Shock Connectivity in the 2010 August and 2012 July Solar Energetic Particle Events Inferred From Observations and Enlil Modeling

Abstract

During periods of increased solar activity, coronal mass ejections (CMEs) can occur in close succession and proximity to one another. This can lead to the interaction and merger of CME ejecta as they propagate in the heliosphere. The particles accelerated in these shocks can result in complex solar energetic particle (SEP) events, as observing spacecraft form both remote and local shock connections. It can be challenging to understand these complex SEP events from in situ profiles alone. Multipoint observations of CMEs in the near-Sun environment, from the Solar Terrestrial Relations Observatory–Sun Earth Connection Coronal and Heliospheric Investigation and the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph, greatly improve our chances of identifying the origin of these accelerated particles. However, contextual information on conditions in the heliosphere, including the background solar wind conditions and shock structures, is essential for understanding SEP properties well enough to forecast their characteristics. Wang–Sheeley–Arge WSA-ENLIL + Cone modeling provides a tool to interpret major SEP event periods in the context of a realistic heliospheric model and to determine how much of what is observed in large SEP events depends on nonlocal magnetic connections to shock sources. We discuss observations of the SEP-rich periods of 2010 August and 2012 July in conjunction with ENLIL modeling. We find that much SEP activity can only be understood in the light of such models, and in particular from knowing about both remote and local shock source connections. These results must be folded into the investigations of the physics underlying the longitudinal extent of SEP events, and the source connection versus diffusion pictures of interpretations of SEP events.

The Solar Energetic Particle Event of 2010 August 14: Connectivity with the Solar Source Inferred from Multiple Spacecraft Observations and Modeling

“…Pre-event particle intensities were low at all three locations, and a week had elapsed since the previous SEP event observed on 2010 August 7 by STEREO-B and near-Earth spacecraft, but not by STEREO-A (Richardson et al 2014). The IP shock associated with this prior SEP event was observed in situ by STEREO-B and near-Earth spacecraft on 2010 August 11 (Bain et al 2016), indicating that transient solar wind structures were absent at the time of the onset of the 2010 August 14 SEP event (see Figure1 in Bain et al 2016).…”

“…The IP shock associated with the CME on 2010 August 14 was observed in situ by STEREO-A at ∼17:50 UT on 2010 August 17 (day of year 229) followed by a magnetic cloud (see Bain et al 2016 and references therein; this is shock S5 in their Figure1). Bain et al (2016) did not identify a corresponding shock at Earth, but a candidate weak shock-like feature passed L1 (specifically, the Wind spacecraft) at ∼23:25UT on 2010 August 17 (day 229).…”

“…Bain et al (2016) did not identify a corresponding shock at Earth, but a candidate weak shock-like feature passed L1 (specifically, the Wind spacecraft) at ∼23:25UT on 2010 August 17 (day 229). The passage of this IP shock by STEREO-A and by L1 was not accompanied by a local enhancement of the energetic particle intensities (Figures 1(c) and (d)).…”

“…Similarly, 4-6 MeV protons observed by STEREO-B/LET were highly anisotropic during the onset of the event but were isotropic throughout the event in similar observations at STEREO-A (cf. Figures7 and8 in Bain et al 2016).…”

Solar signatures and eruption mechanism of the August 14, 2010 coronal mass ejection (CME)

“…Despite its moderate strength, this proton event received a lot of attention as it was the first proton event affecting Earth that was recorded since December 2006 and the only one observed during the year 2010, according to NOAA. 1 Bain et al (2016) performed a detailed study on the SEP events that were observed in August 2010 and combined observations with various modeling techniques to understand the shock connectivity for and interaction between these events.…”