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03/10/2014

Understanding the Northern Lights

   

From earliest times man has sought to understand the brilliant and dynamic light displays seen high in the Arctic and Antarctic skies known as the Aurora Borealis and Aurora Australis, respectively. Continuing a long history of research at Rice seeking to understand the causes and behavior of the aurora, members of the Physics and Astronomy Department have recently made a major breakthrough toward understanding the multi-scale physical processes responsible for this complicated geophysical phenomenon.

 

Rocket and satellite observations in the early years of the space age made it clear that the highly structured auroral forms that dance across the night sky at high latitudes are due to kilovolt energy electrons that rain down on and energize atoms and molecules in Earth’s atmosphere, causing them to emit light. It quickly became clear that the electrons causing the aurora originated from Earth's magnetospheric plasma sheet, a population of hot ions and electrons that extends hundreds of thousand kilometers away from the Sun on the night side of the Earth. What was not clear was why the electrons causing the auroral displays are concentrated in thin sheets that are only tens of kilometers thick (or less). In the 1970s, Rice faculty members Paul Cloutier and Hugh Anderson and their students shot rockets into the aurora to show that the bright auroral arcs correspond to intense electrical currents flowing up along magnetic field lines linking the auroral region to the plasma sheet. In the late 1980s, Professor Patricia Reiff and collaborators used careful analysis of data from a pair of Earth-orbiting spacecraft, one located a short distance above the auroral altitude and one much higher, to show that the electrons in the bright auroral forms were accelerated downward along the magnetic field lines by upward-directed electric fields. Those electric fields allow the magnetic field lines to carry intense electric currents, much like current is driven along a wire. Still unexplained, however, was the question of why the upward current is concentrated in such thin sheets. That question has remained unanswered until very recently, when the latest version of a large computer code developed over many years at Rice provided the likely answer.

 

This code, known as the Rice Convection Model, had its origin as a little computer program written 45 years ago by Dick Wolf (then a Rice assistant professor, now professor emeritus), based on an idea originally suggested by Alex Dessler, the founder of Rice's Space Science Department. Over the years, the original simple little computer program has grown greatly in both length and comprehensiveness, evolving into a major research code that solves a complete set of basic physical equations governing the closed-field-line region of Earth's magnetosphere and its coupling to Earth’s ionosphere. It has developed to the point where its sophisticated grid system extends over a hundred thousand kilometers, while still resolving structures that are only a few tens of kilometers thick in the ionosphere. This greatly improved spatial resolution is what allows the large-scale code to be self-consistently applied to the study of discrete auroral forms, for the very first time.

 

Recent work on the aurora carried out by Research Scientist Jian Yang in collaboration with Rice colleagues Frank Toffoletto, Stan Sazykin, and Dick Wolf, has focused on an important class of aurora called "growth phase arcs", which occur after a sustained period of strong driving of the magnetosphere by its interaction with the solar wind, an energetic stream of ionized particles streaming outward from the Sun. In a paper that was published in the December, 2013 issue of Geophysical Research Letters, Dr. Yang and collaborators have used the Rice Model to simulate the formation of a growth phase arc and have interrogated the simulation results to produce a physical explanation of the mechanisms responsible for the arc. They have shown that very thin sheets of magnetic-field-aligned current, both up and down, form naturally at the earthwardmost edge of the plasma sheet. As the grid size of the simulation is decreased further and further, and the code is run longer and longer, the sheets get thinner and thinner. In Nature, if the sheets get thin enough, still smaller-scale physical processes that the Rice code cannot yet represent take over and govern the detailed properties of the resulting auroral arc. In spite of this limitation the Rice Model is capable of calculating the location and electric current strengths associated with the thin auroral forms.

 

Encouraged by these results, the group is systematically working toward the simulation and explanation of the various types of observed discrete auroral features. Almost all previous quantitative theories of auroral arc formation have focused on the near-Earth aspects associated with the electron energization. Now for the first time, the computational capability exists for treating the various auroral features within the context of a comprehensive model of large-scale magnetospheric dynamics.

 

It has been clear for many years that the changing pattern of discrete auroral structures provides a dynamic image of activity taking place in the distant magnetosphere of the Earth. However it has proven to be very difficult to interpret the auroral images unambiguously. The new capabilities of the Rice Convection Model, which allows computation of discrete aurora within their magnetospheric context, should greatly improve capabilities for inferring what is going on in the magnetosphere from observations of the night sky.

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Figure 1: Model-computed auroral arc as viewed above the pre-midnight northern hemisphere. The configuration is after one hour of solar-wind driving.

 

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Figure 2: Model-computed plasma pressure in the magnetosphere. The sharp pressure gradients in the magnetosphere drive intense currents along magnetic field lines, which cause the bright aurora in the ionosphere.