The Principal Investigator (PI) has assembled an international collaboration to test whether coronal mass ejection (CME) build-up can be ascribed to a four-stage process of magnetic field evolution that ultimately leads to the triggering of a CME. This four stage process consists of (1) canceling of magnetic fields along a polarity reversal boundary; (2) formation of a filament channel concurrent with the ascent of horizontal magnetic field into the chromosphere from cancellation sites; (3) thread-by-thread formation of a filament magnetic field and visible filament when the filament channel attains maximum development; and finally, (4) thread-by-thread building of the filament cavity magnetic field as mass drains from the visible threads, thereby incrementally releasing filament magnetic field into the cavity magnetic field until one of many possible triggering mechanisms initiates the CME.
The PI's team will examine these distinct stages and the mechanisms linking them, taking advantage of the synergy offered by new observations, data analysis, numerical modeling, analytic theory, and laboratory plasma experiments. The PI's team expects this project to provide a greatly improved understanding of how CMEs originate and evolve. This improved knowledge of the CME environment is needed to develop predictive tools so that critical space weather events can be accurately forecast.
To the naked eye the Sun appears as a simple bright disk. However, telescopes reveal that the solar surface is a complex jungle of loop-like structures that sometimes stay the same for days and at other times erupt in just a few minutes. These loops consist of hot, electrically conducting gas called plasma which is embedded in magnetic field. The loops follow the shape of the magnetic field so that the plasma and magnetic field can be considered as a single entity. Why the loops behave the way they do has been an enduring mystery and so is the subject of much current research. Understanding solar plasma behavior is not just a matter of satisfying intellectual curiosity -- when solar loops erupt, they eject magnetized plasma that can strike Earth’s magnetosphere with enough force to knock out spacecraft, communications systems, and even electric power grids. Developing understanding of these phenomena has been elusive. Instead of observing the Sun, the Bellan Plasma Group at Caltech has produced small-scale replicas of these phenomena in a controlled laboratory experiment. This is feasible because the underlying physics, known as magnetohydrodynamics, has no intrinsic scale so that the laboratory experiment can be designed to be governed by the same physics as on the Sun. Magnetohydrodynamics models plasma as a superconducting gas, meaning that magnetic flux is frozen into the plasma frame so when plasma moves, the magnetic field moves with it. In addition, currents flowing in the plasma interact with magnetic fields to produce forces that both accelerate and deform the plasma. The basic building block of magnetohydrodynamics is a set of magnetic field lines twisting around a common axis like a barber pole. This is called a magnetic flux tube and flux tubes are typically arched with ends attached to a surface such as the solar surface. The Bellan lab (see Fig. 1) creates arched plasma-filled, magnetic flux tubes similar to those on the Sun but much smaller and much shorter lived. The lab flux tubes are created in a controlled, reproducible fashion so behavior can be examined in detail. Time and space scales are both about one billion times smaller than on the Sun, but the same equations apply and the geometry is similar. The relevant physical regime is attained using high-speed pulsed power technology (typical parameters: 100 million watts, 10 millionths of a second duration, 50 kiloamps, 2 kilovolts, 5-50 cm plasma dimension). Important outcomes of the research have been identification of two simultaneous, related dynamical behaviors caused by the magnetic forces associated with electric current flowing along a loop. One force, called the hoop force, comes from the current being arched and causes the arch to expand in radius. The other force, called the pinch force, squeezes the loop cross-section, but this squeezing is balanced by the loop internal plasma pressure. If the loop cross-section varies along the loop, the squeezing varies along the loop such that pressure is highest where the cross-section is small. Pressure gradients along the loop then drive plasma flows along the loop, much like squeezing a toothpaste tube at one end causes toothpaste to flow along the tube axis. By injecting different gases at the footpoints of an arched loop in the laboratory, this squeeze-generated flow mechanism has been demonstrated in a vivid, colorful way (see Fig. 2). Since different gases emit light at different wavelengths, i.e., with different colors, using different gases effectively color-codes different parts of the plasma so it becomes possible to distinguish one flow from another. High-speed movies (several million frames per second) show green plasma flowing from one footpoint and red plasma flowing from the other. Together these flows fill up the arched flux tube and do so in such a way that as the hoop force increases the flux tube volume by increasing the flux tube length, the density of plasma in the flux tube remains constant. Collaboration with researchers at Helio Physics who observe the actual Sun has included comparison between lab and solar observations, attempts to explain observed solar behavior using mechanisms observed in the lab experiments, and designing an upgraded lab experiment that addresses an outstanding issue in the solar observations, namely how small loops can merge to create larger loops. Besides being published in scientific journals and presented at scientific conferences, the Caltech lab experiments have been featured in several television science documentaries including the BBC Horizon series (Solar Storms- the Threat to Planet Earth episode, blurb at www.bbc.co.uk/programmes/b01d99vb) and the National Geographic Known Universe Series (Most Powerful Stars episode). The experiments have also been featured in science news web sites (e.g., http://news.discovery.com/space/mini-loops-of-lab-plasma-unravel-solar-secrets-120822.htm). Finally, this work has increased the impact of women in physics as an important contributor to this project was graduate student Eve Stenson (now graduated).
