1.3 Main types and solar sources of the solar wind

As is well known, the solar wind is the continuous outflow of completely-ionised gas from the solar corona (Parker, 19581963Jump To The Next Citation Point). It consists of protons and electrons, with an admixture of a few percent alpha particles and much less abundant heavy ions in different ionization stages. The hot corona typically has electron and proton temperatures of 1 to 2 MK and expands radially outward into interplanetary space, with the flow becoming supersonic within a few solar radii. Because the solar wind plasma is highly electrically conductive, the solar magnetic field lines are dragged away by the flow, and due to solar rotation are wound into spirals. The wind attains a constant terminal speed, and its density then decreases radially in proportion to the square of the radial distance.

Space missions have revealed that there are three major types of solar wind flows: first, the steady fast wind which originates on open magnetic field lines in coronal holes; second, the unsteady slow wind coming from the tips and edges of temporarily open streamers or from opening loops and active regions; and third, the transient wind in the form of coronal mass ejections (CMEs) prevailing during solar maximum. Models for these types of wind have been developed to different levels of sophistication. Subsequently, we discuss the empirical constraints (Marsch, 1999Jump To The Next Citation Point) imposed on the models mainly by Helios (in-ecliptic) and Ulysses (high-latitude) interplanetary in situ measurements, and by the solar remote-sensing observations of the corona made by SOHO.

These observations indicate that the fast solar wind seems to emanate in the polar coronal holes directly from the chromospheric magnetic network (boundaries) (Hassler et al., 1999Wilhelm et al., 2000Xia et al., 2004Wiegelmann et al., 2005Aiouaz et al., 2005), with outward initial speeds of up to 10 km s–1. The open coronal magnetic field (of about 10 G) is anchored in the supergranular network, which occupies merely 10% of the coronal base area. The strong network field (with an average of about 10 – 100 G) is rooted in the photosphere in small, kG-field flux tubes (about 100 km in size). The field in the shape of coronal funnels rapidly expands with height in the transition region and ultimately fills the entire overlying corona. That the solar wind originates in these coronal funnels was recently found by Tu et al. (2005), who identified, by means of correlations between Doppler shifts and the coronal magnetic field as obtained by extrapolation from photospheric magnetogrammes, the source regions of the plasma outflow. The origin of the slow solar wind remains less clear (Schwadron and McComas, 2003), but most likely involves magnetic reconnection, which may lead to transient openings of coronal loops feeding plasma to the slow wind.

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