Time evolution of stream interaction region energetic particle spectra in the inner heliosphere

¹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08540, USA
e-mail: cjjoyce@princeton.edu
² University of New Hampshire, Durham, NH 03824, USA
³ Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵ Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
⁶ California Institute of Technology, Pasadena, CA 91125, USA
⁷ Southwest Research Institute, San Antonio, TX 78228, USA
⁸ University of Texas at San Antonio, San Antonio, TX 78249, USA
⁹ University of Arizona, Tucson, AZ 85721, USA
¹⁰ University of Delaware, Newark, DE 19716, USA
¹¹ University of California at Berkeley, Berkeley, CA, USA
¹² The Blackett Laboratory, Imperial College London, London, UK
¹³ University of Michigan, Ann Arbor, MI, USA

Received: 2 September 2020
Accepted: 30 March 2021

Abstract

We analyze an energetic proton event associated with a stream interaction region (SIR) that was observed at Parker Solar Probe on day 320 of 2018 when the spacecraft was just 0.34 AU from the Sun. Using the Integrated Science Investigation of the Sun instrument suite, we perform a spectral analysis of the event and show how the observed spectra evolve over the course of the event. We find that the spectra from the first day of the event are much more consistent with local acceleration at a weak compression, while spectra from later on are more typical of SIR-related events in which particles accelerated at distant shocks dominate. After the first day, the spectra remain approximately constant, which indicates that the modulation of energetic particles during transit from the presumed source region is weaker than previously thought. We argue that these observations can be explained by a sub-Parker spiral magnetic field structure connecting the spacecraft to a source region in the SIR that is relatively close to the Sun. We further propose that acceleration at weak, pre-shock compressions likely plays an important role in observations of SIR-related events in the inner heliosphere and that future modelling of such events should consider acceleration all along the compression region, not just at the distant shock region.