The overarching goal of this grant was to study the three-dimensional time evolution of magnetic reconnection, the mechanism underlying the energy release in solar flares and many other phenomena. This project was a collaborative grant; we partnered with a team at Montana State University led by Prof. Jiong Qiu to study the time evolution of "two-ribbon" solar flares observationally, in particular how they spread in the direction along the ribbons.  We carried out the theoretical and numerical portion of the project, using massively parallel numerical simulations to study how reconnection spreads in the out-of-plane direction. Previous work on the spreading of reconnection addressed current sheets of uniform thickness along their extent. Our study addressed how reconnection spreads in a current sheet where its thickness changes as a function of distance along its extent, which we call a fluted current sheet.  A reason studying how magnetic reconnection spreads is important is that reconnection is the mechanism allowing large-scale energy release in solar flares, so knowing how it evolves in time feeds into an understanding of how much energy is being released and how many particles get accelerated during the solar flare, which is an important aspect of space weather.

One of the results of the observational portion of this study was that the spreading of ribbons in solar flares was often slower than the typical velocity scale expected in a magnetized plasma, the so-called Alfven speed.  We investigated what can make reconnection spread more slowly than the Alfven speed.  By systematically varying the thickness of a uniform current sheet, we confirmed our expectations that reconnection spreads more slowly for thicker current sheets.  Then we studied spreading in fluted current sheets and found another mechanism for slower spreading. We found the surprising result that spreading occurs more slowly at a current sheet of a given thickness if it is fluted, i.e., if the reconnection begins in a region where the current sheet is thinner. We performed a systematic parametric study for a number of different fluted current sheets, and found that the spreading speed in a fluted current sheet follows a predicable and quantifiable pattern.  We identified a physical mechanism causing the observed speeds.

In addition to the activities directly in support of the project, we performed many other successful studies. We provided numerical support for the first measurement of the electron diffusion region by the Magnetospheric Multiscale (MMS) satellites, published in Science.  We proposed a potential explanation of why the reconnection rate tends to be on the order of 0.1 in normalized units, and an age-old question going back to the discovery of reconnection as the cause of solar flares. A study of the structure of exhausts of reconnection confined to a finite extent was published. There were surprises when there is an out-of-plane (guide) magnetic field: the exhausts open in the out-of-plane direction and are collimated in the inflow direction, the exact opposite of what occurs in two dimensions. We showed solar wind events attributed to extended reconnection likely were, in fact, extended. We presented a model for the reduction of the outflow speed of reconnection as a function of temperature. We successfully implemented and validated kinetic entropy into our particle-in-cell simulation code and provided a physical interpretation of the position space and velocity space kinetic entropies; the velocity space entropy increases and position space entropy during reconnection.

This grant also contributed to the training of one postdoctoral researcher and two graduate student. They led two of the publications. The postdoctoral researcher delivered presentations, including some invited ones. The graduate student received a studentship award.

For outreach, we continued our successful partnership with the local children's science museum, Spark! Imagination and Science Center (formerly the Children's Discovery Museum of West Virginia).  We developed elementary school classroom activity kits about space weather.  We delivered a kit to each elementary school in McDowell, Webster, and Braxton Counties, West Virginia, rural counties which are in dire need of genuine science materials for classrooms.  We did five presentations in 4th grade classrooms. The kits were given outright to the schools for their perpetual use.  In addition, we acquired a terrella for the Space Weather exhibit I previously helped create using other funding.  There was an opening for the event where I was there for the public to "Ask The Scientist". We participated in special events. We invited science communicator Emily Calandrelli (a correspondent on Bill Nye's show) to the event, and she talked about space to the kids.  We participated in another special event for her book signing. We also did outreach for the Great American Eclipse, being filmed for a video distributed online for use for state-wide eclipse activities, and doing public lectures about the eclipse.

Last Modified: 08/17/2019
Modified by: Paul A Cassak