07/23/2012 23:00:00 UTC

This was the famous energetic, very fast event directed at STEREO-A. There were two consecutive prominence eruption/flares starting about 02:20 UT on July 23, seen best in SOHO and STEREO-B observations. The shock hit STEREO-A on July 23, 20:55 UT, followed by two ICMEs, the first starting about 23:00 UT and the second at 01:51 UT on July 24.

- N. Nitta says “It was not an Earth-affecting event, but it was said to have possibly been as geoeffective as the Carrington event had it occurred 9 days earlier [and been aimed at Earth-DW]. I think it is important to understand the interplanetary conditions as disturbed by AR 11520 over an extended period.”

- M. Temmer: “i am currently working together with Nariaki on the complex (two-step) eruption from July 23, 2012 event aiming to simulate the short arrival time and high impact speed by using the analytical drag-based-model. the low density in interplanetary space as well as the high mass of the CME might be the decisive factors for this event to be so fast. The question remains whether the event from July 19 is able to lower the density over several days and as such is able to change the interplanetary conditions. i had during the process of the analysis nice discussions with Ying Liu and Janet Luhmann. We would like to encourage people to take a closer look on this event. We had subjective interpretations of the white-light structures, but most important found no conclusion on the high magnetic field as measured in-situ for both magnetic structures. Is it maybe something intrinsic to the active region? … it would be good to get modelers involved for gaining some deeper insight into the complex eruption process and its in-situ effects.”

[Added by D. Webb]

- M. Temmer: A key element for the extreme character of the July 23, 2012 eruption is a high level of flare energy release over a long time range (max acceleration 2.2km/s^2, ~30 minutes of acceleration phase). Due to an efficient magnetic reconnection process the CME might reach a very high speed (max v=2580+/-280km/s derived from GCS 3D model) that is sustained by the prolonged energy release. The underlying mechanism which is able to build up the energy in the source region as well its conversion into such high kinetic energy is beyond the limit of information from observational data. We encourage modelers to provide further insight into the physics of source regions and reconnection processes related to such extreme events. The very high mass of the CME (1.5x10^16 g) as well as the reduced solar wind density in IP space (1-2/cm^3) due to A) the prior CME from July 19, 2012, B) general low density owing to the low solar activity, may have been the decisive factors for making this event super-fast. In addition, the largely radial orientation of the interplanetary magnetic field, due to its stretching by the CME from July 19, may have reduced the pile-up of solar wind and delayed its replenishment. [Temmer and Nitta, Solar Physics, under review]

REFERENCES:

• Russell, C. et al., ApJ, 770, 38, 2013
• Ngwira, C. et al., GRL, 2013
• Baker, D., Space Weather, 11, 585, 2013
• Liu, D.L. et al., Nature Comm., 5, 4381, 2014.
• Gopalswamy et al., E,P&S, 66, 104, 2014
• Temmer, M. and N. Nitta, Interplanetary Propagation behavior of the fast coronal mass ejection on 23 July 2012, Solar Phys., 290, 919, 2015. Doi: 10.1007/s11207-014-0642-3.
• Liou, K, C.-C. Wu, M. Dryer, S.-T. Wu, N. Rich, S. Plunkett, L. Simpson, C. D. Fry, and K. Schenk, Global Hybrid Simulation of Extremely Fast Coronal Mass Ejection on 23 July 2012, JASTP, 121, 32-41, 2014.
• Riley, P., R. M. Caplan, J. Giacalone, D. Lario, and Y. Liu, 2016, Properties of the Fast Forward Shock Driven by the 2012 July 23 Extreme Coronal Mass Ejection, Astrophys. J., 819, 57, doi: 10.3847/0004-637X/819/1/57.
• Zhu, B.; Liu, Y. D.; Luhmann, J. G.; et al., Solar Energetic Particle Event Associated with the 2012 July 23 Extreme Solar Storm, 2016, Astrophys. J., 827, 146 (study of the SEP event).
• Gopalswamy, N., S. Yashiro, N. Thakur, P. Mäkelä, H. Xie and S. Akiyama, 2016, The 2012 July 23 Backside Eruption: An Extreme Energetic Particle Event?, Astrophysical Journal, 833, 216, doi: 10.3847/1538-4357/833/2/216.
• Liu, Ying D.; Hu, Huidong; Zhu, Bei; Luhmann, Janet G.; Vourlidas, Angelos; Structure, Propagation, and Expansion of a CME-Driven Shock in the Heliosphere: A Revisit of the 2012 July 23 Extreme Storm, 2017, Astrophys. J., 834, 158 (study of the structure, propagation and expansion of the shock).