The pickup He+ velocity distributions embedded in a CME structure

The velocity distribution functions (VDFs) of pickup He⁺ ions near a Coronal Mass Ejection (CME) 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. One of the most eminent examples is the shock injection process. Around interplanetary (IP) shocks, significant abundance enhancements of He⁺ pickup ions over He⁺⁺ ions of solar wind origin (e.g., Chotoo et al., 2000) provided compelling evidence that the source population for the energetic particle population is not the solar wind itself, but rather the pickup ion population that contains more particles beyond the injection threshold due to the difference in VDF. However, due to the lack of high-resolution measurements of pickup ion’s energy and phase space densities, their kinetics are not well understood.

In this study, VDFs of He⁺ pickup ions are investigated for a typical quasi-perpendicular shock observed in the heart 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) was 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 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 10 min cadence.

The IP shock that we focus on was driven by a coronal mass ejection producing a fast mode shock. Our study focuses on two regions: (1) VDFs within the CME sheath in the shock downstream, and (2) VDFs in the magnetic cloud. VDFs are analyzed in terms of particle heating/cooling, acceleration/deceleration, and pitch angle diffusion. The connectivity to the shock will also be investigated. In the far downstream region, correlation between the VDFs and the ambient magnetic field activities (the power spectra and the Alfvénic activity) are discussed in terms of how they modify the He+ pitch angle distributions.

References:

Chotoo, K., et al. (2000), The suprathermal seed population for corotating interaction region ions at 1 AU deduced from composition and spectra of H+, He++, and He+ observed on Wind, doi:10.1029/1998JA000015.

Taut, A., et al. (2018), Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff, doi: 10.1051/0004-6361/201731796.

Galvin, A.B., Kistler, L.M., Popecki, M.A. et al. The Plasma and Suprathermal Ion Composition (PLASTIC) Investigation on the STEREO Observatories. Space Sci Rev 136, 437–486 (2008). https://doi.org/10.1007/s11214-007-9296-x