This investigation explored the relationship between plasma flows beneath the surface of the Sun and flare activity with the goals of helping to understand and predict the flare phenomenon. Flares represent intense releases of energy (including X rays) and can sometimes have damaging consequences on Earth and in space for a variety of critical systems.  Based on prior studies by others, which suggested that flares might be preceded by vortical motions below the Sun which can twist and help destabilize magnetic structures, we followed up with helioseismic methods designed to measure the properties of flows below groups of sunspots with higher spatial resolution.  To do this we utilized observations made with the Helioseismic and Magnetic Imager, an instrument which is part of the Solar Dynamics Observatory (SDO) mission launched by NASA in 2010.

We carried out a survey including 250 of the largest (and most flare prone) sunspot groups observed by SDO during its first five years of operation. Our methodology is based on helioseismic holography, co-developed by the principal investigator, which has been used in a variety of applications of exploring flows and magnetic fields below the surface. Four basic flow parameters: horizontal speed, horizontal component of divergence, vertical component of vorticity, and a kinetic helicity proxy, were mapped for each of the 250 active regions over the ten-day window during which the Sun rotated them across the visible solar disk. Flow indices were derived representing the mean and standard deviation of these parameters and compared with contemporary measures of flare X-rays. A correlation between the flows and X-ray flux was found for several of the flow indices, especially those based on the speed and the standard deviation of all flow parameters. However, their correlation with X-ray flux is similar to that observed with the surface properties of the sunspot groups, including magnetic flux density.  The temporal variation of the flow indices were also studied, and a superposed epoch analysis with respect to the occurrence to 70 medium and large flares was carried out.  Contol analyses, using random times, were also performed.  While the flows evolved with the passage of the sunspot groups across the disk, no discernible precursors or other changes specifically associated with flares were detected.

Other components of the investigation focused on testing helioseismic methods with artificial data derived from numerical computations performed on supercomputers. These models simulate the propagation of seismic (sound) waves through magnetic regions on the Sun. We used two types of models, including (more realistic) dynamical simulations including turbulent convective motions as well as (more flexible) static types of simulations. While, for the most part, our tests confirmed the general sensitivity of helioseismic holography measurements to flows, we found some artifacts in the strongest magnetic regions which can lead to underestimations of outflows from sunspots.  As part of this research we successfully developed and tested a new method for numerically computing magneto-hydrostatic models of the subsurface properties of sunspots based on observations at the surface. These, as well as the numerical codes for wave propagation will be useful for a variety of future investigations.

Our research has advanced our knowledge of the association of solar flares with near-surface flows using the largest survey carried out to date with methods capable of spatially resolving flows on the size of sunspots. Beyond our largely negative findings with respect to flare precursors, it has nonetheless revealed important insight into the general properties of flows around large active regions, and the datasets and methods will be highly useful in other applications.  Helioseismology is one of a number of "extraterrestrial" seismic methods, which are analogous and complimentary to the century-old field of terrestrial seismology. The refinement of the methods carried out in our research and described in the literature provide incentive and support to advancing other seismic methods.  The project has provided support for three young post-doctoral scientists, two of which were hired immediately after graduate school. Solar physics is a critically important discipline concentrating on understanding and predicting our nearest star, but it has historically been carried out by a painfully small number of researchers.  The long-term viability of this field can only be maintained by enticing and employing new students.

Last Modified: 10/13/2016
Modified by: Douglas C Braun