the sun and do not make good conduits for the solar wind plasma into the rest of the solar system. The open field lines, however, spiral out from the sun like a pinwheel as the sun rotates, and along these field lines particles can travel easily, and even get accelerated to form the bulk of the solar wind. Near the Earth's orbit, this plasma has a typical density of a few atoms per cubic centimeter and travels at about 400 kilometers/second. The presence of regions of open field lines on the solar surface at times when these areas are active, produces coronal holes which are easily seen in x-ray photographs. The outward moving plasma can intersect the Earth's orbit and can form solar wind 'sectors' like the arms of a pinwheel. Often, the polarity of the magnetic field inside adjacent sectors is reversed.
The interaction of the solar wind with individual planets is quite complex to follow.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.