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).
This outcomes report is for the period May 2010 through the no-cost extension ending April 2014. The main goals of our study were to probe the following three fundamental questions: Where are the CIR particles seen at 1 AU accelerated? How do these CIR ions propagate throughout the inner heliosphere? How does magnetic connectivity to remote acceleration sites affect CIR-associated particle observations at Earth? The primary outcomes of the study regarding the three main goals were: CIR particles at 1 AU are accelerated locally (near 1 AU) for low energies (< 1 MeV/nuc) and at several AU for higher energy particles. The particles accelerated come from the suprathermal ion pool, that is the energy range above the solar wind, and NOT from the solar wind itself. Other accelerating events such as Solar Energetic Particle events contribute importantly to this pool. Surprisingly, the suprathermal ion pool is always present in the interplanetary medium even when there are no obvious accelerating agents responsible. Low energy particles seen at 1 AU come from close by, while the higher energy particles come from several AU, and propagate inward. Large scale magnetic disconnection in the high solar corona due to magnetic reconnection can be detected uniquely in CIRs, proving for the first time the existence of this reconnection process in the corona. CIRs propagate through the inner heliosphere following magnetic field lines tied to the Sun. These field lines are influenced by the tilt angle of the heliospheric current sheet, and the intensity of the CIR at 1 AU depends critically on the details of the connection between the CIR and the solar source. This often leads to markedly different CIR signatures seen on different spacecraft which connect to slightly different latitudes on the Sun. More details on the outcomes of this study were described many talks at scientific meetings, and in 7 journal publications: 1) We carried out the first systematic evaluation of the changes in Corotating Interaction Region (CIR) energetic particle intensities viewed from multiple spacecraft. We found that the tilt angle of the heliospheric current sheet at the beginning of the period played a key role, and that the main factor controlling the differences in ion intensities is the latitudinal separation between the spacecraft and the tilted CIRs. (see Bu?ík et al., J. Geophys. Res., 116, A06103, doi:10.1029/2010JA016311, June 2011) 2) We surveyed the heavy ion composition of CIRs over the period 1998-2001 in order to see if there was a solar cycle dependence in the composition. We discovered that the heavy ions were much more abundant during solar active periods. This information gives evidence that the CIR major heavy ion species are accelerated out of the suprathermal ion pool in the interplanetary medium, and not from the bulk solar wind. (see Mason et al., Astrophys. J. (Letters), 748, L31, doi: 10.1088/2041-8205/748/2/L31, April 2012) 3) We that the peak He intensity in 73 CIRs was well organized by the compression region trailing edges. These results provide evidence that the low energy He enhancements are from particles accelerated near 1 AU, while the higher energy (> MeV/nucleon) particles are likely accelerated by CIR shocks beyond Earth orbit. (see Ebert et al., Astrophys. J., 749, 73, doi: 10.1088/0004-637X/749/1/73, April 2012) 4) We measured the flow directions (anisotropies) of He ions in CIRs with a reverse shock and a well-formed compression region. These flow measurements showed that He ions can be accelerated at the CIR trailing edge. (see Ebert et al., Astrophys. J. (Letters), 754, L30, doi:10.1088/2041-8205/754/2/L30, August 2012) 5) We measured the heavy ion composition in 50 CIR events and found systematic differences from year to year, for example in 2010 the elemental composition of the CIRs was influenced by sporadic solar energetic particle events. (see Bu?ík et al., Solar Phys., 281, 411-422, doi: 10.1007/s11207-012-0094-6, August 2012) 6) In order to probe the seed population for CIRs we constructed particle spectra from solar wind energies to ~1 MeV/nucleon, and found that a suprathermal tail was continuously present including, surprisingly, periods when there were no obvious nearly accelerating shocks. We reviewed several theoretical mechanisms proposed to explain these ions. (see Mason and Gloeckler, Space Sci. Rev., DOI: 10.1007/s11214-010-9741-0, Space Sci. Rev., 172, 241-251, Nov. 2012) 7) We found a CIR pair where the particle and field data showed that a large scale U-shape magnetic field disconnected from the Sun. One consequence of these magnetic reconnections is the existence of disconnected magnetic field from the Sun. While it has been proposed for sometime, observational confirmation of such disconnected fields are difficult, if not impossible. Our paper therefore, provides the FIRST evidence of the existence of such large scale magnetic field reconnection. (see Wu et al., Astrophys. J., 781:17, doi: 10.1088/0004-637X/781/1/17, Jan., 2014)
