Solar wind from the coronal hole boundaries

Recent studies using in situ observations established that the interface between fast and slow wind in interplanetary space has two distinct parts: a smoothly varying boundary layer flow that flanks fast wind from coronal holes, and a sharper plasma discontinuity between intermediate and slow solar wind. Other studies using in situ observations and modeling have demonstrated the existence of the sub-Parker spiral structure of the heliospheric magnetic field in which the magnetic connection between fast and slow wind created by footpoint motion at the Sun deforms field lines, making them significantly less transverse than the Parker spiral. Here we model the formation of co-rotating interaction regions (CIRs), and by including a coronal hole boundary layer (CHBL) and magnetic footpoint motion across the coronal hole boundary back at the Sun, explain the detailed, characteristic variations in composition and magnetic field orientation observed in interplanetary space. Our model accomplishes this using only two free parameters, with all other quantities derived directly from solar wind observations. Through the model we trace the observed interplanetary variations back to an intrinsic two-part structure in the source of solar wind at the Sun. These parts are (1) a CHBL that encircles the coronal hole and has a smooth transition in the source properties that produce the fast through intermediate speed (∼ 600 km/s) solar wind; and (2) a sharp coronal hole discontinuity separating the distinct sources of solar wind with intermediate speeds and temperatures from slow solar wind. This study establishes the connection between the characteristic variations of the solar wind speed, charge-state composition, and magnetic field orientation observed in situ near 5 AU with their sources in the two-part structure of coronal hole boundaries back at the Sun.