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A Twist in the Tale

Rui Liu, David Alexander, and Holly R. Gilbert
Department of Physics and Astronomy, Rice University

Solar filaments (or prominences) commonly spawn large geomagnetic storms. New observations of writhing filaments may tell us why.  Dynamic events at the Sun can have a profound impact on the magnetic environment of the Earth resulting in a range of effects from enhanced aurora at southerly latitudes to complete loss of function of telecommunication satellites and other space resources. These solar storms can significantly strengthen the radiation environment around the Earth providing a hazard to astronauts working on the international space station. Understanding the source of these storms and the conditions leading up to them is one of the major challenges in solar physics research.

The Solar Group in Rice’s Department of Physics and Astronomy are exploring a class of events known as erupting prominences which are strongly correlated with the onset of Coronal Mass Ejections, or CMEs, a major source of enhanced geomagnetic storms. These prominences are long low-lying structures of cool dense material supported in the otherwise hot solar corona by twisted, “slinky-like” magnetic fields. As these structures twist they reach a critical point where the whole system becomes unstable with the prominence undergoing a dramatic transition and erupting from the Sun, disrupting the surrounding atmosphere and launching a CME, the fastest of which can reach 5 million miles per hour!

Using a combination of observations from both ground- and space-based telescopes, researchers in the Rice University Solar Group have discovered that a key process in causing the prominences to erupt is whether or not the “slinky” structure writhes as it evolves: think of the sudden kink that forms in an elastic band if you twist it too much. Of specific interest are the observations from the Chromospheric Helium Imaging Polarimeter (CHIP) instrument at the Mauna Loa Solar Observatory in Hawai’i. CHIP provides an image of the Sun every 3 minutes in a very narrow wavelength range focusing on emission from neutral Helium. Using the Doppler effect the CHIP instrument allows us to measure the velocity of the mass towards (blue-shifted) and away (red-shifted) from the observer. The distinctive pattern of these velocity shifts shown in the Figure, together with movies of how the mass is moving, allow us to infer that the prominence is in fact kinking. The kinking of the prominence initiates an interaction between oppositely directed magnetic fields in the prominence and the overlying corona causing the whole structure to erupt and form a CME. The system undergoes what is known as a “catastrophic loss of equilibrium”. What’s particularly interesting about these new results is that only the upper part of the prominence erupts as the kinking is so severe it literally cuts the “slinky” in two with the lower part falling back to the Sun. In this case, very little, if any, of the cool dense plasma that makes up the prominence escapes from the Sun. These new observations provide key diagnostic information in our attempts to understand, and ultimately predict, the sources of space weather.