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