Abstract
NASA’s Solar Terrestrial Relations Observatory (STEREO) mission has coincided with a pronounced solar minimum. This allowed for easier detection of corotating interaction regions (CIRs). CIRs are formed by the interaction between fast and slow solar-wind streams ejected from source regions on the solar surface that rotate with the Sun. High-density plasma blobs that have become entrained at the stream interface can be tracked out to large elongations in data from the Heliospheric Imager (HI) instruments onboard STEREO. These blobs act as tracers of the CIR itself such that their HI signatures can be used to estimate CIR source location and radial speed. We estimate the kinematic properties of solar-wind transients associated with 40 CIRs detected by the HI instrument onboard the STEREO-A spacecraft between 2007 and 2010. We identify in-situ signatures of these transients at L1 using the Advanced Composition Explorer (ACE) and compare the in-situ parameters with the HI results. We note that solar-wind transients associated with CIRs appear to travel at or close to the slow solar-wind speed preceding the event as measured in situ. We also highlight limitations in the commonly used analysis techniques of solar-wind transients by considering variability in the solar wind.
Highlights
Interaction regions are formed in the solar wind by the interaction between fast and slow solar-wind streams from the Sun
corotating interaction regions (CIRs) are formed by the interaction between fast and slow solarwind streams ejected from source regions on the solar surface that rotate with the Sun
In a similar superposed-epoch analysis conducted by Davis et al (2012) it appears that the denser material that should correspond to that observed by Solar Terrestrial Relations Observatory (STEREO)/Heliospheric Imager (HI) is not quite associated with the slow solar wind, but rather with material travelling at a speed slightly higher than that
Summary
Interface as the faster and slow material originate from different regions of the Sun, so they are threaded by different magnetic-field lines and cannot mix or flow past one another This interaction region, which has the overall structure of an Archimedean spiral, is called a corotating interaction region (CIR: Gosling and Pizzo, 1999). The faster solar wind that has not yet collided with the slower solar wind forms a less dense region following the density enhancement as the feature corotates with the Sun. It is possible that the stream interfaces can form into shocks further out in the heliosphere (Smith and Wolfe, 1976). We shall make some comparisons with Davis et al (2012) who performed an analysis on a larger number of CIR-associated solar-wind transients
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