The Rising Phase of Solar Cycle 24: General Solar Wind, Large-Scale Solar Wind Structures, and Sector Asymmetry

The solar polar field is presently nearing its reversal, suggesting the approach of solar maximum. However, the sunspot number, solar wind dynamic pressure, and IMF are still weak, similar to conditions in 1998 (the middle of last rising phase), suggesting a peculiar rising phase for Solar Cycle 24. In this presentation, following the study of solar minimum 23/24 in Jian et al. (2011), we first report the variations of solar wind parameters from the beginning of space era to present and compare the current rising phase with that of previous cycles. Secondly, based on our long-term study of large-scale solar wind structures, including interplanetary CMEs (ICMEs), stream interaction regions (SIRs), and their associated shocks at 1 AU from 1995 to present, we compare their properties in this rising phase with those of Cycle 23 and study their possible influence on geomagnetic activity. Thirdly, dividing the solar wind into positive (anti-sunward) and negative (sunward) sectors depending on the IMF polarity, we compare the solar wind parameters of the two polarity sectors from the beginning of Solar Cycle 21 to present. We note that an asymmetry between the two sectors exists for past cycles as reported by Hiltula and Mursula (2007) as well as Erdos and Balogh (2010). The sector asymmetry is more pronounced during the last solar minimum 23/24. Positive polarity solar wind is observed more often at 1 AU than negative polarity in Cycles 21 and 23 and less often in Cycle 22 and likely in this Cycle 24. Using the PFSS and MHD models, we can calculate the polarity distributions closer to the Sun, and they do not always agree with the observations. We closely examine several representative Carrington rotations to find out the reason. From 1-AU observations, the solar wind from the negative polarity sector (currently from northern hemisphere of the Sun) is found to be faster, hotter, and have a smaller proton density than the wind from the positive sector since 2009. This can affect the geomagnetic activity systematically. Comparing with the solar and coronal observations, we look for interpretations of the asymmetry.