Illustration of the magnetic field structure in a stream interaction region (SIR) adapted from Gosling et al. (1976). Within SIRs, Faster solar wind overtakes slower solar wind, forming a compression or shock (red dashed curves). The Parker Spiral magnetic field lines are shown as thin blue curves. With footpoint motion at the Sun, magnetic field lines (black curves) are connected between fast and slow solar wind and form the sub-Parker spiral (Murphy et al. 2002; Schwadron & McComas 2005; Schwadron et al. 2005) in rarefaction regions where the fast solar wind stretches out magnetic field lines in the radial direction.