Polar coronal holes during the past solar cycle: Ulysses observations

Abstract

During its nearly 19-year mission, Ulysses pioneered novel measurements of the three-dimensional heliosphere and particularly in situ observations of high-latitude solar wind from polar coronal holes (PCHs). Winds from PCHs exhibit constant elemental abundances to within the limits of the measurements, indicative of the fact that such winds truly provide a ground state of solar wind composition. However, these solar wind streams show long-term variability in the composition of ionic charge states frozen into the low corona. The C and O freeze-in temperatures measured in high-latitude solar wind have decreased ∼10% as compared to the previous solar minimum and are now around 0.87 and 1.01 MK, respectively. The ionization states of Si and Fe also exhibit a substantial cooling with a reduction of 0.4 and 0.5 charge states, respectively. We show that these observations are indicative of an overall decrease of coronal temperature, forming a trend toward cooler PCH temperature persisting for over 14 years. We support these observations with a detailed and comprehensive description of the data analysis processes relevant for Ulysses SWICS and similar instruments. Copyright 2011 by the American Geophysical Union.