Energetic Particle Increases Associated with Stream Interaction Regions (1912.08244v2)

Abstract: The Parker Solar Probe was launched on 2018 August 12 and completed its second orbit on 2019 June 19 with perihelion of 35.7 solar radii. During this time, the Energetic particle Instrument-Hi (EPI-Hi, one of the two energetic particle instruments comprising the Integrated Science Investigation of the Sun, ISOIS) measured seven proton intensity increases associated with stream interaction regions (SIRs), two of which appear to be occurring in the same region corotating with the Sun. The events are relatively weak, with observed proton spectra extending to only a few MeV and lasting for a few days. The proton spectra are best characterized by power laws with indices ranging from -4.3 to -6.5, generally softer than events associated with SIRs observed at 1 au and beyond. Helium spectra were also obtained with similar indices, allowing He/H abundance ratios to be calculated for each event. We find values of 0.016-0.031, which are consistent with ratios obtained previously for corotating interaction region events with fast solar wind < 600 km s-1. Using the observed solar wind data combined with solar wind simulations, we study the solar wind structures associated with these events and identify additional spacecraft near 1 au appropriately positioned to observe the same structures after some corotation. Examination of the energetic particle observations from these spacecraft yields two events that may correspond to the energetic particle increases seen by EPI-Hi earlier.

• The paper presents a detailed analysis of energetic particle increases linked to SIRs using PSP data from unprecedented inner heliospheric orbits.
• It employs the EPI-Hi instrument to capture proton and helium spectra, identifying power law indices between -4.3 and -6.5 and typical He/H ratios.
• The findings support acceleration models at heliospheric shock regions and offer a foundation for future comparative studies on solar wind dynamics.

Energetic Particle Increases Associated with Stream Interaction Regions

The paper conducted a comprehensive paper of energetic particle increases associated with Stream Interaction Regions (SIRs) observed by the Parker Solar Probe (PSP) during its initial orbits. The PSP, launched in August 2018, offers unprecedented proximity to the Sun, facilitating data collection within regions hitherto unexplored by previous missions. This investigation leverages data from the Energetic Particle Instrument-Hi (EPI-Hi), part of the Integrated Science Investigation of the Sun (ISeIS), focusing on protons and helium particles associated with SIRs.

Methodology and Instrumentation

The EPI-Hi instrument plays a central role in this investigation, capturing data on particle intensities and spectra. Comprising two high-energy telescopes and one low-energy telescope, EPI-Hi detects protons and helium over specified energy ranges. The paper's temporal context includes seven distinct proton intensity increases, sourced primarily from interactions between faster solar wind streams overrunning slower streams, generating SIRs. The resulting shock phenomena commonly manifest in corotating interaction regions (CIRs) as observed in the particle data collected.

Observations and Results

The observations detail SIR-associated particle events, each characterized by relatively weak intensity but prolonged occurrence over several days, with proton spectra following power laws with indices between -4.3 and -6.5. Helium spectra parity yields He/H abundance ratios ranging from 0.016 to 0.031, aligning with past findings related to fast solar wind CIR events (~600 km/s). This concurrence reinforces the hypothesis of source consistency across detected events, particularly within a range of fast and slow wind streams as interpreted by WSA-ENLIL simulations. The analyzed timelines and Parker spiral connections reveal longitudinal relationships, notably identifying two potential corotating events.

Discussion and Theoretical Implications

Hydrodynamic interpretations based on observed patterns suggest that the most likely particle acceleration mechanisms occur at larger radial distances from the Sun. Potentially at these distances, either shocks are sufficiently established or significant compression regions are formed, enabling initial particle acceleration. These phenomena allow energetic particles to propagate back toward PSP's vicinity, resulting in the detected isotropic distributions devoid of strong local acceleration indicators. The paper identifies extended particle presence even past significant solar wind interfaces, supporting an acceleration model consequent to CIR shock strengthening.

Future Research and Developments

These initial findings of SIRs' influence on solar energetic particles provide critical insights into particle dynamics within the heliosphere. For future research, a comparative paper involving 1 au observations and EPI-Lo spectra would extend understanding of radial and longitudinal gradients of energetic particles. As solar activity escalates, continued PSP observations promise to augment this dataset, elucidating the nuanced forces governing solar and heliospheric energetic particle dynamics.

Conclusion

The paper contributes valuable quantitative data on SIR-associated energetic particle events within unprecedented inner heliospheric distances. By employing PSP's capabilities, this detailed analysis correlates heliospheric conditions with SIRs, emphasizing the intricate interplay between solar wind dynamics and particle propagation. This research contributes significantly to astrophysical sciences by illuminating the mechanisms of particle acceleration and distribution near the Sun, forming a solid foundation for future solar and space physics exploration.