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After decades of observations of solar energetic particles (SEP) from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼ 30 • heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit (ICU) that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere.

Context. Following a multi-year minimum of solar activity, a solar energetic particle event on 2020 Nov. 29 was observed by multiple spacecraft covering a wide range of solar longitudes including ACE, the Solar Terrestrial Relations Observatory-A (STEREO-A), and the recently launched Parker Solar Probe (PSP) and Solar Orbiter (SOLO). Aims. Multi-point observations of a solar particle event, combined with remote-sensing imaging of flaring, shocks, and coronal mass ejections allows for a global picture of the event to be synthesized, and made available to the modeling community to test, constrain, and refine models of particle acceleration and transport according to such parameters as shock geometries and particle mass-to-charge ratios. Methods. Detailed measurements of heavy ion intensities, time dependence, fluences, and spectral slopes provided the required test data for this study. Results. The heavy ion abundances, timing, and spectral forms for this event fall well within the range found in prior surveys at 1 au. The spectra were well fitted by broken power law shapes; the Fe/O ratio was somewhat lower than the average of other events. In addition, 3 He/ 4 He was very low, with only the upper limits established here.

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Context. The Solar Orbiter spacecraft cruised in the inner heliosphere during Feb. 2020 -Jan. 2021, moving between~0.5-1.0 au radial distance. The Energetic Particle Detector (EPD) suite operated continuously during this period. Aims. The Suprathermal Ion Spectrograph (SIS) and High Energy Telescope (HET) observations made during intervals in between transient intensity increases were used to determine the low energy ion spectra and composition during quiet times. Methods. Energetic particle spectra and major ion components, including 3 He, were measured over the range~0.1->100 MeV/nucleon. The radial dependence of 4.4 MeV/nucleon 4 He and O was measured. A short interval of extremely low intensities ("super-quiet") was also studied. Results. Spectra measured during the quiet period showed transitions, including galactic cosmic rays (GCRs; > 50 MeV/nucleon), anomalous cosmic rays (ACRs; a few to~50 MeV/nucleon), and a steeply rising "turn-up" spectrum below a few MeV/nucleon whose composition resembled impulsive, 3 He-rich solar energetic particle (SEP) events. The radial dependence had large uncertainties but was consistent with a small gradient. During the super-quiet interval, the higher energy components remained similar to the quiet period, while the approximately flat low energy 4 He spectrum extended downward, reaching~300 keV/nucleon before transitioning to a steeply rising spectrum.