The 2012 July 23 Backside Eruption: An Extreme Energetic Particle Event?

TL;DR

The 2012 July 23 backside CME produced an extremely intense solar energetic particle event with high-energy particles, hard spectra, and shock characteristics similar to ground level enhancement events, indicating an extreme space weather occurrence.

Contribution

This study provides a detailed analysis linking CME speed and SEP spectral hardness, confirming the event's extremity and its similarity to GLE events, which was not previously established.

Findings

The event had a >10 MeV proton flux peaking at ~5000 pfu.

Shock speed exceeded 2000 km/s, typical of GLE events.

Spectral hardness correlated with CME initial speed.

Abstract

The backside coronal mass ejection (CME) of 2012 July 23 had a short Sun to Earth shock transit time (18.5 hours). The associated solar energetic particle (SEP) event had a >10 MeV proton flux peaking at ~5000 pfu, and the energetic storm particle (ESP) event was an order of magnitude larger, making it the most intense event in the space era at these energies. By a detailed analysis of the CME, shock, and SEP characteristics, we find that the July 23 event is consistent with a high-energy SEP event (accelerating particles to GeV energies). The time of maximum and fluence spectra in the range 10-100 MeV were very hard, similar to those of ground level enhancement (GLE) events. We found a hierarchical relationship between the CME initial speeds and the fluence spectral indices: CMEs with low initial speeds had SEP events with the softest spectra, while those with highest initial speeds had SEP events with the hardest spectra. CMEs attaining intermediate speeds result in moderately hard spectra. The July 23 event was in the group of hard-spectrum events. During the July 23 event, the shock speed (>2000 km s^-1), the initial acceleration (~1.70 km s^-2), and the shock formation height (~1.5 solar radii) were all typical of GLE events. The associated type II burst had emission components from metric to kilometric wavelengths suggesting a strong shock. These observation confirm that the 2012 July 23 event is likely to be an extreme event in terms of the energetic particles it accelerated.