Variations of pickup He+ velocity distributions under the CME structures

The velocity distribution functions (VDFs) of pickup He+ ions near Coronal Mass Ejections (CMEs) driven shocks are expected to be extremely variable due to the specific source particle distributions and the interactions with the shock passage, turbulence, and large scale magnetic structures. However, due to the lack of high-resolution measurements of pickup ion energy and phase space densities, their kinetics are not well understood.

In this study, VDFs of He+ pickup ions are investigated for a shock-associated CME passage accompanied with a typical quasi-perpendicular shock observed in the middle of the helium focusing cone. In order to calculate the VDFs of helium ions, pulse height information from each ion event in the PLasma And SupraThermal Ion Compostion (PLASTIC) instrument (Galvin et al., 2008) on the Solar Terrestrial Relations Observatory (STEREO) is utilized. During each electrostatic analyzer energy-per-charge (E/q) step (128 steps, 435.6 ms each), PLASTIC stores 512 raw pulse height events including E/q, time-of-flight, total energy, and arrival direction. This allows us to reproduce partial 3-dimensional VDFs for various ion species with ~2° angular resolution (Taut, A. et al., 2018). Moreover, the concentration of He+ ions in the helium focusing cone increased the count rate significantly, and provided enough counting statistics to achieve unprecedented 10 min cadence.

Our study focuses on three regions: (1) VDFs upstream of the shock, (2) VDFs downstream of the shock within the CME sheath, and (3) VDFs in the magnetic cloud. VDFs are analyzed in terms of particle heating/cooling, acceleration/deceleration, and pitch angle diffusion. Overall, the pickup He+ VDFs keep the ring distribution features. Angular diffusion effects were eminent near the shock, while the velocity diffusion was dominant away from the shock. In the upstream region, shock-reflected particles were observed, along with significant angular diffusion. In the downstream region, suprathermal tail formation was observed and the spectra were consistent with the solar wind compression origin (Möbius et al., 2019). In the magnetic clouds, typical fresh pickup ion features were consistently observed without eminent suprathermal tails. Results advance our understanding of the suprathermal particle sources and acceleration in CME structures.

Galvin, A. B., Kistler, L. M., Popecki, M. A., Farrugia, C. J., Simunac, K. D. C., Ellis, L., et al. (2008). The Plasma and Suprathermal Ion Composition (PLASTIC) Investigation on the STEREO Observatories. Space Science Reviews, 136(1), 437–486. https://doi.org/10.1007/s11214-007-9296-x

Möbius, E., Bower, J., Aly, A., Berger, L., Farrugia, C., Galvin, A. B., et al. (2019). Observation of Suprathermal Tails of He+ Pickup Ions across Solar Wind Compression Regions with STEREO PLASTIC. Journal of Physics: Conference Series, 1332(1), 12011. https://doi.org/10.1088/1742-6596/1332/1/012011

Taut, A., Berger, L., Möbius, E., Drews, C., Heidrich-Meisner, V., Keilbach, D., et al. (2018). Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff. A&A, 611, A61. https://doi.org/10.1051/0004-6361/201731796