What is a Solar Sector Boundary Crossing?

As the solar wind flows away from the Sun, the interplanetary magnetic field (IMF) is carried with it and has a spiral shape. Along the ecliptic plane (direct line between the sun and Earth), the IMF generally has 2 or 4 sectors per solar rotation (27 days) where it is pointed toward or away from the Sun. The surface separating the polarities is called the heliospheric current sheet. A sector boundary crossing occurs when the polarity of the IMF reverses. A well defined sector boundary crossing has a uniform field direction for about 4 days before and after the crossing.