J.G. Luhmann (SSL U of California, Berkeley)

1. The July 2012 STEREO-A Extreme Event SEPs: Solar and Heliospheric Context, 1 AU Observations and a Model J.G. Luhmann (SSL U of California, Berkeley) C.T. Russell (IGPP UCLA), Ying Liu (Chinese Acad. Sciences) D. Odstrcil (GMU, GSFC), M.L. Mays (CUA, GSFC), H. Bain, Y. Li (SSL, University of California) and the STEREO Teams LWS ESWE Meeting 2014, Boulder

2. We consider the context of the July 2012 STA extreme SEP event period. This included a very fast backsided halo CME for Earth that was a frontside halo at STA STEREO Science Center Locator plot (center); SECCHI COR and LASCO C2 images

3. SECCHI images On STEREO-A July 23, 2012 CME speed Near Sun ~3500 km/s ICME speed at STEREO-A ~2000 km/s or greater Major Solar Energetic Particle Events Large interplanetary fields (>100 nT) Described in Liu et al. Nature Comm., 2014

4. The July 23, 2012 event produced SEPs seen at widespread locations STEREO-B STEREO-A MEX backgnd VEX backgnd MESSENGER (above) and ACE and GOES (below)

5. July 23, 2013 Event: Energetic Proton Flux, Magnetic Field and Speed • On July 23, 2013, a strong burst of solar activity began shortly after 0200 UT. • Four times of interest are marked • At about 1630 UT, the speed jumped; the magnetic field increased in several steps. This field behavior was not shock-like. • At 1920 UT, the magnetic field slowly began to drop to very low values while the energetic particle flux rose sharply. This has the characteristics of a slow-mode wave. • The solar wind slowly increased in speed until 2100 UT, when it jumped to about 2400 km/s. The 0.4-0.6 MeV protons and the 4-6 MeV protons increased in flux but the 40-60 MeV protons dropped. • The magnetic cloud was entered at 2300 UT and the energetic proton fluxes all dropped. NOTE: This event was ~1000x more intense than the average SEP event From Russell et al., ApJL, 2013

6. The July 23, 2012 energetic proton fluence at STA was also comparable to that of the largest events of cycle 23. It certainly would have caused a Ground-Level Enhancement (GLE). A fit to July 23rd fluence is remarkably similar to fits to 3 of the 4 largest events of cycle 23. (Mewaldt et al.; Fall AGU 2013 SH33B-2068)

7. Had the July 23, 2012 Proton Event occurred at Earth, it would place 3rd on NOAA’s list of major Solar Proton Events since 1976 08/2328 Note that 8 of the events are from Cycle 23. So far only two Cycle-24 events rank in the top 20. (Mewaldt et al.; Fall AGU 2013 SH33B-2068)

8. SECCHI images On STEREO-A July 23, 2012 CME speed Near Sun ~3500 km/s ICME speed at STEREO-A ~1400 km/s or greater (TBD) Major Solar Energetic Particle Events Large interplanetary fields (>100 nT) and large long- duration south- ward component This was an example of ‘extreme events’ that seem to occur ~once a cycle Somehow conditions conspire to produce these rare exceptional outbursts-but how?

9. The related flare site: AR1520 activity summary (from Hyesook Lee et al., CCMC, personal comm.) This event was not alone!

10. Was it a single or multiple event? The SEEDS search algorithm identifies a second CME in essentially the same sector in STA COR2 difference images that took off a few hours after the fastest CME.

11. Was it a single or multiple event? The SEEDS CME catalog automated search also finds 2 events in LASCO C2 images, supporting the idea of co-located eruptions ~hours apart

12. Cause of minimal slowdown ? LASCO • A series of preceding eruptions occurred on the Sun including a July 19 CME (from the same active region); • The July 23 event was moving through a density depletion region (as low as 1 cm^-3) with radial magnetic fields, created by the earlier eruption..

13. The Images also allowed triangulation and event tracking of such a major event for the first time. STEREO-A STEREO-A SOHO-LASCO Images from the GMU SEEDS site STEREO-B

14. An ENLIL model was produced that captured some of the background behavior in the plasma and field, including the presence of multiple CME shocks

15. ENLIL model results support the suggested influences on the July 23, 2012 event Multiple eruptions are introduced Into the model solar wind several days ahead (Model: D. Odstrcil, GMU)

16. The earlier events merged to form a wide front followed by rarefaction

17. The July 23 event(s) thus traveled outward relatively unimpeded

18. Here time series of the observer-connected shock locations for several sites in ENLIL’s domain are shown. Connected shock strength is indicated by color-fill.

19. This CISM SEP event model (JASR 2010) uses these ENLIL results : -assumes the ENLIL shocks are moving SEP sources -assumes field-aligned transport only, here w/o scattering except close to the shock source -uses guiding center particle tracing on ‘snapshots’ of ENLIL observer-connected field lines-sampled every ~5 minutes

20. STEREO-A SEP Event observations from the IMPACT LET. SEPT and HET instruments (top) and STEREO-A ICME shock connection locations along field lines for 6 CMEs In the ENLIL run (bottom), distinguished by color..

21. In this SEP event model the SEP onsets are determined by the shock connection times and shock strengths. Thus its performance depends on ENLIL’s capturing those accurately.

22. The event character and duration is also significantly affected by the ENLIL field and shock source evolution, Both mirroring in the helio- spheric field and post-shock passage shock connections matter.

23. Magnetic mirroring in the model occurs due to field compressions like these along the observer connected field lines

24. Each of the CME/ICME shocks in ENLIL creates a distinct event. These merge together to produce the final, extreme event

25. The results of the complete event model for STEREO A (bottom), compared with STEREO IMPACT SEP data from LET, SEPT and HET instruments (top).

26. Lessons learned from the July 2012 Observations:: -The ability of a CME to produce an extreme SEP event depends on context (solar, solar wind), including timing (previous and following activity) and proximate (same or nearby active region) and also global activity. -Widespread/Extreme SEP events like this one are likely often affected by multiple CME-created shocks and compressions, by field geometries that allow mirroring/trapping , as well as by SEPs heading back toward the Sun from a still-strong shock source that has already passed. -Looking at SEPs in isolation is probably not going to reveal the physics! One needs to consider solar conditions (even farside) and heliosphere conditions (from corona to well beyond 1 AU) -

27. Lessons learned from the July 2012 model tests: -The ability to model a SEP event at any spacecraft location requires modeling shock source injections and field line connectivity and evolution in sufficient detail to capture all sources and magnetic mirrors affecting the site. A deficient heliospheric background description will prevent the accurate interpretation of the SEP events generation. notes: >> ENLIL does not include the domain inside 20Rs, precluding coronal shock portion inclusion) >> ENLIL’s outer boundary is set at 5.5 AU, and the run at ~20 days duration in order to include the full heliospheric history involved -Widespread/Extreme SEP events like this one are likely often affected by multiple CME-created shocks and compressions, by field geometries that allow mirroring/trapping , as well as by SEPs heading back toward the Sun from a still-strong shock source that has already passed. -