Anomalous cosmic rays within the inner heliosphere: Observations of helium by the High Energy Telescope on board Solar Orbiter

Context. The radial gradients of cosmic rays are key parameters in studies of the transport of particles in space. Solar Orbiter, launched on 2020 February 10, approaches the Sun approximately every half-a-year, with its closest perihelion distance of 0.29 au at the end of 2022 during the nominal mission phase. The two double-ended high-energy telescopes (HET) on board Solar Orbiter measure energetic particles in the energy range between a few MeV/nuc and a few hundreds of MeV/nuc. These particles are dominated by anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) during solar quiet times. Aims. By obtaining the radial gradient of ACR helium in the inner heliosphere, we can advance our understanding of how the transport of cosmic rays is affected by the particle drift effect and the large-scale magnetic field. Methods. We analyzed helium observations at Solar Orbiter/HET between 11.1 and 49 MeV/nuc. Since our study is focused on quiet time measurements, we removed the periods of solar energetic particle (SEP) events. The intensities were averaged over the Carrington rotation period. The helium observations from the Electron Proton and Helium Instrument (EPHIN) on board SOHO were utilized as the baseline to correct the long-term variation caused by the solar modulations. Results. We present the first observation of ACR helium at Solar Orbiter/HET between February 2020 and July 2022 in the inner heliosphere before the sun became fully active. We derived a radial gradient of ACR helium of between 0.3 and 1 au. The averaged radial gradient between 11.1 and 49 MeV/nuc is about 22 ± 4%/au and the averaged value between 11.1 and 41.2 MeV/nuc was raised to 32 ± 8%/au after removing the GCR contribution, which was estimated using a GCR model. In addition, the temporal variation of radial gradients indicates that gradients tend to increase with the enhancement of the solar modulation and the increased tilt angle of the heliospheric current sheet.