Abstract
<title>Abstract</title> <p>The period during early September 2017 showed extraordinary solar activity with several strong solar energetic particle (SEP) events including the ground level event (GLE) beginning September 10th and lasting nearly a week. During this time the Relativistic Electron Proton Telescope (REPT) onboard NASA’s twin-spacecraft Van Allen Probes mission observed energetic helium ions penetrating into the Earth’s magnetospheric polar regions. The Earth’s magnetic field usually shields energetic particles; they can still penetrate provided their “cutoff rigidities” are larger than the local vertical cutoff. These so-called cutoffs are determined by particle rigidities, i.e., momentum per unit charge. The Pulse Height Analyzed (PHA) data from REPT enable identifying helium ions as well as measure their spectra. These PHA data were used to characterize helium ions penetrating into the Earth’s magnetosphere including the temporal evolution of their spectra during first weeks of September 2017. Helium is the second most ubiquitous element comprising SEPs and is critically important from a space weather particle radiation perspective. The REPT observations showed contemporaneous helium injection at high latitudes with the SEP event. The injected helium spectra gradually softened during the course of the event. These observations suggest that most likely no dynamic processes within the magnetosphere further energized the injected helium which gradually escaped the magnetosphere.</p>