Dynamic behavior of almost monoenergetic ions near Earth’s bow shock

Context. Almost monoenergetic ions (AMIs) are a class of energetic ions characterized by narrow spectral peaks and are occasionally observed near Earth’s bow shock and interplanetary shocks. The acceleration mechanisms of AMIs are still unclear.Aims. We used high-resolution measurements from the Energetic Particle Detector (EPD) suite on board Solar Orbiter to investigate the dynamic behavior of AMIs observed near Earth’s bow shock and explore their possible acceleration and formation processes.Methods. We combined data from two EPD sensors, the SupraThermal Electron Proton (STEP) sensor and the Electron-Proton Telescope (EPT), to study AMIs across a broad energy range. We examined their dynamic energy spectra, pitch-angle distributions, and spectral characteristics.Results. The observed AMIs propagate antiparallel to the interplanetary magnetic field (IMF) that points toward Earth’s bow shock. They exhibit either a single spectral peak at ∼20−30 keV or a double-peak feature consisting of a low-energy peak at ∼15−30 keV and a high-energy peak at ∼60−120 keV. These spectral peaks are relatively narrow, with width-to-peak ratios of ∼0.25−0.3, and their peak energies evolve dynamically on a timescale of ∼1−2 minutes. Notably, in the double-peak AMIs, the low-energy and high-energy peaks vary synchronously, maintaining a nearly constant energy ratio of ∼4. In addition, these AMIs do not appear to correlate with local IMF or solar wind conditions.Conclusions. These results suggest that the observed AMIs are likely protons and alpha particles accelerated at/near Earth’s bow shock via surfatron acceleration rather than by bursts of strong potential electric fields.