The solar wind in the heliosphere has two distinct components: the fast with speed > 450 km/s and the slow, with velocity < 450 km/s. Coronal holes, where the magnetic field lines open out into the heliosphere, are the sources of fast wind, while the slow wind is thought to be emanating from closed field regions. However, the source of slow wind is still not clearly understood and is now an interesting topic.

Wang (1994): identified two sources of slow solar wind, the boundaries of large polar coronal holes and small coronal holes. His analysis was based on his theory on flux divergence and solar wind speed, accoring to which, these two sources of slow solar wind have rapidly diverging magnetic fields. The former is responsible for slow solar wind found near the heliospheric current sheet and the latter gives rise to slow solar wind near solar maximum.

Wang et al. (1998): identified the closed coronal loops or the helmet streamer, and the rapidly diverging open flux tubes rooted inside the coronal holes as the two sources of slow solar wind.

Neugebauer et al. (1998): identified intermediate and slow solar wind originatiing from small coronal holes at low latitudes.

Lundstedt (1989): carried out a study of the coronal sources of slow solar wind by mapping the solar wind observed at 1 AU back to the source surface. He used 3-hr averages of solar wind speed and selected those events whose velocity remained below 350 km/s for a minimum of 4 days. In that study, very low hourly values of temperature, ~5 × 10³ K, and density, ~5.2 ions/cm^-1 were found while solar wind speed was moderately low, ~ 311 km/s. He grouped the source locations as 'along the neutral line','crossing the neutral line', and 'inside a warp'. He identified 25 events of which 4 were 'along the neutral line', another 4 were 'inside a warp' and the remaining 17 were found to be 'crossing the neutral line'. He concluded by noting that the slow wind found 'inside a warp' was quite unexpected since slow wind was expected to originate from a region of closed magnetic field.

Ohmi et al. (2004): used the solar wind synoptic maps on the source surface deduced from Interplanetary Scintillation (IPS) techniques to infer the source of slow solar wind. They found the slow solar wind to be originating from an equatorial coronal hole near active regions and from a polar coronal hole that was about to disappear at solar maximum (see also Ohmi, 2003). The slow wind from the equatorial coronal hole had properties similar to that of fast wind from a large coronal hole, i.e., they had uniform magnetic polarity, the ratio of alpha paricles to proton was as large as fast wind, whereas, variances of density, velocity and helium abundances were as small as fast wind from a large coronal hole. On the other hand, properties like density and the ion freez-in temperature were as large as the slow solar wind from the heliospheric plasma sheet.

Poduval and Zhao (2004b):

Mapping of open field regions using PFSS model during solar maximum: CR1961. Top panel: coronal open (colored dotted areas) and closed (areas consisting of blue-red field lines) field regions below 1.25 R_sun. Middle panel: radial extension of the boundaries of open field regions to the source surface at 2.5 R_sun. Bottom panel: two kinds of boundary layers between open field regions at the source surface; bipolar (coincident with the black neutral line) and unipolar boundary layers. The solar wind observed near the Earth during the same period in different speed range as described in the text are superposed in each panel.

The figure on the left is for CR 1961 and the one on the right is for CR 1965.