ON THE ORIGIN OF HIGHLY ALFVENIC SLOW SOLAR WIND

Alfvenic fluctuations are a common feature in the solar wind and are found especially in the trailing edges of fast wind streams. The slow wind usually has a lower degree of Alfvenicity, being more strongly intermixed with structures of non-Alfvenic nature. In the present paper we show the first evidence in the interplanetary space of two different kinds of slow solar wind: one coming from coronal streamers or active regions and characterized by non-Alfvenic structures and the other one being highly Alfvenic and originating from the boundary of coronal holes. The Alfvenic character of fluctuations, either outward or inward, can be studied by means of the normalized cross-helicity, sigma(C), which is an indicator of the v-b alignment. The evolution of sigma(C) toward lower values with increasing radial distance is interpreted both as a decrease of the presence of the outward modes and as a continuous production of inward modes within those regions such as stream shears where some plasma instability is active. On the other hand, the decrease of sigma(C) is often related also to magnetic field and/or density enhancements which specifically act on the destruction of the v-b alignment. In the present analysis we study the role of compressibility presenting both case studies and a statistical analysis over different phases of solar cycle 23. Our findings indicate that the presence of regions of magnetic field compression generally play a major role in the depletion of sigma(C) and thus in the destruction of the v-b alignment.