CAN PLASMA EXPANSION EXPLAIN THE OBSERVED ACCELERATION OF Ne 7+ IONS IN A CORONAL MAGNETIC FUNNEL?

Abstract

Tu et al. measured outflow of Ne7+ at a height of 20 Mm above the photosphere demonstrating that the solar wind originates in coronal funnels footed in the chromospheric magnetic networks. We suggest that when the bottom of a coronal flux tube is populated by the chromospheric plasma consisting of protons as the major ions, Ne7+ as minor ions, and heated electrons in the magnetic networks, large-scale plasma expansion could accelerate the Ne7+ ions to a velocity ~10 km s–1 at a height ~ 20 Mm as measured. Two scenarios are discussed here, one with the bulk heating of the electrons to a temperature T e > 64 eV and another with a small fraction of the electrons heated to high temperatures T eh > 158 eV, appearing as an energetic tail to the electrons' velocity distribution function. In the former scenario, the expansion produces weak ambipolar electric fields distributed along the entire length of the funnel. In the latter one the electric field is concentrated in a double layer. The electric fields accelerate the ions against the solar gravity. The required electron temperatures in the above scenarios are large enough to ionize neon atoms yielding Ne7+.