tailieunhanh - Sources of solar wind over the solar activity cycle

Fast solar wind has been recognized, about 40 years ago, to originate in polar coronal holes (CHs), that, since then, have been identified with sources of recurrent high speed wind streams. As of today, however, there is no general consensus about whether there are, within CHs, preferential locations where the solar wind is accelerated. Knowledge of slow wind sources is far from complete as well. Slow wind observed in situ can be traced back to its solar source by backward extrapolation of magnetic fields whose field lines are streamlines of the outflowing plasma. However, this technique often has not the necessary precision for an indisputable identification of the region where wind originates. As the Sun progresses through its activity cycle, different wind sources prevail and contribute to filling the heliosphere. Our present knowledge of different wind sources is here summarized. Also, a Section addresses the problem of wind acceleration in the low corona, as inferred from an analysis of UV data, and illustrates changes between fast and slow wind profiles and possible signatures of changes along the solar cycle. A brief reference to recent work about the deep roots of solar wind and their changes over different solar cycles concludes the review. | Journal of Advanced Research 2013 4 215-220 Cairo University Journal of Advanced Research REVIEW Sources of solar wind over the solar activity cycle Giannina Poletto INAF - Arcetri Astrophysical Observatory Largo Fermi 5 50125 Firenze Italy Received 4 April 2012 revised 31 July 2012 accepted 16 August 2012 Available online 26 September 2012 KEYWORDS Solar wind Coronal holes Active regions Streamers Doppler dimming Abstract Fast solar wind has been recognized about 40 years ago to originate in polar coronal holes CHs that since then have been identified with sources of recurrent high speed wind streams. As of today however there is no general consensus about whether there are within CHs preferential locations where the solar wind is accelerated. Knowledge of slow wind sources is far from complete as well. Slow wind observed in situ can be traced back to its solar source by backward extrapolation of magnetic fields whose field lines are streamlines of the outflowing plasma. However this technique often has not the necessary precision for an indisputable identification of the region where wind originates. As the Sun progresses through its activity cycle different wind sources prevail and contribute to filling the heliosphere. Our present knowledge of different wind sources is here summarized. Also a Section addresses the problem of wind acceleration in the low corona as inferred from an analysis of UV data and illustrates changes between fast and slow wind profiles and possible signatures of changes along the solar cycle. A brief reference to recent work about the deep roots of solar wind and their changes over different solar cycles concludes the review. 2012 Cairo University. Production and hosting by Elsevier . All rights reserved. Introduction The search for solar wind sources dates back to about 1950 -with early insights from the end of the previous century - and got further motivation from Parker s theoretical work 1 . The shape of the solar minimum corona .