![]() More recent measurements made remotely using the Solar Ultraviolet Measurements of Emitted Radiation instrument on the Solar and Heliospheric Observatory satellite, however, showed that a similar cyclic variation is detectable in the Ne/O ratio in coronal streamers 10, a slow solar wind source candidate. The formation of the slow solar wind is not yet understood, so these observations alone cannot tell us whether this cyclic variation is due to properties of the wind, once fully heated and accelerated, or to cyclic properties detectable in the wind source regions. For example, the abundance ratio of the high-FIP elements Ne and O was enhanced by about 40% in the slowest solar wind streams (<400 km s −1) during the last solar minimum from 2007 to 2010 compared to during the previous solar maximum between 20 9. In situ measurements made by the Solar Wind Ion Composition Spectrometer instrument on the Advanced Composition Explorer observatory confirm that some composition measures vary with the 11-year solar activity cycle. Observations from the Wind spacecraft show that the He abundance in the slow wind is strongly correlated with sunspot number 8, and decreases at solar minimum in the fast wind 6. Recent observations have provided evidence of cyclic variability of elemental abundances in the solar wind 5, 6, 7. This composition enhancement process is known as the FIP effect 3, and it is frequently used as a powerful diagnostic in solar and stellar astrophysical studies 4. Furthermore, the composition appears to vary from atmospheric structure to structure, making it a potential signature of the coronal heating process. ![]() ![]() The mass supplied to the solar wind and the Sun’s outer atmosphere originates in the lower atmospheric layers, yet surprisingly, the elemental composition in the corona and slow solar wind is different than in the photosphere 1: low first ionization potential (FIP) elements that are easy to ionize in the chromosphere are preferentially enhanced (fractionated) by factors of 3–4 2, whereas high-FIP elements that remain neutral in the chromosphere retain their photospheric abundances. Knowledge of the elemental composition of the Sun underpins our understanding of the transport of energy from the deep interior, through the outer atmosphere, and into the heliosphere. ![]()
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