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Failure to Launch: Solar prominences find it difficult to leave home


Erupting solar prominences frequently create solar storms, solar flares and coronal mass ejections, which can cause major geomagnetic effects at the Earth. Coronal Mass Ejections, or CMEs, are large volumes of plasma and magnetic field hurtling from the Sun at up to 5 million miles per hour. When they collide with the Earth they can generate a range of geomagnetic phenomena from enhanced aurorae, seen as far south as Houston, to the loss of satellite function affecting telecommunications. In some cases, such solar storms have resulted in the loss of city power grids. These effects are collectively known as Space Weather (see www.spaceweather.com) .

Using data from two different space-borne telescopes focusing on the Sun, researchers in the Rice University Solar Group have shown that rapid energy release in the solar corona occurs from the dynamic evolution of solar prominences that do not erupt to escape the Sun. The telescopes are on the NASA science missions TRACE (Transition Region and Coronal Explorer) and RHESSI (Ramaty High Energy Solar Spectroscopic Imager). TRACE provides high resolution observations of the hot solar corona with temperatures of around 1-2 million degrees. RHESSI detects high energy X-ray radiation normally found to accompany large releases of magnetic energy in the production of solar flares.

The event studied by the Rice researchers initially showed the classic signs of an erupting prominence: dramatic heating and a rapid rise of the prominence from a height of 5,000 km to over 80,000 km in less than 10 minutes (~300,000 miles/hr). However, rather than continuing on into space the prominence displayed an elaborate kinking motion (see Figure) twisting the axis of the prominence around itself (like those annoying twist ties on children's toys). It seems that this writhing motion prevented the prominence from erupting but at the same time allowed the structure to release a lot of its energy in the form of energetic particle motions and local heating. The writhing brought oppositely-directed magnetic fields from the two adjacent legs of the prominence into contact enabling the release of magnetic energy. This is confirmed in part by the detection of high energy X-ray signatures at the location of the interaction of the prominence legs.

While this is only one event, we have since discovered a number of similar cases and are in the process of analyzing the role that the kinking motions play in determining whether these dynamic solar structures erupt or relax back to a quiescent state. Understanding this process will have important implications for modeling the drivers of Space Weather.

 

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Fig. 1.—Temporal evolution of the active prominence eruption seen in TRACE. The contours show the 12--25 keV hard X-ray emission from RHESSI. The apex of the filament lies approximately 80 Mm above the solar surface.

Citation is: Alexander, D, Liu, R. and Gilbert, H. R.: 2006, Astrophys. J., 653, 719.