Abstract
Context.Solar Orbiter strives to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been in operation, providing excellent data, for just over a year.Aims.EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV nuc−1for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. In addition, we present areas where EPD excels and provides opportunities for significant scientific progress in understanding how our Sun shapes the heliosphere.Methods.We used the solar particle events observed by Solar Orbiter on 21 July and between 10 and 11 December 2020 to discuss the capabilities, along with updates and open issues related to EPD on Solar Orbiter. We also give some words of caution and caveats related to the use of EPD-derived data.Results.During this first year of operations of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10 and 11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as3He, for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10 and 11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and we present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided.
Highlights
Understanding how the Sun accelerates particles to relativistic energies and how these propagate from their acceleration site to fill the heliosphere are among the key questions that Solar Orbiter has set out to answer (Müller et al 2020)
Energetic Particle Detector (EPD) consists of four individual sensors that are served by a common Instrument Control Unit (ICU)
The EPD instrument is described in more detail in Rodríguez-Pacheco et al (2020), where the energy ranges for each sensor and particle species are included, along with the sensors’ field of view; we refer to that
Summary
Understanding how the Sun accelerates particles to relativistic energies and how these propagate from their acceleration site to fill the heliosphere are among the key questions that Solar Orbiter has set out to answer (Müller et al 2020). The EPD instrument is described in more detail in Rodríguez-Pacheco et al (2020), where the energy ranges for each sensor and particle species are included, along with the sensors’ field of view; we refer to that. Pitch-angle information can only be derived with the invaluable data from the Solar Orbiter magnetometer (MAG, Horbury et al 2020) In this first-year overview paper, we provide updates on EPD as well as additional insights into its operation and data products, which are based on EPD’s first year of operation. The format chosen for this is to present a selection of observations from the time period between 28 February 2020 and 28 February 2021 to illustrate EPD data products and provide pertinent information that was not yet available when the instrument paper was written. Appendix A contains a list of all particle enhancements observed in the first year of EPD operations
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