Science Applications of Solar Wind Observations off the Sun-Earth Line (STEREO)

This presentation provides an overview of the solar wind and suprathermal ion measurements that have been possible due to the STEREO off-line observations, including transient solar wind events, recurrent solar wind events, and suprathermal ion populations. When combined with near Earth (e.g. L1) observations, the two STEREO observatories are ideally suited for these longitudinal studies. The early mission included the declining phase of Cycle 23, while the recent observations cover the same phase for Cycle 24. These orbital and cycle phase circumstances present unique conditions for studying the persistence of solar wind parameters over various delta solar longitudes during solar minimum conditions. Prior solar wind persistence studies during solar minimum estimated about 2-3 days robustness (e.g., Opitz et al. 2009 and references therein), although this result depends upon the type of solar wind structure being studied. We look in particular at intervals during the declining phase of Cycle 23 (early mission) and in contrast Cycle 24 (current mission), when solar winds emanating from long-lived coronal-hole structures or from helmet streamers and pseudostreamers are observed both at STEREO and at near-Earth assets (OMNI2). The STEREO data set has not only science, but space weather applications, as the accuracy of L5 monitor 'forecasts' will depend on the persistence of solar wind structures. At 60-degree longitude prior to Sun-Earth Line, solar wind at L5 will be sampled ~ 4 days prior to recurring structure arrival at the Sun-Earth line (e.g., see Simunac et al. 2009 for STB and Jian et al. 2019 for STA and STB as a test bed for a L5 monitor).