A new view of energetic particles from stream interaction regions observed by Parker Solar Probe

¹ University of New Hampshire, Durham, NH 03824, USA
e-mail: nschwadron@unh.edu
² Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
³ California Institute of Technology, Pasadena, CA 91125, USA
⁴ University of Texas at San Antonio, San Antonio, TX 78249, USA
⁵ University of California at Berkeley, Berkeley, CA 94720, USA
⁶ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁷ Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ University of Arizona, Tucson, AZ 85721, USA
⁹ Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA
¹⁰ BWX Technologies, Inc, Lynchburg, VA, 24504, USA
¹¹ University of Michigan, Ann Arbor, MI 48109, USA
¹² Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing of the National Observatory of Athens, Vas. Pavlou and I. Metaxa, 15236 Penteli, Greece
¹³ University of Delaware, Newark, DE 19716, USA
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 7 September 2020
Accepted: 14 November 2020

Abstract

Early observations from the first orbit of Parker Solar Probe (PSP) show recurrent stream interaction regions that form close to the Sun. Energetic particle enhancements were observed on the 320th–326th day of the year 2018, which corresponds to ~1–7 days after the passage of the stream interface between faster and slower solar wind. Energetic particles stream into the inner heliosphere to the PSP spacecraft near 0.33 au (71 solar radii) where they are measured by the Integrated Science Investigation of the Sun (IS⊙IS). The large 6-day time interval over which energetic particles are observed after the stream passage provides a unique perspective on the development of stream interactions within the heliosphere. The long duration of energetic particle enhancements suggests that particles stream in through the inner heliosphere more directly along magnetic field lines that form a sub-Parker spiral structure due to magnetic footpoint motion at the Sun and shearing of the magnetic field in the rarefaction region behind the stream interface. The strong build-up of energetic particle fluxes in the first 3 days after the passage of the stream interface indicates that suprathermal populations are enhanced near the interaction region through compression or other acceleration processes in addition to being diffusively accelerated. The early increases in energetic particle fluxes (in the first 3 days) in the formation of these events allows for the characterization of the acceleration associated with these suprathermal seed populations. Thus, we show that the time history of energetic particle fluxes observed by IS⊙IS provides a new view of particle acceleration at stream interaction regions throughout the inner heliosphere.