The Very Unusual Interplanetary Coronal Mass Ejection of 2012 July 23: A Blast Wave Mediated by Solar Energetic Particles

Abstract

The giant, superfast, interplanetary coronal mass ejection, detected by STEREO A on 2012 July 23, well away from Earth, appears to have reached 1 AU with an unusual set of leading bow waves resembling in some ways a subsonic interaction, possibly due to the high pressures present in the very energetic particles produced in this event. Eventually, a front of record high-speed flow reached STEREO. The unusual behavior of this event is illustrated using the magnetic field, plasma, and energetic ion observations obtained by STEREO. Had the Earth been at the location of STEREO, the large southward-oriented magnetic field component in the event, combined with its high speed, would have produced a record storm.

Solar energetic particle anisotropies and insights into particle transport

“…However, shortly before the arrival of the extreme shock (from ∼19:40-20:15), she finds several small flux ropes with some high frequency magnetic field fluctuations, which is rare. If these high frequency fluctuations had been present ∼13-16 hours earlier when we observed the beam-width oscillations, and if they traveled at the ambient solar wind speed (∼800 km/s at 20:00 [2,4]), they would have been located ∼0.27 AU sunward of the spacecraft at 06:00, and 10 MeV protons would have passed through them only ∼15 minutes before we observed them. More work is needed to determine if the interaction of the particles with these field fluctuations might have produced the unusual anisotropy behavior.…”

“…The active region source of this event was at ∼W133 • , over the limb from Earth but near central meridian from the viewpoint of Ahead, and the event was associated with an x-ray flare of estimated magnitude between M8.2 and X2.5 [5]. The shock or blast wave that hit STEREO-Ahead had a peak magnetic field strength of over 100 nT and solar wind speed exceeding 2000 km/s [2], and was followed by two interplanetary coronal mass ejections (ICMEs) [4]. Only a minor shock (with peak field strength of ∼10 nT) was detected at STEREO-Behind, 124 • of heliolongitude westward from Ahead; it is not clear if this was the flank of the tremendous shock seen at Ahead or associated instead with one of several other CMEs during this period [4].…”

“…The largest solar energetic particle (SEP) event detected so far in solar cycle 24 was observed at the Solar TErrestrial RElations Observatory (STEREO) [1] on 23 July 2012 [2,3,4] when the Ahead spacecraft was 121 • of heliolongitude westward from Earth. The active region source of this event was at ∼W133 • , over the limb from Earth but near central meridian from the viewpoint of Ahead, and the event was associated with an x-ray flare of estimated magnitude between M8.2 and X2.5 [5].…”

25 MeV solar proton events in Cycle 24 and previous cycles

“…activity and the increasing southern activity. Other intervals when sun spots and SEP events from a particular hemisphere are dominant are evident, including a brief interval of enhanced southern activity in 2012 that follows the northern hemisphere activity peak and included the largest SEP event observed (at STEREO A) so far in this cycle (Russell et al, 2013). In addition, there are quasi-periodic ∼ 6-7 month (i.e., data point) variations in the SEP rates in each hemisphere that are also evident in the respective sunspot number and areas and are present during much of the rise and peak phases of Cycle 24, as discussed in more detail by Richardson et al (2016).…”

Sheath-accumulating Propagation of Interplanetary Coronal Mass Ejection

“…The estimated peak > E 10 MeV proton flux measured in situ by STEREO-A was about6.5 10 4 pfu when the interplanetary shock passed the spacecraft (Russell et al 2013;Gopalswamy et al 2014). Liu et al (2014) carried out a comprehensive study of the ICME characteristics using stereoscopic observations and in situ measurements.…”