SOLAR-WIND AND CHROMOSPHERIC NETWORK

A physical model of the transition region, including upflow of the pla sma in magnetic field funnels that art open to the overlying corona. i s presented. A numerical study of the effects of Alfven waves on the h eating and acceleration of the nascent solar wind originating in the c hromospheric network is carried out within the framework of a two-flui d model for the plasma. It is shown that waves with reasonable amplitu des can, through their pressure gradient together with the thermal pre ssure gradient, cause a substantial initial acceleration of the wind ( on scales of a few Mm) to locally supersonic flows in the rapidly expa nding magnetic field 'trunks' of the transition region network. The co ncurrent proton heating is due to the energy supplied by cyclotron dam ping of the high-frequency Alfven waves, which are assumed to be creat ed through small-scale magnetic activity. The wave energy Aux of the m odel is given as a condition at the upper chromosphere boundary, locat ed above the thin layer where the first ionization of hydrogen takes p lace. Among the new numerical results are the following: Alfven waves with an assumed f(-1) power spectrum in the frequency range from 1 to 10(4) Hz, and with an integrated mean amplitude ranging between 25 and 75 km s(-1), can produce very fast acceleration and also heating thro ugh wave dissipation. This can heat the lower corona to a temperature of 5 x 10(5) K at a height of h = 12 000 hn, starting from 5 x 10(5) K at h = 3000 km. The resulting thermal and wave pressure gradients can accelerate the wind to speeds of up to 150 km s(-1) at h = 12 000 km, starting from 20 hn s(-1) at IL = 3000 km in a rapidly diverging flux tube. Thus the nascent solar wind becomes supersonic at heights well below the classical Parker-type sonic point. This is a consequence of the fact that any large wave-energy flux, if it is to be conducted thr ough the expanding funnel to the corona, implies the building-up of an associated wave-pressure gradient. Because of the diverging field geo metry, this might lead to a strong initial acceleration of the flow. T here is a multiplicity of solutions, depending mainly on the coronal p ressure. Here we discuss two new (as compared with a static transition region model) possibilities, namely that either the flow remains supe rsonic or slows down abruptly by shock formation, which then yields su bstantial coronal heating up to the canonical 10(6) K for the proton t emperature.