Coronal Sources and In Situ Properties of the Solar Winds Sampled by ACE During 1999 – 2008
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We identify the coronal sources of the solar winds sampled by the ACE spacecraft during 1999 – 2008 and examine the in situ solar wind properties as a function of wind sources. The standard two-step mapping technique is adopted to establish the photospheric footpoints of the magnetic flux tubes along which the ACE winds flow. The footpoints are then placed in the context of EIT 284 Å images and photospheric magnetograms, allowing us to categorize the sources into four groups: coronal holes (CHs), active regions (ARs), the quiet Sun (QS), and “undefined”. This practice also enables us to establish the response to solar activity of the fractions occupied by each type of solar wind, and of their speeds and O7+/O6+ ratios measured in situ. We find that during the maximum phase, the majority of ACE winds originate from ARs. During the declining phase, CHs and ARs are equally important contributors to the ACE solar winds. The QS contribution increases with decreasing solar activity and maximizes in the minimum phase when the QS appears to be the primary supplier of the ACE winds. With decreasing activity, the winds from all sources tend to become cooler, as represented by the increasingly low O7+/O6+ ratios. On the other hand, during each activity phase, the AR winds tend to be the slowest and are associated with the highest O7+/O6+ ratios, while the CH winds correspond to the other extreme, with the QS winds lying in between. Applying the same analysis method to the slow winds alone, here defined as the winds with speeds lower than 500 km s−1, we find basically the same overall behavior, as far as the contributions of individual groups of sources are concerned. This statistical study indicates that QS regions are an important source of the solar wind during the minimum phase.
KeywordsSolar Wind, sources Solar wind, properties Solar cycle
We would like to thank the anonymous referee for helpful comments. We thank the ACE SWICS, SWEPAM, and MAG instrument teams and the ACE Science Center for providing the ACE data. SOHO is a project of international cooperation between ESA and NASA. Wilcox Solar Observatory data used in this study were obtained via the web site http://wso.stanford.edu courtesy of J.T. Hoeksema. The Wilcox Solar Observatory is currently supported by NASA. This research is supported by the China 973 program 2012CB825601, the National Natural Science Foundation of China (41174154, 41274176, and 41274178), and the Ministry of Education of China (20110131110058 and NCET-11-0305). BL is also grateful to the International Space Science Institute (ISSI) for providing the financial support to the international team on the origins of the slow solar wind.
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