This collaborative research team will investigate the evolution of energy spectra, time-intensity profiles, and charged particle flows along and across the interplanetary magnetic field in co-rotating interaction regions (CIRs) observed during solar cycles 22 and 23. They will use measurements of energetic particles, magnetic fields, and solar wind plasma obtained by the ACE, Wind, and STEREO spacecraft to study these CIR events. This team will also develop a new theoretical model, based on their existing 'Particle Acceleration and Transport in the Heliosphere' (PATH) numerical code, in order to study the time-dependent acceleration and transport of particles associated with CIRs, as well as to probe the 3D structure of CIRs and the evolution of magnetic connections between an observer at Earth and remote CIR locations beyond Earth orbit.
Understanding the properties of solar energetic particle (SEP) events associated with CIRs remains an outstanding problem for the scientific community. This study will yield new information about the 3D structure of CIRs and the configuration of the interplanetary magnetic field, both of which are crucial for the development of future global heliospheric models, which are in turn important for space weather forecasting.
Research results will be presented to the broader community at scientific meetings and at smaller workshops where student participation is encouraged. The team will also publish findings via peer-reviewed journals and web-based monthly electronic newsletters. In this project, several graduate and undergraduate students will be supported at the University of Alabama at Huntsville (UAH) and at the University of Texas at San Antonio (UTSA). This project will integrate and synergize existing research programs at UAH, the Southwest Research Institute (SwRI) in San Antonio, and the Johns Hopkins University's Applied Physics Laboratory (JHU/APL).
Particle acceleration occurs in a variety of environments inside our solar system, such as the Sun’s atmosphere, the interplanetary medium, planetary systems, as well as at the edge of the sphere of influence of our Sun; and in distant locations such as other stars, galaxies, and supernovae explosions. Most of these sites are remote and inaccessible to spacecraft that make direct, concurrent measurements of the energized particle populations and their accelerators. The primary producer of energetic particles inside the orbit of Jupiter during solar activity cycle minimum, corotating interaction regions (CIRs) form when a stream of fast solar wind emerges from a coronal hole that extends to low-latitudes on the solar surface and overtakes a parcel of slow solar wind emitted from the Sun at an earlier time. As the Sun rotates once every 27 days, these different speed winds, or plasmas, can become radially aligned and interact, creating a compression region that also corotates with the Sun. This slow-fast wind interaction becomes stronger with increasing distance from the Sun, and eventually strengthens sufficiently to form shocks at the leading and trailing edges of the CIR compression region that can accelerate particles. The leading edge is the interface between the slow, the compressed and accelerated slow solar wind; and the trailing edge is the interface between the compressed and decelerated fast solar wind and the high speed stream. Our study used simultaneous in situ measurements of the properties of co-rotating interaction regions (CIRs) and their associated accelerated particles near Earth to advance our understanding of the operating energization mechanisms. Our primary motivation was to provide new insights into the universal phenomenon of particle acceleration. We combined high sensitivity particle data from mass spectrometers with magnetic field and solar wind plasma measurements from NASA missions, such as ACE, Wind, and STEREO. Particle measurements consist of elemental composition, energy, spectral, and angular distributions, and time variations of these quantities. We studied particle properties over a broad range of solar longitudes and linked them to properties of the CIRs as observed by the magnetic field and solar wind sensors. For the last 30 years, researchers believed that the CIR-associated particle populations observed near Earth orbit were accelerated by distant shock waves that are stronger further outside of Earth orbit, between Mars and Jupiter. This seemed logical because the interactions between the fast and slow solar wind parcels become more intense the further they are from the Sun. However, our work has conclusively shown that a substantial fraction of the near-Earth CIR energetic ions are in fact accelerated near and inside Earth orbit provided that the interaction between the fast and slow solar wind is sufficiently strong. We also surveyed properties of the compressed magnetic field inside ~150 CIRs that were observed near Earth’s orbit during the last two solar cycles. We found that nearly half of these CIRs contained planar structures wherein the magnetic field is aligned along sheets that are tilted relative to both the Sun-Earth line and the ecliptic plane. Such magnetic structures have been related to a number of different phenomena in the heliosphere such as the sheath ahead of fast interplanetary coronal mass ejections (ICMEs), magnetic sector boundaries, and CIRs. Our research discovered that planar magnetic structures in CIRs are not caused by a unique characteristic in the local plasma or magnetic field. Our results are currently being used by modelers to better understand the role of CIR orientation, topology and evolution and the roles they play in accelerating energetic particles inside Earth-orbit
