Modeling interstellar pickup ion distributions in corotating interaction regions inside 1 AU

Abstract

We present a modeling study of interstellar pickup ion (PUI) distributions in corotating interaction regions (CIRs). We consider gradual compressions associated with CIRs formed when fast speed streams overtake slower streams in the inner heliosphere. For the analysis, we adopt a simplified magnetohydrodynamic model of a CIR. The Energetic Particle Radiation Environment Module, a parallelized particle numerical kinetic code, is used to model PUI distributions using the focused transport equation, including adiabatic cooling/heating, adiabatic focusing, and parallel and perpendicular diffusion. The continuous injection of PUIs is handled as a source term with a ring distribution in velocity space that is produced from the local neutral density obtained from a hot model of the interstellar neutral gas. The simulated distributions exhibit a harder spectrum in the compression region and a softer spectrum in the rarefaction region than that in undisturbed solar wind. As an additional result, a v À5 power law tail distribution above the PUI cutoff speed (a knee in the distribution) emerges for a particular velocity gradient in the CIR. The tail above the PUI cutoff is sensitive to the CIR velocity gradient, and in one observational case studied, this relationship adequately explains the observed spectrum from 2 to 4 times the solar wind speed. This suggests that the velocity gradient associated with the CIR formation can efficiently create a seed population of PUIs before a shock forms even without stochastic acceleration. Thus, local CIR compressions without shocks may play a significant role in the acceleration process as suggested previously. CHEN ET AL.MODELING PICKUP ION DISTRIBUTION 9269 PUBLICATIONS

A living catalog of stream interaction regions in the Parker Solar Probe era

“…These studies have suggested that in the inner heliosphere, the lower-energy component of SIR-and CIR-associated suprathermal ions may be a result of local acceleration, while the higherenergy component is a result of distant shock acceleration (e.g., Ebert et al 2012;Filwett et al 2019). This has been supported by observations of acceleration at SIR compressions within 1 au, where shocks have not yet formed (Chotoo et al 2000;Chen et al 2015). Recent observations by Parker Solar Probe (PSP) at 0.3 au (McComas et al 2019) have further suggested the importance of compression-related acceleration at lower heliospheric distances than where SIR-and CIR-associated shocks typically form (Joyce et al 2020b).…”

Solar wind stream interaction regions throughout the heliosphere

“…(1996) and Chen et al. (2015) discuss models of interstellar pick-up ion acceleration in interaction regions. Accelerated He pick up ions may help to account for the enhanced abundance of He relative to the solar wind evident in the left-hand panel of Fig.…”

A Quarter Century ofWindSpacecraft Discoveries

“…Their result suggests that Fe-rich particles from impulsive solar events contribute to the source of CIR particles. Interstellar pick up ions-interstellar neutrals that are ionized near the Sun-such as He + (Chotoo et al, 2000), may also be accelerated at CIRs (Chen et al, 2015). Reames (2018), using EPACT/LEMT data, concluded that the element abundances of CIR-accelerated ions mirror the solar wind abundances with a modification depending on the mass to charge ratio of the ions.…”