A step forward in understanding the role of shocks for solar energetic electrons

Large and fast Coronal Mass Ejections (CMEs) can drive shock fronts that are known to be efficient accelerators of protons even up to distances of the Earth. However, the role of shocks in accelerating solar energetic electrons is still not understood and highly debated. This topic is one of SERPENTINE’s specific science questions and to tackle it we analyzed in-situ measurements of solar energetic electrons observed by the STEREO spacecraft for 33 events that were accompanied by large and fast CMEs. The shocks driven by these CMEs were previously analyzed by Kouloumvakos et al. (2019), who provided 3D shock characteristics along the whole shock front allowing us to compare the in-situ measurements with the shock regions that were magnetically connected to the spacecraft. Kouloumvakos et al (2019) found already striking correlations between peak intensities of deka MeV protons, especially with the shock’s Alfvénic Mach number, which measures how fast the shock is with respect to small-amplitude waves in the ambient medium. The higher its Mach number, the more efficient the shock should, theoretically, be in accelerating particles.

Our analysis reveals that the peak intensities of high energy electrons at ~1MeV correlate similarly well with the shock parameters as the protons. However, for ~100 keV electrons, the correlation is significantly weaker (see Fig. 1). This suggests that the high-energy electrons are accelerated mainly by the shock, while the low energy electrons are likely produced by a mixture of flare and shock-related acceleration processes.

Fig. 1: Correlation of solar energetic electron peak intensities with the Alfvénic Mach number. Left: <100 keV electrons, right: ∼1 MeV electrons. The Pearson correlation coefficient cc is provided in each figure legend together with its uncertainty range corresponding to a 90% confidence interval (in square brackets) and the corresponding p-value (in round brackets). The p-value means the probability of the correlation to be found just by chance, when in reality the two variables are not actually correlated. On the left, the probability is about one in 700 and on the right panel it is a staggering one in 44 million.
Doing the swap test

We tested our results by swapping the values of STEREO A and B: When peak intensities at STEREO A are related with the Alfvénic Mach numbers at the magnetic footpoint of STEREO B and vice versa, the correlations vanish (see Fig. 2). This strongly supports the causal relationship of the accelerated electrons with the shock and shows that the location on the shock front, to which the spacecraft is connected, matters!

Fig. 2: Same as in Fig. 1 but peak intensities of STEREO A are correlated with the Alfvénic Mach number at the magnetic footoint of STEREO B. The p-values in both cases show high probabilities that there is no correlation in the data.


Dresing, N., Kouloumvakos, A., Vainio, R., & Rouillard, A., ApJL, 925, doi:10.3847/2041-8213/ac4ca7

Kouloumvakos, A., Rouillard, A. P., Wu, Y., et al. 2019, ApJ, 876, 80, doi:10.3847/1538-4357/ab15d7

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