Investigation on Upstream Ion Events from the L1 Point Observation: New Insights

Abstract There has been long-standing controversy regarding the origin and transport of short-lived energetic ion events observed upstream of the Earth’s bow shock. In this work, by adopting a unique method, we study the characteristics of 152 upstream ion events observed by the Suprathermal Energetic Particles (STEP) instrument on board the Wind satellite during 2012–2017. These events are selected manually based on rise and fall of H and 4 He intensities. During nearly 80% of the selected upstream events, the local interplanetary magnetic field appears to be nearly radial. Using the multidirectional measurement capability of the STEP, we compare the 4 He/H and CNO/H ratios as well as the spectral indices of H, 4 He, and CNO in the Earth-facing sectors with those seen in the Sun-facing sectors. This exercise is repeated by applying 10–120 minute lags on the ion intensities measured in the Sun-facing sectors of the instrument. It is found that the composition ratios are well correlated, thus confirming that the origin of the ions seen in the opposite-facing directions is the same. Surprisingly, we find that the spectra measured in the Earth-facing sectors are generally softer during majority of the events as compared to those measured in the Sun-facing sectors, with the mean softening in H, 4 He, and CNO spectra in the solar wind frame varying between ∼0.877 and 1.0, ∼1.068–1.116, and ∼1.4–1.939, respectively. We propose that scattering due to processes like cross-field diffusion could be responsible for producing the harder spectra in the Sun-facing sectors.