STEREO observations of the energetic ions in tilted corotating interaction regions

Abstract

In this paper we examine suprathermal He ions measured by the SIT (Suprathermal Ion Telescope) instrument associated with tilted corotating interaction regions (CIRs). We use observations of the two STEREO spacecraft (s/c) for the first 2.7 years of the mission, along with ground-based measurements of the solar magnetic field during the unusually long minimum of Solar Cycle 23. Due to the unique configuration of the STEREO s/c orbits we are able to investigate spatial variations in the intensity of the corotating ions on time scales of less than one solar rotation. The observations reveal that the occurrence of the strong CIR events was the most frequent at the beginning of the period. The inclination of the heliospheric current sheet relative to the heliographic equator (the tilt angle) was quite high in the first stage of the mission and gradually flattened with the time, followed by a decrease in the CIR activity. By examining the differences between measurements on the two STEREO s/c we discuss how the changes in the position of the s/c relative to the CIRs affect the energetic particle observations. We combine STEREO observations with observations from the ULEIS instrument on the ACE s/c and argue that the main factor which controls the differences in the ion intensities is the latitudinal separation between the two STEREO s/c relative to the tilted CIRs. The position of the s/c is less important when the tilt angle is high. In this case we found that the CIR ion intensity positively correlates with the tilt angle.