DSA activity and efficiency at the PSP S1 quasi-perpendicular shock

Determine whether diffusive shock acceleration was fully active or efficient at the quasi-perpendicular interplanetary shock observed by Parker Solar Probe on September 5, 2022 (S1), given that the measured electron energy spectrum did not extend beyond a Lorentz factor of approximately γ ≈ 2.

Background

The paper analyzes synchrotron radiation and in situ electron distributions for two near-Sun interplanetary shocks observed by Parker Solar Probe, labeled S1 (September 5, 2022) and S2 (March 13, 2023). Diffusive shock acceleration (DSA) predicts electron energy spectral indices that depend on the gas compression ratio; for S1 (quasi-perpendicular, r_gas ≈ 2.5), DSA would suggest δ ≈ 3, but effective participation in DSA generally requires electrons with Lorentz factor γ ≳ 2.

For S1, the observed electron power-law did not extend beyond γ ∼ 2, and the electron distribution showed strong anisotropy, both of which cast doubt on the efficiency of DSA at that shock. The authors explicitly state uncertainty regarding whether DSA was fully active or efficient under those conditions, motivating a focused determination of DSA’s role in electron acceleration at S1.