Four Kinds of Solar Wind

It is customary to divide the solar wind seen during sunspot minimum in distinct fast and slow regimes. Most theoretical modelling has been concerned with the fast wind since the slow wind is evidently inherently transient, associated with the opening and closing of magnetic field lines associated with quiet coronal streamers. The fast wind, associated with polar coronal holes above ∼ 60° latitude, is usually considered to be driven a result of an exponential coronal heating function sufficient to produce the observed ∼ 750 km/sec at large distances. The mass flux is conserved along magnetic flux tubes but cannot be deduced since it is determined at the coronal base, however the free parameter in the heating function must be adjusted so that the density at the base is ∼ 10⁸/cm³ as observed. These constraints, together with a suitable magnetic field model, are sufficient to determine the flow, which exhibits very rapid acceleration. The slow wind is contained in a warped disc, sharply bounded at about ± 15° latitude. It has distinct physical characteristics and has its source in the corona below ∼ 60° latitude. The energy supply to both fast and slow winds and to the quiet corona is evidently the same in each case, namely pico-flare activity in the chromospheric network, which exhibits no significant variation in and out of corona; holes. It is noteworthy that the total energy supply to coronal holes and fast wind (∼ 7x10⁵ erg/cm³) is the same as required for heating the quiet corona and the slow wind. However the total energy, mass flux and magnetic flux contained in the slow wind region is comparatively small. Within the slow wind region is a third component of solar wind flow, namely `bubbles' of magnetic field, produced by reconnection at the tips of coronal streamers, and marking the position of the `equatorial' current sheet. The bubbles have physical characteristics (low temperatures and absence of heat conduction, low helium abundance), which identify them clearly. Within the fast wind region there is a fourth component of the solar wind, namely coronal plumes, which outline the magnetic field configuration and are associated with bright points in the EUV emission marking the chromospheric network. This must be a relatively minor component, which eventually becomes part of the fast wind within a few tens of solar radii. Since the magnetic flux observed above 15° latitude at large distances must all map into the source of the prevailing fast wind, this wind cannot have its origin in plumes and coronal bright points as has sometimes been proposed. Close to the sun, the plume plasma is slow-moving and relatively dense compared to the inter-plume fast wind and presumably relatively cool. It must however eventually accelerate and merge with the fast wind since there is no evidence for plumes at large distances. It is suggested that this is the result of Alfven waves refracting into the plume, accumulating in intensity in the effective wave-guide, and accelerating the plasma until it reached fast wind speeds. Kelvin-Helmholtz instability may also assist the process at distances where the Alfven speed has decreased sufficiently. In this scenario it is possible that the line-of-sight column density of plume plasma is of the same order as the more extensive inter-plume plasma, masking the presence of the latter in some EUV measurements, in particular with respect to its speed and temperature(s).