In this project, the Principal Investigator (PI) will investigate the suprathermal particle distributions for solar wind ions. The PI plans to use existing models developed under prior NSF support to predict the production of energetic neutral atoms in the solar wind and to compare this output with recent observations from the IBEX and Cassini spacecraft. He also plans to study whether his presumed 'compressive turbulence' mechanism for stochastic acceleration of suprathermal solar wind ions might also be able to accelerate galactic cosmic rays in the interstellar medium, or to create seed populations of energetic particles in the solar corona.
As a means to promote the development of a predictive model for the solar wind, the proposer intends to investigate the dynamics of the region surrounding the heliospheric current sheet, to study the relationship between the mass flux of the solar wind and the strength of the Sun's open magnetic field, and to attempt a reconciliation of various theories for embedding open magnetic flux in primarily closed-field magnetic regions. The PI expects this research to be relevant to general astrophysical plasmas as well as to solar physics. He plans to incorporate the new concepts developed in this project into the graduate education program at the University of Michigan, in order to encourage and train the next generation of scientists in solar and heliospheric physics.
NSF Grant 1043012 L. A. Fisk, PI This grant supported research on two topics central to understanding the heliosphere: (1) the acceleration of energetic particles, and (2) the source and acceleration of the solar wind. A general description of project outcomes is provided below. As can be seen in the description, the research resulted in a new mechanism for accelerating energetic particles, which has broad applications to solar and solar wind research, as well as to astrophysics. The research on the solar wind has the prospect of leading to a predictive model for the solar wind, which will be essential for predicting space weather and its ever increasing impact on our technological society. The research is also now part of the core graduate curriculum at the University of Michigan, and since it introduces new concepts is serving to challenge our students to think new thoughts, something we consider essential to their development as productive space researchers. The Acceleration of Energetic Particles Starting in the early 2000s, a series of observations were reported that revealed that superthermal particles accelerated in the solar wind have a common spectral shape: a distribution function that is a power law in particle speed with a spectral index of -5. The common spectral shape became an even more interesting problem when the Voyager spacecraft crossed the termination shock of the solar wind at ~90 AU from the Sun and began exploring the heliosheath. Shortly after crossing the termination shock, both spacecraft observed low-energy ion spectra (up to energies ~3 MeV) with spectral index of -5. Indeed, the spectra at the two Voyagers not only had the same spectral index, but the spectra were also of identical magnitudes even though the two Voyagers are separated by ~100 AU. Further into the heliosheath, Voyager 1 at 120 AU observed the full Anomalous Cosmic Ray (ACR) spectrum, extending up to ~100 MeV, and it is also a single power law spectrum with spectral index -5. The observations of the common spectral shape demanded a new acceleration mechanism. Diffusive shock acceleration, the most commonly invoked acceleration mechanism, cannot account for the common spectral shape for various reasons: There are many observed examples of the common spectral shape where no shocks are present, and also, although diffusive shock acceleration can yield power law spectra, there is no obvious reason why the spectral index should always be -5. Traditional stochastic acceleration does not tend to yield power law spectra, least of all a unique spectral index. The research supported under this grant systematically developed a new acceleration mechanism, a pump mechanism, in which particles are accelerated out of a low-energy core particle population through a series of compressions and expansions of the thermal plasma. The theory has been derived in detail, and now applied to and able to account for the observations of particle acceleration at shocks in the solar wind and the acceleration of Anomalous Cosmic Rays in the heliosheath. The Solar Wind The recent solar minimum between cycles 23 and 24 was unprecedented. Solar activity was at its lowest level in nearly a century. The heliospheric magnetic field decreased by 30% compared to any previously observed minimum, and the mass flux of the solar wind decreased by 20% The coronal electron temperature, as inferred from measurements of solar wind charge states, decreased by about 40% compared to previous minima. Even more dramatic, there was a decrease in the elemental abundances of heavy ions (He, C, O Si, and Fe) relative to H by 50% between the previous maximum and the recent minimum. The current solar maximum appears also to be anomalous, peaking at only ~100 sunspots on the Sun, and is thus a weak cycle. Moreover, preliminary analysis of solar wind charge state data, and thus the corona electron temperature, shows that the corona is cooler by ~20% compared to a similar period in the previous cycle. During the previous and current funding periods for this grant, a model for the behavior of the solar magnetic field and for the origin and acceleration of the solar wind was developed. In this model there are global motions of the open magnetic flux of the Sun, the component of the solar magnetic field that opens into the heliosphere, which can embed open flux into regions containing coronal loops. Reconnection between the open magnetic flux and the coronal loops releases material to form the solar wind. This model has had success in accounting for the behavior of the open magnetic flux and for the solar wind through the unusual solar minimum between cycles 23 & 24. Tests are ongoing as whether the model remains valid though solar maximum, and if successful this model could serve as the foundation for a predictive model of the solar wind.
