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

Abstract

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

Figures (6)

• Figure 1: (a): The orbit trajectories of Solar Orbiter (2020.02.28 - 2022.08.31) in the Stonyhurst heliographic coordinate system with the Earth fixed at 1 au as the blue dot. The color scale of the twisted trajectories represents the order of time since its launch. The first 4 orbits and 34 Solar Orbiter-perspective Carrington rotations are included in this figure. The orange dashed line represents a circle of 0.95 au radial distance. (b): From top to bottom, the panels display the time variation of Solar Orbiter's radial distance, Carrington longitude and heliographic latitude, together with its relative distance ($d_\mathrm{r}$) and longitudinal ($d_\mathrm{lon}$) separation from Earth. Orange segments over the blue line in the top panel indicate periods when the radial distance of Solar Orbiter exceeded 0.95 au, and likewise for the grey and orange bars at the bottom of the figure.
• Figure 2: The quiet time helium spectra of Solar Orbiter/HET for stopping (filled blue diamonds) and penetrating (empty blue diamonds) particles, of ACE/SIS (green circles), and SOHO/EPHIN (orange squares) at L1, as well as Chang'E-4/LND (grey circles) measured at the lunar surface. The helium spectra were averaged over periods when Solar Orbiter was located between 0.95 and 1 au, as given in Tab. \ref{['tab:1AU_period']}. The GCR spectrum predicted from the BON2020 model slaba_badhwaroneill_2020 is given as the dashed line. Error bars indicate the statistical uncertainties.
• Figure 3: From top to bottom: the averaged helium intensity between $\sim$ 10 and $\sim$ 50 MeV/nuc measured by Solar Orbiter/HET (top panel) and SOHO/EPHIN (second panel) over the corresponding Carrington rotations between 2020 February and 2022 September; the intensity ratio of Solar Orbiter/HET to SOHO/EPHIN (third panel); the monthly averaged tilt angle of HCS and sunspot number (fourth panel); Solar Orbiter's radial distance (bottom panel).
• Figure 4: Helium intensity ratios between Solar Orbiter/HET and SOHO/EPHIN as a function of Solar Orbiter’s radial distance during the first three orbits. The dashed lines are the fitted lines for each orbit, which are indicated by colors. The y-axis is shown on a logarithmic scale.
• Figure 5: The radial gradients of ACR helium are shown as a function of energy between 11.1 and 49 MeV/nuc. The blue points are the fitted results as in Fig. \ref{['fig4:ratio_radialgradient']} and the orange squares further exclude the GCR contamination which are estimated from the BON2020 model, accompanied by their error bars. The grey circles represent the radial gradients obtained from PSP Rankin2021ApJ_helium. The dashed lines are the averaged gradient over the corresponding energy range.